Biotreatability of liquors from wet oxidation of sludges and industrial wastewaters

Abstract

As environmental awareness increases, it will become increasingly difficult to discharge pollutants to the environment without treatment. New and improved technologies can only be based on a knowledge of a large number of factors for each situation. These can be divided into: legislative requirements, environmental impacts, economics, and technical feasibility. Meeting all these criteria will often mean that no single technology will be sufficient to completely alleviate the problem. Hence, a combination of technologies can often be employed. Industrial wastewater, which is often very difficult to treat by conventional treatment, and the large volumes of sludge produced by the wastewater industry have opened up the potential for wet oxidation, which could be very effective in destroying many hazardous organic wastes, and also very effective in reducing the volume of sludge to be disposed of. The decant liquor from WO is often very concentrated and contains low molecular weight organic compounds, mostly acetic acid, but which could be treated to some degree by a biological treatment process. The Wet Oxidation (WO) process is recommended for the oxidation of organic effluent with a solids concentration of between 1% to 25% but which are too toxic to be biologically treated or too diluted to be incinerated. This research project was a continuation of a previous study by Luduvice (1992) and, when possible, most of his recommendations were investigated, including the use of pure oxygen instead of air in the reactor, the biotreatability of the heat liquor and an evaluation of the chemical characteristics of the liquor. It was not, however, possible to develop a continuous Wet Oxidation process capable of operating at both subcritical and supercritical conditions. This thesis describes the ability of wet oxidation to treat different organic wastewaters and sludges under conditions which included the stoichiometric requirement of oxygen being provided and with further biological treatment being given to the decant liquor. The organic wastewater and sludges tested were from different origins and characteristics, including paracetamol wastewater, detergent wastewater, from industries plus raw primary sludge and activated sludge from a biological wastewater treatment plant. Tests were carried out at temperatures varying between 1600 C and 3000 C at retention times of 10, 15, 30 and 60 minutes in a 3.78 1 stainless steel reactor. Temperature proved to be the most significant parameter, followed by retention time and oxygen overpressure. A considerable reduction in sludge volume and organic content was obtained in most runs, which in general produced an effluent liquor with a high oxygen demand and relatively stable residual solids. The residual WO solids, when dried were found to be capable of removing colour from a textile-dye wastewater, implying that dried WO sludge may have adsorption properties similar to that of activated carbon. Simplified empirical equations were developed from the experimental data. The equations adequately described the transformation pattern of the organic and inorganic components of the activated sludge in a WO environment. The empirical equations further demonstrate a direct relationship between the influent VTS and the transformed organic and inorganic components in the liquor after WO. The purpose of this study was also to demonstrate the feasibility of reducing the strength of heat treatment liquors to that approximating domestic wastewater. A range of aerobic and anaerobic biological treatment systems was investigated. Aerobic biological processes proved to be very effective and robust in COD and BCOD removal compared to the anaerobic biological processes.