Effect of storage on the performance of methanogenic granules

Abstract

Anaerobic methanogenic granules were stored under anaerobic condition at 4° and 22°C for 1- to 18-month period to evaluate the effect of storage on degradation of volatile fatty acids (VFAs), including acetate, propionate and butyrate, and on methane production. The length of storage period affected the activities of different microbial trophic groups. During storage at 22°C, the degradation rates for all the three acids decreased gradually. At low temperature (4°C), reduction in degradation rates of acetate and propionate was relatively slower than that at 22°C. Reduction in butyrate degradation rate was faster (by 45%) during the first month of storage at 4°C, but the rate declined afterwards. Nevertheless, the granules maintained, although at reduced level, their metabolic activities for all three VFAs even after storage for 18 months. Higher decay coefficients were obtained at 22°C than those at 4°C. For a relatively short period (1–5 months), granules can be stored at ambient temperature (approx. 20–22°C) with limited loss in their VFA degradation rates. However, granules can maintain higher levels of VFA degradation rates when they are stored at low (4°C) rather than at ambient temperature. Reactor studies indicated that the granules can completely recover their original VFA degradation rates in three days when stored for 31 d at 22°C. The granules stored at 22°C for 9 months were used successfully as inoculum to start a laboratory-scale reactor. The original VFA degradation rates of the granules were achieved after 15–20 d of reactor operation at 35°C.