Lessons Learned from Deammonification Startup treating THP Filtrate at two WRFs

Objectives This paper summarizes key lessons learned from startup of the first two U.S. AnitaMox IFAS deammonification systems implemented downstream of thermal hydrolysis (THP) and anaerobic digestion at the WSSC Water Piscataway Water Resource Recovery Facility (WRRF) (Figure 1) and the City of Raleigh's Neuse River RRF (NRRRF) (Figure 2). Both facilities transitioned to Class A biosolids production, increasing sidestream nitrogen loads from dewatering filtrate. Deammonification was incorporated to reliably maintain compliance with effluent discharge limits. Startup at Piscataway WRRF began in June 2024 and startup at NRRRF begin in May 2025. Status WSSC Piscataway WRRF The deammonification facility includes a common equalization tank and two AnitaMox IFAS reactors, each with a dedicated clarifier and RAS pump. Design loading rates are 0.52 kg NH3 N/m³ d (VLR) and 1.4 g NH3 N/m² d (SALR). - Reactor 2 (R2) was seeded July 15, 2024 in MBBR mode. In early September, R2 experienced an acute toxicity event characterized by rapid nitrite accumulation and loss of attached biomass. Loading was reduced, and the reactor transitioned to IFAS mode in October. Over subsequent months, R2 fully recovered and now operates consistently at capacity. - Reactor 1 (R1) was seeded October 15, 2024 and ramped up without performance issues. Both reactors have treated 500-750 kg NH3 N/day with 75-85% ammonia removal, occasionally operating above design SALR (Figure 3). City of Raleigh NRRRF The NRRRF sidestream system consists of one EQ tank and one AnitaMox IFAS reactor with design loading rates of 0.66 kg NH3 N/m³ d VLR and 1.7 g NH3 N/m² d SALR. High filtrate TSS during early startup required holding flows constant until BFP performance improved, which is why SALR was constant for approximately 100 days (Figure 4). Normalized SALR trends show steady progress, and the reactor is currently treating ~315 kg NH3 N/day with 67% removal. Methodology WSSC Piscataway WRRF Historical influent data (TSS, COD, sCOD) and in-reactor nitrogen species were evaluated to understand R2's inhibition event. Batch activity tests performed by Hazen and Veolia assessed anammox and AOB activity and evaluated whether THP filtrate characteristics were inhibitory. Literature was reviewed to compare observed events with documented inhibition mechanisms. City of Raleigh NRRRF Startup challenges included maintaining design temperature during early-season seeding and addressing high filtrate TSS. A rheology study performed by Bucknell University evaluated viscosity, shear-thinning behavior, and mixing needs for polymer conditioning to improve solids capture across the belt filter presses. Findings WSSC Piscataway WRRF Historical Data: R2 destabilized 53 days after seeding, with nitrite accumulation suggesting either temporary overloading or reduced anammox activity. Operators increased dilution, extended anoxic periods, and transitioned to IFAS mode on Day 108. Persistent nitrite variability led to a 'hard reset' on Day 113 by sharply reducing loading. R1, seeded later, exhibited stable startup, indicating the THP filtrate was not inherently inhibitory. Since recovery, both reactors have handled design and above-design loadings without further events. Activity Testing: Visual inspection and biomass density (Figure 5, R1: 30.9 g/m² vs. R2: 11.4 g/m²) indicate R2 lost anammox bacteria, reducing capacity. A variety of anammox activity tests were performed and the main conclusions from the activity testing include: - Anammox activity increased in both reactors from November to December, indicating recovery in R2 and continued growth in R1 (Figure 6) - R1 carriers + bulk liquid showed strong heterotrophic activity (simultaneous nitrite/nitrate removal), suggesting the need for COD fractionation analysis to determine impacts on TIN removal. - AOB activity was present in both reactors, with limited NOB activity maintained by appropriate ammonia residuals. R2 AOB activity remained lower, consistent with prior biomass loss. Corrective Actions that were taken include: - Reduced mechanical mixing and adjusted aeration intervals to stabilize biofilm. - Improved BFP solids capture through optimized polymer and metal salt dosing. - Adjusted THP operation (temperature, HRT) to reduce sCOD to deammonification. Literature indicates sCOD and colloidal organics can inhibit anammox and AOB, and THP parameter adjustments can reduce sCOD release. City of Raleigh NRRRF THP digestate exhibited high viscosity and solids concentration, requiring increased mixing energy to achieve proper polymer conditioning. The rheology study recommended: 1.High energy, adjustable mixing to provide sufficient velocity gradient. 2.Dilution of digested sludge to lower viscosity. 3.Multi point polymer addition to enhance floc formation and reduce demand. Raleigh installed an additional dynamic mixer ahead of the static mixer with dual polymer injection. Filtrate TSS has since remained below 750 mg/L, supporting stable IFAS loading. Significance Sidestream deammonification is essential for managing the increased nitrogen recycle associated with THP MAD systems at both WSSC Water and the City of Raleigh. The startup experience-including data evaluation, activity testing, filtrate quality improvement, and rheology-informed mixing strategies-provides practical guidance for future facilities implementing AnitaMox IFAS for THP filtrate treatment.