Inhibition of the anaerobic digestion process by linear alkylbenzene sulfonates

Linear Alkylbenzene Sulfonates (LAS) are the most widely used synthetic anionicsurfactants. They are anthropogenic, toxic compounds and are found in the primarysludge generated in municipal wastewater treatment plants. Primary sludge is usuallystabilized anaerobically and therefore it is important to investigate the effect of thesexenobiotic compounds on an anaerobic environment. The inhibitory effect of LinearAlkylbenzene Sulfonates (LAS) on the acetogenic and methanogenic step of theanaerobic digestion process was studied. LAS inhibit both acetogenesis from propionate and methanogenesis from acetate and hydrogen and it is shown that the propionate-utilising bacteria are more sensitive to the presence of LAS than the acetoclastic methanogens. It has been proven that the inhibition intensity depends on the solids concentration and thus the term ``biomass specific LAS concentration' has been introduced in order to describe the phenomenon better. Conclusively, it is believed that the inhibitory effect of LAS is the main reason that anaerobic microbial enrichments on LAS have not been succeeded yet. Also, the inhibition caused by LAS on the acetogenic and methanogenic step of the anaerobic digestion process should be seriously taken into consideration when wastewater from a surfactant producing industry is to be treated biologically or enter a municipal wastewater treatment plant that employs anaerobic technology. The upper allowable biomass specific LAS concentration should be 14 mg LAS (gVSS)^-1.