Patent ID: 12258288

In describing the preferred embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description.

Turning initially toFIG.1, one embodiment of a water treatment system500with integrated biogas treatment510and chlorine disinfection520is illustrated. Initial steps in the water treatment system500may include, for example, sedimentation, biological, and filtration processes. Still other steps may be included, but are not shown for ease of illustration. A flow of secondary treated wastewater525is received at a first basin530. Optionally, the secondary treated wastewater525may result from some or all of the initial steps discussed above being performed in the first basin on water to be treated. A first portion532of the secondary treated wastewater525is provided to the biogas treatment process510, and a second portion534of the secondary treated wastewater525is provided to the chlorine disinfection process520which is illustrated as occurring in a second basin540.

With reference also toFIGS.2-4, it is contemplated that the present invention may be integrated into various configurations of water treatment systems. InFIG.2, the secondary treated wastewater525is again supplied to or produced within the first basin530. A first portion532of the secondary treated wastewater is supplied to the biogas treatment process510and a second portion534of the secondary treated wastewater is provided to the second basin540for chlorine disinfection. InFIGS.3and4, it is contemplated that the first basin may be a prior basin or other reservoir in the wastewater treatment process and the first basin is not expressly shown. Rather the secondary treated wastewater525is supplied from the earlier steps in the water treatment system500which are not shown and provided to the illustrated portions of the water treatment system. InFIG.3, it is contemplated that the entire supply of secondary treated wastewater525may be provided as the second portion534to the second basin540. A secondary supply of water536is provided to the biogas treatment process510rather than diverting a first portion532from the secondary treated wastewater525. InFIG.4, again there is no explicitly illustrated basin for the secondary treated wastewater525, however, the secondary treated wastewater525supply is again divided into a first portion532supplied to the biogas treatment process510and a second portion534supplied to the chlorine disinfection process. Because the output of the biogas treatment process is a carbon dioxide water stream518, as discussed in more detail below, where the carbon dioxide water stream518includes water from either the first portion532of the secondary treated wastewater525or from the secondary supply of water536and carbon dioxide dissolved into that water, the first portion532of secondary treated wastewater525that is initially diverted through the biogas treatment510will still be disinfected in the chlorine disinfection process520when the carbon dioxide water stream518enters the second basin540. Although discussed above, secondary treated wastewater525in this description is not intended to be limiting. It is understood that the secondary treated wastewater525may be any water having a high pH value which requires a reduction in the pH level via the regulation process described in more detail below.

AlthoughFIGS.1-4illustrate different embodiments of a water treatment system500integrating biogas treatment510and chloring disinfection520, these embodiments are intended to be illustrative and not exhaustive of different configurations of water treatment systems500. It is contemplated that different arrangements of water treatment systems500may be utilized and the features shown in the illustrated embodiments may be arranged in different combinations without deviating from the scope of the invention.

With reference again toFIG.1, the chlorine disinfection process520receives chlorine at a chlorine injection point550from a chlorine supply. It is contemplated that the chlorine supply may be of multiple different forms, where chlorine is part of a chemical compound or supplied as chlorine gas. However, according to one embodiment the chlorine supply is provided in a mixture form of either sodium hypochlorite (NaClO) or as calcium hypochlorite (Ca(OCl)2). Both sodium hypochlorite supplied in a liquid form and calcium hypochlorite supplied in a solid form are relatively stable and are safer to use for the disinfection process520. Both are less concentrated than pure chlorine gas. As a result, the liquid or solid mixtures do not emit airborne chlorine as readily as pure chlorine gas, reducing the risk to personnel operating the disinfection process. When the chlorine gas, sodium hypochlorite, or calcium hypochlorite is added to water, they generate concentrations of both hypochlorous acid and hypochlorite. With reference to Table 1 below and toFIG.5, the relative concentrations of hypochlorous acid and hypochlorite are shown as the percentage of overall chlorine present for disinfection as a function of the pH level of the water to be treated.

TABLE 1Exemplary Percentages of Hypochlorous acid(HOCl) and Hypochlorite (OCl−)% HOCl% OCl−% HOCl% OCl−pH32° F. (0° C.)32° F. (0° C.)68° F. (20° C.)68° F. (20° C.)4100.00.0100.00.05100.00.097.72.3698.21.896.83.2783.316.775.224.8832.367.823.276.894.595.52.997.1100.599.50.399.7110.0599.950.0399.97

As illustrated inFIG.5and Table 1 above, the percent concentration of hypochlorous acid and hypochlorite generated when the chlorine is injected into the water to be treated is about equal when the water has a pH level of about 7.5. As the pH level decreases, the percent concentration of hypochlorous acid increases and the percent concentration of hypochlorite decreases. The shift in concentration of hypochlorous acid versus hypochlorite is significant because the effectiveness of the two compounds for disinfection is substantially different. Hypochlorous acid has about twenty to one hundred (20-100) times the effectiveness of killing pathogens or causing the pathogens to become inactive when compared to a similar concentration of hypochlorite in water. Consequently, a smaller volume of hypochlorous acid than hypochlorite is required to provide the same level of disinfection of the water to be treated.

As also illustrated inFIG.5and Table 1 above, when the chlorine supply is injected into the water to be treated, the chlorine will take the form of either the hypochlorous acid or hypochlorite and their respective from of the chlorine will be dependent on the pH level of the water. The concentration of hypochlorous acid and hypochlorite will vary as the pH level rises above or goes below 7.5, but the combination of the two pH dependent forms of chlorine will always sum to one hundred percent. Within a range of pH levels from about 6 to about 9, the concentrations change from almost entirely generating one form of chlorine to almost entirely generating the otherform of chlorine in water. For example, at a pH level of 6, the concentration of hypochlorous acid generated as a percent of this first available chlorine form is about ninety eight percent (98%) and the concentration of hypochlorite generated as a percent of this available chlorine form is about two percent (2%). Conversely, at a pH level of 9, the concentration of hypochlorous acid generated as a percent of the available chlorine form is about four percent (4%) and the concentration of hypochlorite generated as a percent of this second available chlorine form is about ninety-six percent (96%). Thus, a small change in the pH level of the water to be treated can have a significant impact on the effectiveness of the disinfection process.

Additionally, a step in the water treatment process500which is not illustrated inFIG.1, involves neutralization of residual chlorine in the treated water560discharged from the chlorine disinfection process520. Neutralization of residual chlorine is performed by the addition of another chemical, such as sodium bisulfite (NaHSO3) or sulfur dioxide gas (SO2). The amount of sodium bisulfite or sulfur dioxide gas required is proportional to the residual amount of chlorine present in the effluent discharge. As a result, if the pH level is controlled to maximize generation of hypochlorous acid, thereby minimizing the amount of sodium or calcium hypochlorite which is required, the amount of sodium bisulfite or sulfur dioxide required to neutralize residual chlorine can be similarly minimized. Thus, regulating the level of pH in the secondary treated wastewater525to be disinfected improves efficiency of the disinfection process520and reduces the volume of chemicals required, and therefore the cost, both at the initial stage (injection of chlorine) and potentially at dichlorination (removal of residual chlorine), the final step of the disinfection process.

Turning next toFIGS.2and3, it is contemplated that the chlorine injection point550may be provided at various locations in the water treatment system500. As shown inFIG.2, the chlorine injection point550is provided at the carbon dioxide water stream518being transferred between the biogas treatment process510and the chlorine disinfection process520. As shown inFIG.3, the chlorine injection point550is at the biogas treatment process510. Injection of the chlorine directly into the carbon dioxide water stream518or into the biogas treatment process510which ultimately generates the carbon dioxide water stream518such that the chlorine is delivered in tandem with the carbon dioxide water stream518may provide additional benefits of hypochlorous acid generation. As discussed in more detail below, the secondary treated wastewater525may have an initial pH in a range between seven and nine (7-9). The carbon dioxide water stream518may have a pH in the range of five to six and one-half (5-6.5). While the carbon dioxide water stream518is used to lower the pH range of the secondary treated wastewater525, injecting the chlorine directly into the carbon dioxide water stream518allows generation of hypochlorous acid where the pH level in the carbon dioxide water stream may result in nearly one hundred percent (100%) of the available chlorine to form the hypochlorous acid. In addition, the biogas treatment system510may utilize a long pipe or coil of piping in the water wash process, which may range up to five hundred feet long and result in contact with the water during the biogas treatment process for times up to or exceeding one to two minutes. This extended period of contact with the water forming the carbon dioxide water stream provides additional mixing time and additional time for the sodium or calcium hypochlorite to generate hypochlorous acid before introduction into the second basin540, providing more thorough and more efficient usage of the sodium hypochlorite or calcium hypochlorite.

