Wastewater treatment control

Wastewater treatment controls that maintain the ORP of a mixed liquor at a relatively stable value during a wastewater treatment process such as an aerated-anoxic wastewater treatment process. The wastewater treatment control may compare a setpoint value of ORP and a measured value of ORP of the mixed liquor, generate a control signal based at least in part on the comparison, and control a biological nutrient removal control parameter using the control signal. Values corresponding to the control signal may be acquired and utilized to adjust the setpoint value of ORP to maintain stable operating conditions of the wastewater treatment process.

FIELD OF THE INVENTION

The invention relates to wastewater treatment control.

BACKGROUND OF THE INVENTION

Many wastewater treatment facilities utilize an activated sludge wastewater treatment process to treat domestic and industrial wastewater. Wastewater containing organic compounds, nitrogen compounds, and/or phosphorus compounds is introduced into one tank or a series of tanks in the presence of biologically active microogranisms, or biomass, to form a mixed liquor. Reductions in organic compounds, nitrogen compounds, and/or phosphorus compounds are achieved by maintaining specific environmental conditions in each treatment tank.

SUMMARY OF THE INVENTION

In one embodiment, the invention may provide a wastewater treatment method. The method may comprise comparing a setpoint value of ORP of a mixed liquor and a measured value of ORP of the mixed liquor, and generating a control signal based at least in part on the comparison. The method may also comprise controlling a biological nutrient removal control parameter using the control signal, acquiring at least one value corresponding to the control signal, and adjusting the setpoint value of ORP using the at least one value.

In another embodiment, the invention may provide a wastewater treatment method. The method may comprise determining a variation of a measured value of ORP from a setpoint value of ORP of a mixed liquor, and controlling operation of a device based at least in part on the determined variation. The device regulates a biological nutrient removal control parameter. The method may also comprise using data corresponding to the step of controlling a device to adjust the setpoint value of ORP so at least one operating characteristic of the device is maintained substantially within an established range of variation.

In yet another embodiment, the invention may provide a wastewater treatment method. The method may comprise mixing wastewater and activated sludge to form a mixed liquor, treating the mixed liquor under aerated-anoxic conditions, and using a control to compare a measured value of ORP and a setpoint value of ORP. The measured value of ORP may be measured using an ORP sensor. The control may generate an output corresponding to the comparison of the measured value of ORP and the setpoint value of ORP. The method may also comprise using the output to generate a control signal, and using the control signal to control operation of an oxygen supply device that regulates a supply of oxygen in the mixed liquor. The method may also comprise acquiring at least one value corresponding to at least one of the output, the control signal, the operation of the oxygen supply device, and a combination thereof, and adjusting the setpoint value of ORP when a predetermined number of values corresponding to the at least one value exceed a first threshold value in a first duration of time and when a predetermined number of values corresponding to the at least one value fail to exceed a second predetermined threshold in a second duration of time.

Further aspects of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings wherein like elements have like numerals throughout the drawings.

DETAILED DESCRIPTION

Biological Nutrient Removal (BNR) wastewater treatment processes generally comprise one or more anoxic zones for removal of nitrogen compounds, and/or one or more anaerobic zones for removal of phosphorus compounds. A number of control parameters affect the performance of a BNR wastewater treatment process. These control parameters can vary with the objectives of the wastewater treatment process and with the design configuration of the BNR wastewater treatment process.

One example of a BNR control parameter can include the supply of oxygen in the mixed liquor. Although dissolved oxygen is typically excluded in both anoxic and anaerobic zones, oxygen-carrying gas may still be supplied to the mixed liquor in these zones as long as the supply of oxygen in the mixed liquor is less than or equal to the biological oxygen demand of the mixed liquor. A negative difference between the supply of oxygen in the mixed liquor and the biological oxygen demand of the mixed liquor is commonly referred to as an oxygen deficit situation. Regulation of the oxygen deficit situation can create anaerobic and anoxic conditions for enhanced biological phosphorous removal and simultaneous nitrification and denitrification. This oxygen deficit situation is commonly referred to as an aerated-anoxic wastewater treatment process.

In an aerated-anoxic wastewater treatment process, maximum process efficiency may not be realized when the supply of oxygen in the mixed liquor is too low (e.g., the supply of oxygen in the mixed liquor is not less than or equal to the biological oxygen demand of the mixed liquor). Generally, overall costs for the wastewater treatment system increase when the process efficiency is not optimized. Similarly, the wastewater treatment process may fail when the supply of oxygen in the mixed liquor is too high (e.g., the supply of oxygen in the mixed liquor exceeds the biological oxygen demand of the mixed liquor). Process failure often provides unacceptable results (e.g., high levels of undesirable compounds in the effluent). Accordingly, regulation of the supply of oxygen in the mixed liquor may be required.

