Patent Description:
In wealthier countries, disposal companies or governmental agencies dry the waste (in particular the fecal sludge/biosolids) using fossil fuel and land apply it as "natural fertilizer" or dispose of it in landfills. Some countries dispose of it in the oceans as well, though this practice is prohibited in most countries. Other countries incinerate the biosolids which generates horrible air quality concerns.

Since the US Clean Air Act of <NUM>, incineration of biosolids in the United States has been phased out due to air emission rates well in excess of permittable limits. Nearly no new incinerators have been permitted in the last two decades. In <NUM>, approximately <NUM>% of all biosolids produced are land applied and <NUM>% of all biosolids are landfilled. On November <NUM>, <NUM>, the U. EPA's Office of Inspector General (OIG), an independent review branch of the EPA, released a report reviewing EPA's biosolids regulatory program. The report identified <NUM> pollutants that cause severe environmental and human health concerns when leached into the ground and waterways that the EPA is not sufficiently monitoring. Upon reading the report, states independently placed moratoriums on land application of biosolids and landfills are no longer accepting biosolids for disposal creating the impossible situation of unstoppable biosolids production with no disposal options. <CIT> teaches the use of steam heating to dry biomass which is used as a feedstock. <CIT> teaches the use of solar energy to reduce the moisture content in a sludge. <CIT> teaches the use of an external heat source to heat a belt upon which sludge is dried. <CIT> teaches a two-stage method for the drying of sludge. <CIT>teaches automatic speed control to dry animal manure in a conventional stacked belt drying system.

Thus, there exists a need for a human biosolids treatment solution that can handle human biosolids, remove its moisture, break down harmful chemicals, produce a biochar with desirable properties, with a minimal environmental footprint.

A biosolids treatment facility that treats human biosolids to produce a low carbon ash (LCA), dried Class A biosolids, and/or carbon rich biochar products (HCAB), where the feed rate into a dryer is modulated in response to a measured moisture content of blended human biosolids and recirculated dried Class A product.

As described herein, a gasifier may produce LCA and HCAB, while the dryer produces the Class A biosolids. As mentioned, discussed herein, the dryer's Class A biosolid may be conveyed back to the gasifier, which converts all carbon in the overall system to energy, with LCA as the only byproduct. This is made possible using the plume abatement heater, the absence of which leaves a functioning system where excess energy is recovered in solid form with the gasifier producing LCA and the dryer produces Class A biosolids OR the gasifier produces only HCAB and no Class A product.

While biosolids are mostly discussed herein, the treatment system is configured to use comingled materials generated by a wastewater treatment facility (WWTF) as well, including grit and screenings, fats/oils/grease (FOG), anaerobically digested biosolids, biosolids post thermal hydrolysis treatment, primary/secondary waste activate sludge, non-activated sludge in varying percent of composition as fuel sources for the gasifier without need for supplemental fuel.

The novel biosolids treatment process and combination of equipment presents an alternative to the land application, incineration, composting, or landfilling of biosolids, all of which are increasingly problematic in biosolids management. The process, using the facility, converts dewatered municipal biosolids ("sludge cake") generated at wastewater treatment plants (WWTPs) to renewable energy and three value added products (low carbon ash, high carbon biochar, and Class A dried biosolids) preferably without the use of supplemental fuels (natural gas, oil, biomass, etc.), which are not required.

An initial design of a first treatment facility's footprint was approximately <NUM> square meters (<NUM> acres) with site disturbance limited to the foundation footers and slab required for the storage and process buildings. A negative pressure one-story building houses the biosolids gasification and drying sections. The enclosed facility is not subject to leaching, runoff, or wind. There are no outfall pipes that discharge to surface streams or drainage channels and wastewater discharge from the facility may be routed to a wastewater treatment plant.

Ancillary equipment and structures located adjacent to the gasifying and drying process building may include mechanical equipment for receiving, storing, and conveying biosolids into the building. Conveyance and storage equipment for the Class A products may also be housed in the ancillary building.

Air emissions from the facility are very low to begin with and are further controlled by a wet scrubber and activated carbon filter. Air emissions from the facility are similar in content to combustion of natural gas.

