Propionic acid accumulation and degradation during restart of a full-scale anaerobic biowaste digester

The methane formation rate of 300 m3 of sludge from a full scale biowaste reactor, that was stored without feeding for six weeks during a maintenance period, was about 60% of the methanogenic activity before maintenance. The 300 m3 sludge was then pumped back into the biowaste reactor. On the third day, after refilling of the stored biowaste suspension, anaerobic conditions were obtained and feeding was started by addition of 36.1 m3 of fresh biowaste suspension (=11.3 tons biowaste). The pH dropped from originally pH 7.7 to pH 7.3 and later on to pH 6.8, which was considered the minimum allowed pH for methanogenesis to recover. Maximum concentrations of acetate (1.78 g l−1), n-butyrate (0.57 g l−1) and n-valerate (0.44 g l−1) accumulated during the following days with feeding of 11.8 tons on day 5 and twice 6.5 tons on days 7 and 9, respectively. Thereafter, acetate, n-butyrate and n-valerate were degraded completely, whereas the concentration of propionate was still increasing. Propionic acid was the dominant fatty acid during the restart period and reached its maximum concentration of 6.2 g l−1 17 days after start of feeding. This high level of propionate was degraded completely in about 5 days with maximum degradation rates of 2.14 g l−1 d−1, and the pH of the anaerobic sludge increased from 7.1 to 7.4. During restart, the methane content of the biogas increased successively to 65%. Samples that were taken at different time intervals during the restart phase of the methane reactor showed different fatty acid degradation capabilities. After 10 days, when acetate and n-butyrate still accumulated in the methane reactor the maximum acetate degradation rate was 1.52 g l−1 d−1 and the n-butyrate degradation rate was 0.51 g l−1 d−1. Oxidation of n-valerate caused an increase of propionate, which was degraded after a lag phase of 6 days with a maximum rate of 0.6 g l−1 d−1. In the samples taken after 16 and 23 days, the propionate degradation rate increased to 1.42 g l−1 d−1 and 1.55 g l−1 d−1, respectively, and the lag phase for propionate degradation was reduced or had disappeared completely. The maximum propionate degradation rate was measured in the methane reactor in the fourth week after restart. The synthrophic propionate oxidizing bacteria were apparently the most suffering bacteria during sludge storage. If the propionate oxidizing bacteria could be kept active and the propionate degrading activity of the biowaste suspension of 6.16 g l−1 d−1 before the maintenance period could be maintained, then accumulation of 6.2 g l−1 propionate in the methane reactor after restart could be avoided and full activity reached even earlier.