Research On Synchronous Treatment Of Coking Wastewater And Excess Sludge By EGSB Reactors

The components are very complex in coking wastewater, in which contains volatile phenol, cyanide, thiocyanate, NH3-N, and refractory polycyclic and heterocyclic aromatic pollutants such as indole, pyridine and' quinoline,etc. Coking wastewater is a typical industrial wastewater containing toxic and refractory compounds. Treatment of coking wastewater is a difficulty problem in the pollution control of industrial wastewater in China.The activated sludge processes such as A2/O process and improved A2/O processes are the mainstream treatment methods of coking wastewater. However, the effluent quality of these biochemical treatment processes are becoming difficult to meet the increasing effluent discharge standards, and also faced with the problems of large land area occupation, high operation cost and a large quantity of excess sludge generation, etc.The two-stage micro-aerobic Expanded Granular Sludge Bed (EGSB) reactors were used to treat actual coking wastewater, operated continuously for 228d with complete sludge retention. Pollutant removal performances of the system were analyzed, and the synchronous treatment of coking wastewater and excess sludge by EGSB reactors was investigated.The two-stage EGSB reactors were seeded with granular sludge which was used to treat coking wastewater in the laboratory. After 76d, the volumetric loading rate (VLR) of EGSB I reached 4.04kgCOD/(m3·d), the removal efficiency of COD was stable at 80.0% or more, the quantity of small granular sludge at the bottom of the EGSB reactors increased, the color of sludge changed from shining black to grey black, and MLVSS/MLSS was improved. The reactors were started successfully by treating actual coking wastewater.Under the conditions of temperature of 27.5 to 31.5? in the reactors, hydraulic retention time (HRT) of 24h, liquid upflow velocity (vup) of 2.7m/h, DO of 0 to 0.01mg/L at the reactor inlets, the two-stage EGSB reactors could operate efficiently and be steady for long-term.After the two-stage EGSB reactors were steadily operated, the system could remove 97.7% of thiocyanate, 88.3% of cyanide, 75.9% of COD, 94.1% of BODs, 82.1% of NH3-N,91.0% of NO3--N, and 75.0% of TN, with average effluent concentrations of 5.9mg/L,0.07mg/L, 178mg/L, 16mg/L, 7.4mg/L, 6.0mg/L and 31.2mg/L, respectively. Volatile phenol could be removed thoroughly, and it had not been detected in the effluent. The effluent concentrations of volatile phenol, cyanide, BODs and NH3-N were less than the direct discharge limits of "Emission Standard of Pollutants for Coking Chemical Industry" (GB 16171-2012). The volatile phenol, thiocyanate, cyanide, COD, NO3--N and TN were mainly removed in the EGSB I,while the NH3-N was removed in the EGSB II. The UV-Vis absorption spectra showed that most of polycyclic and heterocyclic aromatic compounds could be removed by EGSB I, but the degradation of benzene derivatives was limited in this reactor. However, benzene derivatives were removed or oxidized into some intermediates partially in EGSB II.During the whole operation process, the MLSS were high in the two-stage EGSB reactors with 22.4 to 38.6 g/L for EGSB I and 20.9 to 30.4 g/L for EGSB II. MLVSS/MLSS of the two-stage EGSB reactors were 0.356 to 0.489 and 0.344 to 0.434, respectively. A high treatment efficiencies of pollutants in the coking wastewater were obtained in the system although they have lower sludge activity than other biological treatment processes. The average apparent sludge yield of EGSB I was 0.01kgMLVSS/kgCOD, at a low level.It was feasible that the two-stage EGSB reactors were used to treat coking wastewater with complete sludge retention under the micro-aerobic condition, because of its micro-aerobic condition, low sludge loading, infinite long sludge retention time (SRT) and developed microbial ecological chain. And the feasibility of synchronous treating coking wastewater and excess sludge by EGSB reactors was proved in theory.