Improving Aerobic Digestion by Prethickening, Staged Operation, and Aerobic–Anoxic Operation: Four Full‐Scale Demonstrations

Abstract

Four full-scale aerobic digesters were studied to determine the effects of prethickening, staged operation, and aerobic-anoxic operation on digester performance. Prethickening is expected to enhance aerobic digestion by reducing hydraulic flow rates, thereby allowing increased solids retention time (SRT) values and temperature elevation. Staged operation increases the plug flow characteristic of the digester, resulting in increased solids stabilization and pathogen destruction. Aerobic-anoxic operation results in denitrification, which can provide approximately neutral pH that enhances nitrification, digestion, and nitrogen removal. Results demonstrate that staging enhances digestion and reduces the SRT necessary to produce stabilized solids (i.e., one with a volatile solids reduction efficiency greater than 38% or a specific oxygen uptake rate less than 1.5 mg O 2 /g volatile suspended solids⅐h). Effective pathogen destruction is also observed. Aerobic-anoxic operation at several facilities resulted in maintenance of approximately neutral pH values, enhanced digestion, near-complete nitrification, and nitrogen removal. Prethickening resulted in autoheating at the Paris, Illinois, site, caused by the high concentration of biodegradable volatile solids in the digester feed, and at the Los Lunas, New Mexico, facility, which was specifically designed to maintain specified temperatures. The oxygen transfer capacity of the first stage must be adequate to meet the increased process oxygen requirement in that stage to allow increased digestion rates. A combination of low temperature and high solids concentration may interfere with oxygen transfer capacity. Excessive foaming can occur if adequate oxygen transfer capacity is not provided. Excellent performance can be achieved at SRT values in the 20-to 40-day range at operating temperatures of 20°C or greater. Water Environ. Res., 72, 260 (2000).

Biosolids Management in the Future

“…The additional process also decreased odors, filtrate strength and phosphate, and foaming in filtrate. Daigger and Bailey (2000) showed that staged operation and aerobic-anoxic operation improve performance of aerobic digestion. Staged operation (with several aerobic-digestion tanks in series) increases solids stabilization and pathogen destruction.…”