Removal and destruction of toxic micropolluting organic compounds from waste waters by a combined NF and SCWO process

Supercritical Water Oxidation (SCWO) has been proposed almost 20 years ago as a potential waste treatment process since it can be used to destroy any organic polluting material dissolved or suspended in aqueous solutions or sludge. In fact, since oxygen or air and supercritical water are completely miscible, and organic compounds are quite soluble in supercritical water, complete oxidation of such compounds can be achieved. As a result, all the polluting compounds are converted to harmless gaseous or liquid substances such as carbon dioxide and water. SCWO usually operates at a pressure of about 25 MPa and at a temperature between the critical temperature of water and 850 degreesK. Pure oxygen, air or hydrogen peroxide can be used as oxidizing agents. In comparison with other waste purification processes, SCWO is characterized by high investment and operating costs, but application of SCWO may result in an ultimate solution of the waste disposal problem. This is very important when dealing with very dangerous polluting compounds which are usually present in waste waters as micropolluting substances. In this paper a new wastewater purification process for the removal and destruction of toxic substances is described and discussed. This process is based on the coupling of a nanofiltration (NF) section with a SCWO section in order to reduce both the investment and operating costs, while retaining final destruction of toxic substances and high level of purification.