Turning again toFIG.1, a pH sensor580is included in the second basin540for the disinfection process520. The pH sensor580generates a signal582corresponding to the pH level of the water present in the second basin540. The signal582is provided to a controller570which is configured to regulate the pH level in the second basin540during the disinfection process520. According to one embodiment of the invention, the controller570is an industrial controller such as a programmable logic controller (PLC). The controller includes a control program, or series of instructions, stored in non-transitory memory, such as a hard-drive, an optical drive, a magnetic drive, a solid-state drive, or the like. The control program, or series of instructions, is executed by a processor in the PLC. It is contemplated that a single storage device, or multiple storage devices, may be provided in the PLC. Similarly, a single processor or multiple processors or processing cores, configured to execute synchronously or asynchronously, may be provided. The industrial controller570receives input signals, such as the signal corresponding to the pH level in the second basin540, and generates output signals to control operation of the water treatment system500. It is further contemplated that a single controller570may control the entire system or separate controllers570may be provided at different portions of the water treatment system500. For example, a first controller570may control the biogas treatment process510, a second controller570may control upstream steps in the water treatment process, and a third controller570may control the disinfection process520and manage the pH level in the second basin540. Each controller570may be in communication with the other controller to transfer signals corresponding to operation of the water treatment system500as required by each controller.

According to the embodiment illustrated inFIG.1, it is contemplated that a pump and valve or other flow control device may be provided at the output of the biogas treatment process510or along the piping carrying the carbon dioxide water stream518from the biogas water treatment process to the disinfection process520. The controller570receives the signal corresponding to the pH level in the second basin540and adjusts a flow rate of the carbon dioxide water stream518to achieve a desired pH level in the second basin540. As discussed in more detail below, it is contemplated that the carbon dioxide water stream518may be pressurized and/or chilled as a result of the biogas treatment process510. Pressurization and/or refrigeration of the water used in the biogas treatment process510allows for supersaturation of carbon dioxide in the carbon dioxide water stream518. Different pressure levels and temperatures as well as differing amounts of carbon dioxide being removed from the biogas during the biogas treatment process510may result in varying levels of carbon dioxide present in the carbon dioxide water stream518. A second pH sensor584may be provided to measure the PH level of the carbon dioxide water stream518. The second pH sensor584also generates a signal586provided to the controller570, and the controller570may further adjust the flow rate of the carbon dioxide water stream518as a function of both the pH level in the second basin540and of the pH level of the carbon dioxide water stream518.

Alternately, or in addition, to the pH sensor584one or more additional sensors may be provided to measure process variables. For example, a sensor such as an oxidation reduction potential (ORP) sensor may be used to determine the relative amount of chlorine disinfection capability present in the water. An ORP sensor in a chlorine disinfection system provides a scaled indication of the amount of hypochlorous acid present, which is directly related to the pH level of the water being treated. The higher the value of a feedback signal from the ORP the greater the oxidation potential for disinfection purposes and the lower the value of the pH. As a result, the feedback signal from the ORP sensor may be provided to the controller570in addition or in place of the feedback signal from a pH sensor584. The feedback signal for the ORP sensor corresponds to a disinfecting strength present in the second basin540as it detects the ability of a solution to act as a reducing agent which is a function of the relative levels of hypochlorous acid and hypochlorite as illustrate inFIG.5. The feedback signal from the ORP sensor may be used by itself or combination with the feedback signal from the pH sensor to control injection of the carbon dioxide water stream518into the second basin540to increase the level of hypochlorous acid as a percent of the available chlorine available for disinfection.

With reference again toFIG.2, it is contemplated that the carbon dioxide water stream518may be delivered into the second basin540with multiple injection devices or nozzles590. Each nozzle590is located at a different depth within the second basin540. When the carbon dioxide water stream518enters the water to be treated, the carbon dioxide is released from carbon dioxide water stream518. The amount of carbon dioxide carried within the carbon dioxide water stream518will vary as a function of the pressure at which the carbon dioxide water stream518is maintained. When the carbon dioxide water stream518is released into the basin540, the water in the basin to be treated is no longer under pressure and the extra carbon dioxide is released from the pressurized stream in the form of small gaseous bubbles within the water to be treated. The gaseous bubbles of carbon dioxide rise to the surface of the basin. A portion of the carbon dioxide mixes into the secondary treated wastewater525present in the second basin540and a portion of the carbon dioxide is released into the atmosphere. The efficiency of the mixing process is controlled in part by the amount of time the carbon dioxide is present in the basin540. Therefore, releasing carbon dioxide at a greater depth allows more time for the carbon dioxide to be in contact with and, therefore, to mix and absorb into the water in the basin540, causing a greater reduction in the pH level of the secondary treated wastewater525present in the second basin540. One or more injectors, or nozzles, can be used at different depths within the basin540or, alternately, a nozzle at or above the surface of the basin may be activated to inject or spray the secondary treated wastewater525into the second basin540as a function of the pH level in the basin540and/or the pH level present in the secondary treated wastewater525. The controller570may receive the feedback signal, or signals,582,586corresponding to the pH level in the second basin or in the carbon dioxide water stream and regulate operation of the nozzles to achieve a desired pH level.

According to still another aspect of the invention, it is contemplated that not all of the carbon dioxide water stream518produced by the biogas treatment510process may be required to regulate the pH level in the disinfection process520to a desired pH level. As discussed in more detail below, various embodiments of the biogas treatment process510may be configured to release excess carbon dioxide. The controller570may receive the feedback signal, or signals,582,586corresponding to the pH level in the second basin or in the carbon dioxide water stream and regulate operation of the carbon dioxide aeration or stripping process300to achieve a desired pH level.

Integration of the biogas process510into a water treatment system500results in the production of a purified methane stream, as discussed in more detail below, and a more efficient water treatment process by use of carbon dioxide supersaturated in water, which is a by-product of the biogas process510. According to an exemplary application, a wastewater treatment plant anaerobic digester produces 250 cubic feet per minute (cfm) of biogas where the biogas has a typical carbon dioxide (CO2) concentration of thirty-five percent (35%) on a volume basis and the balance of the biogas is methane. Based on the CO2 gas concentration and rate of production of biogas, the anaerobic digester would produce the equivalent of about 14,500 pounds (lbs.) per day or 7.25 tons per day of CO2. In this example, about ninety-five percent (95%) of the CO2 gas from the raw biogas is dissolved by the water wash process resulting in about 13,100 lbs or 6.5 tons per day of CO2 available for pH adjustment in the downstream chlorination disinfection process520. However, in an exemplary application, about seventy-five percent of the dissolved CO2 water may mix effectively to adjust the pH level in the disinfection process. As a result, about 9,800 lbs. or 4.9 tons of CO2 per day can be beneficially utilized from this biogas treatment process510.

The controller570measures a pH level in the disinfection process520of about 7.4, which results in approximately an even division of hypochlorous acid and hypochlorite being generated as available chlorine for disinfection. For the exemplary application, a desired pH level of 6.8 is selected. Based on chemical equilibrium and assuming an alkalinity of 100 mg/l in the secondary treated wastewater525, about 26 mg/l of additional CO2 is needed to lower the pH from 7.4 to 6.8 at a temperature of 65 deg F. assuming the secondary treated wastewater has an initial CO2 concentration of 8 mg/l. With reference to Table 1 andFIG.5, lowering the pH level in the secondary treated wastewater from 7.4 to 6.8 would result in an effective increase in hypochlorous acid availability from about 55% to 80%. Based on the total additional 9,800 lbs per day of dissolved CO2 from this integrated biogas treatment process510, adjusting the pH level from 7.4 to 6.8, and using a pre-determined chlorine dosage rate (for that particular pH), about 45.19 million gallons per day of secondary treated wastewater525could be treated.

The above-described minor adjustment in the pH level allows for approximately a twenty-five percent (25%) reduction in the amount of chlorine required to achieve the desired disinfection due to increased levels of hypochlorous acid present. In addition, there could be approximately a twenty-five percent (25%) reduction in the amount of sodium bisulfite required to neutralize residual chlorine. This reduction in the pH level by using the carbon dioxide water stream518can result in significant annual cost savings for disinfection chemicals used in the water treatment plant operations.

Turning next toFIG.6, an exemplary biogas treatment system utilized in one embodiment of the present invention is illustrated. A biogas stream10is provided as an input to the system, where the biogas may be produced, for example, from an anaerobic decomposition process. The anaerobic decomposition process may, for example, convert food waste, sewage, animal manure, landfill waste and the like into biogas. The biogas primarily includes methane and carbon dioxide with a lesser percentage of other constituents, such as nitrogen, oxygen, and hydrogen sulfide. Methane is typically present in a concentration of fifty to sixty-five percent (50-65%) by volume and carbon dioxide is typically present in a concentration of thirty-five to fifty-five percent (35-50%) by volume. The disclosed water wash process employed by the biogas treatment system removes the carbon dioxide and other trace constituents, such as hydrogen sulfide and siloxanes, resulting in a purified biogas stream having a methane content of up to about ninety-eight percent (98%) and carbon dioxide content to about two percent (2%). The resulting purified biogas stream may be used as a replacement fuel for natural gas, for example, in a compressed natural gas vehicle engine or other natural gas fuel energy applications. Although the invention will be discussed with respect to a water wash process for treating biogas, it is understood that the system may be used to treat other gas mixtures in which the relative solubility of one gas in the mixture is substantially higher than the other gas in the mixture.

