Numerical Simulation And Optimization Of A Triple Oxidation Ditch Process

With great seasonal variations of inlet wastewater quantity and quality in South China, a triple oxidation ditch process was investigated in this paper to solving the problems of high energy consumption.The key point of triple oxidation ditch process energy saving was determined based on the analysis of process characteristic energy consumption status. The reactor building methods for numerical simulation of triple oxidation ditch process were proposed and the mathematical models of triple oxidation ditch process were built. The distribution and variation law of dissolved oxygen and pollutants concentration were simulated and analysed. The influence factors on effluent quality of triple oxidation ditch process were simulated and studied. And an energy saving optimization scheme was proposed.The characteristics of inlet and energy consumption status were analysed, the potimization of aeration system was determined to be the key point of the energy saving control. The reactors were built according to the partial plug-flow, integral mixing and sequence batch characteristics of the triple oxidation ditch process. The alternate operation mode was also considered. The mathematical models were then built with ASM2D as simulation mechanism and GPS-X software as simulation platform. The sensitivity of the chemometrics and kinetic parameters were analysed to calibrate the models and make the simulation results reflented the actual condition well. The differences between the simulated and measured results for average removal rates of COD, ammonia nitrogen, total nitrogen and total phosphorus were 0.3%, 1.8%, 0.9% and 1.5%.The distribution and variation law of dissolved oxygen and pollutants concentration were analysed by the numerical simulated methods. The concentration gradient of dissolved oxygen was obviously in space. And the nonlinear and periodicity characteristics of the dissolved oxygen and pollutants in time domain were verified. Partial organics were removed in the anaerobic tank, the residual organics were diluted and degraded in the oxidation ditch. NH₄⁺-N was mianly removed in the middle ditch of the triple oxidation ditch. The denitrification reaction mainly took place in the A stage and D stage of the reaction side ditch. Phosphorus removal was jointly accomplished by anaerobic phosphorus release of the phosphorus accumulating bacteria in the anaerobic tank, phosphorus release and uptake periodically in the reaction side ditch and phosphorus uptake in middle ditch. The sludge concentration in the middle ditch was relatively stable and variated periodic in the reaction side ditch. The influence factors on effluent quality were temperature, pH, substrate concentrations, hydraulic retention time, dissolved oxygen concentration, sludge concentration and sludge retention time. Adjusting the quantity of gas supply, recycling sludge and excess sludge, optimizing the operation parameters such as controlling the dissolved oxygen concentration, the sludge concentration and the sludge retention time could enhance the adaptability of the system to resisting hydraulic and pollutant load, control the effluent quality and operation energy consumption when the water quantity and quality varied greatly.The whole year was devided into dry season, rainy season and transition season according to the seasonal dynamic characteristics of the influent flow. The whole day time was devided into 8 control times according the hourly characteristics of the influent flow, and a discrete energy saving optimization scheme was proposed based on them in the 3 different seasons. The running conditions of the aeration system, sludge recycling syste m and sludge discharge system were optimized with the variation of influent flow as feedforward mian control index and dissolved oxygen as feedback auxiliary control index. The feasibility of the scheme was proved by numerical simulation methods.Implemented the optimization schemes in a triple oxidation ditch process form January to June 2010. The ranges of COD, ammonia nitrogen, total nitrogen and total phosphorus concentration in the effluent were 10.5 to 45.6mg/L, 0.1 to 4.8mg/L, 3.5 to 14.8mg/L, 0.1 to 0.9mg/L and3.0 to 19.0mg/L when the concentration ranges of these polutnets in the influent were 149.2 to 1340.6mg/L, 12.4 to 35.4mg/L, 23.8 to 77.8mg/L, 2.4 to 13.1mg/L and 82.0 to 1953.7mg/L. The effluent quality could meet the first class of B standard (the ammonia nitrogen and total nitrogen concentration could meet the the first class of A standard) with implementing the scheme. The power consumption per unit and power consumption per kg BOD removed decreased 6.7% and 14.9% after optimization controlling. The energy saving effect of the optimization scheme was significant.