Using a Partial-mix, Single-jet Mixing System to Improve Solids Retention in Anaerobic Sequencing Batch Reactors

Abstract

A mixing design for Anaerobic Sequencing Batch Reactors (ASBR) was evaluated to increase solids retention while treating dilute wastewater such as swine manure. The concept behind the proposed design consists in partially mixing the reactor contents to maintain two separate regions: a solids-concentrated layer at the bottom and a solids-free layer near the top of the reactor. Injection of clarified liquid to the bottom of the reactor provides reactor mixing. This results in a less aggressive mixing system, which prevents aggregate disruption and particle size reduction increasing effluent quality and the solid retention time. The partial-mix design provided improvement in effluent quality, organic matter removal efficiency, and solids retention time, while maintaining stable operation. The partial-mix system was able to effectively decouple the hydraulic and solids retention times, which allows the treatment at higher organic loading rates and improved reactor volumetric efficiency. Fed a mixture of dilute swine manure and raw glycerol, and operating at 35�C, a 6-day HRT, and a loading rate of 0.87 g COD L-1 reactor day-1, solids retention times up to 700 days were achieved, along with organic matter removal efficiencies above 90% COD and 80%VS. Effluent VSS concentration decreased 67% compared to a fully mixed ASBR design. While biogas yield did not show a significant increase, the increased ability to retain solids in the reactor using partial-mix design allowed maintaining high organic matter removal efficiencies while operating under low HRTs.