Countercurrent absorption system and method with clarified digestate for H2S removal in biogas.

Abstract

A countercurrent absorption system and method for removing H2S from a biogas stream, using clarified digestate from an anaerobic digestion process as the absorbent medium. The system comprises at least one gas-liquid contact tower or module, a raw biogas inlet, a treated biogas outlet, means for distributing or recirculating the clarified digestate, an associated storage volume or tank, a feed of fresh clarified digestate, and a renewal or purge outlet. The method comprises contacting the biogas with the clarified digestate in a countercurrent flow, preferably operating under anoxic conditions without deliberate dosing of air or oxygen into the absorption zone, renewing at least a portion of the digestate to maintain absorption capacity, and removing biogas with a lower H2S concentration.The invention allows the replacement or reduction of external alkaline reagents, including caustic soda, by means of a clarified internal stream from the anaerobic plant.