Maximizing Energy Recovery from Sludge Kitchener WWTP Case Study

摘要

The Kitchener Wastewater Treatment Plant (WWTP) is a conventional secondary treatmentfacility, with a rated capacity of 122,745 m~3/d. An upgrade project is being completed to providereliable and efficient operation in the long term and improve effluent quality. With continueduse of mesophilic anaerobic digestion at the Kitchener WWTP, there is opportunity to recoverenergy from the sludge, while reducing operating costs by realizing revenue from the energy.Five different options for digestion and energy recovery that could be included in the KitchenerWWTP upgrades project were evaluated in this study.Currently, a portion of biogas generated is burned in gas boilers, to create heat to maintain thedigester temperature. In the summer, most of the gas is flared, while in colder winter months,more gas is required to generate heat for the digesters, and some heat is also used for buildingheating. Based on a review of technologies, five options were developed for evaluation for theKitchener WWTP with a goal of maximizing the recovery of energy from the inherent energyvalue of biosolids. All of the options incorporate recovering energy from gas for heat (baselinecase) or for electricity generation using cogeneration in a combined heat and power (CHP)facility. Primary sludge thickening, and thermal hydrolysis process (THP) for sludge preconditioning,were evaluated in the different options, to maximize energy recovery.Detailed energy and cost evaluations of the 5 options were completed. Implementation of a CHPfacility to generate electricity and heat from digester gas will realize a benefit of approximately$600,000 per year at current flow and $1.0 million at design flow, resulting in a 20 life-cycle costadvantage in the range of $11 to $14 million, relative to no CHP facility. Implementation of aCHP facility with primary sludge thickening will further reduce heat input requirements fordigesters thereby increasing net energy recovery. This option also provides advantages in termsof greater digester contingency capacity and less potential for odour generation from primaryclarifiers.To maximize the value from the biosolids, the option of purifying a small portion of the gas foruse as vehicle fuel was also evaluated. The option was based on using excess heat availableapproximately nine months per year, which would be equivalent to up to 1,000 L/d of gasoline.The analysis showed that more energy can be recovered at higher value, approximately 220%more, if purified gas is used to replace gasoline in vehicles. However, if gas were simplyinjected into the natural gas distribution system at a sale price equivalent to the natural gaspurchase price of $9 per GJ, the gas value would drop to approximately 50% of the equivalentvalue of the CHP and heat recovery option.