Some initial processing of the biogas stream may occur prior to supplying the biogas stream to the water wash system. An optional hydrogen sulfide (H2S) removal process15such as an iron sponge type system may be inserted in series with the biogas stream10to perform an initial removal of hydrogen sulfide present in the biogas stream. Because hydrogen sulfide is corrosive, removal of the gas at an initial stage limits the effects of the gas on the system components through the water wash process. Optionally, hydrogen sulfide may be removed in the off-gas exhaust output from the stripping process. The biogas stream may also be passed through a filter20to remove particulate content. In addition, carbon dioxide has increased solubility characteristics with decreasing temperature and increasing pressure. The biogas stream is, therefore, passed through a compressor25to achieve an elevated pressure. The pressure range of the compressed biogas stream30may be between ten and two hundred pounds per square inch gauge (10-200 psig). According to one embodiment of the invention, the pressure range of the compressed gas is between about sixty and one hundred pounds per square inch gauge (60-100 psig). The compressed biogas may also be chilled, for example, to between thirty-five and sixty-eight degrees Fahrenheit (35-68° F.). The compressed and/or chilled biogas stream30is provided as an input to the water wash process.

The water wash process utilizes water to remove the carbon dioxide from the biogas stream. According to the illustrated embodiment, water is provided to a holding tank40from which a water stream50is provided to the water wash process. Water provided to the holding tank40may be chilled and/or under pressure to facilitate the water wash process. As discussed above, secondary treated wastewater525from a water treatment system500in which the biogas treatment process is integrated may be a source of water. The secondary treated wastewater525may be supplied to the holding tank40or directly input to the first riser110. Optionally, the holding tank40may incorporate a chiller and/or a compressor to chill or pressurize the water prior to supplying it in the water stream. The water, for example, may be chilled to between thirty-five and sixty-eight degrees Fahrenheit (35-68° F.) and pressurized to mix with the compressed biogas stream30at about the same input pressure of the compressed biogas stream. The carbon dioxide has significantly more solubility in water than methane and the solubility is further improved with increased pressure and reduced temperature. Thus, providing a chilled and/or pressurized water stream50and a compressed and/or chilled biogas stream30into the absorption risers110enhance the absorption of carbon dioxide from the biogas and into the water and, thereby also provide a super-saturated carbon dioxide water stream518from the biogas treatment process510.

Although discussed herein with respect to a water-wash process, it is contemplated that a carbon dioxide water stream518may be generated from any suitable biogas treatment process. For example, a membrane separation process may receive a stream of biogas as an input and pass this stream of biogas through a membrane. The membrane separates the methane from the carbon dioxide. The resulting carbon dioxide may be channeled to the pH reduction process discussed above. According to one embodiment, the carbon dioxide resulting from the membrane separation process may be introduced into water to form a carbon dioxide water stream518. Additionally, the water into which the carbon dioxide is introduced may be pressurized and/or chilled to generate a super-saturated carbon dioxide water stream518.

The water wash process begins with an absorption process100that has multiple absorption risers110operatively connected together to remove the carbon dioxide from the compressed biogas stream30. Referring also toFIG.10, each absorption riser110includes multiple pipes. In the illustrated embodiment, the absorption riser110includes an outer pipe112, a first inner pipe122, and a second inner pipe132. According to the illustrated embodiment, each of the pipes is concentric to the others. Optionally, the first inner pipe122and the second inner pipe132may be positioned adjacent to each other or extend downward at different locations within the outer pipe112. The outer pipe112has a first end114, a second end116, and a first length, L1. The first inner pipe122has a first end124, a second end126, and a second length, L2. The second inner pipe132has a first end134, a second end136, and a third length, L3. According to one embodiment of the invention, each of the absorption risers110are installed in a vertical orientation, such that the first ends114,124,134of each pipe112,122,132are generally positioned at the top of each absorption riser110. The first inner pipe122extends for the second length, L2into the outer pipe112such that the compressed biogas stream30may be delivered into a lower segment of the absorption riser110. According to the illustrated embodiment, the first inner pipe122is cylindrical and open at the second end126. The compressed biogas stream30flows from the first inlet140and exits at the second end126of the first inner piper122. The second inner pipe132extends for the third length, L3, through the first inner piper122, beyond the second end126of the first inner pipe122, and into the outer pipe112. The second inner pipe132is cylindrical and the second end136of the second inner pipe132includes a check valve between the interior of the outer pipe112and the interior of the second inner pipe132.

Each absorption riser110includes a set of inlets and outlets to allow water and biogas to flow into and out of the riser110. A first inlet140receives the compressed biogas stream30and is located on the first end114of the outer pipe112. The first inlet140is in fluid communication with the first end124of the first inner pipe122and establishes a flow path for the compressed biogas stream30into the absorption riser110. The first inner pipe122extends into the absorption riser110for the length, L2, of the inner pipe122. According to the embodiment illustrated inFIG.10, the second end126of the first inner pipe122terminates at a dispersion element144proximate the second end116of the first inner pipe122. A second inlet145receives the water stream50and is located on the first end114of the outer pipe112. The second inlet145is in fluid communication with the first end114of the outer pipe112to dispense the water stream50from the top of the absorption riser110. As will be discussed in more detail below, the water stream50is dispensed at the top of the interior of the absorption riser110via the second inlet145and the compressed biogas stream30is dispensed at the bottom of the interior of the absorption riser110via the first inner piper122, and the compressed biogas stream30passes up through the water stream50within the absorption riser110. As the water stream50falls to the bottom of the absorption riser110it mixes with the biogas stream and the carbon dioxide within the compressed biogas stream30is dissolved into the water. Although small amounts of methane may be absorbed in the water, the majority of the methane remains unabsorbed and rises to the top of the absorption riser110. Because carbon dioxide is removed from the compressed biogas stream30as it interacts with the water stream50, the flow of biogas resulting from mixing with the water will be referred to herein as a purified biogas stream162. Similarly, because the water stream50absorbs carbon dioxide from the compressed biogas stream30, the resulting water stream will be referred to herein as a mixed water stream166.

According to the embodiment illustrated inFIG.11, the second end126of the first inner pipe122simply terminates within the outer pipe112without a dispersion element144located proximate the second end126of the first inner pipe122. The second end126of the first inner pipe122may be configured to disperse the compressed biogas stream30into water flowing within the outer pipe112. The dispersion may be achieved, for example, via a series of holes127located along the length of the first inner pipe122as shown inFIGS.20-22, via a nozzle, or series of nozzles positioned at the second end126, or via other dispersion methods which are integrally formed with the first inner pipe122. Optionally, a series of nozzles may be located along the length of the first inner pipe122, where each nozzle disperses a portion of the biogas stream within the outer pipe112. According to still another option, multiple inner pipes122may be provided, where each inner pipe122includes a series of holes127or nozzles spaced along the length of each inner pipe122to facilitate dispersion of the biogas stream30throughout the interior of the outer pipe112. It is contemplated that an optional dispersion element146may still be located within the outer pipe112at a location between the inlet of the water stream and the inlet of the compressed biogas stream within the absorption riser110if desired for further mixing of the two streams.

A first outlet160located at the first end114of the outer pipe112provides a flow path161for the purified biogas stream162to exit the absorption riser. The first outlet160is in fluid communication with and receives the purified biogas stream162from the interior of the outer pipe112. A second outlet165is also located at the first end114of the outer pipe112and provides a flow path167for the mixed water stream166. The second outlet165is in fluid communication with the first end134of the second inner pipe132. The mixed water stream166enters the second end136of the second inner pipe132and travels up through the second inner pipe132to the second outlet165. According to the illustrated embodiment, each of the outer pipe112, first inner pipe122, and second inner pipe132are concentric about a central axis. The second inner pipe132is located within the first inner pipe122, which is, in turn, located within the outer pipe112. As discussed above and for purposes of illustration inFIG.10, the first end114,124,134of each pipe112,122,132ends at substantially the same point. It is contemplated that in various embodiments the first end124,134of each of the first inner pipe122and the second inner pipe132may extend for a short distance beyond the first end114of the outer pipe112to facilitate connections between each pipe and an inlet or outlet. It is further contemplated that an inlet140,145or outlet160,165may be positioned along and enter the outer pipe112via a side wall proximate the end of the absorption riser110. For example, the first inlet140is shown connecting generally orthogonally to a wall of the first inner pipe122beyond the first end114of the outer pipe and the second inner pipe132extends through an end wall of the first inner pipe122to connect to the second outlet165. Alternately, the first inlet140or second outlet165may include a fixture connected to the first end114of the outer pipe112and comprise the necessary connections to establish the fluid flow paths from the inlet and outlet to the inner pipes extending into the outer pipe112.

With reference again toFIG.10, each absorption riser110may also include one or more dispersion elements located within the flow path to facilitate mixing of the compressed gas stream30with the water stream50. A first dispersion element149is located in the flow path147of the water stream50as it exits the second inlet145, and a second dispersion element144is located in the flow path142of the compressed gas stream30as it exits the second end126of the first inner pipe122. Each dispersion element144,149is operable to distribute either the compressed gas stream30or the water stream50throughout the interior of the outer pipe112. According to the illustrated embodiment, each dispersion element144,149is a diffuser plate, where the diffuser plate extends around the first inner pipe122, forming a disk within the interior or the outer pipe112. The diffuser plate includes multiple holes extending through the plate which allow the water and gas to flow through. The holes are distributed around the surface of the disk such that water and gas flow through and are distributed throughout the interior of the outer pipe112.

