IMPROVEMENT OF A GRASS-CLOVER SILAGE-FED BIOGAS PROCESS BY THE ADDITION OF COBALT

Batch assays were performed with samples from a silage-fed mesophilic biogas process accumulating acetate to examine if the addition of sing le trace elements (iron, nickel, cobalt and molybdenum) or a mixture o f trace elements could improve the process. The results from the batch assays led to the addition of cobalt to reach a concentration of 0.2 mg L-1. This made it possible to increase the organic loading rate (OL R) above that in a parallel process not receiving any extra cobalt. Pr oblems with low gas production and decreasing pH were overcome by the daily supply of this single element in small amounts. Acetate conversi on to methane was improved, which was confirmed by increased specific methanogenic activity (SMA) with acetate as a substrate. As a conseque nce, an OLR of 7.0 g VS L-1 day(-1) was achieved at a hydraulic retent ion time (HRT) of 20 days with an equivalent increase in methane produ ction. Without cobalt addition, an OLR of 5.0 g VS L-1 day(-1) with an HRT of 20 days was reached after 70 weeks operation. Copyright (C) 19 97 Published by Elsevier Science Ltd.