The waste treatment process involves the thermal drying of dewatered biosolid to produce a Class A biosolid that can be used as a fertilizer, soil conditioner, and/or a renewable fuel product. Class A biosolids meet the requirements established under <NUM> Pa. Code §<NUM>(a) in regard to pathogens and <NUM> Pa. Code §<NUM>(b )(<NUM>)-(<NUM>) relating to vector attraction reduction, (also found in US EPA Part <NUM> Biosolids Rule, Chapters <NUM>, <NUM>, and <NUM>), are nonliquid, and are unrecognizable as human waste.

The process may also produce a renewable fuel coproduct with as-is energy value ≥ <NUM>,<NUM> kilojoule/kilogram (<NUM>,<NUM> Btu/lb) as defined in <NUM> Pa. Code §<NUM> as a beneficial use product. Further, the biosolids gasifying drying facility satisfies the time-temperature requirement of Regime B as identified in Table <NUM>-<NUM> of US EPA Part <NUM> Biosolids Rule, Chapter <NUM> for Class A pathogen reduction.

The facility reduces the attractiveness of biosolids to vectors by drying the biosolids to > <NUM>% solids content as identified in Table <NUM>-<NUM>, Option #<NUM> of US EPA Part <NUM> Biosolids Rule, Chapter <NUM> for Class A vector attraction reduction. The system can process dewatered biosolids cake as wet as <NUM>% total solids content.

The unloading process and storage may be maintained at a negative pressure and minimal onsite storage in a building maintained at a negative pressure may ensure all odors are contained inside the building thereby minimizing nuisance complaints.

To put the system in context, it may be helpful to review the process from the step of biosolids sludge delivery to the treatment system, it being understood that the delivery steps may be performed in many other ways.

<FIG> illustrates an overview of the sludge receiving and metering facility <NUM>, which includes the sludge receiving building <NUM> (that receives the trucks) and sludge metering building <NUM> (where certain measurements are taken the sludge is prepared before treatment). The sludge receiving building includes the rollup door <NUM> that allows trucks to access the sludge receiving building <NUM> and in particular the building's interior unloading bay <NUM>. The sludge receiving and metering facility <NUM> includes an elevating conveyor <NUM> that conveys sludge received from the trucks upwards to a metering building conveyor <NUM> that spans the buildings <NUM>, <NUM> via a port <NUM> into the sludge metering bins <NUM> located in the sludge metering building <NUM>.

Within the metering building <NUM>, sludge may be tested and stored before final transfer (which could be done via a treatment facility conveyor <NUM> (which may be a screw conveyor) that exits through a treatment port <NUM> between the metering building <NUM> and the treatment facility/building <NUM>. Arrows indicate the direction of material flow. Within the metering building <NUM>, technicians may test the sludge for chemical composition, moisture content, weight, volume.

The metering bins <NUM> may be graduated indicating the volume at different levels. A technician may record the sludge level before and after filling each metering bin <NUM>, calculating the displaced sludge volume, and thus the mass flow rate entering the treatment building <NUM> can be calculated with the bulk density of the material.

As shown in <FIG> (a simple overview) and 2B/2C (a more detailed and thorough view) treatment facility conveyors <NUM> deliver the sludge to the treatment building <NUM>, and in particular a conventional biosolids mixer <NUM> (paddle, pug mill, pin mixer, or other) within a treatment building <NUM> where wet biosolids cake (average of <NUM>% solids, ranging from <NUM>% to <NUM>% solids) is mixed with recirculated dried Class A product (average of <NUM>% solids) from an output <NUM> of the dryer <NUM>, to create a blended feed mixture (mixed biosolids) ranging between <NUM>% to <NUM>% total solids. The mixed biosolids are conveyed (via conveyor 222a and vertical lift conveyor 222b) to the direct contact, rotary drum dryer <NUM> for heat treatment that yields Class A product and also sent to the gasifier <NUM> via conveyors 222c and mixers to the gasifier metering bin <NUM> as the exclusive fuel source for generating the thermal energy needed to dry the mixed biosolids to ><NUM>% total solids. The mixed biosolids are the sole energy source for the evaporation of water from the mixture. The gasifier <NUM> described herein may be of the type described in <CIT>.