With reference again toFIG.11, it is contemplated that one or more of the dispersion elements are optionally included within the absorption riser. It is contemplated that other methods of distributing the compressed gas stream30and/or the water stream50within the absorption riser may be utilized without deviating from the scope of the invention. For example, one or more sparging tubes may be operatively connected to the second inlet145or to the second end126of the first inner pipe122and arranged within the interior of the outer pipe112to distribute the water and gas throughout the interior of the outer pipe112. According to still another embodiment, spray nozzles may be operatively connected to the second inlet145or to the second end126of the first inner pipe122to discharge the water and gas as a mist throughout the interior of the outer pipe112. An additional dispersion element146, may be included within the combined streams if desired for further mixing of the water stream50with the compressed gas stream30. According to still other embodiments, various combinations of dispersion elements may be utilized. Each dispersion element distributes the water and gas in finer jets, flows, or droplets to increase the surface area of water and gas present within the outer pipe112. The increased surface area of water and gas increases the area at which the water and gas may contact each other and thereby increasing the area across which carbon dioxide may transfer from the compressed biogas stream30to the water stream50.

It is further contemplated that each absorption riser may include packing material within at least a portion of the interior of the outer pipe112to further enhance the mixing of the compressed biogas stream30with the water stream50. InFIG.10, an additional dispersion plate146is shown. One or more additional dispersion plates146may be distributed along the length of the interior of the outer pipe112to continually redistribute the gas and water as they travel through the interior of the pipe. With reference also toFIGS.14and15, other packing material may be inserted into the outer pipe112. InFIG.14, a flexible material170is rolled into a coil and inserted between the inner periphery of the outer pipe112and the outer periphery of the first inner pipe122. According to one embodiment of the invention, the flexible material170is a netting material, such as a geonet, including multiple holes throughout the material. As the water and gas pass through the absorption riser110, the netting and the multiple holes create numerous flow paths and opportunities for collisions and, thereby, increasing contact surface area between the water and biogas for transfer of the carbon dioxide from the biogas to the water. InFIG.15, a mesh material180may be formed into a basket or bag and is used to contain another bulk material182within the mesh. The bulk material is preferably a material that allows the water and gas to flow through while increasing contact between the water and gas. Optionally, the bulk material may be a medium that has absorptive characteristics such as activated carbon or zeolites which may further aid in the removal of trace constituents from the compressed biogas stream30. The mesh and bulk materials180,182may be inserted into and removed from the interior of the outer pipe112as a unit. Both the flexible material170and the mesh and bulk material combination180,182facilitate cleaning of the packing material. The flexible material170may be removed and unrolled for cleaning. The mesh and bulk material180,182may be pulled out of the outer pipe112and the bulk material spread out for cleaning. Once clean, the flexible material170may be rolled back into a coil and inserted back into the outer pipe112. Similarly, the bulk material182may be placed back into the mesh material180and inserted into the outer pipe112.

With reference again toFIG.6, it is contemplated that multiple absorption risers110may be connected in series. The effect of connecting the absorption risers110in series is to create an overall longer length of pipe greater than the length of a single riser through which the compressed biogas stream30interacts with the water stream50, allowing for a greater concentration of carbon dioxide to be transferred from the compressed biogas stream30to the water stream50. One of the absorption risers110is designated as an initial absorption riser in the system and receives the initial input of the compressed biogas stream at the first inlet140and the water stream50at the second inlet145. The first outlet160of the initial absorption riser is connected to the first inlet140of another absorption riser110and the second outlet165of the initial absorption riser is connected to the second inlet145of the other absorption riser110. This sequence of connections repeats for each absorption riser in the system until a final absorption riser is reached. At the final absorption riser, the first inlet140still receives the biogas stream from the first outlet160of the preceding absorption riser and the second inlet145receives the water stream from the second outlet165of the preceding riser. However, the first outlet160of the final absorption riser provides the purified biogas stream162and the second outlet165of the final absorption riser provides the mixed water stream166. As the biogas and water streams progress through each absorption riser, the concentration of carbon dioxide in the biogas stream is incrementally reduced and the concentration of carbon dioxide in the water stream is incrementally increased from the starting level at the initial absorption riser to the final levels at the final absorption riser.

With reference next toFIG.7, it is also contemplated that multiple absorption risers110may be connected in parallel. Each of the compressed biogas stream30and the water stream50are split and portions of each stream are supplied to each riser. As illustrated, the compressed biogas stream30is provided to the first inlet140of each absorption riser110, and the water stream50is provided to the second inlet145of each absorption riser110. The first outlet160of each absorption riser is connected to a junction at which the purified biogas stream162from each absorption riser is combined and delivered from the system. Similarly, the second outlet165of each absorption riser is connected to a second junction at which the mixed water stream166from each absorption riser is combined and may be transferred for further processing. To achieve comparable purifying performance to the serial connection discussed above, the volume of biogas introduced into each absorption riser110may be split between each absorption riser while the volume of water introduced into each riser remains the same. Thus, a greater volume of water per unit is available for interaction with the same volume of biogas, allowing a greater percentage of the carbon dioxide to be removed in a single absorption riser than when the entire flow of biogas enters a single riser.

According to still another aspect of the invention, it is contemplated that the absorption risers110may be connected in a combination of serial and parallel connections. For example, two or three absorption risers110may be connected in series as a set of absorption risers with multiple sets of absorption risers connected in parallel. Alternately, the biogas stream30may enter a first absorption riser110and pass through subsequent absorption risers in series and the water stream50may be supplied to the absorption risers in parallel, thereby maximizing the transfer of carbon dioxide from the biogas stream to the water stream at each riser.

In addition to determining whether to connect the absorption risers110in series or parallel, a number of other design criteria are considered when configuring the water wash system. As previously discussed, the gas and/or water stream may be cooled or compressed. Further, the diameter and length of each absorption riser110is evaluated. In addition, the material from which the absorption riser is constructed must be determined.

Existing water wash systems typically utilize a single stainless steel vessel with a height ranging from twenty to sixty feet and a diameter up to six feet. The size of the vessel, the materials from which it is constructed and the weight of the water and biogas within the vessel further requires structural considerations such as a reinforced concrete footing to support the weight and horizontal stabilization members to prevent tipping.

In contrast, the absorption risers110of the present system are constructed from a non-metallic material and, preferably, are constructed of a plastic or reinforced resin material. According to one embodiment of the invention, the risers are made from a polyethylene material, such as high density polyethylene (HDPE) or medium density polyethylene (MDPE), or from a polyimide material. Optionally, the risers may be made from polyvinyl chloride (PVC) or fiberglass. The materials are lighter and less expensive than existing materials, reducing system costs and making construction easier.

With reference next toFIG.16, an exemplary installation of one embodiment of the present invention at a farm is illustrated. The farm includes an anaerobic digester11to break down animal waste created on the farm. The raw biogas stream10output from the anaerobic digester11is provided to an initial processing stage12. With reference also toFIG.6, the initial processing stage12may include the hydrogen sulfide cleaner15, filter20, and compressor25. The initial processing stage, therefore, removes hydrogen sulfide from the raw biogas stream10and then filters and compresses the biogas stream, providing a compressed biogas stream30to a series of absorption risers110, a flash riser210, and an air stripping riser310.

Each of the risers110,210,310are installed in a trench13and substantially below grade. The diameter of each riser is preferably in the range of four to thirty inches (4-48 in.) and the length may be, for example twenty feet (20 ft.). The trench may be dug using conventional excavation methods and each riser inserted within the trench. Optionally, an auger may be used to drill individual holes into the ground and each riser is inserted into one of the holes. The top of each riser is at or above grade to provide for connection of tubing and fittings for transmitting biogas and/or water to and from each riser. After each riser is installed within the trench13or hole, the trench or hole may be back-filled so the earth surrounds each riser. The earth surrounding each riser provides a number of benefits, such as protection from ultraviolet radiation in outdoor installations, insulation for the chilled water, and physical support for each riser when it is filled with biogas and water. In alternate embodiments of the invention, it is contemplated that the risers may be installed below grade, above grade, or a combination thereof. When either a portion or all of a riser is installed above grade, it is contemplated that one or more exterior sleeves may cover the portion of the riser above grade. Each sleeve may provide ultraviolet (UV) ray protection, insulation, support, or a combination thereof for the portion of the riser that is above grade and no longer protected, insulated, or supported by the ground. According to still other embodiments of the invention, a riser may be submerged in water, where the water similarly provides some UV ray protection, insulation and support for the submerged risers. Optionally, one or more exterior sleeves may be used in combination with submerging each riser to further protect, insulate, or support each riser.

As the name implies, the water wash system requires a supply of water by which the carbon dioxide is removed from the biogas stream. In some applications, such as a waste water treatment system, there may be a continuous supply of water, such as the secondary treated wastewater525. In the illustrated embodiment, a holding tank40is provided to supply the water. Water may be drawn from a pond or lake or otherwise be supplied from a well or from a municipal water supply. As previously discussed, the water may be chilled and/or compressed prior to being pumped to the absorption riser110.

