Influence of ammonia in the anaerobic digestion of food waste

Abstract

Ammonia toxicity was investigated in mesophilic (37 °C) and thermophilic (55 °C) digesters using high and lownitrogen food wastes (FW). Mesophilic inoculum was successfully acclimated to thermophilic conditions by a step change in temperature followed by incremental increases in organic loading rate (OLR). Digestion performance and stability were monitored via volatile fatty acid (VFA) profiles, alkalinity, specific methane production (SMP) and volatile solids (VS) destruction. High-nitrogen mesophilic digesters stabilised by day 70 and responded well to increases in OLR, with a SMP of 0.45 L CH4 g−1 VS, stable pH, and VFA<0.2 g L−1. Thermophilic digesters fed on high and low-nitrogen FWs showed almost identical responses to acclimatisation. Behaviour then deviated, with high-nitrogen digesters accumulating VFA. Stable pH could be maintained for up to 310 days before eventual failure at total ammonia nitrogen (TAN) concentration>5.0 g N L−1, although accumulation of propionic and other longer-chain VFA began at TAN ∼3.5 g N L−1. The low-nitrogen digesters showed no VFA accumulation, and had a SMP of ∼0.39 L CH4 g VS L−1 day−1 with 91% VS destruction. After 384 days the TAN concentration was increased from ∼0.7 g N L−1 by urea addition. This resulted in progressive VFA accumulation in one digester when TAN reached ∼3.5 g N L−1, while stable operation at very low VFA was possible at up to ∼2.5 g N L−1 in the second digester. The results confirmed acclimatisation to thermophilic conditions was possible on a far shorter timescale than to high TAN concentrations.

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Identifiers

ORCID for Sonia Heaven: orcid.org/0000-0001-7798-4683

ORCID for Charles Banks: orcid.org/0000-0001-6795-814X

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