Study On The Treatment For Toxicity Reduction Of Nitro Aromatic Industry Wastewater

By seizing the new demands of watershed quality with the goal of human health and ecosystem safety, a whole-process technological strategy for toxicity reduction of nitro aromatic industry wastewater, based on the whole-process control and toxic reduction of the wastewater, was proposed in this dissertation. Specifically, the whole-process control system was mainly composed of novel zero-valent iron (ZVI)-Fenton integrated pretreatment, biological process and advanced treatment.The pretreatment stage determines the overall treatment efficiency and operation costs, and forms a choke point of the wastewater treatment system. However, so far, there are few efficient and cost-effective pretreatment methods for nitration wastewater. In this study, a novel ZVI-Fenton integrated system was designed for the pretreatment of nitration wastewater, and the effects of nitrobenzene (NB) transformation and removal on wastewater toxicity reduction in this system were investigated in detail. The completely-mixed operation mode of novel ZVI reactor, not only effectively avoided the agglomeration and clogging engineering problem, but also made it easy to control the reduction efficiency of NB. The reduction extent of ZVI could be controlled by adjusting the initial ZVI concentration and/or the pH of solution in the novel ZVI reactor. Although the increasing reduction efficiency of NB was helpful for decreasing the toxicity of wastewater, the wastewater would still remain extremely toxic due to the existence of hazardous substances with reduction-resistant properties.Compared with ZVI reduction, Fenton oxidation can degrade a wider range of toxic pollutants in the wastewater, but probably form more toxic by-products in the oxidation process. For example, while conventional Fenton process attained the NB removal efficiency of 97.0% and brought the yield of 1,3-dinitrobenzene (1,3-DNB) of 5.0%. Since the toxicity of 1,3-DNB is about 30 times that of NB, effectively inhibiting 1,3-DNB formation in Fenton oxidation of NB is essential for significantly decreasing the toxicity of Fenton-treated NB wastewater. However, the novel ZVI-Fenton integrated process with the NB reduction efficiency of 48.1% could lead to a significant relative decrease of 92% in 1,3-DNB (from 5% to 0.4%). The inhibition of 1,3-DNB formation might be caused by the scavenging action of·NO2 resulting from reduction products and ferrous iron generated in ZVI pretreatment.To verify the feasibility of application of novel ZVI-Fenton integrated process, a comparison of typical pretreatment methods was investigated for the treatment of nitration industrial wastewater from a large chemical factory. Novel ZVI-Fenton integrated process had superior treatment performance to ZVI column and conventional Fenton oxidation. The integrated system accomplished the toxicity unit (TU) removal of 83%, however, ZVI reduction and traditional Fenton oxidation only removed TU of 35% and 59%, respectively. In addition, the total cost of the ZVI-Fenton integrated system was only about one third that of enhanced Fenton oxidation.Currently, a 2000 t/d wastewater treatment project, based on the whole-process control techniques with the pretreatment of the novel ZVI-Fenton integrated process, has been applied in China. The TU of the pretreated wastewater decreased from 6.3 (very toxic) of untreated solution to 1.77 (toxic), meanwhile BOD5/CODcr increased up to 0.45. High-efficiency biological processes were used for further biodegradation of the wastewater, but the biologically treated wastewater effluent was still toxic (TU was 1.49) due to the remaining toxic pollutants. Advanced treatment was often necessary to ensure the quality safety of the effluent. Combined enhanced coagulation and adsorption (bi-functional resin, activated carbon)/ion exchange processes not only led to further removal of DOC but also achieved nontoxic discharge (TU was below 1.0). Disinfection, as an alternative advanced treatment, might easily form disinfection by-products of public health concern; therefore it should be coupled with adsorption technology (bi-functional resin, activated carbon) to remove by-products for nontoxic effluent.