| Hexavalent chromium-containing wastewater(HCCWW), which was harmful to natural environment and human being health, has widespread sources and great toxicity. At present, physicochemical methods are the major ones to treat this wastewater, such as adsorption, ion exchange, and chemical precipitation. However, these methods will produce secondary pollution and own the shortcoming of high cost. In recent years, people started to adopt new methods to deal with HCCWW. Sulfate-reducing bacteria (SRB) available to treat wastewater were a highlight in this field. SRB can reduce sulfate to hydrogen sulfide, which would produce insoluble metal sulfides with heavy metals. So the heavy metals in the HCCWW will be drastically cleared by this characteristic. Technical research of HCCWW by SRB limits pure bacteria. Although good results can be obtained at lab and pilot scales, this method should be modified because there were some problems need to be resolved such as exacting operation terms, large loss of bacteria, and so on.Activated sludge, as a good carrier, was able to provide a stable environment for bacteria growth and benefit for decreasing the loss of bacteria and resisting the change of HCCWW. It's traditionally known that SRB was a strict anaerobic bacterium, so only anaerobic sludge with finite sources can be used as inocula, which seldom were used to treat HCCWW. Some researches in recent years revealed that SRB could survive with oxygen and even use oxygen to grow. In order to solve these problems, activated sludge should be anaerobically modified. The Relation of quantities of SRB and rate of sulfate removal, best conditions of SRB growth, efficiency of chromium removal by sludge, occurrence and distribution of chromium in sludge, technical process and dynamics of Cr(VI) removal were investigated based on the successfully modification of activated sludge. Finally the mechanism of chromium removal by sludge was discussed.Anaerobic sludge containing high concentration SRB was obtained by anaerboically modification of activated sludge. The study on the relation of rate of sulfate removal and growth of SRB in sludge proved that the rate of sulfate reduction can act as an index to assess the conditions of SRB growth.In batch experiments, the ability of sulfate reduction of the modified sludge was researched by monitoring rateof sulfate reduction. Best conditions of SRB culture were yielded as follows: i.e. 3 g/Lsulfate, COD/sulfate ratio of 1.45, ferrous ion free, blasting N2 at pH of 7, with addition of sodium lactate. Under the conditions mentioned above, the semi-continuous experiment was carried out with the result that reactor can get more than 90% rate of 3 g/L sulfate removal after only 5 days culture. Therefore the modified sludge can be activated quickly for removing high concentration of sulfate at low COD/sulfate ratio.Effects of concentration of hexavalent chromium, sulfate and COD, hydraulic retention time and occurrence of other heavy metals on ability of sludge of chromium removal were investigated in the continuous experiments. The results indicated that the sludge can effectively remove 200mg/L Cr(VI) in wastewater at the conditions of 1g/L sulfate, 2g/LCOD and 16h HRT, and that at same time the sulfate concentration in the effluent was superior to National standard, and that below 20mg/L, other metals, i.e. Fe, Cu, Zn, Ni, had no great effect on the Cr removal and also can be effectively removed by sludge. The process of chromium removal by sludge was due to the cooperation of various microbes, out of which SRB played the most important role. Dynamical equation of degradation of substances and sulfate removal was built for the process of Cr removal by anaerobic sludge. The equation as follow:The sludge that treated HCCWW was investigated by sequential extraction method in order to understand the effect of concentration of sulfate and Cr (VI) and occurrence of other metals on the transportation of chromium in the reactor. The results revealed that the concentration of sulfate and Cr (VI) c... |
PDF Full Text Request