Biomethane potential evaluation of co-digestion of sewage sludge and organic wastes

• Co-digestion with organic wastes significantly increased methane yield.
• Maximum co-digestion ratios were identifiable for most solid/slurry co-substrates.
• Solids free co-substrates resulted in a synergistic effect regarding VS/COD removal.
• Solid co-substrates resulted in additional VS and COD residuals in the final biosolids.

Co-digestion of organic rich wastes and wastewater sludge to enhance biogas production has become an attractive economic possibility for water utilities. The suitability of the organic rich waste depends on its ability to produce biogas as well as its influence on the overall anaerobic digestion process. Biomethane potential evaluation was conducted to screen seven organic wastes and dehydrated algae. All co-substrates increased the bio-methane yield by three to six times compared with conventional anaerobic digestion of sewage sludge. Maximum co-digestion ratios were identifiable for most solid co-substrates including algae (6% wt/wt), undiluted food waste (5% wt/wt), bakery waste (5% wt/wt), and diluted commercial food waste (10% wt/wt). On the other hand, the maximum co-digestions ratio of beverage reject and sewage sludge was 10% (wt/wt). With the exception of fat-oil-grease, all solids free liquid co-substrates evaluated in this study showed a notable synergistic effect, to enhanced removals of total solids, volatile solids (VS) and chemical oxygen demand (COD) during anaerobic digestion. The increase in COD removal when co-digesting wastewater sludge and liquid waste was from 2 to 41%. Conversely, the co-digestion of most solid co-substrates resulted in additional VS and COD residuals in the final biosolids. Elevated concentrations of sulphur and phosphorous in all food waste co-substrates suggest that control measures to address H2S in biogas and the accumulation of phosphorus in sludge centrate may be necessary during full scale operation. Data presented here provide the basis for subsequent pilot scale evaluation of anaerobic digestion of these organic rich wastes and wastewater sludge.