OPTIMIZING BIOLOGICAL PHOSPHORUS REMOVAL FROM AN SBR SYSTEM MIDDLEBURY, VT. Paul Klebs, Senior Applications Engineer Aqua-Aerobic Systems, Inc.

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1 OPTIMIZING BIOLOGICAL PHOSPHORUS REMOVAL FROM AN SBR SYSTEM ABSTRACT MIDDLEBURY, VT Paul Klebs, Senior Applications Engineer Aqua-Aerobic Systems, Inc. The Middlebury Wastwater Treatment Plant, located in Middlebury, Vermont utilizes a Sequencing Batch Reactor (SBR) system to process their wastewater. The SBR technology was chosen because of its ability to efficiently provide wastewater with low values of BOD and TSS, with the added ability to perform nutrient removal without the need for supplemental carbon addition or separate anoxic mixing tanks. In operation of the plant, Middlebury operators have seen the low effluent BOD and TSS values they expected, but are also achieving very high removals of phosphorus biologically. The system utilizes minimal amounts of metal salts and is achieving low effluent Total Phosphorus values. The Middlebury SBR system consists of a four-basin SBR designed to treat an average flow of 2.72 MGD, and a peak flow of 4.6 MGD. The system utilizes SBR basins that are each 82 x55 with an 11.2 LWL and an 18.0 HWL. Each SBR basin is designed to operate 5 cycles per day. The influent for the system was designed for a BOD of 388 mg/l, TSS of 258 mg/l, TKN of 40 mg/l, and Total P of 6 mg/l. The effluent required by the design was 30 mg/l BOD, 30 mg/l TSS and 0.8 mg/l Total P. Actual influent data (March 2002) showed an average flow of 1.13 MGD, with influent BOD of 387 mg/l, TSS of 271 mg/l and Total Phosphorus of mg/l. Effluent from March 2002 showed a BOD of 5.3 mg/l, TSS of 6.2 mg/l and Total Phosphorus of 0.36 mg/l. Further influent and effluent data is displayed in Table 1. page 1 of 6

The SBR technology was chosen because of its ability to efficiently provide wastewater with low values of BOD and TSS, with the added ability to perform nutrient removal without the need for

2 Table 1 SBR MONTHLY PERFORMANCE DATA INFLUENT EFFLUENT DATE FLOW BOD 5 TSS P BOD 5 TSS P ** 2001 (MGD) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC AVERAGE DESIGN ** 0.8 mg/l design requirement for Total P with chemical addition, 3 mg/l w/o chemical addition. Upon startup of the facility, it was determined that the plant was receiving a much higher load of influent phosphorus than the system was originally designed for. The original plant design was based on an influent Total P of 6 mg/l. Actual influent data from the first month of operation (June 200) indicated that the influent Total P averaged mg/l and the effluent Total P averaged 4.5 mg/l. The influent phosphorus was greater than the design value due to the SBR accepting influent from a dairy and a brewery, both of which discharged greater phosphorus loads to the Middlebury SBR than were anticipated. At the time the data was compiled, the alum addition system was off-line; therefore the phosphorus removal observed was due to biological page 2 of 6

970 518 412 20.3 6.1 6.5 0.4 OCT 0.850 496 19.0 7.1 7.4 0.4 NOV 0.810 456 390 6.6 10.8 0.5 DEC AVERAGE 1.038 437 341 17.8 6.6 9.7 1.0 DESIGN 2.720 388 258 6 30.0 30.0 0.8 ** 0.

3 uptake. The SBR system removed approximately 19.6 mg/l of phosphorus via biological uptake. Middlebury s SBR utilizes distinct treatment phases in order to achieve its targeted phosphorus removal. Each phase and its effect on phosphorus removal is described below. Mix Fill Phase The Mix Fill phase represents a non-aerated fill stage where the SBR reactor is completely mixed. Due to the absence of D.O. in the reactor, denitrification is capable of occurring during the Mix Fill phase. As a result, the residual level of nitrate nitrogen that previously existed in the supernatant layer is depleted to a near-zero concentration level. At the start of the Mix Fill phase, the effective mixing of the biomass with the influent wastewater in an anoxic environment results in a substantial release of phosphorus from the cell mass to the liquid medium. A typical monitoring program would indicate a steady increase in the concentration of phosphorus during the Mix Fill phase. The duration of the Mix Fill phase for the Middlebury plant is 45 minutes. React Fill Phase The React Fill phase represents an aerated fill stage. The basin continues to be completely mixed. The introduction of oxygen converts the reactor from an anoxic environment to an aerobic environment. The concentration of total nitrogen present in the reactor steadily declines as the React Fill phase is completed due to the ability to nitrify during aerated periods of React Fill and to denitrify during non-aerated periods of React Fill. During the initial period of the React Fill phase, the onset of aerobic conditions in the reactor allows the microorganisms to "take in" phosphorus. Therefore, the phosphorus that was previously released into solution page 3 of 6

Due to the absence of D.O. in the reactor, denitrification is capable of occurring during the Mix Fill phase.