The water stream50and compressed gas streams each enter the top of each absorption risers110in a series arrangement as also shown inFIG.6. A portion of the carbon dioxide is transferred from the compressed biogas stream30to the water stream50in each absorption riser110. The compressed biogas stream30travels down a pipe to the lower portion of the absorption riser and the water stream50enters the top of the absorption riser. The compressed biogas rises and the water falls within each absorption riser110, creating contact between the two streams. The partly purified biogas stream exits a first outlet160at the top of the initial absorption riser, and the mixed water stream166is internally pumped from the bottom of the absorption riser110to the top and exits a second outlet165also at the top of the absorption riser. Each subsequent absorption riser110in the series receives the partly purified biogas stream and mixed water stream from the prior absorption riser at the inlets and transfers additional carbon dioxide from the biogas stream to the water stream. The final absorption riser110contains the purified biogas stream which exits at the first outlet160. According to the illustrated embodiment, the purified biogas stream162is provided to a storage tank14from which it may be used as a fuel. According to other embodiments and as illustrated inFIGS.6-9, the purified biogas stream162may undergo some additional processing prior to use. For example, a first moisture removal vessel26and/or a subsequent desiccant dryer27may be provided to remove water from the purified biogas stream162. Still other processing steps may be provided for polishing the gas to remove, for example, trace constituents or additional carbon dioxide still remaining in the biogas stream162.

With reference also toFIGS.25and26, two exemplary polishing processes28are illustrated. InFIG.25, the polishing process28uses a liquid polishing agent82which is a carbon dioxide (CO2) absorbent. According to one aspect of the invention, the polishing agent is an alkali material comprised of a hydroxide compound such as calcium hydroxide, sodium hydroxide, or potassium hydroxide. Optionally, the polishing agent may be an oxide compound such as calcium oxide. For purposes of discussion calcium hydroxide will be discussed in the form of a liquid polishing agent82, however, this is not intended to be limiting. The calcium hydroxide is introduced into a first holding tank80. The calcium hydroxide may be delivered directly to the first holding tank80in a liquid form as a mixture or solution. Optionally, the calcium hydroxide may be first introduced into the holding tank80in a solid form, for example, as powder and water, or other suitable liquid carrier, also introduced into the holding tank80. A mixer may be used to combine the granules and the water into a solution suitable for delivery to the polishing tank85. The liquid calcium hydroxide82is then delivered to an inlet83on the polishing tank85.

Within the polishing tank85, the polishing agent interacts with the purified biogas stream162to further remove any CO2 remaining in the biogas. The purified biogas stream162is delivered from the absorption riser110to the polishing tank85via a second inlet31. Within the polishing tank85a perforated pipe88may be used to distribute biogas87throughout the polishing tank85. Optionally, other distribution methods such as a nozzle or mixing element may be located within the polishing tank85to distribute the biogas87. The biogas87interacts with the calcium hydroxide, Ca(OH)2, to remove CO2 remaining in the biogas87. The calcium hydroxide, Ca(OH)2, interacts with the carbon dioxide (CO2) to generate calcium carbonate, CaCO3, and water, H20, forming a slurry89that settles to the bottom of the polishing tank85. Further refined biogas87rises to the top of the polishing tank85and exits via an outlet33to storage or to a dryer27, if present.

InFIG.26, the polishing process28uses a solid polishing agent92. Similar to the liquid polishing agent82, the solid polishing agent is a hydroxide compound such as calcium hydroxide, sodium hydroxide, or potassium hydroxide. For purposes of discussion calcium hydroxide will again be discussed as the solid polishing agent92, however, this is not intended to be limiting. A cannister90is provided in which the solid polishing agent92is located. It is contemplated that the cannister90and solid polishing agent92may be provided in combination as a replaceable unit, where the cannister90is changed out after a predefined volume of biogas87has passed through the cannister90. Optionally, the cannister90may be a fixture in the treatment system and the solid polishing agent92may be removed and replaced within the cannister90after a predefined volume of biogas87has passed through the cannister90. The purified biogas stream162is delivered from the absorption risers110to an inlet31on the cannister90. Within the cannister90a perforated pipe88may be used to distribute biogas87throughout the cannister90. Optionally, other distribution methods such as a nozzle or mixing element may be located within the canister90to distribute the biogas87. The calcium hydroxide92is provided in a granular form, allowing the biogas87to flow through the solid polishing agent92. In a manner similar to that discussed above with the liquid polishing agent82, the biogas87interacts with the solid polishing agent92, calcium hydroxide Ca(OH)2, to remove CO2 remaining in the biogas87. The calcium hydroxide, Ca(OH)2, interacts with the carbon dioxide (CO2) in the biogas87to generate calcium carbonate, CaCO3, and water, H20, forming a slurry89that falls to the bottom of the cannister90.

After a predefined volume of biogas87has passed through the cannister90, the ability of the calcium hydroxide to further react with the CO2 in the biogas87will be depleted and the slurry in the cannister90will need to be cleaned out. As previously indicated, the cannister90and solid polishing agent92may be provided as a unit and the depleted cannister90with the slurry may be removed and a new cannister90and solid polishing agent92may be inserted. Optionally, the cannister90may have one or more openings by which the slurry may be removed and new solid polishing agent92introduced into the cannister90for further polishing of the purified biogas stream162.

With the biogas treatment process510integrated into a water treatment system500, the mixed water stream166from the last riser100in series connected risers or from each riser100in parallel connected risers, provides the source of the carbon dioxide water stream518for controlling the pH level of the disinfection process520. The mixed water stream166may be supplied directly or first be discharged into a holding tank or other such water holding feature for subsequent discharge. An excess volume of mixed water166, beyond that required for control of the pH level, may be discharged and the carbon dioxide allowed to dissipate naturally or to be aerated to facilitate the release of excess carbon dioxide.

In other applications, however, it may be desirable to recycle and reuse the water in which the carbon dioxide was dissolved. The water wash system may then include a flash process200, an air stripper process300, or a combination thereof. According to the illustrated embodiment inFIG.6, both a flash process200and an air stripper process300are included. The mixed water stream166from the final absorption riser110is provided as an input to a flash riser210in the flash process200. As will be discussed in more detail below, the flash riser210separates residual methane dissolved in the mixed water stream166. The first outlet260is connected back to the initial processing stage12such that the methane extracted from the mixed water stream166may be recovered in subsequent processing and a carbon dioxide water stream is output from a second outlet265of the flash riser210to a second inlet340of an air stripping riser310in the carbon dioxide stripper process300. A fan75discharges air into the first inlet345of the air stripping riser310. As will be discussed in more detail below, the air stripping riser310separates the carbon dioxide from the water stream and the carbon dioxide is output from a first outlet360. The reclaimed water may be used again within the water wash system and is pumped from the second outlet365of the air stripping riser310back to the holding tank40.

Optionally, when the biogas treatment system510is integrated into the water treatment system500as shown inFIGS.1-4, a flash process200may be provided to recover methane that may escape the absorber process100further purifying the mixed water stream166before it is used as the carbon dioxide water stream518. A valve590regulated by the controller570may be used to divert a portion of the carbon dioxide water stream518to the stripper process300, also shown as carbon dioxide aeration inFIG.4. Excess carbon dioxide which is not required for regulating the pH level in the disinfection process520may be directed to the stripping riser310and separated from the water stream.

Referring again toFIGS.6and7, each of the illustrated systems includes both a flash riser210and an air stripper riser310. With reference also toFIG.12, an exemplary flash riser210is illustrated. During the absorption process, a small amount of methane may be absorbed into the water stream. This methane is referred to herein as the “slip gas.” The flash riser210is configured to remove the slip gas from the mixed water stream166and return this methane to the supply for subsequent processing. The remaining water stream is passed on to the air stripping riser310where the carbon dioxide may be removed and the water reclaimed for subsequent use.

Each flash riser210includes multiple pipes. In the illustrated embodiment, the flash riser210includes an outer pipe212, a first inner pipe222, and a second inner pipe232. According to the illustrated embodiment, each of the pipes is concentric to the others. Optionally, the first inner pipe222and the second inner pipe232may be positioned adjacent to each other or extend downward at different locations within the outer pipe212. The outer pipe212has a first end214, a second end216, and a first length, L1. The first inner pipe222has a first end224, a second end226, and a second length, L2. The second inner pipe232has a first end234, a second end236, and a third length, L3. According to one embodiment of the invention, each of the flash risers210are installed in a vertical orientation, such that the first ends214,224,234of each pipe212,222,232are generally positioned at the top of each flash riser210. The first inner pipe222extends for the second length, L2into the outer pipe212and the mixed water stream166is delivered into the flash riser210. According to the illustrated embodiment, the first inner pipe222is cylindrical and open at the second end226. The mixed water stream166flows from the first inlet250and exits at the second end226of the first inner piper222The second inner pipe232extends for the third length, L3, through the first inner piper222, beyond the second end226of the first inner pipe222, and into the outer pipe212. The second inner pipe232is cylindrical and the second end236of the second inner pipe232includes a check valve between the interior of the outer pipe212and the interior of the second inner pipe232.

