Optimization Of Operating Conditions For Co-digestion Of Sewage Sludge And Kitchen Waste

Management of sewage sludge is one of the main environmental issues in China,since the amount of sludge has been increasing dramatically during the past decades.Anaerobic digestion can achieve removal of organic pollutants in sludge, andsimultaneously produce methane energy. However, the C/N ratio of sludge is very low,which limits its digestion performance. Kitchen waste contains great amount oforganics and has much higher C/N ratio than sludge. Anaerobic digestion of kitchenwaste alone will lead to accumulation of organic acids and eventually inhibitanaerobic digestion process. Co-digestion of sewage sludge and kitchen waste cancontribute to the balance of nutrients in the substrate, dilution of toxic components inthe sludge, neutralization reaction between volatile fatty acids (VFA) and ammonia inorder to maintain the stability of the pH value in the fermentation process, which canimprove the digestion efficiency.In this study, effects of addition of kitchen waste on mesophilic anaerobic sludgedigestion were investigated in batch experiments. Methane yield, methane content ofbiogas, and VS removal efficiency were examined to select an optimal mixing ratio ofsewage sludge and kitchen waste for enhancement of digestion. Variation of organicmatters in the mixed liquor such as soluble carbohydrate and soluble protein were alsoinvestigated. Results showed that the optimal TS ratio between sewage sludge andkitchen waste was4:1, with a methane yield of592.65mL/g-VS and the VS removalefficiency of30.83%, which were47%and55%higher than those of sole sludgedigestion, respectively.The CSTR was used to investigate co-digestion of sewage sludge and kitchenwaste in a one-stage fermentation process. The sewage sludge and kitchen waste weremixed by TS ratio of4:1and1:1, respectively, and the sole sewage sludge digestionand sole kitchen waste digestion served as controls. The organic loading rate (OLR)was gradually increased by changing HRTs. Results showed that the VFA andammonia produced could maintain the pH in a certain range, which was preferable bymethanogenic bacteria. The optimal operating conditions were TS ratio betweensewage sludge and kitchen waste of4:1and OLR of1.06g-VS/(L·d). The biogas andmethane production rates were32.8%and34.6%higher than those of sole sludgedigestion, respectively. Under this optimal condition, the VS removal efficiency was 50.30±2.48%and the methane yield was1160.15±58.16mL/g-VS.The resultrs showed that adding kitchen waste can effectively improve themethane yield and VS removal efficiency for anaerobic digestion of sewage sludge.The optimal TS ratio between sewage sludge and kitchen waste was4:1. The optimaloperating conditions were HRT of20days and OLR of1.06g-VS/(L·d).