Research Of Coking Wastewater Advanced Treatment By Four-stage Bio-reactor Combination Process

Coking wastewater was the side product of coking and gas-made process in coal chemical industry. And coking wastewater is a kind of wastewater with hard-biodegradation and high concentration organic contaminants which heavily polluted the water environment.Coking wastewater contains high concentration of organic contaminants, many kinds of hard-biodegradation substance, and this led to the high treatment difficulty and high cost. To make the effluent stably meet the discharge standard and reduce the operating cost were being the common difficult problem in coking wastewater treatment field.Taking the great demand of national economy and serious problem of environmental pollution into account, and aiming at the treatment situation of coking wastewater, this paper began with the research of immobilized predominant bacterium and then explored the in-situ intensification coking wastewater treatment technology. The four-stage bio-reactor combination process was developed through the research of single optimization bio-technology which suited to the treatment situation so as to meet the COD and NH₄⁺-N discharge demand. Through the kinetic study of the core reactor, it was expected that some guidance was provided to the application of the process. Research provided the technical support to the reuse of coking wastewater by studies on the decoloration of the coking wastewater after the determined process treatment and the selection of economical efficient absorbent for advanced decoloration treatment in coking wastewater treatment, through the intermediary product during the coking.The predominant hybrid bacterium which were screened from the aerobic tank and anoxic tank, which were immobilized in the new modified carriers, were distributed into the anaerobic-aerobic two-stage SBR reactors individually to enhance the bio-treatment of coking wastewater. The experimental results showed that predominant hybrid bacterium exhibited obvious degradation advantage to the treatment of hard-biodegradable organic contaminants in coking wastewater. The removal rate for COD and NH₄⁺-N were up to 80.4% and 68.4%, respectively, which were improved by 20% and 38% compared to that without predominant hybrid bacterium. It clearly indicated that bioaugmentation of coking wastewater treatment by predominant hybrid bacterium was key technology to the in-situ intensification for the current coking wastewater treatment plant.It was found in the research of optimal operating condition of some typical coking wastewater treatment processes that there were abundant biological phase in the single hybrid reactor, which behaved high shock resistance and high MLSS of 16600 mg/L in the reactor. The treatment capacities of the reactor for COD and NH₄⁺-N in coking wastewater were improved by 30% and 55%, respectively, compared to conventional activated sludge aerobic tank. The ratio of BOD5/COD was increased from 0.27⁰.30 to 0.32⁰.40 by hydrolysis-acidification process, that is averarge 26.3%, and this improved the treatment efficiency of the following aerobic reactor. The combination of anaerobic hydrolysis and the aerobic reactor made the COD removal efficiency increased by 12%, that from 58% of the conventional activated sludge to 70% of the aerobic reactor in the process. And the anoxic micro environment in the anaerobic reactor made efficiency of denitrification improved by 16.5% in the combined process compared to the single hybrid process. The hard-biodegraded organic contaminants in coking wastewater were efficiently degraded by many kinds of bacteria due to the pre-treatment of hydrolysis and two-stage aerobic in the combination of anaerobic hydrolysis reactor, aerobic activated sludge reactor and hybrid bio-reactor. Especially, in the combination of aerobic activated sludge reactor and the hybrid bioreactor, the removal rate for COD and NH₄⁺-N reached up to 90% and 85%, and the effluent concentration of COD and NH₄⁺-N were lower than 100 mg/L and 15 mg/L, respectively.The study showed that the optimal process for coking wastewater treatment was combination of anaerobic hydrolysis, anoxic reactor, aerobic activated sludge reactor and hybrid bio-reactor. The high concentration hard-biodegraded organic contaminants in coking wastewater were removed through anaerobic hydrolysis and two-stage aerobic in the combined process. NH₄⁺-N was removed through aerobic nitrification and anoxic denitrification. These made the effluent meet the discharge standards. The ratio of HRT in the different stages in the combined process was 1:1.2:1.9:0.8. Because of the high MLSS and the strong degrade performance in the hybrid bioreactor, DO concentration was above 3 times than that in activated sludge reactor. The sludge load of COD and NH₄⁺-N were 1.2 and 1.4 times in the two stage- aerobic units of the combined process than that in single conventional aerobic activated sludge reactor. The removal rates for COD and NH₄⁺-N in coking wastewater were up to 90% and 85%, respectively, and denitrification efficiency was above 75% under the optimal operating condition in the combined process.The kinetic study on biodegradability of coking wastewater through the bioaugmentation hydrolysis acidification showed that the half rate constant (Ks) decreased from 101.998 of the conventional activated sludge system to 71.899, and the maximum specific substrate degradation rate (K) increased from 0.00224 to 0.00304. These indicated that the biodegradability of coking wastewater was greatly improved. Through the kinetic study on the reaction process of coking wastewater in anaerobic hydrolysis-aerobic process, the second order kinetic equation of COD removal in coking wastewater was drawn out, and the rate constant was 0.0298.The research on adsorption efficiency of activated carbon, fly ash, coke and modified semi-coke implied that decoloration efficiency of the modified semi-coke was 92.3%, and it was 1.06 times of activated carbon, 1.94 times of fly ash, 1.14 times of coke, respectively. Compared in facileness, cost in preparation and operating cost of the absorbents indicated that semi-coke had the similar absorbing effect with activated carbon, and the operating cost was only 60% of activated carbon. Semi-coke was a new kind of absorbent for coking wastewater advanced treatment which had broad perspective in industrial application.The remarkable advantages of semi-coke asorbent were regeneration of the semi-coke asorbent after saturation adsorption and reuse of the coking wastewater after the advanced treatment.Through engineering design and economy calculation and analysis about the four-stage combination process for coking wastewater treatment, the conclusion could be drawn out that the construction cost was about RMB 4200 yuan/m3, and the operating cost including decoloration was RMB 4.15 yuan/m3 meeting level I of the discharge standard. The results of the research reached the engineering objective of advanced technology and economic feasibility.