Study On Supercritical Water Oxidation Of Dyeing Wastewater

Dyeing wastewater is one kind of refractory industrial wastewater. Because of the characteristics of the wastewater such as large water volume, high concentration of organic pollutant, deep chromaticity, high alkalinity, changeable water quality and complex ingredient, it is difficult to remove organic substances and colority from the wastewater completely by conventional means.Supercritical water oxidation(SCWO) is a new and effective technology for treatment of organic waste. This technology has the advantages of high organics destruction efficiency, short reaction time and friendly environment, the effluent after treatment can discharge or reuse since it could meet the national discharge standard. In this dissertation, the dyeing wastewater obtained from a printing and dyeing mill was treated by SCWO. The effect of reaction conditions on the degradation of total organic carbon(TOC) and ammonia nitrogen was investigated in the SCW oxidation and pyrolysis processes. Based on the experiments, the kinetic equations for TOC degradation efficiency about the oxidation and pyrolysis processes in SCW were established. Besides, the effect of operation parameters of the self-built salt separator in which salts in the wastewater were removed on total dissolved solids(TDS) removal efficiency were discussed. The possibility of effluent reuse after SCWO treatment was studied and the operation cost of SCWO process was calculated simply. The application of SCWO technology in industry was discussed too. Through above analysis, the main conclusions are as follows:1. The effect of temperature on TOC and ammonia removal from the wastewater in SCWO process is the most remarkable. The changes of temperature and residence time have greater effect on the TOC degradation of smaller excess oxygen reaction. The degradation of ammonia nitrogen is difficult at low temperatures, it can be degraded only at high temperatures in SCWO process.2. For the SCW pyrolysis, temperature is the most important influence factor in TOC degradation too and the effect of the residence time can not be ignored, but degradation of ammonia nitrogen is not be found in this process.3. It can be seen from the comparison pyrolysis with oxidation reactions in SCW, the changes of temperature have greater influence on TOC pyrolysis and the influence of oxidant on TOC conversion is obvious at a lower temperature, but degradation of ammonia nitrogen may remain unaffected by the extension of residence time and adding oxidant in the lower temperature.4. A kinetic equation based on TOC removal of SCW pyrolysis was established by pseudo-first order kinetics. Another kinetic equation for TOC removal of SCWO was built up by multiple nonlinear regression when excess oxidant was added. Based on the above results, the third kinetic equation for TOC removal of SCWO was set up by using Runge-Kutta algorithm at the condition with stoichiometric oxygen supply to explore the influence of oxidant concentration on TOC conversion. Their expressions were given as follow: SCW pyrolysis (pseudo-first order kinetics): SCWO (multiple nonlinear regression): SCWO(Runge-Kutta algorithm):5. For the desalination process in salt separator, temperature has the most significant effects on the desalination efficiency. The process of desalination of the wastewater in supercriticality may be completed in50.76s. Under a certain temperature, the content of TDS in effluent after desalination is affected by the initial salt(TDS) concentration of the wastewater.6. The wastewater treated by SCWO can meet the national emission standard and be used for rinsing processes, but it can not be used in dyeing and printing process.7. After a simple calculation, the operating cost using SCWO for treating the dyeing wastewater is about20.25yuan/m3when CODCr of the wastewater is3000mg/L and hydrogen peroxide is used as oxidant.8. SCWO technology will have a broad prospect in industrial application in the future, but some respects such as the choice of raw material, design of reactor, optimization of process conditions and utilization of reaction releases energy should be studied deeply, so we will be able to overcome the salt deposition, avoid the material corrosion, save operating cost. The SCWO technology will eventually be accepted.