The District of Columbia Water and Sewer Authority (DC Water) have recently installed a state of the art biosolids processing facility at its Blue Plains wastewater treatment works. The facility, the first of its kind in North America, will generate approximately 10 MW electricity by thermally hydrolyzing and digesting site-generated sewage sludge. In addition, it will produce Class A biosolids cake which further reduces DC Water’s carbon and environmental footprint.
In order to optimize the use of this facility, DC Water are investigating the impact of co-digesting other organic waste materials to further increase renewable energy and provide financial benefits for its rate-payers. Tests looking specifically at food waste show it to be a good candidate for codigestion.
When compared to two control reactors in a laboratory study, a digester supplemented thermally hydrolyzed food waste to similarly processed sludge. The increase in COD loading resulted in an increase in biogas output consistent with the loading rate, and improved biodegradability of the food waste fraction. This increased the total COD destruction and volatile solids destruction (by mass balance) compared to the controls.
Due to high degradability, the solids yield produced from the food waste was much lower than that from sludge. As the food waste had a higher carbon:nitrogen ratio, its addition resulted in a statistically significant drop in pH, which helped reduce free ammonia levels, known to limit the anaerobic digestion process, although not found to be inhibitory in the trials.
It was determined that up to 600 m3 of the foodwaste could be added daily prior to the need for a major infrastructure upgrade.
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