Enhanced Methane Production from Pilot-Scale Anaerobic Digester Loaded with Rice Straw

A novel co-digestion approach was evaluated to determine if agricultural and industrial waste residues could efficiently degrade straw without pretreatment. Untreated rice straw was co-digested with pig wastewater and anaerobic sludge from the pulp and paper mill treatment process in a pilot-scale digester (1 m3) operated in dry, mesophilic conditions. The total weight ratio of dry straw to pig wastewater to sludge was 1 to1.25 to 0.5. The experiment was performed for a total of 153 days, however, the optimum period to balance the maximum energy output with the minimum retention time was determined to be 93 days. The addition of paper mill sludge accelerated VFA formation and gas production when compared to another pilot-scale digester operated under the same conditions without the sludge material. The straw in the pilot-scale digester with the sludge yielded 231 LCH4/kg VS within a 93-day digestion cycle compared to 189 days without the sludge. Daily leachate recirculation (0.2m3/m3 straw-day), however, was not adequate for internal mixing and homogenization of the digester material. With adequate mixing, this co-digestion approach could enhance methane production and reduce the digestion time for untreated rice straw in a farm-scale digester.