Troubleshooting Nitrification Process Upsets with a Robust Testing Protocol

Introduction & Approach Wastewater utilities must often navigate biological process upsets due to the vulnerability of microbial processes to toxicity. Nitrification is particularly susceptible to impact from inhibitory substances (Bye et al., 2012; Xiao et al., 2015). This paper describes three WWTPs where deterioration in nitrification performance was troubleshooted successfully. Information in this paper is intended to support industry efforts to navigate process upset events at WWTPs. For each facility, operating data including flows, NH3, TN, TKN, airflows, DO, MLSS, and SRT were assessed. Additionally, WWTP influent and waste streams were analyzed for typical (ammonia, TKN, COD, sCOD) and potentially toxic (QACs, surfactants, amines, isothiazolones, phenols, peracetic acid, metals) compounds. Aerobic biomass activity was evaluated through specific oxygen uptake rate (sOUR) and substrate uptake rate (SUR) testing (Adani et al., 2001). Results and Conclusions Case Study 1 WWTP1 is a BNR facility with capacity of 7 mgd annual average (AA) flow and effluent TN limit of 8 mg/L AA. The treatment process contains aerobic basins and secondary clarifiers with no primary treatment. WWTP1 experienced loss of nitrification in 2022 and 2023, evidenced by elevated ammonia concentrations in the effluent. In 2022, sampling of the plant influent, as well as in the collection system indicated high QAC and zinc concentrations. Batch testing was performed on plant biomass spiked with the influent and collection system streams, showing inhibition of nitrification in the plant biomass as well as in a control biomass. Figure 1 shows data suggesting the continued presence of toxic substances in the plant. No increase in nitrification activity was observed using decanted biomass, suggesting a continued influx of toxicant to the WWTP. Increasing the aerobic volume in service resulted in partial recovery. The process ultimately recovered as influx of toxic substances in the plant influent abated and viable nitrifier inventory increased (1.5 months). In 2023, analyses of plant data indicated that the likely cause of elevated nitrogen in the effluent was nitrifier washout due to over-wasting. The facility was able to recover nitrification by reducing wasting rate and building up nitrifier inventory. The batch testing approach was found to be robust in determining the cause of the inhibition and informing recovery efforts through data analysis in combination with sOUR and SUR tests. A decision tree was provided to support future recovery efforts, outlining checks and proactive actions for prevention and timely resolution of issues (Fig 2). Case Study 2 WWTP 2 is a BNR facility with treatment capacity of 7.5 mgd AA and effluent TKN and TN limits of 3 mg/L (monthly) and 8 mg/L AA, respectively. The process includes primary clarifiers, BNR basins, secondary clarifiers, and gravity sand filters. In February 2022, the facility experienced near complete loss of nitrification. Influent characterization did not indicate toxicity in the plant influent. However, sOUR and SUR analyses on decanted biomass resulted in a clear and immediate increase in activity, suggesting the presence of inhibitory compounds in the liquid biomass matrix (Figure 3). sOURs did not change when using influent from WWTP2 versus a control facility, indicating that carbon removal was not impacted. Consequently, the facility increased wasting rate to accelerate washout of the embedded toxicant(s) from the process. The basins were then re-seeded with a fully nitrifying biomass from a nearby facility and a decision tree was provided (Figure 4). The chosen strategy of biomass wasting and re-seeding were successful in helping the facility recover nitrification. Case Study 3 WWTP 3 is BNR facility with Treatment capacity of 15 mgd AA and seasonal NH3-N limits (Winter: 18.5 mg/L; Summer: 6.5 mg/L). The process includes primary clarifiers, BNR basins, secondary clarifiers, and disinfection. In October 2020, the facility experienced a process upset characterized by slime growth, loss of nitrification, low DO, high SVIs, and elevated effluent cBOD, TSS and NH3-N. Operational data, microscopy, sampling, and toxicity testing indicated excessive sulfur may have caused the issues. Additionally, a number of industrial dischargers were identified, that had potentially caused previous process upsets. To combat future upsets, the following were recommended: (1) Routine examination of MLSS to identify early signs of deteriorating performance. (2) Addition of a selector zone for the selection of floc forming bacteria. (3) RAS chlorination (4) Installation of SentryTM probes to provide real-time monitoring of influent wastewater. Recommendations A comprehensive and robust protocol can help identify and address the process upset at WWTPs. The protocol proposed in this study includes review of facility performance leading up to and during the performance deterioration, waste stream characterization, and biomass activity testing. periodic sampling of non-routine constituents such as surfactants that are common nitrification toxicants, proactive monitoring of influent characteristics using sensors and sampling to provide early warning of potential detrimental impacts are recommended. Prompt response and robust testing can provide clear insight into causes of process upsets.