Performance And Optimization Of High-Solids Anaerobic Digestion Of Sewage Sludge

Anaerobic digestion(AD) can reduce the amount of sludge solids and transform organic matter into biogas. However, the feeding TS of conventional low-solids anaerobic digestion(CAD) is only 3~5%, needing large volume and space occupation, and corresponding heating consumption. To overcome the above shortcomings, high-solids anaerobic digestion(HSAD) is possibly a good option. However, the low organic content, high viscosity and low mass and heat transfer efficiency of high-solids sludge would have a negative effct on the performance of HSAD. By far, the research on HSAD is very rare. In order to support a practical project of HSAD in Kunming City, the performance of HSAD with feeding low organic content sludge were studied in a long-term experiment, and low temperature thermal pretreatment was tested to improve the performance of HASD. The main results are as following:(1) Compared with CAD, HSAD of sludge need a longer digestion time to achieve a similar performance. The blocked mass transfer is the main factor impacting the performance of high-solids batch anaerobic digestion. Due to the inadequate agitation, intermediate products accumulated, and high concentration of free ammonia is the key inhibitor deteriorating HSAD.(2) HSAD can evolve successfully from low-solids anaerobic digestion, as the solids content of feeding sludge increased. The system changed gradually from an unstable state to a stable state. The concentration of volatile fatty acids(VFAs) in the HSAD digester was between 200 and 400 mg/L, and the concentration of free ammonia nitrogen(FAN) is lower than 250 mg/L. No intermediate products accumulated in the digester equipped with an enhanced stirrer. Consequently, HSAD exhibited similar performance to low-solids anaerobic digestion. For the anaerobic digestion of low organic content sludge,the organic removal rate was almost proportional to the organic content in feeding sludge.(3) During low temperature thermal pretreatment, the organic substances in sludge solids were dissolved effectively and the viscosity of high-solids sludgewas decreased. The high concentration of VFAs and low p H generated by thermal hydrolysis had no negative effect on HSAD. After pretreatment at 70 °C for 30 min, biogas production and organic degradation rate increased by 14.5%and 9.5%, respectively, and sludge retention time can be shortened from 30 d to15~22 d.(4) The energy consumption on stirring is the main energy consumption of HSAD. When the organic content of feeding sludge was higher than 40%, most of the produced biogas can be used to generate electricity. The energy output of HSAD system decreased by 5.6% after applying pretreatment at 70 °C. However,the increase in organic removal rate reduced the energy consumption of subsequent sludge drying. Therefore, low temperature thermal pretreatment at70 °C is feasible.(5) Due to the application of thermal pretreatment, the digested sludge discharged from the HSAD digester required more polyacrylamide to obtain similar dewatering performance. Conditioning with ferric trichloride resulted in a better dewatering effect. The total nutrients and heavy metals in the digested sludge met the demand of landscaping use. The main inorganic substances in the digested sludge were silicon dioxide and aluminium trioxide, and thus, the digested sludge can be used in ceramics manufactruring after mixing with a certain amount of clay.