Effects of a long-term periodic substrate perturbation on an anaerobic community

The response of a glucose-fed anaerobic community to a long-term (>400 days) periodic substrate perturbation was examined. A completely mixed “mother” reactor operated at steady state with a 10-day hydraulic retention time (HRT) and constant 8 g/liter glucose feed served as the source of organisms for a “daughter” reactor. The daughter reactor was initially operated in the same manner as the mother reactor, i.e. at a 10-day HRT and a constant 8 g/liter glucose feed. After reaching steady state, the daughter reactor was subjected to a periodic organic loading pattern in which the influent glucose concentration was alternately varied from 16 to 0 g/liter (mineral media only) on a 6-day cycle. A rapid accumulation in glucose fermentation intermediates occurred immediately after initiating the perturbation. Effluent chemical oxygen demand (COD) increased from ca 400 to a peak level of 6000 mg/liter, methane content of the biogas decreased from 50 to less than 20% and the pH decreased from 7.0 to 6.2. This was followed by a 230-day metastable “steady state” during which the effluent COD concentration fluctuated about a new median value of 5000 mg/liter and the acetate and butyrate concentrations varied in a cyclic, inverse pattern. At the end of the metastable period, volatile fatty acid concentrations decreased rapidly. Within ca 40 days, a new steady state was established. These observations indicate that the anaerobic system was able to adapt to the periodic substrate perturbation through a long-term change in microbial community.