Doing More with Less: Enhanced Biological Phosphorous Removal in a Small System

Small utilities often turn to chemical phosphorous removal when needing to meet stringent phosphorous limits, however, operators at a water resource reclamation facility (WRRF) in New Hampshire are showing that creative aeration can be an attractive alternative. This small WRRF, not designed for biological nutrient removal, achieved greater than 90% phosphorous removal by implementing cyclic aeration in their aeration basins. With needed facility and collection system upgrades in the near future, a better understanding of the process was needed to ensure future performance. Historical process data, additional sampling, and process modeling were used to determine that the aeration strategy used leads to low-DO and anaerobic zones in the aeration basin, thus sustaining enhanced biological phosphorous removal (EBPR).
Introduction
Wolfeboro is a popular vacation destination with a population of approximately 6,400 on Lake Winnipesaukee in New Hampshire. The Town of Wolfeboro's WRRF was built in the 1970's and was designed as an extended aeration activated sludge process. Figure 1 shows an aerial photo of the WRRF which has a design capacity of 0.6 mgd and receives an average daily flow of 0.28 mgd. In 2007 the facility started utilizing cyclic aeration in their aeration tanks in response to tighter effluent nitrogen limits of 10 mg/L TN and 5 mg/L Ammonia as N. While this process change was successful in meeting nitrogen limits, unexpectedly, luxury uptake of phosphorous was also observed as a result. The Town suspected that the luxury uptake of phosphorus in the process was due to EBPR which requires anaerobic and anoxic/aerobic sequential zones and a source of volatile fatty acids (VFAs). In anaerobic zones, phosphorus accumulating organisms (PAOs) consume VFAs and release orthophosphate, then in subsequent anoxic/aerobic zones PAOs store phosphorous in excess of what is required for growth, thus removing dissolved phosphorus across the system. VFAs are typically a product of fermentation in EBPR processes however if the conditions are right VFAs can form in collection systems. Not fully understanding the mechanisms under which luxury phosphorus uptake is occurring, Town staff feared modifying any parameters of their treatment process or lift station operations should they inadvertently interfere with the EBPR process.
Objectives
This evaluation sought to understand the mechanism of phosphorous removal in the Wolfeboro WRRF to support future optimization and planning efforts. Approach Our approach included historic data review, additional sampling, and modeling. Ten years' of process data were evaluated to determine long term wastewater characteristics and establish process performance. Supplemental sampling was conducted to further characterize influent wastewater and the treatment process. A full plant model was developed and calibrated using the data. The model was subsequently used to identify potential explanations for the EBPR found at this plant.
Results
The historical data evaluation confirmed luxury phosphorous uptake with influent total phosphorous (TP) averaging 9.9 mg/L and effluent TP averaging 0.8 mg/L. Daily influent wastewater was sampled for five days and showed an influent VFA concentration of approximately 50 mg COD/L which indicates a significant VFA source in the collection system. Supplemental sampling also included monitoring of key process parameters within the basin to better understand the environmental conditions created by the cyclic aeration in the flow-through basins. Twice daily for one-week, dissolved oxygen (DO), pH, oxidation-reduction potential, nitrate concentration, and orthophosphate concentration were measured along the length of the basin during air on and air off cycles. The process model was developed in BioWin using annual average influent characterization and each of the narrow aeration basins of the WRRF were modeled as six aeration zones in series (AER1 through AER6). The facility controls DO in the aeration basins using DO probes at the effluent end of the basin and a DO set-point of 2 mg/L during air-on cycles. This aeration strategy was modeled by using a DO set-point of 2 mg/L for AER 6, and a controller was used to set the airflow of AER1 through AER5 equal to that of AER6. The process flow diagram and aeration strategy is presented in Figure 2. A controller was also used to adjust the waste activated sludge (WAS) wasting rate to maintain a mixed liquor suspended solids (MLSS) of approximately 4,500 mg/L (the observed annual average MLSS). This resulted in a modelled solids retention time of approximately 10.5 days. WAS is stored in two aerated storage basins and occasionally polymer is added, sludge settled, supernatant decanted and returned to the head of the plant, then thickened solids are hauled to an incineration facility. These storage basins were modeled as aerated digesters with a low DO set-point. While the influent VFAs would decrease the required anaerobic retention time for EBPR, an anaerobic zone is still required for selection of PAOs. As shown in Figure 3, the model predicted that the aeration strategy used by the WRRF would lead to low DO conditions on the influent end (approximately 0.25 mg/L in AER1 during air-on cycles) of the basin and increasing DO concentration along the basin meeting the DO target at the effluent end of the basin (where the DO probe is located). This occurs because the airflow is set for the entire basin based on where the oxygen demand is the lowest (the effluent end of the basin). This low DO environment limits nitrification and therefore allows for an anaerobic zone to quickly develop during air-off cycles due to high BOD loads in the influent and denitrification. This anaerobic zone was confirmed by the orthophosphate results of the supplemental sampling shown in Figure 4 which demonstrates an anaerobic gradient in the basin through PAOs releasing more orthophosphate at the influent end of the basin, and decreased release along the basin.
Conclusions
This evaluation of the Wolfeboro WRRF luxury phosphorous uptake determined: - The flow-through cyclic-aeration strategy applied here can sustain both biological nitrogen reduction and EBPR. - A DO gradient and development of anaerobic zones at the influent end of aeration basins were predicted by the process model because of aeration control using a DO probe at the effluent end of the basin. - Supplemental sampling results indicated high influent VFA concentrations which likely bolstered EBPR at this WRRF. Utilization of the process described here may make EBPR a more attractive, cost effective option for similar small WRRFs that are required to meet nitrogen and phosphorous effluent limits.