Study On Landfill Leachate Treatment By A Composite Biological System Under Low Dissolved Oxygen Conditions Coupled With Advanced Oxidation Process

At present,sanitary landfill is a technology commonly used in the treatment of municipal solid waste（MSW）.As a by-product of MSW in the process of sanitary landfill,landfill leachate can cause long-term potential harm to environment,animals,plants and human beings if handled improperly.Due to the characteristics of mature landfill leachate with high concentrations of refractory organic matters and ammonia,low C/N ratio and large changes in water quality and quantity,it is difficult to reach the discharge standard by using separate physicochemical methods or biological treatment technologies.The low oxygen environment can promote the hydrolysis and acidification of complex compounds,and the persulfate advanced oxidation technology has attracted much attention due to its efficient decontamination ability.However,there are few reports on the combination of these two technologies for the treatment of landfill leachate.In view of this,a combined process of a pilot-scale low dissolved oxygen composite biological system（LDOCBS）coupled with persulfate（PS）advanced oxidation technology was applied to treat landfill leachate.The low dissolved oxygen composite biological system consisted of a rotating biological contactor（RBC）and four aeration tanks with gradient aeration（1#-4#）.The operational conditions and decontamination mechanism for long-term stable operation of LDOCBS were systematically studied,the denitrification and carbon-removing functional bacteria were screened,and the advanced treatment performance and decontamination mechanism of different PS activation methods for landfill leachate treatment were studied.The main research contents and conclusions are as follows:（1）This research constructed a pilot-scale continuous-flow LDOCBS.The operation characteristics and mechanism of LDOCBS in the start-up stage were studied from the aspects of selection of RBC fillers,decontamination performance,removal of dissolved organic matter and changes of microbial community structure.The braided fiber material was conducive to the adhesion of microorganisms and had a good film-hanging performance.The maximum removal rates of LDOCBS for COD,NH₄⁺-N and TN during the start-up phase were 85.65%,99.92%and 84.06%,respectively.LDOCBS had strong adaptability to the shock loading,which could effectively resist the change of load and improve the biodegradability of landfill leachate.Each single tank had strong ability to degrade organic matter and could realize simultaneous nitrification and denitrification.In terms of nitrogen removal,RBC and 4#mainly played the role of denitrification,and 1#,2#and 3#played the role of nitrification.This system facilitated the enrichment of denitrifying bacteria.As the most representative denitrifying bacteria,Thauera,Arenimonas,Azoarcus and Hydrogenophaga played key roles in organic matter degradation and nitrogen removal.（2）On the basis of the successful start-up of LDOCBS,the effects of different factors on the decontamination efficiency and the changes of microbial community structure under different conditions were investigated by adjusting the hydraulic retention time（HRT）,nitrated liquid reflux ratio,operating temperature,the types of carbon sources and C/N ratio.The optimal HRT,reflux ratio,operation temperature,C/N ratio and external carbon source of the system were 7 d,200%,35℃,5:1 and sodium acetate,respectively.Under the optimal conditions,the maximum removal rates of COD,NH₄⁺-N and TN by LDOCBS were 89.57%,99.78%and 85.88%,respectively.All the factors influenced the removal of COD and TN greatly.The lowest C/N ratio that the system could tolerate was 3:1.The effects of HRT,nitrated liquid reflux ratio and types of carbon source on the nitrification process were relatively small.When the C/N ratio was 10 or the operating temperature was 50℃,the nitrification efficiency all decreased obviously,but the inhibition of high temperature on the nitrification process was less than that of high C/N ratio.Under the optimal conditions,the diversity of microbial community structure was high.Proteobacteria and Bacteroidetes were the dominant phylum,andβ-proteobacteria was the absolute dominant class.The variety of AOB and NOB were more abundant.Compared with other conditions,Nitrosomonas as a typical AOB had the highest abundance（1.20%-1.60%）under the optimal conditions.The denitrifying bacteria DNB（30.92%-40.71%）were mainly composed of Thauera,Azoarcus,Hydrogenophaga,Pseudomonas,Arenimonas and Thiopseudomonas.（3）25 strains of denitrification and decarburization functional bacteria were successfully screened from the stable running LDOCBS.According to nitrogen and carbon removal performance and colony morphology,15 strains were selected for 16S r RNA gene identification and it was found that the strains belonged to Firmicutes and Proteobacteria.Compared with other strains,B1 2#,D0 1#,D0 2#,D2 2#and D4 1#had better nitrogen and carbon removal efficiency.With a C/N ratio of 10 and a NH₄⁺-N concentration of 100 mg/L,the removal rates of NH₄⁺-N were 45.60%,55.10%,50.00%,61.70%and 40.10%,respectively,and the removal rates of COD were 96.00%,93.00%,97.00%,96.00%and 88.00%,respectively.Additionally,the selected strains had a wide range of adaptability to p H value,C/N ratio and NH₄⁺-N concentration,and had a strong suitability to the environment.（4）The LDOCBS effluent was treated with Artemisia argyi stem biochar supported nanoscale zero-valent iron activated PS（ASB@n ZVI/PS）and ultrasound enhanced electro-activated PS（US/E/PS）,respectively.The decontamination efficiency and mechanism of the two persulfate activated methods were compared by single-factor impact experiments and multi-factor coupling experiments based on response surface method.The results showed that the US/E/PS system and the AS600@n ZVI/PS system had higher removal for color,but the removal rate of color and the removal of organic matters in the former were significantly higher than that in the latter.In ASB@n ZVI/PS system,the optimal pyrolysis temperature of ASB was600℃,the optimal mass ratio of ASB/n ZVI was 5.22 and the optimal dosage of ASB@n ZVI was 2.10 g/L.The US/E/PS system had the best performance when the ultrasonic power was 600 W.The two systems had the same optimal PS dosage and p H value,which were about 30 mmol/L and 5.40,respectively.The radical inhibition experiments found that the main radicals in both systems were SO₄^·-,and·OH and O₂^·-also played a role.