Characterisation of soluble residual chemical oxygen demand (COD) in anaerobic wastewater treatment effluents

Anaerobic wastewater treatment processes cannot usually achieve discharge levels of COD without some form of post-treatment due to the high levels of soluble residual COD in the effluent. However, there is very little information in the literature on the nature of this material, and hence the aim of this work was to characterise effluents from several different anaerobic processes by their: molecular weight (MW) distributions (using the techniques of ultrafiltration and size exclusion chromatography); biodegradability (both aerobic and anaerobic) and activated carbon adsorption characteristics. The MW distribution data indicated that the majority (up to 89% w/w) of the material present in the effluents was in the low MW range (i.e. MW<1 kDa), although there was a significant amount (up to 22% w/w) in the high MW range (i.e. MW>300 kDa). Differences in the distributions were observed according to reactor type, with effluents from anaerobic baffled reactors (ABRs) containing a higher proportion of high MW material than effluents from other reactor types. Aerobic degradation of the effluents was significantly more successful than anaerobic, despite the fact that the low MW material was found to be the most difficult to degrade aerobically. Activated carbon adsorption revealed that the low MW material was more difficult to adsorb than the high MW compounds. The data was fitted to Freundlich, Langmuir and BET isotherms and it was concluded that the Freundlich model was the most accurate for predicting removal efficiencies especially with regard to scale up. Some generic patterns emerged with respect to the characterisation of the effluents and these will assist in decisions regarding post-treatment. Nevertheless, in order for these generic patterns to be confirmed more work is needed on characterising effluent samples from anaerobic treatments.