EFFECT OF TWO-STAGE ANAEROBIC DIGESTION PROCESS ON DIGESTATE STABILITY

Anaerobic digestion is used to stabilize organic materials through the production of bio-fuels and bio-products, as digestate usable in agricolture as fertilizer and soil conditioner. Digestate can be considered as an indicator of treatment efficiency in anaerobic digestion plants and the evaluation of digestate stability index can be useful for determining process performance. In this work, two different configurations of anaerobic co-digestion process were considered: a conventional one-stage reactor and its implementation in a two-stage process with a preliminary fermentative phase called dark fermentation. Process performances were determinated for each configuations relatively to outgoing digestate stability index, assessed by SOUR test. The objective of this paper is the evaluation of which anaerobic co-digestion configuration can reach the preferable performance in terms of digestate biostabilization. The results indicate that using dark fermentation as a preliminary biological treatment for anaerobic co-digestion process can improve the outcoming digestate stability. Two-stage process allows a total SOUR reduction equal to 33.4% and 40.6% respectively for a hydraulic retention time of 1.5 and 3.0 days in the fermentative reactor. Compared to the conventional one-stage process, with a stabilization efficiency in the amount of 6.5%, two-stage configuration determines the best performances obtaining a biologically more stable digestate.