Research On Transformation Of Organic Matters And Community Structure During Biological Treatment Process Of Coking Wastewater

Coking wastewater contains phenols, benzene series, indole, quinoline, pyridine, amines and PAHs etc. refractory organic pollutants, and other inorganic substances such as ammonia, sulfide, thiocyanate, cyanide. Because of carcinogenicity, teratogenicity and mutagenicity, coking wastewater not be treated properly could cause serious harm to the ecological environment. To protect the ecological environment, Chinese government has developed more stringent emission standards "Emission standard of pollutants for coking chemical industry 16171-2012", requiring direct emissions COD≤80 mg/L, ammonia≤10 mg/L, indirect emissions COD≤150 mg/L, ammonia≤25 mg/L. Ideas of the integration process, including phenol removal, ammonia evaporation, cyanide removal, degreasing, biological treatment, coagulation sedimentation and advanced oxidation, are used for coking wastewater treatment both at home and abroad. Biological nitrogen removal process becomes the core subunit of coking wastewater treatment due to the high treatment efficiency and cost-effective advantages. Organic components are major contributors to COD in coking wastewater, and community structure determines the efficiency of coking wastewater treatment. Relevance understanding of the microbial diversity and function of community structure are still very limited, this restricts the efficiency of coking wastewater treatment system considerably. To this end, the research on the relations between the migration and transformation of organic components and community structure during biological treatment of coking wastewater is the key for improving wastewater treatment efficiency, reducing processing costs and protecting the ecological environment.In this study, the different full-scale coking wastewater treatment plants with anoxic and oxic process(A/O) were studied. Firstly, GC-MS was used to analyze the degradation and transformation of organic components during biological wastewater treatment process, and high-throughput sequencing technology was applied to investigate microbial communities and biodiversities which were responsible for organic matters degradation, to identify the dominant microorganisms. Secondly, the cumulative effect of PAHs adsorption on sludge and adsorption mechanism during coking wastewater biological treatment process were revealed, and PAHs ring hydroxy dioxygenase gene which is the key start gene for PAHs degradation was conducted cloning expression and diversity analysis. And finally the relationship between the biological community structures and treatment process was explored. The main results obtained are as follows:1. The main organic matters in coking wastewater influent are phenols, indole, quinoline, pyridine, PAHs and long-chain alkanes. Phenol, indole, quinoline and pyridine, accounting for 61.70, 13.63, 7.71 and 2.30% respectively, are the most important contributor to the COD. After A2/O process, along with organics removal their peak areas are reduced slowly, and along with organics degradation, their complexities are decreased gradually. A2/O bioprocess plays an important role in phenol, indole, quinoline and pyridine removals. Quinoline is degraded completely after anaerobic digestion, while indole is removed thoroughly after anoxic reaction.Through anaerobic, anoxic and aerobic degradation, phenol and pyridine disappeared effectively. Phenol, indole, quinoline and pyridine are not the reason for high COD effluent. Refractory organic pollutants, including long-chain alkanes, benzene series, esters and alcohols are detected in coking wastewater effluent, are the main contributor for high effluent COD. The effluent, COD<200 mg/L, NH3-N<15 mg/L, volatile phenols<0.5 mg/L, and cyanid<2 mg/L, can not meet emission standards for the coking chemical industry and must be treatment in-depth.2. Biological community structures and biodiversities of the three subunits of A2/O process, which is applied to remove phenol, indole, quinoline and pyridine substances, are different. The OUTs classification number of community structures for anaerobic, anoxic and aerobic sludge is 638, 790 and 947, respectively. The number of shared OUTs is 92, accounting for only 4.66% of the entire biological community, Shannon index is 4.656, 5.246 and 5.642, respectively, indicating that aerobic sludge biodiversity is the most. Community structure composition and abundances of microorganism show that phylum Proteobacteria, accounting for 84.64, 62.73 and 83.24%, is the highest in relative abundance in three coking wastewater sludge communities, the corresponding most dominant orders in three samples are Pseudomonadales, Syntrophobacterales, and Burkholderiales, respectively. While genus Pseudomonas, Desulfoglaeba, and Diaphorobacter is the dominant bacterium, respectively. It is speculated that microorganisms capable of degrading phenol, indole, quinoline and pyridine are Desulfoglaeba, Thauera, Diaphorobacter, Thiobacillus, Desulforegula and Pseudomonas.3. 