Impact of Series Digestion on Process Stability and Performance

One of the most common solids stabilization processes used by municipal wastewater treatment plants (WWTPs) is mesophilic anaerobic digestion. The process typically is operated at a hydraulic retention time (HRT) ranging from 20 to 40 days and an organic loading rate of 0.01 – 0.15 lb VS/cf/day. The most common operational configuration of anaerobic digesters is parallel operation of two or more tanks, where digester feed is split equally between tanks and each tank operates with approximately the same solids loading rate and HRT. Several methods for improving digester performance and biogas production have been reported, and one such method, series digestion, involves operating two tanks in a series configuration. In series operation, the digester feed to sent to the first digester, the digested sludge from the first digester is transferred to the second digester, and digested sludge from the second digester is transferred to the solids disposal/dewatering process.Series digestion can offer improved digestion performance and increased gas production relative to parallel operation. The conversion of two digesters from parallel operation to series operation will double the solids loading rate to the first digester in series (relative to parallel operation) while maintaining the equivalent HRT through the digestion system. Digested sludge from the first digester in series will pass on to the second digester in series for further stabilization and polishing. Series digestion has reported an increase in volatile solids reduction (VSR), gas production, and gas quality, and in several cases, series digestion will improve the stability of the digestion process, as measured by alkalinity, volatile acid, and pH. By placing the digesters in series, the process approaches plug-flow design and offers improved process performance, and a design with tanks in series will reduce the potential for short circuiting and improved pathogen reduction.This paper will review the latest finding on the performance of series digestion in literature and compare several full-scale WWTP digesters operated in both parallel and series digestion configurations. Recent literature and studies have reported improved performance of laboratory bench scale reactors operated in series digestion configurations. Full-scale digester performance of series digestion relative to parallel digestion indicates an increase in 5% VSR, often saving the facility significant biosolids processing and hauling costs. Conventional knowledge of anaerobic digestion has often considered longer HRTs will provide greater process stability. Investigations into some full-scale WWTP digester operations have indicated excessively long HRTs can impair digester performance, and in these cases, reduction of digester HRT (increase in solids loading) through series digestion can dramatically improve digester performance and stability.