The biosolids gasifier <NUM> heats the wet mixed biosolids using self-sustaining heat recovery from the system in an oxygen deficient chamber where the biosolids are converted to volatile gases (syngas) and low carbon ash or high carbon biochar, depending upon process purpose. The ash/biochar is automatically conveyed from the system as a beneficial byproduct for use as a soil amendment, fertilizer, or activated carbon filter media (as described below).

The volatile syngas is channeled into a separate chamber downstream from the gasifier <NUM> to an oxidizer <NUM> where controlled combustion is achieved. Combustion occurs when adequate air is introduced to the syngas in the oxidizer balancing the combustion equation resulting in exothermic energy release achieving temperatures ranging between 982C-1200C (1800F-2200F).

The hot flue gases leave the oxidizer <NUM> and enter a flue gas tempering chamber (blend box) <NUM> where ambient air and moist recirculated dryer exhaust are mixed with the hot flue gas to achieve a controlled "tempered" flue gas temperature in the range of 370C-760C (700F - 1400F), depending on biosolids characteristics and process purpose.

The blend box <NUM> is designed to mix the hot oxidizer flue gas (> 1093C (2000F)) with ambient air (+/- 21C (70F)) and moist recirculated dryer (dryer <NUM>) exhaust (+/- 240F) to a set target temperature (370C-760C) (700F - 1400F)). The target temperature is achieved by controlling the induced ambient air with a temperature-controlled damper operating from a thermocouple placed in a strategic location within the blend box <NUM> and the recirculated dryer exhaust with a variable frequency drive (VFD) on a recirculation fan motor that allows the operator to vary the flow rate of the recirculated dryer exhaust. Together, these two control points enable an operator (or automated operating controls but discussed herein as operator) to achieve accurate and stable temperature control exiting the blend box <NUM>.

Contained within the blend box <NUM> is a shell and tube air-to-air heat exchanger designed to preheat ambient air for use in the gasification process (this gasifier air preheater is within the blend box <NUM>, with visible inlet/outlets 237in <FIG>)) and further lower the flue gas temperature to a "target" temperature. Furthermore, the tubes of the gasifier air preheater cause flue gas turbulence ensuring complete blending of gases for a uniform gas temperature exiting the blend box <NUM>.

When sludge characteristics and process purpose allow, a customized two-pass, crossflow, plume abatement heat exchanger <NUM> (plume abatement HX) is installed between the gasifier air preheater and the dryer inlet. The tempered flue gas exiting the blend box <NUM> is used to heat the dryer exhaust in two steps. First, after passing through the mechanical cyclone (2050a, 2050b) and then the wet scrubber <NUM> of the emission train (each removing further particulate, following the arrows and piping as shown), the saturated exhaust from the wet scrubber <NUM> is routed through piping 281a to the plume abatement HX <NUM> where excess energy, not needed for the evaporation of moisture from the mixed biosolids, is used to heat the exhaust gases to a controlled temperature thereby dropping the relative humidity of the flue gas stream prior to entering, through additional piping 281b, a activated carbon filter (ACF) <NUM>. Low relative humidity gas flow maximizes the performance of activated carbon. High temperature flue gas minimizes ACF performance so accurate temperature control of the heated gas may be a design consideration.

After the ACF <NUM>, the flue gas passes through the piping 281c in a second pass of the plume abatement HX <NUM> to superheat the exhaust to minimize plume before routing the exhaust through piping 281d to the final discharge point <NUM> to atmosphere.

The heat source for the plume abatement HX <NUM> is the tempered flue gas exiting the blend box <NUM>. In the same manner that it is important to control temperature entering the ACF <NUM>, control of the temperature of the hot flue gas may be maintained while entering the dryer <NUM>. The dryer inlet temperature may be controlled using a system of dampers in the flue gas stream that direct flow through the plume abatement HX <NUM>. In this manner, accurate control of both gas streams (entering the ACF <NUM> and entering the dryer <NUM>) may be controlled to target temperatures set by the operator. Target temperatures entering the dryer <NUM> may range between 316C-593C (600F - 1100F). The direct contact dryer <NUM> may be conventional to industry.

