The impact of increasing organic loading in two phase digestion of food waste

Abstract

This paper examines the impact of increasing organic loading in a two phase anaerobic digestion system treating commercial food waste. The first phase is a series of sequentially fed leach bed reactors (LBRs). The second phase is an upflow anaerobic sludge bed (UASB). Leachate from the leach beds, form the influent to the UASB. Effluent from the UASB is re-circulated over the leach beds. Flow rates corresponded to 1 volume of leachate per effective LBR volume per day. The theoretical organic loading rate (OLR) of the UASB is based on the conversion of volatile solids (VS) in the LBR to chemical oxygen demand (COD). The experiment was set up such that the theoretical OLR would rise from 7.1 to 8.8 to 11.8kgCODm−3day−1.The system operated effectively at the lowest organic loading rate producing 384LCH4kgVS−1 which corresponded to 72% of the value obtained in a BMP test. COD conversion efficiency was recorded at 75%. The accumulation of COD over the life of the experiment led to a situation whereby the volumetric OLR (product of COD concentration in the leachate by the flow rate) was over twice the theoretical OLR at the end of the experiment (24.3kgVSm−3day−1 versus 11.8kgVSm−3day−1). At the highest loading rate total ammonia nitrogen (TAN) reached levels of 4500mgL−1 with pH levels of 8.15. This resulted in significant reduction of methane production.