In many wastewater treatment processes, the supply of oxygen in the mixed liquor is regulated using dissolved oxygen measurements (e.g., as the dissolved oxygen concentration of the mixed liquor increases from a desired value, the supply of oxygen in the mixed liquor is decreased, and as the dissolved oxygen concentration in the mixed liquor decreases from a desired value, the supply of oxygen in the mixed liquor is increased). However, in an aerated and anoxic wastewater treatment process, the dissolved oxygen concentration in the mixed liquor is typically zero. Thus, other techniques must be utilized to regulate the supply of oxygen in the mixed liquor.

Another example of a BNR control parameter can include the feed rate of chemicals to the mixed liquor. In one instance, enhanced biological nitrogen removal processes (i.e., denitrification) require the presence of an organic carbon compound in the mixed liquor. This requirement may be provided by the wastewater and/or may require addition of a supplemental organic compound, such as methanol, to the mixed liquor. In another instance, enhanced biological phosphorus removal processes release phosphorus in an anaerobic zone while consuming low molecular weight volatile fatty acid compounds. The volatile fatty acid compounds may be derived from the wastewater and/or added to the anaerobic zone. Similar to the supply of oxygen control parameter, maximum process efficiency may not be realized when the feed rate of chemicals to the mixed liquor does not produce desired concentrations of chemical in the mixed liquor.

Another example of a BNR control parameter include internal recirculation flow rates that recycle the mixed liquor to other zones of the BNR wastewater treatment processes. The mixed liquor may be recycled in some embodiments to enhance the biological removal of nitrogen and/or phosphorus compounds. Yet other examples of BNR control parameters include the return rate of activated sludge, modulation of influent flow, and the like.

The invention provides a new strategy for regulating BNR control parameters for a wastewater treatment process that includes at least one of an anoxic zone, an anaerobic zone, and a combination thereof (e.g., an aerated-anoxic wastewater treatment process). The BNR control parameters may be regulated by controlling a setpoint value of Oxidation-Reduction Potential (ORP). ORP is a parameter that can be measured, for example, by measuring the electro-potential difference between an inert indicator electrode and a standard reference electrode. While the measurement of ORP is relatively straightforward, interpretation of the values of ORP in a wastewater treatment process may be limited by many factors. Therefore, the invention controls the setpoint of ORP using data corresponding to at least one of the BNR control parameters (e.g., variation of the data corresponding to the BNR control parameters). Such control optimizes the reliability and the efficiency of the BNR wastewater treatment process and accounts for variation in the measured value of ORP of the mixed liquor due to changing conditions in the BNR wastewater treatment process.

FIG. 1schematically illustrates one example of a wastewater treatment system10for practicing activated sludge wastewater treatment processes according to the invention. The system10includes a first aeration zone or tank12, a second aeration zone or tank14, a third aeration zone or tank16, and a settling tank or clarifier18. A wastewater influent is introduced into the first aeration tank12via a conduit20. The wastewater generally contains a combination of organic compounds, nitrogen compounds, and/or phosphorous compounds. The wastewater may be subjected to screening and/or a preliminary sedimentation treatment to remove large particulate materials prior to introduction into the first aeration tank12. An activated sludge is introduced into the first aeration tank12via a conduit22. A majority of the activated sludge is recycled from the clarifier18. The wastewater and the recycled activated sludge are mixed (e.g., homogeneously) in the first aeration tank12to form a mixed liquor. Generally, the wastewater and the activated sludge are mixed by air bubbles generated when an oxygen-containing gas (e.g., air) is introduced into the first aeration tank12via an aeration device24.

In the illustrated embodiment, the oxygen-containing gas establishes a supply of oxygen in the mixed liquor that is less than or equal to the biological oxygen demand of the mixed liquor. For aerated and anoxic wastewater treatment processes, a concentration of dissolved oxygen in the mixed liquor is maintained at a value as close to zero as possible. However, because of changing conditions in the wastewater treatment processes, the concentration of dissolved oxygen in the mixed liquor may periodically fluctuate to a value slightly above zero. For example, in some embodiments, the dissolved oxygen concentration in the mixed liquor may periodically fluctuate to a value that is less than 1.0 mg/l and typically less than 0.5 mg/l. In other embodiments, the dissolved oxygen concentration in the mixed liquor may periodically fluctuate to a value higher than 1.0 mg/l.