4 (during the Mix Fill phase) is now taken back into the cell mass. The phosphorus present in the influent is also taken in by the biomass. The anoxic periods during the React Fill and React phases are not long enough to allow a re-release of phosphorus from the biomass into the liquid medium. Therefore, the effluent from the reactor will contain a low concentration of total phosphorus. The duration of the React Fill phase for the Middlebury plant is 27 minutes. React Phase Due to the true batch nature of the SBR system, wastewater is not entering the reactor during the React phase. The React phase represents an aerated non-fill phase. The direct-drive mixer continues to operate and completely mix the reactor, and the aeration system continues to be cycled on and off. This alternately creates aerobic and anoxic conditions. Phosphorus that was taken up in the React Fill phase, continues to remain in the cell mass during the React phase. The duration of the React phase is 117 minutes for the Middlebury plant. Biological Phosphorus Removal The basic principle of biological phosphorus removal in the Middlebury SBR system is to expose bacteria to alternating aerobic and anoxic to anaerobic conditions. Under anoxic to anaerobic conditions such as those found in the Mix Fill phase, some bacteria (especially Acinetobacter) have the ability to take in organic substrate, such as BOD 5. In order to obtain the energy to incorporate BOD 5 into the microbial cell under anaerobic conditions, the bacteria release phosphorus into the wastewater. When aerobic conditions are restored at the beginning of the React Fill phase, the organic substrate that has been taken in is converted to energy and cell mass. This allows the bacteria to take in phosphorus, and after being exposed to anaerobic page 4 of 6

Therefore, the effluent from the reactor will contain a low concentration of total phosphorus. The duration of the React Fill phase for the Middlebury plant is 27 minutes.

5 conditions, the bacteria can take in more phosphorus then they need to fulfill their nutrient needs. This will lead to an excess of phosphorus in bacteria exposed to anoxic to near anaerobic conditions followed by aerobic conditions. The biomass in the SBR may contain 3-5% phosphorus in a system designed to perform biological phosphorus removal. This is compared to approximately 1-3% phosphorus in the biomass of a system not designed for enhanced biological phosphorus removal. It is this enhanced biological removal that allows Middlebury to remove approximately 20 mg/l of total phosphorus from the influent without the use of metal salt addition. Due to the nature of it s design, the SBR utilized at Middlebury is uniquely suited to perform biological phosphorus removal, as well as to achieve high quality effluent relative to BOD and TSS. Details of the operation of the Middlebury SBR system and the effect on effluent quality are described below. Factors affecting the biological phosphorus removal capability of the SBR system at Middlebury are: 1. Nitrate and Dissolved Oxygen Concentrations in the Mixed Fill phase 2. Effluent Suspended Solids 3. Availability of Organic Substrate 4. Sludge Age Control of nitrates in the SBR at Middlebury is performed via the use of the non-aerated Mix Fill phase, as well as non-aerated periods of React Fill and React. Control of effluent suspended solids occurs via adjustment of the Settle phase in the SBR process. Effluent quality also page 5 of 6

The biomass in the SBR may contain 3-5% phosphorus in a system designed to perform biological phosphorus removal.

6 benefits from the fact that the SBR is quiescent during the Settle and Decant phases, leading to perfect settling conditions. The Settle phase at the Middlebury plant has a 45-minute duration. Organic substrate that is utilized in the biological phosphorus removal process is available in the form of the influent BOD. The Middlebury plant is designed to nitrify and denitrify. This is due to the fact that very low nitrate concentrations are required in the reactor as the Mix Fill phase begins. This requires operating at a longer sludge age than would be required for systems that only perform BOD and TSS removal because the growth rate of nitrifying bacteria compared to those that perform organic removal is slower. The design sludge age for the Middlebury system is 9.2 days. page 6 of 6

This is due to the fact that very low nitrate concentrations are required in the reactor as the Mix Fill phase begins.

Phosphorus Removal. Wastewater Treatment

Phosphorus Removal. Wastewater Treatment Phosphorus Removal In Wastewater Treatment by Derek Shires (512) 940-2393 Derek.Shires@ett-inc.com Why do we care? Eutrophication of surface water - Especially reservoirs Maximum agronomic uptake - Limiting