In Situ Methane Enrichment in Methanogenic Energy Crop Digesters

Abstract

A simple in situ technique to enrich digester offgas, which normally contains 30-50% carbon dioxide (CO,) by volume, was developed to take advantage of the differing solubilities of CO, and methane (CH,). Dissolved CO, was removed from the digester in a recycled leachate stream and gas stripped in an external stripper. Bench-scale systems easily enriched the remaining digester offgas to over 95% CH,, and CH, purities in excess of 98% were achieved. Quantitative evaluation of system variables defined the effects of leachate recycle rates and alkalinity on the resulting offgas methane contents. Offgas CH, contents correlated well with the ratio of CO, transport capacity to CO, production. This ratio was termed the alkalinity/CO, ratio, a variable representing the cumulative effects of leachate recycle, leachate alkalinity and digester gas production. Operation at alkalinities of 2 and 4 g 1-l (as CaCO,) was more effective than 8 g I-’ for the specific mode of operation used in this study, as elevated alkalinities required extensive lag times between feeding and initiation of stripping, resulting in blow-by of CO, into the digester offgas, lowering the offgas CH, content. Excessively high recycle rates led to digester pH levels above 8. I, resulting in volatile fatty acid accumulation and lowered CH, production rates.