High solid concentration feedstock improving performance of continuous anaerobic digestion of food waste

Abstract: The effect of feedstock solids concentration on the performance of mesophilic anaerobic digestion of food waste (FW) was investigated in this assay using single wet continuous experiments. FW was made into two kinds of feedstocks, one has a high solids concentration of 19.0% and the other has a low solids concentration of 9.5%. The effects of the two different solids concentrations of feedstocks on the mesophilic anaerobic digestion of FW for biogas production were compared. A total of six digesters were designed in present assay, and were equally divided into two groups. One group was fed with high solids concentration of feedstock, and the other group was fed with low solids concentration of feedstock. The three digesters of each group were operated under three different organic loading rates (OLR) of 1.78, 2.54, and 3.56 g/(L·d), respectively. The test lasted for 50 days. The biogas volume, methane content and the pH value of effluent were monitored each two days, and the total solids (TS), volatile solids (VS), ammonia concentration of the effluent were monitored each week.The results showed that the digesters fed with high solids content feedstock could obtain a higher methane production rate (mL/g-VS) than those fed with low solids content feedstock in the same OLR, the pH of all the high solids feedstock trials had been maintained above 7.20, and the trials of low solids feedstock had been maintained above 7.15 only observed in the former two lower OLRs, the pH of the third digester operate under OLR of 3.56 g/(L·d) decreased to below 6.5 at 10th day leading to biogas production failure. The ammonia concentrations of all digesters had been maintained bellowed the initial inhibition concentration of 2700 mg/L and the toxic concentration of 3000 mg/L reported by previous literatures. In addition, the interior TS concentration of each digester fed with high solids content feedstock was lower than 10% which agreed with the request of wet anaerobic digestion.All the findings of this assay suggested that the digesters applied high solids contend feedstock could obtain a better performance of biogas production than the group adopted low solids contend feedstock. There are three conclusions could be summarized as follows: 1) Under the same OLR, the high solids group could obtain higher methane production rate and volumetric biogas production rate than the low solids group; 2) Under the same OLR, the TS concentrations in the interior of the high solids group were obviously higher than the other group, which was helpful to maintain a higher cell content in the digester; 3) Under the same OLR, the process of anaerobic digestion using high solids feedstock not only can save the dilution water used to decrease the solids concentration of raw materials but also can reduce the discharge of biogas effluent, which could decrease the comprehensive cost of biogas production. This study provided technical reference for the design of actual biogas engineering on the aspect of how to dilute the raw FW and furthest save the dilution water.