Inhibition of Total Gas and Methane Production in Anaerobic Digestion by Various Toxicants

Heavy metal inhibition of anaerobic digestion of municipal sewage sludge has been investigated over the past 50 years. Many assays began with steady-state conditions and observed the rate at which total gas or methane production decreased due to toxicant inhibition. These methods required considerable time and labor. Modified Warburg respirometers and serum bottle assays reduced much of the effort, but each method has limitations in terms of complexity, cost, and accuracy. This series of experiments had two objectives: 1) develop a quick, inexpensive, and easy method to measure methanogenic activity in anaerobic digesters, and 2) determine median inhibitory concentrations (IC50) of selenium, copper, chromium, zinc, mercury, lead, and cobalt to total gas and methane production in batch anaerobic digesters. Preliminary experiments indicated a linear increase in gas production during community recovery from oxygen inhibition. The rate of increase was relatively independent of substrate quality during the first 6 hr as long as no nutrients were limiting. Toxicants introduced immediately after reestablishment of anaerobic conditions reduced the rate of gas production recovery as a linear function of toxicant concentration. Anaerobic sludge samples taken from the primary digester of the local wastewater treatment plant were supplemented with cow manure, dry dogfood, and ammonium acetate prior to immediate use. Five liter portions in 10 x 70 cm (4 x 28 in) PVC sewer pipe segments were agitated on a rotary shaker and toxicants were added immediately after reestablishment of anaerobic conditions. Total gas production rates (determined through displacement of 10% NaCl solution in 50 mL burets) and percent methane in headspace gases (determined by gas chromatography) were measured 6 hr later. IC50 values for total gas and methane production were determined from linear least squares line equations derived from inhibition as a percent of the control. Results indicated that chromium, selenium, copper, and zinc were relatively equal in terms of toxicity to total gas production and methanogenesis. Calculated IC50 values for reduction of total gas production were 123, 133, 115, and 122 mgL-1, respectively. IC50 values for reduction of methane in headspace gas were 102, 118, 114, and 170 mgL-1, respectively. Correlation coefficients (r) for the line equations of these four toxicants ranged between 0.9733 and 0.9980. Mercury was slightly less toxic with IC50 values of 286 and 254 mgL-1 for reduction of total gas and methane production, respectively. Line equation correlation coefficients were 0.9677 and 0.9919 for these values. Lead and cobalt were the least inhibitory elements examined and median inhibitory concentrations were greater than the maximum concentration tested (400 mgL-1). Extrapolated IC50 values for lead were 410 and 405 mgL-1 for total gas and methane production, respectively. Cobalt showed very little inhibition with IC50 values estimated at 551 and 1106 mgL-1 for total gas and methane production, respectively. Correlation coefficients for lead and cobalt line equations used to calculate IC50 values ranged between 0.8331 and 0.9609