Inhibition of biogas production by phenol, catechol and caffeic acid over methanogenic industrial sludge

Phenol, catechol and caffeic acid are important natural and anthropogenic pollutants. They are associated with the industrial production of olive oil, wine, coffee, pulp and paper. The toxicity of these compounds could be problematic for the anaerobic treatment of wastes discharged by such industries. This study aims at examining the toxicity of these compounds on biogas production of industrial anaerobic sludge and their structure-toxicity relationship. Toxicity assays were carried out in serum bottles by measuring biogas production at different concentrations of aromatic compounds. The biodegradation of the test compounds was minimised by the direct use of unacclimated sludge. Different levels of toxicity were observed, with phenol being the most toxic compound and caffeic acid the least. 404, 319 and 123 milligrams of caffeic acid, catechol and phenol per gram of VSS of sludge inhibited gas production by 50%. A structure-toxicity relationship was not found owing to the variety of microorganisms contained in the sludge and the close values of apolarities. Results Conclusions •Caffeic acid was found to be the least toxic compound and phenol the most. •404, 319 and 123 milligrams of caffeic acid, catechol and phenol per gram of VSS of sludge inhibited gas production by 50%. •The toxicity of the aromatic compound decreases as the number of substitution increases. •Structure-toxicity correlation was not found owing to the close values of apolarity among the compounds and probably because of the variety of microorganisms in the inoculum. •The information reported herein is valuable for anaerobic biodegradability tests and to estimate these substances toxicity in operating an anaerobic digester. For contaminated soils, marine sediments or any other matrices containing anaerobes special standard toxicity tests must be done. •The highest rate of biogas production occurred during the first 72 hours, figure1. •No diauxic growth could be observed owing to the short time of the experiment that neither allowed the biodegradation of the compounds nor the adsorption onto the sludge to build up more toxic concentrations. •The percentage of inhibition, I, of every set of bottles against the controls, was calculated and plotted versus the logarithm of concentration of test compound as shown in figure 2. The linear correlation coefficient, R 2 , for phenol, catechol and caffeic acid was 0.9928, 0.9973 and 0.9816 respectively. •404, 319 and 123 milligrams of caffeic acid, catechol and phenol per gram of VSS of sludge inhibited gas production by 50%. •These results did not show any correlation between toxicity effects and apolarity (Figure 3). •It might be due to two facts: the Log P values of the three compounds are very close or the multi culture of microorganisms contained in the sludge or both.