Each flash riser210includes an inlet and outlets to allow water and gas to flow into and out of the riser210. A first inlet250receives the mixed water stream166and is located on the first end214of the outer pipe212. The first inlet250is in fluid communication with the first end224of the first inner pipe222and establishes a flow path for the mixed water stream166into the flash riser210. The first inner pipe222extends into the flash riser210for the length, L2, of the inner pipe222. According to the illustrated embodiment, the second end226of the first inner pipe222terminates at a perforated coalescing disk246proximate the second end216of the first inner pipe222. The mixed water stream166is dispensed into the flash riser210at the second end226of the first inner pipe222. The pressure within the flash riser210is reduced such that the slip gas present in the mixed water stream166is desorbed and released within the outer pipe212. The remaining water stream, however, continues to hold the carbon dioxide previously absorbed from the compressed biogas stream30. The output water stream from the flash riser will be referred to herein as the CO2 water stream266.

A first outlet260located at the first end214of the outer pipe212provides a flow path261for the slip gas262(i.e., the methane removed from the mixed water stream166) to exit the flash riser210. The first outlet260is in fluid communication with and receives the slip gas262from the interior of the outer pipe212. A second outlet265is also located at the first end214of the outer pipe212and provides a flow path267for the CO2 water stream266. The second outlet265is in fluid communication with the first end234of the second inner pipe232. The CO2 water stream266enters the second end236of the second inner pipe232and travels up through the second inner pipe232to the second outlet265. According to the illustrated embodiment, each of the outer pipe212, first inner pipe222, and second inner pipe232are concentric about a central axis. The second inner pipe232is located within the first inner pipe222, which is, in turn, located within the outer pipe212. As discussed above and for purposes of illustration inFIG.12, the first end214,224,234of each pipe212,222,232ends at substantially the same point. It is contemplated that in various embodiments the first end224,234of each of the first inner pipe222and the second inner pipe232may extend for a short distance beyond the first end214of the outer pipe212to facilitate connections between each pipe and an inlet or outlet. It is further contemplated that an inlet250or outlet260,265may be positioned along and enter the outer pipe212via a side wall proximate the end of the flash riser210. For example, the first inlet250is shown connecting generally orthogonally to a wall of the first inner pipe222beyond the first end212of the outer pipe and the second inner pipe232extends through an end wall of the first inner pipe222to connect to the second outlet265. Alternately, the first inlet250or second outlet265may include a fixture connected to the first end214of the outer pipe212and comprise the necessary connections to establish the fluid flow paths from the inlet and outlet to the inner pipes extending into the outer pipe212.

According to the illustrated embodiments inFIGS.6-9, the carbon dioxide water stream518is shown being connected to the second outlet265of the flash riser210. If a system does not include a flash riser, the carbon dioxide water stream518may be connected to the second outlet165of the absorption riser(s)110.

Each air stripping riser310also includes multiple pipes. In the illustrated embodiment, the air stripping riser310includes an outer pipe312, a first inner pipe322, and a second inner pipe332. According to the illustrated embodiment, each of the pipes is concentric to the others. Optionally, the first inner pipe322and the second inner pipe332may be positioned adjacent to each other or extend downward at different locations within the outer pipe312. The outer pipe312has a first end314and a second end316. The first inner pipe322has a first end324and a second end326. The second inner pipe332has a first end334and a second end336. According to one embodiment of the invention, each of the air stripping riser310are installed in a vertical orientation, with the first end314of the outside pipe positioned at the top of the air stripping riser310. The first ends324,334of each inner pipe322,332are generally positioned at a flange311located within the air stripping riser310. When the air stripping riser310is used in conjunction with the absorption risers110and/or the flash riser210, it is contemplated that the flange311on the air stripping riser310is located at the same height as the first end of the absorption riser110and/or flash riser210. The first inner pipe322extends downward for a length into the outer pipe312. The first inner pipe322receives an air flow70from a fan75at a first inlet345and delivers the air flow70proximate the bottom of the air stripping riser310but above a level at which water may be present in the bottom of the air stripping riser310. According to the illustrated embodiment, the first inner pipe322is cylindrical and open at the second end326. The air flow70is passed from the first inlet345and exits at the second end326of the first inner piper322. The second inner pipe332extends through the first inner piper322, beyond the second end326of the first inner pipe322, and into the outer pipe312. The second inner pipe332is cylindrical and the second end336of the second inner pipe332includes a check valve between the interior of the outer pipe312and the interior of the second inner pipe332.

Each air stripping riser310includes a set of inlets and outlets to allow water and gas to flow into and out of the riser310. A second inlet340of the air stripping riser310receives the CO2 water stream266from the flash riser210. Optionally, if no flash riser210present, the second inlet340of the air stripping riser310may receive the mixed water stream166output from the absorption risers110. The second inlet340is located proximate the top of the air stripping riser310. According to the illustrated embodiment, a first intermediate pipe341and a second intermediate pipe342each extend from the second inlet340into the air stripping riser310. The first intermediate pipe341extends upward and enters the air stripping riser310proximate the first end314of the outer pipe312. The second intermediate pipe342enters the air stripping riser310proximate the flange311and the first ends324,334of the first and second inner pipes322,332. The first intermediate pipe341is in fluid communication with a first nozzle343that sprays the CO2 water stream266into the top of the air stripping riser310and the second intermediate pipe342is in fluid communication with a second nozzle344that sprays the CO2 water stream266into the air stripping riser310at a midpoint along the air stripping riser310. The dual entry points for the CO2 water stream266define separate segments of the air stripping riser310that may then interact with the air flow70entering the air stripping riser310to remove the carbon dioxide from the CO2 water stream266.

As previously indicated, air flow70is provided at the first inlet345and into the first inner pipe322, establishing a flow path for the air flow70into the air stripping riser310. The first inner pipe322extends into the air stripping riser310for a length and, according to the illustrated embodiment, the second end326of the first inner pipe322terminates at a dispersion element349proximate the second end316of the first inner pipe322. The air flow70is dispensed into the air stripping riser310at the second end326of the first inner pipe322as illustrated by the air flow path367. The pressure within the air stripping riser310is further reduced from the flash riser210and is preferably maintained at or near atmospheric pressure. The reduction in pressure reduces the solubility of carbon dioxide in water facilitating the release of the carbon dioxide from the CO2 water stream266within the outer pipe312. The air flow70is pumped into the bottom of the air stripping riser310such that the air flow70rises counter to the CO2 water stream266being sprayed into the top of the riser310. The air flow70interacts with water droplets to facilitate release of the carbon dioxide and further carries the carbon dioxide toward the top of the air stripping riser310.

A first outlet360located at the first end314of the outer pipe312provides a flow path361for the carbon dioxide362removed from the CO2 water stream266to exit the air stripping riser310. The first outlet360is in fluid communication with and receives the carbon dioxide362from the interior of the outer pipe312. A second outlet365is located proximate the first end324of the second inner pipe232. As illustrated, the second inner pipe332is connected to a ninety degree bend pipe337and to a short outlet pipe338such that it extends out the side of the outer pipe312. The second outlet365provides a flow path367for the reclaimed water stream366. The second outlet365is in fluid communication with the first end334of the second inner pipe332. The reclaimed water stream366enters the second end336of the second inner pipe332and travels up through the second inner pipe332to the second outlet365. According to the illustrated embodiment, each of the outer pipe312, first inner pipe322, and second inner pipe332are concentric about a central axis. The second inner pipe332is located within the first inner pipe322, which is, in turn, located within the outer pipe312. The first end324,334of each inner pipe322,332ends proximate the flange311located within the outer pipe312. The first inlet345and the second outlet365are connected to the first inner pipe322and the second inner pipe332, respectively, and extend out through a wall of the outer pipe312. Although the first end314of the outer pipe312extends for some distance beyond the flange311, it is contemplated that in various embodiments the second inlet340may run directly into the outer pipe with a single intermediate pipe and the first end314of the outer pipe312may be positioned proximate the flange311. Optionally, the first end224,234of each of the first inner pipe222and the second inner pipe232may extend up to or for a short distance beyond the first end314of the outer pipe312without deviating from the scope of the invention.

It is further contemplated that each air stripping riser310may include packing material within at least a portion of the interior of the outer pipe312to further enhance the release of the carbon dioxide from the CO2 water stream266. InFIG.13, additional dispersion plates349are shown spaced apart within the outer pipe312. One or more additional dispersion plates may be distributed along the length of the interior of the outer pipe312to continually redistribute the air flow70and CO2 water stream266as they travel through the interior of the pipe. It is also contemplated that packing material similar to that used in the absorption riser110may be inserted into the air stripping riser310. With reference again toFIGS.14and15, a flexible, porous material170may be rolled into a coil and inserted between the inner periphery of the outer pipe and the outer periphery of the first inner pipe. According to one embodiment of the invention, the flexible material170is a netting material, such as a geonet, including multiple holes throughout the material. As the CO2 water stream266passes through the air stripping riser310, the netting and the multiple holes create numerous flow paths and opportunities for separating the CO2 water stream266into more droplets and, thereby, increasing the surface area of the water stream exposed to the air, facilitating release of the carbon dioxide into the air. InFIG.15, a mesh material180may be formed into a basket or bag and is used to contain another bulk material182within the mesh. The mesh and bulk materials180,182may be inserted into and removed from the interior of the outer pipe as a unit. Both the flexible material170and the mesh and bulk material combination180,182facilitate cleaning of the packing material. The flexible material170may be removed and unrolled for cleaning. The mesh and bulk material180,182may be pulled out of the outer pipe and the bulk material spread out for cleaning. Once clean, the flexible material170may be rolled back into a coil and inserted back into the outer pipe. Similarly, the bulk material182may be placed back into the mesh material180and inserted into the outer pipe.