A portion of the dried Class A product (average of <NUM>% solids) may be recirculated from the dryer outlet <NUM> as discussed above for blending with the inbound wet biosolids cake. The ratio of dry to wet biosolids for the mixer is determined by an inline moisture sensor <NUM> on the discharge of the mixer <NUM> and manually checked by periodic operator testing.

After much testing, the inventors found that flexibility to accurately control the mixer output to a range between <NUM>% to <NUM>% solids content is necessary to treat all varieties of municipal sludge with keeping below the sticky phase of the sludge, a phase that makes the operation challenging to manage. This mixed sludge quality is determined by the target fuel specifications for the gasifier <NUM>.

Target temperature flue gas exits the blend box <NUM> and plume abatement HX <NUM> before entering a direct contact dryer <NUM> where it is the heat source for evaporating the water from dewatered mixed biosolids ranging between <NUM>% to <NUM>% solid content (same blended biosolids used for the gasifier fuel). The dried product may be conventionally sold as Class A product that is ≤ <NUM>% MC (<NUM>% DM). The system may recycle the dried Class A product for mixing with incoming dewatered biosolids cake at ranging from <NUM>% solids to ><NUM>% solids content.

The blended biosolid mixture ("mixed biosolids") may be used as the exclusive fuel for the gasifier <NUM> and the input biosolids to the direct contact dryer <NUM>. With the plume abatement HX <NUM> installed, the closed-loop system is estimated to be +/- <NUM>% efficient for energy repurposing.

The evaporated water from the biosolids pass from the dryer <NUM>, through a high efficiency cyclone collectors 2050a, 2050b (<NUM>, <NUM> in <FIG>) to capture <NUM>% of particulate matter larger than <NUM> micron (PM10), then through a three-stage wet scrubber system <NUM> equipped with alkaline treatment to ensure the capture of the majority of the remaining PM and treat for residual odors and SO2 prior to emitting to the atmosphere. The clean water vapor plume is drawn from the system using an induced draft fan <NUM> that blows the moist exhaust through an activated carbon filter <NUM> to perform final pollutant polishing prior to exiting the facility stack <NUM>.

The treatment process produces three types of beneficial coproducts. The process begins by mixing dewatered biosolids cake (ranging from <NUM>% water content to <NUM>% water content) with recirculated dried Class A biosolids (+/- <NUM>% water content) to create a resulting mixed biosolids (ranging between <NUM>% water content to <NUM>% water content) for use as the exclusive gasifier fuel and the inlet material to the dryer. The three coproducts generated from the system are:.

As described herein, the gasifier <NUM> may produce LCA and HCAB, while the dryer <NUM> produces the Class A biosolids. The dryer's Class A biosolid may be conveyed back to the gasifier <NUM>, which converts all carbon in the overall system to energy, with LCA as the only byproduct. This is made possible using the plume abatement heater, the absence of which leaves a functioning system where excess energy is recovered in solid form with the gasifier producing LCA and the dryer produces Class A biosolids OR the gasifier produces only HCAB and no Class A product.

The treatment system gasifier/oxidizer arrangement <NUM>, <NUM> uses mixed biosolids as the exclusive fuel source for the treatment process. Volatile syngas, predominantly hydrogen and carbon monoxide, is produced in the gasifier <NUM> and pulled into the oxidizer <NUM> by the draft that is established and maintained by an induced draft (ID) fan <NUM> located at the end of the process following the wet scrubber and before the activated carbon filter. The production of syngas is an exothermic reaction resulting in temperatures (><NUM> F) adequate to sustain the carbon conversion operation and kill all pathogens.

In operation, the mixed biosolids fuel may remain in the gasifier <NUM> at temperature > 482C (<NUM> F) for <NUM> - <NUM> minutes. The gasifier <NUM> operates under a controlled draft.

The resulting ash from the gasifier <NUM> is one of the stated beneficial use products of the process. The ash may be stored in a covered <NUM>-yard roll-off container providing weeks of storage. Once full, the roll-off container may be replaced with an empty <NUM>-yard roll-off without stopping the process.

