Field test of methane fermentation system for treating swine wastes

A methane fermentation system for treating swine wastes was developed and successfully demonstrated in a field test plant (0.5 m³/d). The system was composed of a screw-press dehydrator, a methanogenic digester, a sludge separator, an oxidation ditch (OD) and composting equipment. A performance evaluation was carried out regarding physical pre-treatment using the screw-press dehydrator, methane fermentation for pre-treated slurry, and post-treatment for digested effluent by OD. Total solids (TS) and chemical oxygen demand (CODCr) removal by the screw-press pre-treatment were 38% and 22%, respectively. Properties of the screenings were as follows: water content 57%, ignition loss 93%, specific gravity 0.33. The pretreated strong slurry was digested under mesophilic conditions. Digestion gas (biogas) production rate was 25 m³/m³-slurry (NTP) and methane content of the biogas was 67%. COD_Cr removal of 65% with methane fermentation treatment of the slurry operating at 35°C was observed. No inhibition of methane fermentation reaction occurred at the NH₄⁺-N concentration of 3,000 mg/l or less during methane fermentation by the system. Mass balance from the present pilot-scale study showed that 1m³ of mixture of excrement and urine of swine waste (TS 90 kg/m³) was biologically converted to 25 m³/m³-slurry (NTP) of biogas (methane content 67%), 100 kg of compost (water content 40%, ignition loss 75%), and 0.80 m³ of treated water (SS 30-70 mg/l).