According to still another embodiment of the invention the diameter and/or length of an absorption riser110may make the insertion of a material within the absorption riser challenging. Therefore, it is contemplated that the packing material may be a porous structure or material that is poured, blown, or pumped into the absorption riser110. Initially, a mesh filter or grate may be inserted at a particular depth or length within the absorption riser to prevent the porous structure or material from passing beyond a certain point within the absorption riser110. The filter or grate has openings of a smaller size than the size of individual members of the packing material, such that the packing material is stopped by the filter or grate while allowing the biogas and water streams flow through and around the packing material and filter or grate. It is further contemplated that the filter or grate may be connected to a cable or rod to facilitate cleaning or maintenance on the absorption riser. The cable or rod may have mixing devices attached or it may be used to pull the filter or grate out of the absorption riser110which, in turn, would pull out the packing material, allowing the packing material and/or the interior of the absorption riser to be inspected or maintained.

The carbon dioxide362extracted from the CO2 water stream266in the air stripping riser310may be vented directly from the first outlet360into the atmosphere. However, the potential exists that the carbon dioxide362stream may also include other contaminants. Therefore, it may be desirable to discharge the carbon dioxide362into the environment in another manner such that further processing may be performed on the carbon dioxide stream362. Referring next toFIGS.17-19, three exemplary off-gas discharge methods are illustrated. InFIG.17, the carbon dioxide362is carried through a discharge pipe400into a bio-filter material405. The bio-filter material is mounded above the ground410and the discharge pipe400is perforated along the length extending into the bio-filter material. The carbon dioxide362is vented into the bio-filter material as shown by the arrows420. InFIG.18, the carbon dioxide362is carried through a discharge pipe400for some distance above the ground410and is then buried below the ground410. The discharge pipe400is perforated along the length extending below the ground, and the carbon dioxide362is vented into the ground as shown by the arrows420. InFIG.19, the carbon dioxide362is carried through a discharge pipe400into a water reservoir415formed in the ground410. The water reservoir415may be naturally occurring such as a pond or lake or may be constructed by digging an area dug out of the ground410. The discharge pipe400is perforated along the length extending under the water, and the carbon dioxide362is vented into the water reservoir as shown by the arrows420. According to still another embodiment of the invention, it may be desirable to provide a thermal oxidization unit and the carbon dioxide362and other trace constituents may pass through the thermal oxidization unit prior to release into the atmosphere. Any of the exemplary off-gas discharge methods may be utilized to regulate a level of carbon dioxide available for adjusting the pH level of the disinfection process520. The controller570receives inputs from sensors corresponding to operating conditions within the water treatment process, such as pressure, temperature, pH levels, flow rates, and the like and supplies the appropriate carbon dioxide water stream518to achieve a desired pH level in the second basin540.

Referring next toFIGS.8and9, an exemplary biogas treatment system incorporating another embodiment of the present invention is illustrated. As discussed above with respect toFIG.6, a biogas stream10is provided as an input to the system, where the biogas may be produced, for example, from an anaerobic decomposition process. Some initial processing of the biogas stream may occur prior to supplying the biogas stream to the water wash system. An optional hydrogen sulfide removal process15such as an iron sponge type system may be inserted in series with the biogas stream10to perform an initial removal of hydrogen sulfide present in the biogas stream. The biogas stream may also be passed through a filter20to remove particulate content. In addition, carbon dioxide has increased solubility characteristics with decreasing temperature and increasing pressure. The biogas stream is, therefore, passed through a compressor25to achieve an elevated pressure. The pressure range of the compressed biogas stream30may be between ten and two hundred pounds per square inch gauge (10-200 psig). According to one embodiment of the invention, the pressure range of the compressed gas is between about sixty and one hundred fifty pounds per square inch gauge (60-150 psig). The compressed biogas may also be chilled, for example, to between thirty-five and sixty-eight degrees Fahrenheit (35-68° F.). The compressed and/or chilled biogas stream30is provided as an input to the water wash process.

Similar to the embodiment illustrated inFIGS.6and7, the water wash process illustrated inFIG.8utilizes water to remove the carbon dioxide from the biogas stream. In the embodiments illustrated inFIGS.6and7, however, the water and biogas streams flow in opposite directions (i.e., counter-current) to each other through the absorption risers110. In the embodiments illustrated inFIGS.8and9, the water and biogas streams flow in the same direction (i.e. concurrent) to each other through an absorption riser110. According to the illustrated embodiment, water is provided to a holding tank40from which a water stream50is provided to the water wash process. Water provided to the holding tank40may be chilled and/or under pressure to facilitate the water wash process. Optionally, the holding tank40may incorporate a chiller and/or a compressor to chill or pressurize the water prior to supplying it in the water stream. The water, for example, may be chilled to between thirty-five and sixty-eight degrees Fahrenheit (35-68° F.) and pressurized to mix with the compressed biogas stream30at about the same input pressure of the biogas stream.

In the biogas treatment system ofFIG.8, a single absorption riser110is provided. Referring also toFIG.23, the absorption riser110includes multiple pipes. In the illustrated embodiment, the absorption riser110includes an outer pipe112, a first inner pipe122, and a second inner pipe132. According to the illustrated embodiment, each of the inner pipes122,132is concentric to the outer pipe112. Optionally, the inner pipes122,132may be positioned at different locations (e.g., along the interior wall) within the outer pipe112. The outer pipe112has a first end114and a second end116. The first inner pipe122has a first end124and a second end126. The second inner pipe132has a first end134and a second end136. The outer pipe112also includes a first segment117, a second segment118, and a third segment119. It is contemplated that the absorption riser110may be buried within the ground or submerged below water. Optionally, the outer pipe112and the inner pipes may be formed of a flexible material and may extend in one or more rows or be coiled up and placed, for example, on the ground in a substantially horizontal configuration.

If the pipes are buried or submerged, the first segment117extends downward where the first end114of the outer pipe112may be located at a surface level. The first inner pipe122is located within the first segment117, where the first end124of the first inner pipe122is generally positioned at the first end114of the outer pipe112. The second segment118extends generally in a horizontal direction, and the first inner pipe122also includes a horizontal segment128that extends, at least for a portion of the horizontal direction, within the outer pipe112. The horizontal segment128includes a plurality of perforations127located along the length of the horizontal segment128from which the compressed biogas stream30may be released into the water flowing through the outer pipe. It is contemplated that the perforations127may also be located along the descending portion of the inner pipe122within the first segment117of the outer pipe112. The third segment119extends upward back to the surface level. The second inner pipe132is located within the third segment119of the outer pipe112. The second inner pipe132is cylindrical and the second end136of the second inner pipe132is located proximate the transition between the second segment118and the third segment119of the outer pipe112. The first end134of the second inner pipe132is located proximate the second end of the outer pipe. The second end136of the second inner pipe132includes a check valve between the interior of the outer pipe112and the interior of the second inner pipe132, where the check valve is controlled to allow the mixed water stream to enter the second inner pipe132and be drawn up and out of the absorption riser110through the second inner pipe.

If the pipes are laid out in rows or coiled up and placed on the ground, the first segment117extends for a portion of the absorption riser110and the first end114of the outer pipe112is located at a first end of the absorption riser110. The first inner pipe122is located within the first segment117, where the first end124of the first inner pipe122is generally positioned at the first end114of the outer pipe112. The second segment118extends for a second portion of the absorption riser110, and the first inner pipe122also extends at least for a portion of the second segment118. The first inner pipe122includes a plurality of perforations127located along the length of the first inner pipe within the second segment118from which the compressed biogas stream30may be released into the water flowing through the outer pipe112. The third segment119extends for a third portion of the outer pipe112, and the second inner pipe132is located within the third segment119of the outer pipe112. The second inner pipe132is cylindrical and the second end136of the second inner pipe132is located proximate the transition between the second segment118and the third segment119of the outer pipe112. The first end134of the second inner pipe132is located proximate the second end of the outer pipe. The second end136of the second inner pipe132includes a check valve between the interior of the outer pipe112and the interior of the second inner pipe132, where the check valve allows the mixed water stream to flow in one direction to enter the second inner pipe132and be drawn up and out of the absorption riser110through the second inner pipe.

In the biogas treatment system ofFIG.9, the single absorption riser110ofFIG.8may be divided into two portions, where a first portion111includes the first segment117and the second segment118of the absorption riser110as discussed above with respect toFIG.8. The second portion113includes the third segment119of the absorption riser110as discussed above with respect toFIG.8. Referring also toFIG.22, the absorption riser110includes multiple pipes. In the illustrated embodiment, the absorption riser110includes an outer pipe112, a first inner pipe122, and a second inner pipe132. According to the illustrated embodiment, each of the inner pipes122,132is concentric to the outer pipe112. Optionally, the inner pipes122,132may be positioned at different locations (e.g., along the interior wall) within the outer pipe112. The outer pipe112is divided into two lengths, where the first length112A of the outer pipe has a first end114A and a second end116B. The outer pipe112also includes a first segment117and a second segment118extending within the first length112A of the outer pipe112. A third segment119extends for the second length112B of the outer pipe112. The first inner pipe122has a first end124and a second end126, where the first end124is proximate the first end114A of the first length112A of the outer pipe. It is contemplated that the absorption riser110may be buried within the ground or submerged below water. Optionally, the outer pipe112and the inner pipes may be formed of a flexible material and may extend in one or more rows or be coiled up and placed, for example, on the ground in a substantially horizontal configuration.