In the illustrated embodiment, the aeration devices24of the system10each include a plurality of conventional diffusers26mounted to conduits34in a grid-like array. Oxygen-containing gas may be supplied to the diffusers26via the conduits34under pressure through a manifold32. The oxygen-containing gas flows through a plurality of perforations in a membrane of the diffuser26to from a plurality of air bubbles. Air bubbles rising from the diffusers26serve the dual functions of providing the necessary mixing action for the mixed liquor and establishing a supply of oxygen that is less than or equal to the biological oxygen demand of the mixed liquor. In some embodiments, mechanical mixing and/or mechanical aerators may be utilized to supplement or replace the mixing provided by the aeration devices24.

The mixed liquor flows by gravity from the first aeration tank12to the second aeration tank14, and from the second aeration tank14to the third aeration tank16. The environmental conditions of each of the first, second, and third aeration tanks12,14, and16are controlled to optimize the efficiency and the reliability of the overall wastewater treatment process. In the illustrated embodiment, aerated and anoxic wastewater treatment processes are carried out in each of the first aeration tank12and the second aeration tank14. The mixed liquor is transferred from the third aeration tank16through a conduit36into the clarifier18. The activated sludge settles in the clarifier18and a clarified effluent or supernatant is withdrawn from the upper portion of the clarifier via a conduit38for further treatment prior to disposal or reuse. A portion of the settled activated sludge withdrawn from the bottom portion of the clarifier18is recycled by a pump40through the conduit22back to the first aeration tank12as illustrated inFIG. 1. Another portion of the settled activated sludge is removed via a conduit42. In some embodiments, enhanced BNR may be obtained by recycling a portion of the mixed liquor from at least one of the first aeration tank12, the second aeration tank14, the third aeration tank16, and a combination thereof to an aeration tank12,14, and16other than the next aeration tank in the BNR wastewater treatment process sequence. For example, with reference to the pump44and the conduit46shown in dotted lines inFIG. 1, a portion of the mixed liquor of the second aeration tank14and/or the third aeration tank16may be recycled by the pump44through the conduit46to the first aeration tank12.

The first, second, and third aeration tanks12,14and16, can be a single tank or basin divided into three separate zones by partitions or walls as illustrated inFIG. 1, or can be completely separate tanks or basins connected by suitable conduit means. The illustrated wastewater treatment process represents a continuous wastewater treatment process. In other embodiments, the wastewater treatment process represents a batch wastewater treatment process. It should be understood that wastewater treatment processes according to the invention may be performed in other wastewater treatment systems and the wastewater treatment system10is merely shown and described as one such example.

FIG. 2schematically illustrates a first wastewater treatment control100according to the invention.FIG. 3schematically illustrates a second wastewater treatment control200according to the invention.FIG. 4schematically illustrates a third wastewater treatment control300according to the invention. Similar components of the wastewater treatment controls100,200, and300are indicated using like reference numerals in the drawings. It should be understood that aspects of the invention may be utilized in other types of wastewater treatment controls and the wastewater treatment controls100,200, and300are merely shown and described as three such examples.

Each wastewater treatment control100,200,300is designed to maintain the ORP of the mixed liquor at an established setpoint value of ORP. ORP values of the mixed liquor generally tend to remain relatively stable when maximum process reliability and efficiency are obtained. Accordingly, maintenance of the ORP of the mixed liquor at the setpoint value of ORP ensures the BNR wastewater treatment process is operating reliably and efficiently. However, the ORP of the mixed liquor generally includes a time varying response to condition changes in the wastewater treatment process (e.g., a change in the volume and/or concentration of the mixed liquor, a change in wastewater treatment process operating variables (e.g., a recycle rate of activated sludge, chemical concentrations in the mixed liquor), a change in oxygen supply device operating conditions, and the like). Therefore, control parameters of the BNR wastewater treatment process must be monitored and adjusted to maintain the ORP of the mixed liquor at a relatively constant value. Further, the setpoint value of ORP may need to be adjusted if the monitored control parameters of the BNR wastewater treatment process become unstable.

