Ann Arbor Tackles Stringent Effluent Phosphorus Limits with a Cost-Effective Side-Stream Phosphorus Removal Solution

BACKGROUND The Ann Arbor WRRF is a 29.5 MGD advanced secondary treatment wastewater facility owned and operated by the City of Ann Arbor, MI. The liquids treatment includes headworks, primary clarification, enhanced biological phosphorus removal (EBPR) , nitrification, final clarification, granular media filtration, and UV disinfection prior to discharge to the Huron River. The solids treatment has been dependent upon the ability to land apply the biosolids for nearly half of the year. During this period, primary sludge (PS) was thickened using gravity thickeners and waste activated sludge (WAS) was thickened by a gravity belt thickener (GBT). Together thickened PS (TPS) and thickened WAS (TWAS) were combined and the biosolids were lime stabilized prior to hauling to local fields. During the rest of the year, when land application was not possible, PS and TWAS were combined in holding tanks before being dewatered by centrifuges. Centrate from the centrifuges was returned to the plant influent, and the dewatered biosolids were hauled to a landfill for disposal. Combining PS and TWAS without the addition of lime, caused phosphate removed by the EBPR process to release into solution and concentrate in the dewatering centrate. For these months, this returned phosphate load reduced the efficiency of the EBPR process, and ultimately increased the effluent Total Phosphorus (TP) concentrations. Ann Arbor is facing challenges associated with increasingly stringent National Pollutant Discharge Elimination System (NPDES) permit limits for effluent phosphorus, with May to December effluent loadings limits reduced from 200 to 50 - 60 lbs total phosphorus (TP)/d. In addition, land application has become more difficult (i.e., farm field availability, transportation issues) causing Ann Arbor to dewater and landfill solids year-round until a longer term biosolids management strategy is developed. To consistently meet the new NPDES permit limits for phosphorus, the City of Ann Arbor engaged a team of consulting engineers to study options for treatment of the high phosphorus centrate side stream resulting from centrifuge dewatering of WRRF sludge. METHODOLOGY The first task in this study was to develop a whole plant process model, which was used to understand the impacts of centrate phosphate loading and evaluate the efficacy of possible mitigation approaches. The project team then identified four alternatives to be considered for mitigation of centrate phosphate loading concerns: 1.Segregated and aerated storage of WAS to prevent phosphorus release. 2.Lime addition to sludge holding tanks to precipitate phosphorus prior to dewatering. 3.Phosphate recovery from the dewatering centrate. 4.Chemically enhanced primary treatment (CEPT) to minimize secondary phosphorus loading. The alternatives were evaluated based on:

*Ability to meet the NPDES limit for TP loading

*Capital and operating costs

*Ease of implementation and operation

*Secondary impacts to plant processes, such as EBPR Following evaluation, bench scale testing was performed for preferred alternatives to validate modeling results and examine operational feasibility. This resulted in one preferred alternative being implemented and optimized at full scale. FINDINGS Simulation results from the whole plant model (Figure 1) demonstrate Alternatives 1, 2 and 4 show the most promise in meeting the strictest year-round final effluent TP load (50 lb/d). Cost analysis, however, showed that the chemical demand associated with Alternative 4 would add nearly $2 million in annual operating cost for Ann Arbor, whereas Alternatives 1 and 2 could be implemented with the fewest operational and infrastructure changes. These alternatives also presented no negative impacts on other plant operations (e.g. EBPR). Benchtop testing for Alternative 1 (Figure 2) demonstrated that it is unlikely that aged PS and aerated TWAS could be mixed for more than a few minutes without releasing more than 20% of the stored phosphate into the centrate stream. Testing observations also identified potential full-scale difficulties with aerating TWAS. Testing for Alternative 2 (Figure 3) predicted a lime dosing rate of 2.3% (18% lime slurry) would provide 90% PO4-P removal in the centrate. Based on this analysis, Alternative 2 — Lime addition to sludge holding tanks, was implemented at the full scale using existing assets in 2023, one year ahead of permit limit reductions taking effect. The plant has met their new NPDES limits (Figure 4) and has been able to optimize the lime dosing operation. Since the initial lime dosing rate of 2.3% was exceeding the targets of 90% PO4-P removal in the centrate (Figure 5), systematic reductions in dosing rate to 1.6% have been completed, resulting in a final sidestream PO4-P removal cost of less than $0.25/lb TP. Under the lower dosing rates, targeted PO4-P removal and daily plant effluent TP limits are still being met on an average basis, however, there is increased variability in daily centrate PO4-P removal rates. This variability has been linked to the final centrate pH, which is thought to be primarily influenced by pre-liming sludge age. The City and its team of consulting engineers is currently investigating and optimizing the sludge holding and lime dosing strategy to reduce this variability. Findings from this optimization will also be included in the final paper.