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

摘要

Microbial anaerobic biodegradation of phthalate esters (PAE's) is a well-known process.rnInitial microbial breakdown of the ester bond releases long chain alcohols (LCA) and arnphthalic acid (PA) ring. The LCA and the PA ring can be further mineralized to CO2 andrnCH4. Complete and rapid mineralization is experienced with PAE's with short esterrnhydrocarbon chains. PAE's with long ester chains are only partially mineralized atrnextremely low degradation rates are therefore considered to be recalcitrant. Moreover,rnthey have been observed to inhibit methanogenesis. There are few reports on the relativerndegradation rates of phthalate esters and no studies have been made on the effect ofrnfeeding a combination of recalcitrant and biodegradable PAE's into anaerobic digestersrntreating wastewater sludge. The present study was conducted with wastewater sludgernfrom two different locations, the Los Angeles Bureau of Sanitation's Hyperion TreatmentrnPlant located in Playa del Rey, CA, USA and the Lundtofte Wastewater Treatment Plantrnlocated in the city of Lyngby, Denmark. The anaerobic degradation of di (2-ethylhexyl)rnphthalate (DEHP), the most common persistent PAE found in wastewater, di-ethylrnphthalate (DEP) and di-butyl phthalate (DBP), two common PAE's with short esterrnchains, was studied. Microbial degradation rates were evaluated from kinetic batchrnexperiments. Degradation rates indicated that DEP and DBP were degraded two orders ofrnmagnitude faster than DEHP. Removal of DEHP or a combination of DEHP and DBPrnwere studied in bench scale digesters. DEHP removal was measured at a concentrationrnsimilar to the one found in wastewater sewage (10 ppm). Removal of DEHP combinedrnwith DBP was measured at high concentrations (100 ppm) to simulate conditions foundrnin industrial effluents. DEHP degradation in both cases was always poor, andrnaccumulation of DEHP was observed. However, when high concentrations of DEHP andrnDBP were added, accumulation of DEHP was correlated with inhibition of the microbialrndegradation of DBP and with process instability of the test digester. Inhibition of thernDBP removal was completely reversed after DEHP addition was discontinued, but biogasrnproduction never recovered to the level observed in a control digester. Other processrnparameters of digester performance were not affected by DEHP accumulation. Thesernresults are similar to the toxic effects of long chain fatty acids on sludge digestion,rnsuggesting that DEHP or its degradation products affect all the microbial populations inrnthe anaerobic bioreactor. Our results imply that high levels of DEHP or other recalcitrantrnPAE's in wastewater sludge are likely to affect methanogenesis and removal ofrnbiodegradable PAE's in sludge digesters.