16 PAHs, including naphthalene, acenaphthene, acenaphthylene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, dibenzo[a,h]anthracene, and benzo[g,h,i]perylene, are monitored in the anaerobic, anoxic and aerobic sludge samples in A2/O process, respectively, the total concentrations of 16 PAHs are 2201.633-581.600 μg/g. At 25 ℃ at 6 g/L MLSS on a rotary shaker(150 rpm), adsorption processes of 2-4 ring PAHs naphthalene, phenanthrene, and pyrene on activated sludge show a great tendency in the first 30 min, follow by slow sorption reaction up to the equilibrium time after 1 h. Quantitative determination and time profile of adsorption show that cumulative effect of PAHs adsorption on biological sludge is found.4. At 25 ℃ at 6 g/L MLSS on a rotary shaker(150 rpm), the study of different concentrations of PAHs shows that the processes of PAHs adsorption on activated sludge closely follow the pseudo-second-order kinetic model, experimental qc values are very close to the qe calculated values, and the model’s correlation coefficient is>0.99. Linear, Langmuir, and Freundlich isotherms, are employed to describe PAHs adsorption equilibrium,both Linear and Freundlich adsorption models are able to adequately show good correlation. Temperature, PAHs concentrations and sludge concentration affect PAHs adsorption. Higher initial concentration, PAHs concentrations in the aqueous solution rapidly decrease and the amount of sludge adsorption increases with prolonged adsorption time. Higher temperature, the partition coefficient kd increases, indicating that the temperature is conducive to PAHs adsorption. More sludge concentration, PAH removal efficiency is improved better. However, the sludge concentration is greater than 6 g/L, the adsorption efficiency changes inconspicuously. The adsorption efficiencies of PAHs are not affected by p H because PAHs are non-polar, non-ionizable aromatic compounds and do not occur in aqueous solution. The concentration of benzo[a]pyrene, in three sludge samples is 126.719,54.386 and 50.847 μg/g, respectively. PAHs carcinogenic risk assessment indicates that Benzo(a)pyrene equivalent concentration for indeno[1,2,3-cd]pyrene(Inp), benzo[a]pyrene(Ba P), benzo[k]fluoranthene(Bk F), benzo[b]fluoranthene(Bb F), benzo[a]anthracene(Ba A), and dibenzo[a,h]anthracene(DBA) is all more than 1.0 μg/g, and the order of carcinogenic health risk is Ba P>DBA>Bb F >Inp>Ba A>Bk F.5. The pseudo-second-order model indicates that chemisorption dominates in the adsorption process. By comparing infrared analysis of adsorption of PAHs on activated sludge, it is found that the wavelength at 2,920 cm-1 has a blue shift after adsorption of phenanthrene and pyrene. After adsorption of naphthalene, phenanthrene, and pyrene, the wavelength at 900 cm-1 shifts to red, the wavelength at 1,076 cm-1 has a blue shift. So, it is speculated that PAHs could bind to the benzene/fused aromatic ring and C–O in carbohydrate, and at the same phenanthrene and pyrene could bind to C–H in fatty substances. The specific sorption coefficient Kd for naphthalene, phenanthrene, and pyrene is 0.072, 1.406, and 27.753, respectively. A positive correlation between specific sorption coefficient Kd and octanol-water partition coefficient kow is found,(R2=0.920), this indicates that sludge adsorption has relationship with the highly hydrophobicity of PAHs. BET characterization shows that N2 adsorption/desorption isotherm of activated sludge belongs to type III, the surface area and pore volume of sludge are very small, only 1.76 m2/g and 0.01 cm3/g, respectively, result of SEM image indicates that activated sludge has a rough and no porous surface, they could not be the main contribution. The percent values of carbon, hydrogen, oxygen, nitrogen, and sulfur are 48.24, 5.55, 30.03, 7.89, and 3.10 %, respectively. Those results suggest that hydrophobic interaction, distribution, and chemisorption are mainly responsible for the adsorption of PAHs.6. After extracting total DNA of the active sludge sample, primers are used for ring hydroxylating dioxygenase gene(RHD) PCR amplification using total DNA as the template, and significant amplification products are found using primers RHDa-GN-610 F and RHDa-GN-916 R. PCR products of the samples are conducted DGGE analysis and a clear separation of the strips is found. Eight representative bands of fingerprint are selected for PCR amplification, and significant amplification products appear, this further confirms the presence of different RHD genes in activated sludge. After TA cloning, 4 sequences are tested successfully and the length is 305 bp, 298 bp, 334 bp and 294 bp, respectively.7. 6 sludge samples from the anoxic basin and aerobic basin of A/O, O/A/O and A/A/O process, respectively, yield 67,845 effective sequence tags. The biodiversities of the samples are different, OTUs classification number is 802-1769, Shannon index is 5.246-6.232, the biodiversity of anoxic basin from Hebei coking wastewater plant is the most. Community structures from different A/O systems have differences and similarities, there are 10 phyla shared by community structures from the different process subunits of different A/O bioprocesses, which are Proteobacteria, Chloroflexi, Chlorobi, Bacteroidetes, Planctomycetes, Acidobacteria,Nitrospira, Verrucomicrobia, Firmicutes and Actinobacteria. The corresponding most dominant order is Burkholderiale in four samples, accounting for 18.43-33.1%. orders, such as Sphingomonadales, Rhizobiales, Rhodocyclales, Legionellales, Xanthomonadales, Hydrogenophilales, Sphingobacteriales, Caulobacterales and Rhodobacterales, are shared by them. The number of genus in each sample is 136-197, the number of shared genera in three anoxic sludge samples is 48, that of shared genera in three aerobic sludge samples is 71, while the shared genera in 6 samples is only 33. The most predominant genera shared by 6 sludge samples are Diaphorobacter, Thiobacillus and Thauera.8. The form of reflux, A/O process and sludge type have great effect on microbial community. Because Liaoning coking plant uses A/A/O process, anoxic basin is biofilm sludge, aerobic basin is activated sludge, and the form of reflux is the supernatant recycle, the differences of the two community structures from the anoxic basin and aerobic basin are obvious. While Hebei coking wastewater plant and Hubei coking wastewater plant apply the A/O process and O/A/O system, respectively, the form of reflux is mixture return, so the community compositions have higher similarity. Wastewater quality indices of COD, SCN-, phenols, oils, CN-, NH3-N, temperature and p H impact on the distribution of community composition, but COD, volatile phenol and temperature are the three most significant deterministic factors.