Effect of Diurnally Cyclic Temperature on the Performance of a Continuous Mix Anaerobic Digester

Abstract

Problem statement: Concentrated livestock production is a major contributor to environmental problems through the production of large quantity of husks on relatively small land area. Anaerobic digestion represents a valuable means of cover by these husks into methane while reducing manure pollution problems. Methodology: An experimental apparatus, including data acquisition and control system, was developed and used to investigate the effects of two diurnally cyclic temperature ranges (20-40 and 15-25°C) and four levels of hydraulic retention times (25, 20, 15 and 10 d) on the performance of anaerobic reactors operated on screened dairy manure. Results: The reactor temperature exhibited a lag relative to the chamber temperature. For the 20-40°C temperature cycles, the average lag at the maximum chamber temperature was 3.75 h while the lag at the minimum chamber temperature was 4.37 h. For the 15-25°C temperature cycle, the average lag at maximum chamber temperatures was 3.61 while the lag of the minimum chamber temperature was 4.34 h. The effluent solids content, total Kjeldahl nitrogen and ammonium nitrogen concentrations were not adversely affected by the reactor diurnally cyclic temperature. The observed values of these parameters compared fairly well with the values obtained by other researchers. The effluent total solids, volatile suspended solids, TKN, NH₄-N and methane content of the biogas diurnally cyclic patterns were out of phase with the diurnally cyclic pattern of the reactor temperature by about 12 h under most of the investigated operating conditions. The pH and the carbon dioxide content of the biogas all exhibited a diurnally cyclic pattern which was in phase with the reactor diurnally cyclic temperature. The reductions in total solids, total suspended solids, fixed suspended solids, total Kjeldahl nitrogen, ammonium nitrogen and methane yield were all significantly affected by the diurnal temperature range and hydraulic retention time. Conclusion: The measured reactor pH followed a diurnally cyclic pattern which was in phase with the reactor temperature. At the operating temperature cycle of 20-40°C, the reactor pH ranged from 6.80-7.54 while VFA concentration ranged from 44.7-154.8 mg L^-1. At the operating temperature cycle of 15-25°C, the pH ranged from 6.30-6.60 while the VFA concentration ranged from 151.0-1187.0 mg L^-1.