Effects of Co-Substrates and Mixing Ratio on the Anaerobic Digestion of Navel Orange Waste

Abstract

The Gannan region is the largest navel orange planting area in the world and has the largest production in China. However, about 5 million tons of navel orange waste (NOW) produced annually. NOW has a great environmental risk because of its high content of organic matter and moisture. Anaerobic digestion of NOW with high nitrogen content waste is a promising alternative to treat these wastes. The effect of swine manure (SM), waste active sludge (WAS) as co-substrates and different mixing ratio were examined in three batch-scale studies. In the first investigation, co-digestion of NOW with SM resulted low methane yield and high concentration of VFAs. In the second investigation, NOW was co-digested with WAS, the methane yield was improved by 260% when the mixing ratio of NOW to WAS (VS/VS) was shifted from 1:2 to 2:1. In the third investigation, the co-digestion of NOW with SM and WAS was conducted. Co-digestion of three substrates has higher methane yield than that of previous two studies, with the exception of equal amounts of NOW with co-substrates (mixing ratio of NOW to SM to WAS was 2:1:1). The highest methane yield of all experiments was 0.20 m3 kg-1VS added while the mixing ratio of NOW to SM to WAS was 1:2:1. It seemed to obtain stable digestion performance, the mixing ratio of co-substates to NOW should not be lower than 1:1. WAS was a better co-substrate than SM, as WAS was capable to supply more organic nitrogen to create positive synergistic effects.