The duration of product exposure to elevated temperatures within the gasifier <NUM> kills all pathogens for Class A product and may thermally destroy Per- and polyfluoroalkyl (PFAS) compounds that are extremely harmful to human health to trace limits. The solids content of ><NUM>% satisfies the VAR requirement for Class A product. The gasifier ash may be tested for metals content to prove their qualification as Class A land application product. In the oxidizer <NUM>, the volatile syngas is mixed with ambient air to balance the combustion equation and oxidation occurs, reaching temperatures in excess of 1093C (<NUM> F), which may thermally destroy vapor phase PFAS compounds to trace limits.

A controlled and consistent temperature entering the dryer <NUM> is achieved by mixing the hot flue gas from the oxidizer (> 1093C (2000F)) with ambient air and recirculated moist flue gas from the dryer <NUM> exhaust in a tempering chamber called the blend box.

Target dryer inlet temperature may be achieved with a temperature-controlled damper on the ambient air inlet. Ambient air supply for the blend box comes from the sludge receiving and storage and metering buildings <NUM>, <NUM> thereby establishing the negative pressure to achieve odor control.

The tempering air flow into the blend box may result in air changes every hour in the sludge storage rooms that exceed limits imposed for office space and hospitals.

In addition to benefiting from its temperature cooling properties, recirculated flue gas is conventional practice for reducing NOx emissions. Once in the dryer <NUM>, the tempered flue gas comes in direct contact with the mixed biosolids evaporating the moisture to a dried solids condition of > <NUM>% satisfying Class A requirements.

The dried biosolids product is one of the stated beneficial use products of the process described herein. The Class A dried biosolids product may be stored in a <NUM> cubic meter (<NUM>-yard) roll-off container providing <NUM> days of storage. Once full, the roll-off container is replaced with an empty <NUM> cubic meter (<NUM>-yard) roll-off without stopping the process.

The duration of product exposure to elevated temperatures (mean temperature is > 232C (<NUM> F)) within the dryer is <NUM> minutes satisfying the pathogen kill requirements for time and temperature for Class A product. The solids content of ><NUM>% satisfies a VAR requirement for Class A product. Dried biosolids may be tested for metals content to prove their qualification as Class A product. Furthermore, a proximate analysis may be performed on the dried solids to confirm the energy content is ≥ <NUM>,<NUM> kilojoule/kilogram (<NUM> Btu/pound) satisfying the requirement to sell the product as a renewable fuel.

Air emissions are controlled with a high efficiency mechanical cyclone collector to capture ><NUM>% of particles larger than <NUM> microns. The flue gas stream exits the cyclone to a <NUM>-stage wet scrubber where the final particulate matter may be captured prior to passing through an activated carbon filter prior to emitting to the atmosphere. The <NUM>-stage wet scrubber is equipped with bleach and caustic soda to scrub odors and sulfur emissions, if necessary. The total wastewater from the process is scrubber blowdown which is routed to the headworks of the WWTP for processing.

The treatment process satisfies the time-temperature requirement of Regime B as identified in Table <NUM>-<NUM> of USEPA Part <NUM> Biosolids Rule, Chapter <NUM> for Class A pathogen reduction and reduces the attractiveness of biosolids to vectors by drying the biosolids to > <NUM>% solids content as identified in Table <NUM>-<NUM>, Option #<NUM> of USEPA Part <NUM> Biosolids Rule, Chapter <NUM> for Class A vector attraction reduction.

Beneficial products produced from the treatment facility are Class A products for land application and renewable fuel coproducts.

There may be two chemical storage tanks for the operation of a wet scrubber, which may not exceed <NUM> liter (<NUM> gallons) each. One tank may contain <NUM>% concentrated solution of sodium hypochlorite (NaOCL, bleach) and the other may contain a <NUM>% concentrated solution of caustic soda (NaOH, sodium hydroxide).

The sodium hypochlorite and caustic soda storage containers may be mounted above a containment basin. This basin nay be sized for full leakage potential and contain the material from both tanks if they both emptied at the same time.

