Design of Sidestream Treatment for Bioaugmentation at the City of Richmond, VA WWTP

Purpose and Background: In 2005, The City of Richmond began implementation of its Nutrient Reduction Program (NRP). This program is in response to a law passed in March 2005 requiring municipalities to upgrade treatment facilities to comply with the waste load allocation for total nitrogen and total phosphorus. The Bioaugmentation Process Facility at the City of Richmond WWTP is a unique facility that will employ the first full scale installation of the InNitri® process while taking advantage of existing tank volume. This process will provide treatment of the ammonia-laden recycle dewatering centrate stream from the anaerobic digestion system. The purpose of the facility is to promote the growth of a completely separate nitrifier biomass that would be available to safeguard the reliability of the nitrification process in the mainstream bioreactor on a regular basis and shorten the recovery period of the mainstream biological process following a major process disruption event such as extreme cold, high hydraulic flow, or both. Reliability of the nitrification process is critical to sustaining effective removal of total nitrogen. Although the City of Richmond's goal for this project is to protect against nitrification upset, discussion of the facility design is relevant to other WWTPs that are required to maintain reliable nutrient reduction when treating sewage collected from combined systems or in cold weather climates. Design of the Bioaugmentation Process Facility is complete and is currently under construction.Preliminary Evaluation of Sidestream Treatment Processes: During preliminary design both in-situ and external types of bioaugmentation were considered. Generally, sidestream treatment and bioaugmentation processes are employed at WWTPs primarily for process performance improvements particularly as it relates to process stability. The City of Richmond recognized that an external sidestream process provided the best opportunity in meeting its goal of as a safeguard. Because of the configuration of the existing plant infrastructure, the InNitri® system emerged as the process most capable in meeting the objectives within the confines and limits of the physical plant.Application of InNitri® for Bioaugmentation: The sidestream treatment of the centrate produced by dewatering of biosolids and subsequent augmentation of the mainstream treatment process in accordance with the InNitri® process will have a two-fold benefit for the Richmond WWTP. First, use of the bioaugmentation will provide a safeguard for the mainstream treatment process, ensuring the constant presence of an active nitrifying microbial community. A major disruption of this nitrifying microbial community could jeopardize the mainstream treatment process's ability to comply with the total nitrogen waste load allocation. Due to the WWTP location within the floodplain and its service of a partially combined sewer system, potential disruption events have been identified. Cold temperatures coupled with high hydraulic loads (due to combined sewer service) tend to wash-out solids from the mainstream treatment process. Also, a major flooding event from the James River could completely inundate the mainstream treatment process (see Figure 1) resulting in massive washout. During winter months, full recovery of the nitrifier biomass from any major disruption event could take several months due to significantly colder influent wastewater temperatures which hinder nitrifier biomass growth (see Figure 2). Minimizing this recovery time is essential to comply with the waste load allocation. The sidestream process, then, promotes the growth of a completely separate nitrifier biomass that is then “wasted“ directly to the mainstream reactor to bolster the nitrification biology and enhance recovery. Second, this application of bioaugmentation involves the equalization and nitrification of the dewatering centrate prior to its discharge to the head of the mainstream treatment process. The mainstream treatment process will be subject to more stable demands which could increase system longevity and reliability.Major Elements of Design: The City of Richmond uses centrifuges to dewater anaerobically digested biosolids over a typical period of 8-12 hours per day. The centrate flows via gravity to the head of the primary sedimentation tanks. The conduit that conveys the centrate is routed adjacent to former sludge holding tanks, which are rarely used and available for modification and use. Thus, the Bioaugmentation Process Facility was designed to operate within the physical constraints of existing space as opposed to requiring additional tank volume. GPS-X modeling software was used to optimize the process design to meet the goal of promoting biomass growth. Physical tank dimensions, centrate characteristics (defined through sampling, see Table 1), and temperature were important modeling inputs. Incorporated into the Bioaugmentation Process Facility bioreactor design were both aerobic and anoxic zones to provide for recovery of alkalinity during the denitrification. The balance between alkalinity recovery via denitrification with supplemental carbon versus addition of magnesium hydroxide for alkalinity will be discussed. This abstract will discuss bases for design for essential equipment such as tapered fine bubble aeration with turbo blowers, anoxic zone mixing using hyperboloid mixers, and internal recycle pumping for alkalinity recovery.Conclusion: Bioaugmentation is required to maintain long term compliance with the total nitrogen waste load allocation for the City of Richmond WWTP. This project will be the first full scale installation of InNitri® Process. Design of the facility was optimized for biomass growth and ease of operation.