Lessons Learned from an Anaerobic Digestion Improvements Project at the Jackson Pike WWTP in Columbus, Ohio

Introduction: This project involved planning the upgrades to the digester improvements at the Jackson Pike Wastewater Treatment Plant (JPWWTP) in Columbus, Ohio. Figure 1 is an aerial of the JPWWTP. This paper provides insights and lessons learned where Jacobs served as the consulting engineer working with the JPWWTP managers, engineers, operators, and maintenance staff (City). The JPWWTP has a design capacity of 68 MGD with a peak wet weather secondary treatment capacity of 150 MGD. There are six 1.0 MG digesters with membrane covers and a pumped mixing system. The design average sludge flow to the digesters is 249,000 gpd at 5.3 percent solids with a volatile solids loading of 87,000 lbVS/day. Figure 2 is a solids process flow diagram with flows and loads for average conditions. Six 1.0 MG mesophilic anaerobic digesters provide solids stabilization and produce digester gas. A digester gas-fueled cogeneration system has been designed with construction set to begin in 2021. The cogeneration system is projected to provide about half of the total electricity used at the plant. Biosolids from the plant are made into a popular organic mulch and soil enrichment product (Com-Til) at the Compost Facility operated by the Division of Sewerage and Drainage. The key driver for this project is the current digester system nearing the end of its life and is ready for an upgrade. For example, the existing membrane digester covers have a relatively short life of six to 8 years and two are currently in need of replacement. Figure 3 is a picture of an existing digester cover. An aged, custom 'trombone valve' mixing system circulates 1,000 gpm with a 20 hp pump providing only 1.4 turnovers per day. Figure 4 is a picture of the trombone valve mixing equipment. With the inadequate mixing, grit accumulates at the bottom of the digesters requiring frequent cleaning every six years when the covers are replaced. Digester improvements are needed for the digester covers, mixing, and heating systems. Planning: The entire solids processing system was evaluated during the planning period. The flows and solids loadings to the digesters were determined from historical data of primary and secondary sludge production and thickening. Future flows and loads due to growth will be routed to the sister plant (Southerly). Existing primary and secondary sludge holding tanks provide flexibility in feeding the digesters. Sludge is pumped through grinders before entering the digesters. Digested biosolids are pumped to storage tanks before dewatering. Class B Biosolids are trucked to the Compost Facility where a soil enrichment product is made for beneficial use. Since Class B biosolids are beneficially used, producing Class A Biosolids through anaerobic digestion is not required. High performance anaerobic digestion processes were evaluated including mesophilic, acid-gas, various configurations of thermophilic, and thermal hydrolysis. Considerations of alternate digestion processes included: - Digester gas production and quality - Digester capacity - Ability to accept co-digestion feedstocks - Dewatering performance Planning was conducted through a series of collaborative workshops for the digestion improvements. The focus of the improvements was on the digester covers, digester mixing, and digester gas storage. The overall objectives were clarified at the beginning of the planning period. A cost benefit comparison was conducted using multi-criteria analysis. Non-economic objectives were compared to the economic objective of lower life cycle cost. Non-economic criteria included operation & maintenance, process reliability, regulatory, sustainability, and community considerations. Specific objectives were identified and prioritized for each component as follows: Digester Covers - O&M: life expectancy, odor containment - Process Reliability: compatibility with digestion processes (e.g., thermophilic) - Sustainability: digester gas containment, digester gas storage Digester Mixing - O&M: grit entrainment, maintenance accessibility, sensitivity to liquid level, energy use - Process Reliability: scum/foam management, ragging, redundancy, mixing effectiveness - Sustainability: digester gas production Preliminary Design: A preliminary design was completed for each of the different options for digester covers, digester mixing, and digester gas storage. The options were ranked based on non-economic criteria. Life cycle costs were determined based on capital cost and operating cost estimates. The options were then compared taking life cycle cost and non-economic benefits into account. The following options were considered: Digester Covers: - Floating - Gas Floating - Carbon Steel Fixed - Stainless Steel Fixed - Concrete Fixed Digester Mixing: - Existing trombone valve pumped - Pumped jet and nozzle - Draft tube - Linear motion mixers The digester covers, mixing, and gas storage options were considered as part of an overall evaluation. Continuing with mesophilic digestion was selected with the ability to convert to thermophilic digestion in the future if additional capacity were required. Sludge withdrawal from the digesters for removal of foam and floatables from the surface were considered along with removal of grit from the bottom. Attention was given to the gas dome for the distance between the digester gas piping and the maximum sludge level to avoid foam in the gas piping. Biogas use and heat supply were covered by a separate cogeneration project. Replacement of the water to sludge heat exchangers was addressed. Project Results: The results of the cost benefit comparison of the digester covers and mixing are summarized in Tables 1 and 2. The digestion improvements solution with the most benefits for the life cycle cost is concrete covers with linear motion mixers. A combination of four concrete covers and two membrane covers was ranked second. Stainless steel covers ranked third. For mixing, linear motion mixers ranked first and the existing mixing system ranked second. However, the existing system would require new pumps and nozzles for adequate mixing. The pumped nozzle hydraulic mixing system would provide adequate mixing but would have a higher energy demand. Gas storage would be provided by gas membrane holders at ground level or on top of the concrete covers. The results of the multi-criteria analysis enabled the City to consider the pros and cons of the different options and make an informed decision on the digestion improvements. Lessons learned from this digestion improvement project will provide guidance to managers, engineers, operators, and maintenance staff of other water resource recovery facilities that are planning on upgrading their anaerobic digestion facilities.