Experience Gained and Lessons Learned from Three Years of Full-Scale Bioaugmentation Trials at Winnipeg's North End Plant

In August 2008 one of North America's first full-scale high-strength centrate nitrogen treatment facility implemented at the City of Winnipeg's North End Water Pollution Control Centre (NEWPCC). The continuously fed treatment facility consists of two parallel sequencing batch reactors (SBRs) designed to remove 1,600 kg/d of the total nitrogen from reject water associated with digested and dewatering sludge through a combination of biological nitrification and methanol-induced denitrification. The City recognized that additional nitrogen reduction could potentially be achieved through bioaugmentation of the main stream process with nitrifiers contained in the waste activated sludge (WAS) from the centrate SBR facility. As such, the City supported and participated in full-scale plant-wide nitrogen removal research at the NEWPCC, led by the University of Manitoba as part of a 2009 National Science and Engineering Research Council (NSERC) of Canada research grant.The Centrate SBR provides nitrifier seed to the NEWPCC main stream activated sludge high purity oxygen (AS HPO) process. The goal of the full-scale trials is to induce stable and predictable nitrification in the main stream process at a solids retention time (SRT) less than that required by conventional means in a cost-effective and practicable manner. The NEWPCC main plant has with three independent process trains treating on average about 180 ML/d. The SBR waste activated sludge (WAS), “the nitrifier seed” can be directed to HPO train #1 or #2 to bioaugment nitrification in the main stream process. The goal is to induce plant-wide nitrification by maximizing nitrifier seed to the HPO reactors, applying nominal increases to the main plant at SRT, and minimize the additional costs associated with achieving regulatory driven effluent ammonia and total nitrogen limits. Since 2009, focus was given to bioaugmentation by maximizing nitrifier seed (WAS) from the SBRs, directing the nitrifier biomass from SBRs in the HPO reactors to induce and sustain nitrification at low SRT in the HPO reactors, and controlled adjustment of HPO operating parameters. WAS from centrate SBR facility was supplied to only one of the HPO trains in order to concentrate the nitrifier biomass to maximize the influence of bioaugmentation and assess its performance relative to the other trains.The NEWPCC treatment process was deigned to remove suspended solids and carbonaceous matter and are intentionally operated at and SRT of 2.5 d to avoid nitrification. Biosolids treatment consists of mesophilic anaerobic sludge digesters and centrifuge dewatering. Reject water from the centrifuges is sent to the centrate treatment facility (twin SBRs, each with a volume 5800 m3) for nitrogen treatment. The operational cycle of each SBR is 12h with a 6hr offset between tanks. WAS “nitrifier seed” is withdrawn from the SBR prior to the settling phase and sent to one of HPO trains with the intent to induce nitrification in HPO train. The other HPO trains are used as control reactors to assess bioaugmentation performance.The plant-wide nitrogen bioaugmentation optimization trails in 2009 focused on achieving nitrification in HPO trains by increasing HPO's SRT up to 4.5 days and maximizing the WAS from the SBRs to HPO train #1. At this SRT, all HPO trains were nitrifying able to remove more ammonia than required to comply with the future load limits for the most stringent month of August. However, it was believed that raising the SRT to 4.5 d were responsible for operational foaming in secondary clarifiers and increase in filaments/bulking. In 2010, HPO trains were operated at less than 2.6 days of SRT to prevent these problems from reoccurring, especially the train receiving the nitrifier seed. To induce nitrification without increasing HPO's SRT up to 2009 level, bioaugmentation was conducted by transferring SBR WAS to HPO Train #2, with WAS from HPO Train#1 sent to the SBR to help build biomass in the SBR. An improvement in ammonia removal in Train #2 (seeded train) relative to Train #3 (un-seeded train, reference) was observed (Szoke et al., 2011).The plant-wide nitrogen optimization in 2011 focused on enhancing the bioaugmentation efficiency. SRT of all HPO trains was operated at less than 2.5 days, even lower level than 2010 in an effort to prove that nitrification may be possible while HPO trains were operated at low SRT. The SBR SRT was lowered as much as possible to generate the greatest amount of WAS possible while maintaining full nitrogen removal. This increased WAS biomass provided more nitrifier seed to a HPO train #1 in a same configuration as 2009. The second adjustment made was to recycle WAS from HPO Train #1 as frequently as possible to the SBR to encourage a stable feedback loop (a natural selector for AOBs and NOBs) since July of 2011.