Effect of a Trace Element Addition Strategy on Volatile Fatty Acid Accumulation in Thermophilic Anaerobic Digestion of Food Waste

Abstract

A trace element (TE) supplementation strategy previously shown to be effective in mesophilic conditions was tested for thermophilic digestion of source segregated domestic food waste. Inoculum from a mesophilic anaerobic digester treating municipal wastewater biosolids was successfully acclimated to thermophilic temperature (55 °C) with food waste as a substrate. Four laboratory-scale digesters were maintained at a loading of 2 g VS l−1 day−1 with one pair receiving TE supplementation. Two more pairs of digesters were incrementally loaded to 3 and 4 g VS l−1 day−1, respectively, and also received TE. All digesters performed well for the first 3–4 months of operation, but volatile fatty acid (VFA) concentrations in those without TE showed no recovery from an initial small accumulation. On continued operation, VFA concentrations increased in all digesters, especially those at higher loading rates or without TE supplementation, reaching >30 g l−1. Under these meta-stable conditions, a deliberate disturbance to daily feeding (cessation then resumption) led to acetic acid accumulation, a fall in pH and a sharp increase in the ratio of partial to intermediate alkalinity. Increases in VFA corresponded to an increasing total ammonia nitrogen concentration which appeared to become inhibitory at ~2,500 mg N l−1. While TE supplementation delayed the onset and reduced the rate of VFA accumulation, it was unable to sustain stable digestion of this substrate in thermophilic conditions.