Partial oxidation by ozone to remove recalcitrance from wastewaters - A review

Xenobiotic organics produced by modem industrial processes resist conventio nal biological wastewater treatment. Ozone pretreatment to improve biodegra dation via partial oxidation is a potential solution for this recalcitrance . Ozone forms no additional sludge, removes colour and has two modes of act ion that can be utilised according to the wastewater consitituents. The pre ozonation of a variety of wastewaters and model compounds is reviewed here. Of the industrial processes examined, the associated recalcitrant organic compounds were generally halogenated heterocyclic or nitrogenous aromatics, aliphatic polymers or polyaromatics. initial optimisation of ozone dose en sured the occurrence of partial oxidation where over 90% parent compound tr ansformation was required to give best results in the subsequent biological process. Ozonation intermediate and end-products included ketones, aldehyd es and organic acids that were more biodegradable than the parent compounds . Biodegradability improvement was assessed using TOC, GOD, BOD, BOD/COD ra tio and OUR. Actual process feasibility and performance was evaluated using various combined ozonation-biodegradation trials; textile dying and finish ing effluent being one area of application. Kinetic studies of chemical and biological stages facilitated process optimisation and can assist scale-up . The main factors affecting ozonation performance were pH, the nature and concentration of oxidisable organics, ozone dose, competition between the t arget compound and biodegradable by-products, the presence of oxidant scave ngers, and the efficiency of ozone mass transfer. The formation of toxic or recalcitrant by-products has been found to occur in some cases, necessitat ing knowledge of wastewater constituents and trials on a site-by-site basis .