Advances in ozonation and biodegradation processes to enhance chlorophenol abatement in multisubstrate wastewaters: a review

Abstract

Chlorophenolic pollutants are considered a serious environmental threat when disposed of without proper treatment. Moreover, increasingly stringent regulations towards effluent quality result in a growing interest in upgrading conventional wastewater treatment to reduce pollutant concentrations to acceptable levels. Among the wide range of treatment options, biological and ozone-based oxidation processes are already well defined for the degradation of chlorophenolic compounds. The main objective of this review is to highlight relevant knowledge gaps and research needs on chlorophenol degradation by applying biological and/or ozone-based oxidation techniques. The main research trends towards chlorophenol biodegradation are the use of specific cultures adapted to the pollutant and co-metabolic biodegradation. More recently, novel biological processes such as biomass immobilization and granular activated sludge were reported. These techniques possess the ability to protect the microorganisms from higher toxic pollutant concentrations. Only a few authors reported the use of mixed cultures and the effects of acclimated biomass to enhance the biodegradation process. However, conventional biological processes do not always achieve satisfactory results, opening the possibility for ozone-based oxidation technologies to degrade the refractory compounds into more biodegradable intermediates. The research objectives in many studies on chlorophenol ozonation consider the degradation of single pollutants and/or the enhancement of the oxidative potential using catalysts or UV light. Only a few attempts were reported investigating chlorophenol degradation in multiple substrate solutions. Also, the need for an additional toxicity and/or biodegradability assessment using microorganisms representative for activated sludge is clear; all the more when assessment of effects on a totally different microbial community is desired.