Maximizing Energy Recovery from Sludge Kitchener WWTP Case Study

摘要

The Kitchener Wastewater Treatment Plant (WWTP) is a conventional secondary treatment facility, with a rated capacity of 122,745 m~3/d. An upgrade project is being completed to provide reliable and efficient operation in the long term and improve effluent quality. With continued use of mesophilic anaerobic digestion at the Kitchener WWTP, there is opportunity to recover energy 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 Kitchener WWTP upgrades project were evaluated in this study. Currently, a portion of biogas generated is burned in gas boilers, to create heat to maintain the digester 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 building heating. Based on a review of technologies, five options were developed for evaluation for the Kitchener WWTP with a goal of maximizing the recovery of energy from the inherent energy value of biosolids. All of the options incorporate recovering energy from gas for heat (baseline case) 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 CHP facility 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 cost advantage in the range of $11 to $14 million, relative to no CHP facility. Implementation of a CHP facility with primary sludge thickening will further reduce heat input requirements for digesters thereby increasing net energy recovery. This option also provides advantages in terms of greater digester contingency capacity and less potential for odour generation from primary clarifiers. To maximize the value from the biosolids, the option of purifying a small portion of the gas for use as vehicle fuel was also evaluated. The option was based on using excess heat available approximately 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 simply injected into the natural gas distribution system at a sale price equivalent to the natural gas purchase price of $9 per GJ, the gas value would drop to approximately 50% of the equivalent value of the CHP and heat recovery option.