An initial setpoint value of ORP105is established (e.g., manually established by an operator, automatically established using an algorithm) and provided as an input to a control110. The control110compares the setpoint value of ORP and a measured value of ORP of the mixed liquor115. The illustrated value of ORP105and115each include a value between −1000 millivolts and +1000 millivolts. Each measured value of ORP115may be obtained using any suitable means (e.g., measured using an ORP sensor). The control110generates an output that corresponds to the comparison of the measured value of ORP115and the setpoint value of ORP105. In one embodiment, the output of the control110corresponds to a variation or deviation of the measured value of ORP115from the setpoint value of ORP105. The variation may include at least one of the value of variation, the rate of variation, and a combination thereof. As illustrated inFIG. 4, the control110may include a proportional-integral-derivative (PID) control. In other embodiments, other types of controls (e.g., controls having other transfer functions) may be utilized to generate an output that corresponds to a comparison of the measured value of ORP115and the setpoint value of ORP105.

A control signal120is generated using the output of the control110. The relationship between the control signal120and the output of the control110may be a linear relationship or a non-linear relationship. In some embodiments, the output of the control110may be utilized as the control signal120. The control signal120is utilized to control a BNR control parameter.

In one embodiment, the BNR control parameter includes operational characteristics of an oxygen supply device125. The oxygen supply device125may include any device that is able to alter the supply of oxygen in the mixed liquor. With reference toFIG. 2, the oxygen supply device125may include at least one aerator. Aerators or aeration devices alter the supply of oxygen in the mixed liquor by supplying oxygen-carrying gas to the mixed liquor. With reference toFIG. 3, the oxygen supply device125may include at least one liquid level weir. Liquid level weirs alter the supply of oxygen in the mixed liquor by changing the oxygen delivery capability of aeration devices (e.g., by changing the immersion depth of a mechanical aeration device by increasing or decreasing the liquid level relative to the pre-existing immersion depth). With reference toFIG. 4, the oxygen supply device125may include at least one valve. Valves alter the supply of oxygen in the mixed liquor by regulating the flow of air into the treatment tanks. In other embodiments, operation of other types of oxygen supply devices125may be controlled.

Operation of an oxygen supply device125may be controlled in a number of ways. For example, operation of an aeration device may be controlled by at least one of controlling a position of a valve that controls the flow of oxygen-carrying gas to an aeration device, controlling a rate at which oxygen-carrying gas is provided to an aeration device (e.g., using a flow meter), controlling a sequencing process of a plurality of values that are associated with aeration devices, controlling a depth of immersion of an aeration device in the mixed liquor, and a combination thereof. Operation of an aeration device may also be controlled by at least one of controlling a speed of a motor (e.g., a variable speed motor) associated with an aeration device, controlling a staging process of a plurality of drives that are associated with aeration devices, controlling an on/off cycle of an aeration device, and a combination thereof. Operation of a liquid level weir may be controlled, for example, by at least one of controlling an actuator utilized to position the weir, controlling the rate at which the weir changes the liquid level of the mixed liquor, and a combination thereof. Operation of a valve may be controlled, for example, by at least one of controlling a position of a valve that controls the flow of influent and/or activated sludge into the treatment tanks, controlling a sequencing process of a plurality of values that are associated with the flow of influent and/or activated sludge into the treatment tanks, and a combination thereof.

Devices that control other BNR control parameters may be similarly controlled. For example, operation of a device that control the concentration of a chemical in the mixed liquor may be controlled by at least one of controlling a rate at which a chemical is provided to the mixed liquor (e.g., using a flow meter), controlling a sequencing process of a plurality of values that are associated with the flow of the chemical, and a combination thereof. Operation of a device that controls the flow of influent wastewater may be controlled by at least one of controlling a rate at which wastewater influent is provided to the mixed liquor (e.g., using a flow meter), controlling a sequencing process of a plurality of values that are associated with the flow of wastewater influent such that the wastewater is transferred to and from other locations (e.g., storage tanks) as necessary, and a combination thereof. Operation of a device that controls the recycle rate of activated sludge may be controlled by at least one of controlling a rate at which recycled sludge is provided to the mixed liquor (e.g., using a flow meter), controlling a pump motor associated with the flow of recycled sludge, and a combination thereof. Operation of a device that controls the recycle rate of mixed liquor may be controlled by at least one of controlling a rate at which mixed liquor is removed from and provided to aeration tanks (e.g., using a flow meter), controlling a sequencing process of a plurality of values that are associated with the flow of recycled mixed liquor, controlling a pump motor associated with the flow of recycled mixed liquor, and a combination thereof. It should be understood that the above is only a representative listing of some of the BNR control parameters that may be utilized with the invention.

