Performance of temperature-phased anaerobic digestion (TPAD) system treating grocery food waste and agro-waste

Temperature-phased anaerobic digestion (TPAD) process was developed at Iowa State University (ISU) in the early 1990s. The TPAD process combines the mesophilic and thermophilic digestion processes in a temperature-phased treatment system and thus offers the advantages of rapid digestion of complex waste coupled with production of Class A biosolids and higher biogas generation. More importantly, TPAD process enhances the oil and grease biodegradation that is found in high fat-content waste streams such as post-consumer food waste streams rich in meats, dairy products, and fatty greases which are regarded as rate limiting steps in mesophilic digestion alone. The degradation of wax-coated cardboard, which is also a part of the grocery store waste, could be enhanced at thermophilic condition. In this study, the feasibility of anaerobic digestibility of food waste together with wax-coated cardboard was investigated using a bench-scale TPAD system operating at semi-continuously feeding mode. The TPAD system was subjected to varying total solids (TS) contents of 2.23 to 8.19% and varying solid retention time (SRT) of 15 to 30 days. The influent feed consisted of 95% food waste and 5% wax-coated cardboard (by weight). At TS content below 8.19% corresponding to a system volatile solids (VS) loading in the range of 1.39 to 4.07 VS/L/d; the VS removal efficiency was in the range of 68.3 to 71.5%. Under these loading conditions, the overall methane yield was 0.51 to 0.53 L/g VS destroyed. The maximum VS destruction of 71.5% was achieved at a TS content of 3.91%. The feed pH was low in the range of 4.5-5.0 due to fruits and vegetables in the waste, which required supplementation of significant amount of alkalinity to maintain an optimal pH of 7.0 to 7.3 in the thermophilic reactor. The food waste was found to contain enough nutrients and trace elements and thus enhanced the biodegradability of wax-coated cardboard