If the pipes are buried or submerged, the first segment117extends downward where the first end114A of the first length112A of the outer pipe112may be located at a surface level. The first inner pipe122is located within the first segment117, where the first end124of the first inner pipe122is generally positioned at the first end114A of the first length112A of the outer pipe112. The second segment118extends generally in a horizontal direction, and the first inner pipe122also includes a horizontal segment128that extends, at least for a portion of the horizontal direction, within the outer pipe112. The horizontal segment128includes a plurality of perforations127located along the length of the horizontal segment128from which the compressed biogas stream30may be released into the water flowing through the outer pipe. It is contemplated that the perforations127may also be located along the descending portion of the inner pipe122within the first segment117of the outer pipe112. Optionally, a series of nozzles or other gas injectors may be located along the length of the inner pipe122, where each nozzle disperses a portion of the biogas stream within the outer pipe112. According to still another option, multiple inner pipes122may be provided, where each inner pipe122includes a series of perforations127or nozzles spaced along the length of each inner pipe122to facilitate dispersion of the biogas stream30throughout the interior of the outer pipe112. The second segment118may further include a rising section, where at least the first length112A of the outer pipe and, optionally, the inner pipe122extend for a distance back toward the surface level.

If the pipes are laid out in rows or coiled up and placed on the ground, the first segment117extends for a portion of the absorption riser110and the first end114A of the outer pipe112is located at a first end of the absorption riser110. The first inner pipe122is located within the first segment117, where the first end124of the first inner pipe122is generally positioned at the first end114of the outer pipe112. The second segment118extends for a second portion of the absorption riser110, and the first inner pipe122also extends at least for a portion of the second segment118. The first inner pipe122includes a plurality of perforations127located along the length of the first inner pipe within the second segment118from which the compressed biogas stream30may be released into the water flowing through the outer pipe112. It is contemplated that the perforations127may also be located along the portion of the inner pipe122within the first segment117of the outer pipe112. The third segment119extends for a third portion of the outer pipe112, and the second inner pipe132is located within the third segment119of the outer pipe112. According to one embodiment, the first and second segments117,118defining the first length112A of the outer pipe112may be located generally horizontally, for example, on the ground and the second length112B of the outer pipe112may extend vertically, for example, buried or submerged below a surface level of ground or water.

The second length112B of the outer pipe112defines a substantially vertical pipe having a first end114B and a second end116B. The second end116A of the first length112A of the outer pipe is connected proximate the first end114B of the second length112B of the outer pipe. The second inner pipe132is located within the third segment119of the outer pipe112, which corresponds to the second length112B of the outer pipe. The second inner pipe132has a first end134and a second end136. The second inner pipe132is cylindrical and the second end136of the second inner pipe132is located proximate the second end116B of the second length112B of the outer pipe. The first end134of the second inner pipe132is located proximate the first end114B of the second length112B of the outer pipe. The second end136of the second inner pipe132includes a check valve between the interior of the second length112B of the outer pipe112and the interior of the second inner pipe132, where the check valve allows the mixed water stream to flow in one direction to enter the second inner pipe132and be drawn up and out of the absorption riser110through the second inner pipe.

With reference again toFIGS.8-9and20-22, the absorption riser110includes a set of inlets and outlets to allow water and biogas to flow into and out of the riser110. A first inlet140receives the compressed biogas stream30and is located on the first end114of the outer pipe112. The first inlet140is in fluid communication with the first end124of the first inner pipe122and establishes a flow path for the compressed biogas stream30into the absorption riser110. As previously indicated, the first inner pipe122extends into through the first segment117and into the second segment118of the outer pipe112, and the second end126of the first inner pipe122. A second inlet145receives the water stream50and is located on the first end114of the outer pipe112. The second inlet145is in fluid communication with the first end114of the outer pipe112to dispense the water stream50from the first end of the absorption riser110. The water stream50flows through to the second end of the absorption riser. The compressed biogas stream30is dispensed into the water flow from the perforations127in the first inner pipe122and flows toward the second end of the absorption riser.

As the compressed biogas stream30and the water stream50flow along the horizontal portion of the absorption riser110the two streams mix and the carbon dioxide within the compressed biogas stream30is dissolved into the water. Although small amounts of methane may be absorbed in the water, the majority of the methane remains unabsorbed. The methane is lighter than the mixed water stream containing carbon dioxide. With reference toFIGS.8and20, the methane rises to the second end116of the absorption riser110. The mixed water stream166including the carbon dioxide absorbed from the biogas stream is heavier than the methane and remains at the end of the horizontal segment of the absorption riser at the transition to the upward segment. A first outlet160located at the second end116of the outer pipe112provides a flow path161for the purified biogas stream162to exit the absorption riser110. The first outlet160is in fluid communication with and receives the purified biogas stream162from the interior of the outer pipe112. With reference toFIGS.9and22, the flow in the first length112A of the outer pipe created by the water stream50and the biogas stream30establish a flow through the first length112A of the outer pipe112and into the second length112B of the outer pipe. The mixed water stream then falls to the second end116B of the second length112B of the outer pipe and the methane rises to the first end114B of the second length112B of the outer pipe. A first outlet160located at the first end114B of the second length112B of the outer pipe112provides a flow path161for the purified biogas stream162to exit the absorption riser110. The first outlet160is in fluid communication with and receives the purified biogas stream162from the interior of the outer pipe112. A second outlet165is also located proximate the first outlet160and provides a flow path167for the mixed water stream166. The second outlet165is in fluid communication with the first end134of the second inner pipe132. The mixed water stream166enters the second end136of the second inner pipe132and travels up through the second inner pipe132to the second outlet165. As discussed above and for purposes of illustration inFIG.9, the first ends114,124, of the outer pipe112and the first inner pipe122end at substantially the same point. Similarly, the first end134of the second inner pipe132and the second end116of the outer pipe112end at substantially the same point. It is contemplated that in various embodiments the first end124,134of each of the first inner pipe122and the second inner pipe132may extend for a short distance beyond either end of the outer pipe112to facilitate connections between each pipe and an inlet or outlet.

Because the interaction of the water stream and the biogas stream is reduced when the streams are travelling in the same direction rather than travelling in opposite directions, transfer of carbon dioxide from the compressed biogas stream30to the water stream50occurs at a reduced rate per length of travel. Thus, the embodiment illustrated inFIG.8is better suited for applications in which a lengthy horizontal segment of the absorption riser is available. It is contemplated that each pipe of the absorption riser may be formed of a flexible material and coiled or laid out in rows alternating back and forth adjacent to each other. According to one embodiment, the absorption riser may extend along the ground in a generally horizontal orientation. According to another embodiment, the absorption riser110may be routed into a pond, lake, or other available water source. The water may provide some protection and/or insulation for the absorption riser110. The absorption riser may extend in numerous configurations, such as a straight line, a curved path, an alternating back-and-forth route, or a combination thereof to increase the length of the horizontal segment. It is contemplated that the horizontal segment of the absorption riser110may extend for one hundred feet or longer before the absorption riser110transitions to the upward segment. Optionally, a portion, or all, of the absorption riser may include an external sleeve to provide further protection and/or insulation. The sleeve may further provide weight to the absorption riser110if it is installed in an underwater application to reduce buoyancy and to help keep the absorption riser110along the bottom of the pond, lake, or other water source.

To increase the interaction between the water stream50and the biogas stream30, it is contemplated that one or more mixing elements may be included along the length of travel. The mixing elements may be powered, for example, a rotating member within the pipe, or preferably, may be passive mixing elements. The passive mixing elements are configured to disrupt a linear flow path through the pipes, causing the water stream50and the biogas stream30to intermix their flows along the length of the absorption riser. With reference toFIGS.20and22, it is contemplated that a dispersion plate149may be included within the outer pipe112. The dispersion plate may be located proximate the inlets to disperse a flow across the interior of the absorption riser110. Optionally, one or more dispersion plates149may be located beyond the second end126of the first inner pipe122to encourage mingling of the two flows. With reference also toFIGS.23and24, the interior of the absorption riser110may have different configurations. InFIG.23, the horizontal segment128of the inner pipe is illustrated with perforations127distributed around the pipe. The compressed biogas stream30escapes through the perforations127into the water stream50flowing in the same direction through the pipe. Optionally, and as shown inFIGS.22and24, the horizontal segment118of the outer pipe112may include packing material190within at least a portion of the interior of the horizontal segment118to further enhance the release of the carbon dioxide from the water stream50. The compressed biogas stream30may be discharged into the horizontal segment118in advance of the packing material190so that the compressed biogas stream30and the water stream50travel through the packing material190and, thereby increase contact between the two streams. A partially mixed stream55is illustrated as continuing on along the horizontal segment of the absorption riser110. It is further contemplated that a combination of the two embodiments may be utilized in which the horizontal segment128of the inner pipe extends into a segment of the outer pipe112that has packing material190located therein.

It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.