Resilience in Action: Withstanding High-strength Sulfate Containing Wastewater, The Case of Korea's First Full-scale PN-Anammox

Introduction Partial-Nitritation/Anammox (PN/A) processes are increasingly being selected as energy and carbon positive options in the treatment of ammonium-rich side-streams from digestate. Two-stage, sequential PN/A systems were the first to be developed and subsequently, single-stage PN/A (where PN and Anammox reactions occur simultaneously in a single reactor) processes were developed. Although the later possesses the advantage of footprint reduction, these processes have lower HRT and require a balance in aeration and loading such that DO concentrations remain high enough to drive nitritation, but low enough to avoid inhibiting Anammox metabolism, which proves to be a persistent challenge. In general, the highly sensitive nature of the anammox process requires that all plausible inhibiting factors (such as heavy metals, sulfate, etc.) are curbed within practical thresholds. A Two-stage PN/A (AMX(R)) process recently reported by Jung et al. (2019) reduces the HRT required in dedicated PN reactors and improves the system's resilience to high ammonium, COD and solids loads. This technology was therefore employed for the reject water treatment within B City's N-treatment plant in Korea, making it the first and only full-scale domestic application in Korea. This paper discusses the resilience of this AMX technology and adopted strategies in overcoming unforeseen high levels of Sulfate (SO42-) in the influent streams. High sulfate concentrations inhibit anammox and reduces nitrogen removal efficiency (E. Rikmann et al., 2016). Therefore, the exact effects of high strength sulfate on a full-scale PN/A process and the resulting steps to long-term operation to stabilize nitrogen removal efficiency in a practical, economical, and innovative way is presented in this work. Methods Fig. 1.1 shows the industrial treatment processes at B City's N-WWTP with a capacity of 184,000 m3·d-1. A combined 778 m3·d<sup?-1 of concentrated sludge and food waste sludge are introduced into the anaerobic digester. After approximately 30 days in the digester, the sludge is further dewatered into 123 m3·d-1 of sludge cake. Meanwhile, the 817 m3·d-1 of filtrate from the dewatering process, is channelled to the Two-stage AMX(R) (Fig. 1.2) process, which is tailored for treating digester effluents with elevated nitrogen levels. Highest measured sulfate levels exceeded the reported anammox threshold by 90%. Results and Discussion The operation for the full scale PN/A process was segmented into three distinct phases: seeding and SO42- control, continuous nitrogen load increase, and performance validation phase. In Table 1.1, the summarized key operating factors and performance results for each phase is tabulated. In Phase I, the unforeseen high sulfate concentration wastewater (Fig. 1.3) fed into the Two-stage AMX(R) process prevented a quick start-up and acclimation of the anammox. According to S. Zhou et al, 2022, in the absence of sulfate reducing ammonium oxidizing bacteria (sulfammox), sulfate levels exceeding 100 mg/L can instantly inhibit any species of anammox bacteria. Therefore, to prevent complete microbial activity loss and intuitively introduce additional biomass, the energy currency of all living cells, ATP was quickly monitored alongside the specific anammox activity (SAA). By measuring SAA, intermittent biomass was gradually added in a timely and economically manner to prevent further SO42- shock and activity loss. SO42- control was simultaneously carried out. Consequently, the anammox reactor HRT was extended beyond its design, while the NLR dipped far below its design value of 0.87 kgN·m-3·d-1. By phase II, as the sulfate concentration in the influent was controlled and kept below 300 mg·L-1, the nitrogen load in the Anammox reactor was gradually increased. HRT was shortened to 2.1 days (design HRT: 1.5 days). This led to an average NLR of 0.57 kgN·m-3·d-1, with a nitrogen removal efficiency of 81%. In Phase III, the NLR reached its design value of 0.87 kgN·m-3·d-1, and performance was evaluated over roughly a month. The operating HRT and average NLR during this phase stood at 1.5 days and 0.89 kgN·m-3·d-1, respectively. The nitrogen removal efficiency averaged 84.6%, surpassing the operational goal of 82%.By analyzing the data of the existing S-WWTP and K-WWTP, provided by B-City, the N-WWTP was identified to have reduced operating costs by 83%, capital expenditures by 64- 93%, and process footprint by 70-90% by applying the Two-stage AMX(R) technology, compared with conventional technologies. In monetary terms, energy costs were reduced by 60%, chemical costs by 99%, and sludge treatment costs by 49%, saving more than KRW 1.5 billion ($1.15M) per year in OPEX. Construction costs were reduced by KRW 1 billion to up to KRW 7.5 billion ($5.73M). Conclusion This paper summarized the effects of high strength sulfate on the full-scale anammox process and the resulting long-term stable nitrogen removal efficiency by simultaneously counteracting the sulfate and phasing the nitrogen loads in a pragmatic but intuitive and repeatable manner. By employing innovative techniques like direct ferric addition and microbial specific activity monitoring at specific intervals, anammox reseeding was done to prevent complete activity loss in a unique but economical fashion. The resilient AMX technology achieved the designed NLR of 0.87 kgN·m-3·d-1 and exceeded the nitrogen removal performance target of 82% reducing nitrogen loads.