In the event of a spill, the contents within the containment basin may be pumped into a secure container and properly disposed in an approved landfill. Operator protection is provided with a pull chain activated, heated shower with integral eye wash station located immediately adjacent to the chemical containment basin. The shower may drain to the headworks of WWTP.

There may be discharge water produced from the wet scrubber, which may be routed directly to a wastewater treatment facility (WWTP).

A facility such as the one proposed may be chosen based on a desire to harness the currently land applied or landfilled dewatered sludge cake produced by a WWTP. There are more than <NUM>,<NUM> WWTP in the United States and all have a sludge disposal problem.

The process shown figuratively in <FIG> produces three types of beneficial use products from different process apparatuses. The project may not generate a waste other than blowdown from the three-stage wet scrubber that may be discharged to the headworks of the WWTP for treatment. Said another way, the process may produce beneficial use products (BUPs), not wastes.

Dewatered biosolids cake ranging between <NUM>% water content to <NUM>% water content may be mixed with self-generated dried Class A biosolids at < <NUM>% water content to create a resulting mixed sludge for the gasifier and the dryer. This mixed sludge may be introduced into the gasifier at high temperatures (482C - 871C (900F - 1600F)) where pyrolysis occurs, and volatile gases are removed and oxidized. After moving through the gasifier, two BUPs can result depending on the energy content of the biosolids cake:.

The thermal drying process may include of a direct contact rotary drum dryer with all associated product collection equipment. The mixed sludge that is sent to the dryer exits the dryer at <NUM>% water content which may then be recycled back to the process to be incorporated with the dewatered biosolids cake via a mixer. A volume of recycled dried product can be adjusted for processing efficiencies. In this event, the non-recycled portion of the dried Class A biosolids is collected for sale as fertilizer or as a renewable energy fuel in the form of alternative coal. If no market is available, the Class A product can be used as landfill cover.

<NUM>) Class A Biosolids- This product is conveyed from the dryer outlet to Class A Biosolids bin where it may be temporarily stored before sold for beneficial uses.

Water blowdown from the wet scrubber may be sent directly back to the headworks of the WWTP to be treated. Water vapor from the drying process may exit the system via the exhaust stack and be sent to atmosphere.

One point worth noting is that the prior art systems have fed the dryer with a fixed flow rate of sludge and changed the firing rate of fossil fuel burners to react to the evaporation rate needed in the dryer. The system here can work differently than this by fixing the firing rate of the gasifier since the energy conversion through gasification must be controlled and then vary the feed rate to the dryer based on the available energy from the gasifier. This one control change is not done in the drying industry.

Another notable unconventional control logic difference is that the induced draft (ID) fan <NUM> is variable frequency driven (VFD) to control draft in the gasifier. The ID fan increases and decreases speed to maintain a constant draft in the gasifier when flue gas temperature controllers (induction air damper in blend box <NUM> and dryer exhaust recirculation fan in blend box) change the total system flue gas flow. A VFD ID fan is critical to maintain desired gasification performance whereas conventional biosolids drying systems have a fixed speed ID fan <NUM> because a controlled draft is not important with fossil fuel burners.

Claim 1:
A biosolids treatment system that treats human biosolids to produce beneficial use products including low carbon ash, high carbon activated biochar, and Class A biosolids with a solids content ≥ <NUM>%, the system comprising:
variable feed conveyors (222a, 222b) that convey a mixed biosolid feed into a dryer (<NUM>) and a gasifier (<NUM>), wherein the mixed biosolid feed comprises a mixture of dried biosolid output from the dryer (<NUM>) and wet biosolid that has not been in the dryer (<NUM>);
wherein said dryer (<NUM>) dries the mixed biosolid feed to a predetermined moisture content to create dried biosolid output (<NUM>) wherein a moisture sensor (<NUM>) measures the predetermined moisture content which is controlled by varying a speed of the variable feed conveyors; and characterized in that
said gasifier uses the mixed biosolid feed as its exclusive fuel source and converts the biosolid feed into usable thermal energy for system use and at least one of the beneficial use products.