Previous studies on the microbial degradation of individual phthalic acid esters (PAEs) have demonstrated that the compounds with short ester hydrocarbon chains are easily biodegraded and mineralized, but PAEs with long ester chains are less susceptible to degradation and some of them are considered recalcitrant. Moreover, they inhibit methanogenesis. However, studies have not been made on the effect of feeding a combination of recalcitrant and biodegradable PAEs into anaerobic digesters treating wastewater sludge. The present study was conducted with wastewater sludge from the Los Angeles Bureau of Sanitation's Hyperion Treatment Plant. Di (2-ethylhexyl) phthalate (DEHP), the most common persistent PAE found in wastewater, and di-n-butyl phthalate (DBP), a common PAE with short ester chains, were sorbed into the sludge fed to a bench-scale digester for a period of 12 weeks. DEHP degradation was always poor, and accumulation of DEHP was correlated with inhibition of the microbial degradation of DBP and with process instability of the test digester. Inhibition of the DBP removal was completely reversed after DEHP addition was discontinued, but biogas production never recovered to the level observed in a control digester. Other process parameters of digester performance were not affected by DEHP accumulation. These results are similar to the toxic effects of long chain fatty acids on sludge digestion, suggesting that DEHP or its degradation products affect all the microbial populations in the anaerobic bioreactor. Our results imply that high levels of DEHP or other recalcitrant PAEs in wastewater sludge are likely to compromise methanogenesis and removal of biodegradable PAEs in sludge digesters.
