Toxicity of linear alkylbenzene sulfonate in anaerobic digestion: influence of exposure time

The inhibition of anaerobic digestion by the anionic surfactant linear alkylbenzene sulfonate was studied. The development of bacterial activity during several weeks was monitored by performing batch degradation tests with acetate and propionate. In the first phase the decay of activity without surfactant addition was studied. After the addition of the surfactant an immediate inhibition was detected. Subsequently, the degradation activity continued to decrease with increasing exposure time. Both, the immediate inhibition and the rate of the subsequent decrease of activity were dependent on the surfactant concentration. A kinetic model is presented that describes this behavior. A surfactant concentration of 14 mg L−1 causes a 50% immediate inhibition of acetate degradation (27 mg L−1 in the case of propionate degradation). Additionally, each 12 mg L−1 of surfactant increases the decay rate of acetate degradation activity by a factor 10 (23 mg L−1 for propionate degradation). A deviation from this model occurs at low surfactant concentrations (<3 mg L−1), where a slight stimulation of bacterial activity was observed. The above-mentioned concentrations refer to measured surfactant concentrations. These were substantially lower than the nominal concentrations (added surfactant per volume). This discrepancy is explained by adsorption of the surfactant to the biomass. Finally, the importance of the presented kinetic model and the significance of surfactant toxicity for anaerobic digestion are discussed.