Long term solids handling alternatives for New York City water resource recovery facilities (WRRFs)
Authors: Ramalingam Krish , Fillos John , Katehis Dimitrios 
Published: November 2022

New York City Department of Environmental Protection, DEP, owns and operates 14 waste water treatment plants which treat a combined average dry weather flow of 57 m3/sec. (1.3 billion gallons per day). The plants have been upgraded periodically to address more stringent discharge permit limits with the last upgrade been step-feed biological nitrogen removal, BNR, achieved in 2013. More recently Mayor Bill de Blasio issued “2018 Strategic Plan: Enriching our Legacy” effectively requiring a conversion of the treatment plants into resource recovery facilities, (WRRFs), extracting water, energy and nutrients (such as nitrogen & phosphate) which will drive them towards energy and carbon neutrality in the near future.
Current practice among the 14 WRRFs in New York City includes thickening the combined primary and secondary sludge in gravity thickeners to approximately three percent solids, followed by anaerobic mesophilic digestion to produce methane, subsequently dewatering the digested sludge to a cake of approximately 30% solids prior to disposing the cake to landfills. Anaerobic digestion produces about 3.6 billion cubic feet of anaerobic digester gas, ADG, of which only 35% is used beneficially satisfying 15% of DEP’s total energy usage. To achieve the above stated goal of energy & carbon neutrality, DEP needs to explore alternatives that would maximize the amount of energy that can be extracted from the sludge solids and adopt technologies and strategies to optimize its beneficial use. One alternative for the future that may be deliberated is modifying and forsaking sludge handling by facilitating incineration as an energy recovery approach. A more realistic approach at this time is to take advantage of the existing digesters and introduce a sludge pretreatment process to enhance production of anaerobic digestion gas. The pretreatment that will be discussed in this presentation will be thermal hydrolysis, for which bench scale experiments are being carried out at the City College of New York (CCNY) Environmental Engineering laboratory in New York.
The research study undertaken at CCNY is a multi-prong approach which includes a series of batch and bench scale experiments to document the degree of hydrolysis achievable for different feed stock which included primary sludge, digested sludge and a combination of both primary and secondary sludge at pre-determined temperatures and contact times, and select the most promising conditions for methane production assessment using the biochemical methane potential, BMP, method. These findings are being used to operate multiple semi-continuous flow anaerobic digesters, with one as a control and two additional digesters operating on sludge previously hydrolyzed. The semi-continuous flow reactors would provide valuable data that would be used for design and operating criteria, and make available a more realistic estimate of methane enhancement due to hydrolysis and energy auditing.
Results from the bench-scale experiments that were used to select the most appropriate time and temperature combination will be presented in addition to data from the on-going long term semi-continuous reactors. Data will show the degree of hydrolysis achieved at each of the selected contact times and the eventual selection of an appropriate contact time and subsequently the optimized temperature at which the process will be carried out for the long term reactors. Additionally, the relationship between degree of hydrolysis, temperature and gas production, using the BMP method will also be shown. The data collected from the semi-continuous flow anaerobic digestion reactors will become the basis of improved design criteria as well as providing guidelines for operation and maintenance of the digesters. It is anticipated that this approach and the additional steps being undertaken to optimize the remaining processes would enable the City to achieve energy neutrality.