Although the BNR control parameter may be controlled to maintain a relatively stable value of ORP of the mixed liquor, the setpoint value of ORP105may need to be adjusted if the BNR control parameter becomes unstable. A BNR control parameter may become unstable if the initial setpoint value of ORP105was incorrectly established and/or if condition changes in the wastewater treatment process require a change in the setpoint value of ORP105. The wastewater treatment controls100,200, and300each adjust the setpoint value of ORP105when the frequency and/or the amplitude of a variation of values corresponding to the control signal120falls outside of a predefined range. In the illustrated embodiment, the predefined range establishes both maximum and minimum levels of variation. In other embodiments, other mathematical solutions (e.g., standard deviation functions, Fast Fourier Transforms, and the like) may be utilized to adjust the setpoint value of ORP105using the values corresponding to the control signal120.

Values corresponding to the control signal120are obtained and utilized to adjust the setpoint value of ORP105. With reference toFIG. 2, the values corresponding to the control signal120include values corresponding to the control signal120. With reference toFIG. 3, the values corresponding to the control signal120include values corresponding to the output of the control110. With reference toFIG. 4, the values corresponding to the control signal120include values corresponding to the BNR control parameter (e.g., operational characteristics of a device that regulates a BNR control parameter, the operational characteristics including, for example, a speed of a motor, a position of a valve, a depth of immersion, a flow rate, a recycle rate, a concentration, a volume, and the like).

With reference toFIGS. 2–4, the values corresponding to the control signal120are provided as inputs to a rolling average function130. The rolling average function130determines a rolling average value of the already sampled values corresponding to the control signal120. A first comparator135receives an input A representative of the current value corresponding to the control signal120and an input B representative of the current rolling average value. The first comparator135generates an output representative of the difference between the value of the input A and the value of the input B. The output generated by the first comparator135is provided as an input to an absolute value function140. The absolute value function140determines an absolute value of variation (i.e., the absolute value of the difference between the value of the input A and the value of the input B).

A second comparator145receives an input C representative of the absolute value of variation and an input D representative of a first threshold value (e.g., thirty millivolts). The second comparator145generates a first trigger output when the value of the input C is greater than the value of the input D.

The first trigger output is utilized to increment a first counter150. The first counter150is incremented until a first counter threshold is reached. If the first threshold level is reached prior to a first timer160resetting the first counter150, the first counter150generates an output which is utilized to initialize an ORP setpoint adjustment value155. In the illustrated embodiment, the first timer160generates a first timer output which resets the first counter150when a first time duration is met so that if the BNR control parameter is not sufficiently unstable, no action is taken due to the first counter150. The first timer output is also utilized to reset the first timer160. The illustrated ORP setpoint adjustment value155adjusts the setpoint value of ORP105by lowering the setpoint value of ORP105by, for example, ten millivolts. In other embodiments, the ORP setpoint adjustment value155may alternatively adjust the setpoint value of ORP105.

A third comparator165receives an input E representative of the absolute value of variation and an input F representative of a second threshold value (e.g., five millivolts). In the illustrated embodiment, the second threshold value is less than the first threshold value. The first threshold value establishes the maximum level of variation and the second threshold value establishes the minimum level of variation. In other embodiments, the thresholds may be alternatively established. The third comparator165generates a second trigger output when the value of the input E is greater than the value of the input F.

The second trigger output is utilized to increment a second counter170. The second counter170is incremented until a second counter threshold is reached. If the second threshold level is reached prior to a second timer180resetting the second counter170, the logic high output which is normally provided to an AND gate175is changed to a logic low output. In the illustrated embodiment, the second timer180generates a second timer output which resets the second counter170when a second time duration is met so that if the BNR control parameter is sufficiently unstable, an action is not taken due to the second counter180.

The illustrated second timer180generates a second timer output which resets the second counter170when the second time duration is met. The second timer output is also utilized to reset the second timer180and to generate an AND output at the AND gate175if the counter170is providing a logic high output (i.e., the second counter threshold has not been reached). The AND output is utilized to initialize an ORP setpoint adjustment value185. The illustrated ORP setpoint adjustment value185adjusts the setpoint value of ORP105by raising the setpoint value of ORP105by, for example, five millivolts. In other embodiments, the ORP setpoint adjustment value185may alternatively adjust the setpoint value of ORP105.

The circuitry of the wastewater treatment controls100,200, and300may be implemented via software, hardware, or a combination thereof.