Improving conversion of Spartina alterniflora into biogas by co-digestion with cow feces

Anaerobic mono-digestion of Spartina alterniflora (S. alterniflora) at initial volatile solid loading (VSL) of 4.0%, 6.0% and 8.0% as well as co-digestion of S. alterniflora and cow feces at cow feces proportions from 12.5% to 87.5% of volatile solid (VS) were investigated. A maximum methane yield of 0.19 L CH4/g VSadded from mono-digestion of S. alterniflora was obtained at initial VSL of 6.0% and the average methane content was 63.29%. Furthermore, co-digestion of S. alterniflora and cow feces in varying proportions was investigated at constant initial VSL of 7.0%. The addition of cow feces decreased the carbon-to-nitrogen (C/N) ratio from 21.82 to 14.19 in the feedstock. Co-digestion improved the biodegradability of S. alterniflora, and thus increased the methane yield by 7.09–44.26%, leading to a lower volatile fatty acid (VFA) concentration of less than 800 mg/L (only about one third of S. alterniflora digested alone ) in the solution and higher pH value in comparison to S. alterniflora digested alone. Although the VFA concentration was lower than S. alterniflora digested alone, the biogas yield was increased and the biogas yield peak was advanced for about 15 days.