Ohio Wastewater Utility Achieves Effective Biosolids Management and Asset Optimization Using Advanced Anaerobic Digestion

INTRODUCTION The Kenneth W. Hotz Water Reclamation Facility (KWH WRF) located in Valley city, Medina, south of Cleveland, Ohio, USA, treats 9 MGD (34 mld) of wastewater serving a population equivalent of 250,000 pe. The three facilities (KWH, Hinckley, and Chippewa Lake) services approximately 35,000 homes and businesses in the cities of Medina and Brunswick, along with several townships in Ohio. This includes liquid food waste from a local food manufacturer from DAF float collected downstream of the manufacturer and additional food waste that is trucked from the manufacturer. The conventional wastewater treatment plant (wwtp) is equipped with primary clarification, a Bio-P process, secondary clarification, filtration, disinfection and biosolids system and handling. In 2018, KWH WRF underwent a $35 million upgrade to reduce its energy demands from an aging old wet oxidation (WAO) process (supplied by Zimpro Process), while reducing its overall operational expenditures. The assets upgrade was made possible using the design build and energy savings performance contract (ESPC) between KWH and Black & Veatch. Two years after its installation, this paper discusses the achievement on an innovative biosolids systems management to achieve circular economy while reducing costs by the KWH WRF. An increase in capacity of the plant to handle higher loads of sludge while reducing its energy demands, was achieved using advanced anaerobic digestion (AAD) as its core technology. AAD makes use of thermal hydrolysis process (CambiTHP) as a pre-treatment and mesophilic anaerobic digestion (MAD). The investment eventually reduced the plant's annual operating costs by approximately 50% (around USD 1.7 million/yr). The installed CambiTHP is also connected to the state-of-the-art CambiPLUS (Process Live Update and Support) platform. PLUS analysis process data online and provides insight into process performance and health, and highlights any issues that might require operator attention. METHODS Former Process Description The site's former sludge line consisted of two sludge streams: one from primary sludge and from secondary or waste activated sludge (WAS). The primary sludge line had systems for thickening, sub-critical wet-air oxidation (WAO), and plate & frame dewatering presses to separate ash and the filtrate prior to sending the filtrate to sidestream treatment. The WAS line had a similar flow. Process Upgrades KWH underwent an extensive revamp where the new sludge line no longer ran on WAO but utilized AAD with CambiTHP as pretreatment. The new system joined the two streams of thickened primary and secondary sludge to go through one treatment stream consisting of the following: screening, pre-dewatering, a cake bin (that can also receive imported sludge cake from other sites), CambiTHP, two MAD tanks, sidestream enhanced biological phosphorus removal system (SEBPR), a struvite removal system (magnesium-ammonium-phosphate, MAP), and post-dewatering. The upgrade resulted in sidestream centrate nitrification/denitrification with SEBPR. Thermal hydrolysis for energy and resource recovery The unit capacity of THP installed prior to MAD at KWH is 23 tDS/d. The system is a compact B2 system, which can be fitted in a 40 ft container system consisting of one pulper, four reactors, and one flash tank. The system is fed with 16.5% DS with primary, secondary solids and food waste. The THP system is operated is a batch process though the feed in and out is continuous. The operating temperatures and pressure are 160°C to 180°C at about 6 bar, respectively, to treat sludge in reactors prior to releasing it in a flash tank. This process disintegrates sludge particles for easier and faster digestion using steam. THP results in an increase in dry solids content and improved dewaterability after digestion, therefore reducing biosolids volumes. The former two existing natural gas electric generators are used to burn the digester gas and operate with a combined heat and power (CHP) operating system. The CambiPLUS platform, is a cloud-based solution that does not interfere with the existing SCADA system, but it's a detailed supplement to it in terms of parameters valuable for optimizing and maintaining continuity and reliability of the process. Notifications and visualized process data, estimated parameters and calculated performance indicators such as energy recovery status, power consumption, plant throughput, and health status of valves and piping, provide an instant overview of process health and performance. PLUS is adaptable according to the users needs and enables monitoring of the THP installation from any place with internet access. RESULTS AND DISCUSSION In a conventional MAD, hydrolysis is a rate limiting step and THP aids increasing the biodegradability of the feedstock using steam. At KWH wwtp, higher biogas volume is generated and it is estimated that the biogas will fuel approximately 30% of the energy requirement and reduce the facilities costs by 50%. In addition, the heat from the natural gas electric generators is recovered to provide the energy needed to generate the steam required for the THP process. With the new process upgrades, KWH was able to increase the loads to plant with imported sludge, raising their sludge intake volume by 60% versus the previous year (Table 1.1). Still, the increase in biosolids output that year was lower than expected, at only 26%. This is aided by the THP capability to improve dewaterability after digestion. With the THP the site's indigenous sludge and the Hinckley plant' sludge, all of the biosolids produced met the Class A requirements under the US Environment Protection Agency's Biosolids Rule, which are free from pathogens and pathogen regrowth and the biosolids has been land applied. Table 1.1. KWH average sludge intake and biosolids production after upgrades Year Sludge Received (t WS/yr) Biosolids Output (tDS/yr) 2017 1928 2018 2111 2019 6278 1260 2020 10042 1591 In addition, the electricity costs decreased by around 38% in the first year (2019) after the upgrades. The increase in imported sludge in 2020 generated more biogas for the plant, offsetting a portion of the electricity costs even further. The results from CambiPLUS platform and other data will be presented at the meeting. The struvite recovery system after AAD and before post dewatering recovered phosphorus intro struvite crystals that can be sold as a fertilizer and the remaining biosolids produced is land applied as a soil conditioner. CONCLUSIONS KWH underwent an extensive plant upgrade by adapting to apply the state-of-the-art disruptive technologies such as THP+MAD (AAD), struvite recovery, and energy recovery systems. Replacing the energy-intensive WAO system with AAD reduced natural gas costs, doubled the loading capacity of the plant to accept more imported sludge, produced more bioenergy, which offset the energy costs. The biosolids generated are Class A and are land applied. Struvite recovery resulted in less maintenance on pumps, pipes, solids process equipment and produced a product as a fertilizer. The KWH wwtp achieved assets optimization, resource recovery thus building on the circular economy model.