Similarity of first-order rate constants for methane from food wastes in batch and continuous feed systems

Abstract

Evaluation of the suitability of food wastes for anaerobic digestion is made difficult because of variable and uncertain degradation parameters. Simpler tests that can estimate degradation kinetics can make it quicker and less expensive to assess suitability. The purpose of the research was to examine the correspondence of first-order rate constants between laboratory testing of batch and continuous-feed systems. The feedstock for the experiments was a food waste developed to simulate residential collection. Batch tests were conducted using 120 g of food waste with 1.5 L of digested sewage sludge as inoculum. Continuous feed tests were conducted in 30 L digesters with daily feed/wasting and a hydraulic retention time of 30 days. Methane production was fit to a first-order rate equation with a resulting 0.22 d-1 rate over 13 reactors. The overall fitted methane yield for batch tests varied greatly from 0.15 to 0.56 L CH4/g VS, although no signs of souring were seen. The inferred rate constant for the continuous tests were 0.09 d-1 when using only the yield data from the 8 of 13 tests above 0.35, and 0.3 when using all data. The influence of inclusion of a lag time term is explored, but not found to greatly impact on results. The results indicate broad similarity in rate constants between the two systems. They also indicate that a large number of batch tests are needed, and researchers should be reluctant to remove non-souring batch test results when fitting parameters.