Integrated chemical and biological treatment process to remove colour compounds from wastewater

Abstract

Coloured wastewater, particularly from non-natural sources, is aesthetically unacceptable, hinders light penetration, damages the quality of the receiving streams. and may be toxic to biological wastewater treatment systems, to food chain organisms and to aquatic life. Large amounts of coloured wastewater that contain reactive azo dyes are generated especially from the textile industry, but there are also other sources of colour discharges to sewers and the environment. The treatment of reactive azo dyes has become a major concern since they are difficult to remove by means of conventional treatment methods. Furthermore, many break-down products of azo dyes are toxic and resistant to elimination by biological treatment.To effectively treat such wastewater, often a combined chemical and biological oxidation process is required, as purely biological treatment is usually very slow or not possible. Advanced oxidation processes (AOPs) can improve the biodegradability of refractory organic compounds through the use of short-lived, very reactive hydroxyl radicals (0H*). Nevertheless, total mineralization is quite difficult to achieve except when long reaction times are applied in the system. Biological processes are far more efficient and cost-effective for smaller molecules which are usually produced from the AOPs operation. The main objective of this study was to investigate the removal process of a reactive azo dye (C.I. Reactive Red 195A) using the integrated UV/MH2O2 and aerobic biological process aiming at minimal chemical and energy consumption, while reducing the toxic or inhibitory compounds to enable rapid and near complete biological degradation.In this PhD thesis, the colour and soluble chemical oxygen demand (sCOD) removal performance of various treatments, such as aerobic biofilm processes, UV irradiation, H2O2 addition and combinations of these approaches, were investigated. In addition, the degradation products and toxicity effects of the integrated UV/MH2O2 and aerobic biofilm process were also evaluated. To evaluate the practical applicability of the process, the main operating costs for the treatment process were also estimated.Using only an aerobic biological reactor as single treatment method showed insignificant dye and sCOD removals. This was due to the non-biodegradable nature of the dye. Additionally, neither H2O2 addition nor UV irradiation alone were able to considerably reduce the dye and sCOD concentrations. Only the combined process of UV/MH2O2 gave significant removals for both parameters. In this work, the UV radiation source was a low-pressure mercury arc lamp (60W emitting at 253.7nm). Using the combined UV/MH2O2 process, the effects of initial hydrogen peroxide dosage, dye concentration, pH and temperature were examined to determine the optimum operating conditions of the treatment process. Complete decolourization of 100 mg/L dye was achieved in the relatively short time of 20-30 minutes irradiation. Faster decolourization was achieved at low pH and high temperature. The removal rate increased with increasing initial concentration of H2O2 up to an optimum value (approximately 900 mg H2O2/L). However, for practical applications an initial pH of 8 (or lower), temperature of 30oC, and a H2O2 dosage of about 3 times the dye concentration was identified as the most appropriate conditions for the chemical treatment. The decolourization reaction was found to follow first order kinetics with respect to the dye concentration. However, even after near complete removal of the active dye from the solution, approximately 60% of the COD was still remaining, indicating only partial breakdown of the dye molecule. Besides, the electric energy per order (Eeo) value of the system to obtain near complete decolourization was quite high (48.6kWh/m3/order) due to the required high UV dose, representing high electrical power costs for the treatment process.