Anaerobic Digestion of Onion Wastes Using a Continuous Two-Phase Anaerobic Solids Digestion System

Abstract

The digestion of onion waste was conducted in a two-phase anaerobic solids digester. Thedigestibility of the onion waste was studied using batch and continuous digesters at mesophilic temperature(35 C). The C:N of onion waste was determined to be 20:1. The two-phase anaerobic digestion was firstperformed under batch conditions with 200 g (wet weight) of onion waste. Biogas yield of onion waste was0.34 - 0.47 L CH4 / g VS fed, with methane yields ranging from 50 - 80%. Total solids (TS) and volatile solids(VS) reduction were 46% and 54%, respectively. The continuous digestion of onion waste was studied at threeorganic loading rates (OLR) from 0.5 to 2.0 g VS/L/d. The pH in the biogasification reactor was controlledabove 7.0 by the addition of hydrated lime. The average methane yield was determined to be 0.35, 0.32, and0.32 L CH4/gVS fed for OLR of 0.5, 1.0, and 2.0 g VS/L/d, respectively. The results were similar to the yieldsdetermined in the batch digestion study. The two-phase system allowed the flexibility of adjusting the loadingrate of organic acids to the methanogens, which made pH control possible without disruption of the continuousoperation of the digester system.