A New Strategy to Control NOB in the Mainstream Anammox Process using Centrate from Anaerobic Digester

Introduction Since its discovery in the 1980s, significant research continues to be conducted in Anammox process where, anaerobic ammonium-oxidizing bacteria utilize nitrite as an electron acceptor to oxidize ammonia to N2 gas, with about 10% of nitrogen being converted to nitrate (1). While nitrite is essential for anammox, nitrogen in wastewater primarily exists in the form of ammonium, thus ammonia-oxidizing bacteria (AOB) are employed to initially oxidize a portion of the ammonia to nitrite. Nitrite-oxidizing bacteria (NOB) however, compete with AOBs by oxidize nitrite rapidly to nitrate and hinder the anammox process. In side-stream treatment, effective NOB suppression is possible due to elevated influent temperature, pH, and NH4 concentration. These merits however do not exist in mainstream. To overcome these limitations and achieve stable partial nitritation (PN), various methods to suppress NOB have been employed (2; 3). The goal of this research is to improve the performance of PN in the mainstream process with an innovative and repeatable strategy to control growth of NOB, by employing available resource at a treatment facility with minimal process disruption and using supernatant from an anaerobic digester (AD) presents the best solution for this. AD supernatant offers the advantages of; warm temperature and environment to instantaneously inhibit NOB as well as alkalinity to buffer pH changes. Method A lab-scale reactor was operated for 2 years to derive basic and optimized operational parameter Based on these results; a pilot plant was operated at the city of LA's Hyperion WWTP for 8 months. Lab-scale (Proof of concept): Three different media types with diverse surface areas (M-1, M-2, M-3) were used in the lab-scale reactors. The innovative NOB control protocol involved a 100% fill and decant cycle (fig 1.1a), DO/NH4 ratio target was maintained as 0.2. Instant introduction or AD supernatant when effluent nitrate concentrations consistently exceeded 5mg/L. Monitoring of free ammonia (FA) and free nitrous acid (FNA) was done to help stimulate the growth and performance of AOBs. Pilot Scale: Based on the optimized results from lab-scale operation, a 1.3 m3 pilot plant operation (fig 1.1b) was carried out in a similar manner using M-2 (highest PN performance media) and the secondary effluent of the Hyperion WWTP as the influent, with an average concentration of 50+/-3.5 mgNH4-N/L for about 250 days. Two (2) different supernatant exposure methods were tested in the pilot based on pH control. Results Fig 1.2 shows the effluent nitrogen concentrations over the operational period. The 100% fill and decant cycle strategy significantly helped in shortening the start-up phase relative to other mainstream processes (2). To optimize the PN performance and economically control NOBs, two different inhibition methods (mainly based on initial supernatant pH control) were adopted to monitor the time required between each inhibition reaction before effluent NO3-N exceeded the set mark of 5mg/L, at which point optimal PN is hindered. It was identified that employing exposure method I (no initial pH control) took fewer days and enhanced the inhibitory potential of NOB and help maintain effluent NO3-N levels below 5mg/L for longer periods relative to exposure method II. Supernatant exposure method II involved controlling initial pH of supernatant to 8.5 by dosing with phosphoric acid and this pH level maintained for first 10 hrs prior to an inhibition reaction. Since pH and temperature determine the reactors instantaneous FA and FNA concentrations, these were monitored for the two distinct exposure methods. Fig. 1.3 shows drastic reduction in NH4 and FA in the exposure method I. Thus, for mainstream PN processes at plants where AD is available, direct application of the AD supernatant without pH control in a regular and recurrent manner can successfully inhibit NOB while improving the performance of AOB in treating higher N-loads. To confirm the effectiveness of the inhibition reaction with AD supernatant a validation of AOB performance and a corresponding effective NOB inhibition was carried out for 20 hrs using NH4-N and NO2-N as the batch experiment substrate, respectively (Fig. 1.4). The minimal nitratation (fig. 1.4b) that occurred at relatively low NO2-N substrate levels confirms the successful inhibition and absence of NOBs after employing the innovative supernatant exposure inhibition reaction. Summary and Recommendation This research successfully achieved a stable mainstream PN operation at high N loading rate both in the lab-scale and pilot scale by employing a reliable, economical, and repeatable NOB inhibitory method, using AD supernatant. By implementing a 100% fill and decant sequence, relying on elevated AD supernatant temperatures, regular inhibition reaction could be carried out when effluent NO3-N recurrently exceeds 5 mg/L. This new strategy utilizes available plant resources, thereby reducing cost of employing alternative NOB inhibitory techniques. However, other control measures must be selected at those plants that do not have digesters. The next phase of this research involves direct application in the full-scale plant, identification of inhibitory sequence for a smooth operation in a full-scale plant as well as investigation of alternative environmentally friendly control measures for plants without ADs.