Biodegradation and Toxicity of Phthalate Esters during the Anaerobic Digestion of Wastewater Sludge

Microbial anaerobic biodegradation of phthalate esters (PAE's) is a well-known process. Initial microbial breakdown of the ester bond releases long chain alcohols (LCA) and a phthalic acid (PA) ring. The LCA and the PA ring can be further mineralized to CO2 and CH4. Complete and rapid mineralization is experienced with PAE's with short ester hydrocarbon chains. PAE's with long ester chains are only partially mineralized at extremely low degradation rates are therefore considered to be recalcitrant. Moreover, they have been observed to inhibit methanogenesis. There are few reports on the relative degradation rates of phthalate esters and no studies have been made on the effect of feeding a combination of recalcitrant and biodegradable PAE's into anaerobic digesters treating wastewater sludge. The present study was conducted with wastewater sludge from two different locations, the Los Angeles Bureau of Sanitation's Hyperion Treatment Plant located in Playa del Rey, CA, USA and the Lundtofte Wastewater Treatment Plant located in the city of Lyngby, Denmark. The anaerobic degradation of di (2-ethylhexyl) phthalate (DEHP), the most common persistent PAE found in wastewater, di-ethyl phthalate (DEP) and di-butyl phthalate (DBP), two common PAE's with short ester chains, was studied. Microbial degradation rates were evaluated from kinetic batch experiments. Degradation rates indicated that DEP and DBP were degraded two orders of magnitude faster than DEHP. Removal of DEHP or a combination of DEHP and DBP were studied in bench scale digesters. DEHP removal was measured at a concentration similar to the one found in wastewater sewage (10 ppm). Removal of DEHP combined with DBP was measured at high concentrations (100 ppm) to simulate conditions found in industrial effluents. DEHP degradation in both cases was always poor, and accumulation of DEHP was observed. However, when high concentrations of DEHP and DBP were added, 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 PAE's in wastewater sludge are likely to affect methanogenesis and removal of biodegradable PAE's in sludge digesters.