Free ammonia

Thermophilic anaerobic digestion of municipal solid wastes (MSW) was conducted for 6 weeks in a lab-scale semi-continuous digester for investigating the start-up operation of anaerobic digestion treating MSW. The feedstock was a mixture of simulated food waste, paper waste, and in some cases, cow manure. Organic loading rate (OLR) was adjusted 1.14 ～ 4.00 gVS/kg-sludge/day to biogas production. At the initial stage of thermophilic anaerobic digestion, pH value gradually increased from 8.2 to 8.3 on 17th day from start-up. Free-ammonia concentration in the digester also gradually increased from 361.6 to 412.5 ppm. On 17th day, inhibition of free-ammonia occurs, and biogas generation stopped. After gas generation stopped on day 17, pH and free-ammonia concentration were adjusted to the proper level by adding paper and deionized water. As a result, gas generation restarted on day 21. Then the amount of food waste charged in the digester was decreased and that of paper waste was increased to adjust the total amount of nitrogen invested in the digester. Afterwards, pH value was settled around 7.8, and a stable digestion was achieved till the end of experiment. Inhibition of free-ammonia could be suppressed by adjusting the amount of nitrogen invested in the digester.