Study On Pollutants Transformation And Microbial Community Succession In Sludge Treatment Wetlands

With the implementation of national sustainable development strategy and the continuous promotion of the urbanization process,exploring economical and effective sludge treatment technology has become a new challenge.As a sludge ecological stabilization technology,sludge treatment wetlands(STWs)have the advantages of low energy consumption and low cost,and are a promising sludge treatment method.Microorganisms are believed to the main mechanisms of pollutants removals in STWs used for sludge treatment.The in-depth study of the relationship between the dynamic changes of microbial community structure and sludge stabilization is of great significance for understanding the essence of STWs technology and improving sludge stabilization performance.Therefore,in view of the above problems,three pilot-scale STWs units—STW1 with aeration tubes,STW2 with aeration tubes and reed planting,and STW3 with reed planting—were constructed and operated under feeding followed by resting periods.And sludge dewatering and stabilization,antibiotics removal,and the change in the microbial community in three STWs were investigated under different operation conditions to reveal the relationship between microbial community structure and sludge stabilization performance,which is of great guiding significance for promoting reasonable design of STWs.The specific research contents and main conclusions are list as follows:(1)The plant growth characteristics,TS(Total solids),COD,NH4+-N and TP(total phosphorus)content of three sludge treatment wetlands in different operation periods were studied.The results showed that plant growth density reached the maximum in the second year of operation with the stable operation of STWs system.The sludge volume was reduced by more than 97%in all STWs,the sludge TS concentration increased from mean 0.65%in the influent to 29.2%?36.6%of STWs in resting period.The STWs were quite efficient in COD and NH4+-N removal,with mean removal efficiencies 82.5%and 52.6%,respectively.Mean TP(total phosphorus)removal was lower(27.9%).Samples of influent sludge and sludge accumulated in the STWs were analysed for organic matter(VS(Volatile solids)),nutrients(TKN(Total kjeldahl nitrogen)and(TP(Total phosphorus)).The results showed that the VS content reduced from mean 60%in the influent to 39.1?47.5%in the STWs,while the TKN and TP concentrations decreased from 51.2 mg/g and 10.6 mg/g in the influent to in STWs decreased to 24.3?35.6 mg/g DW and 5.3?6.5 mg/g DW,respectively.The planted STW2,with aeration tubes,presented the highest removal rate of pollutants during the whole experiment.Besides,with the steady operation of STWs system,the degree of sludge stabilization in STWs has become higher and higher.(2)The concentration characteristics of ciprofloxacin(CIP),azithromycin(AZM)and oxytetracycline(OTC)in three sludge treatment wetlands at different operation periods were investigated by Liquid Chromatography-Mass Spectrometry(LC-MS/MS).The results showed that the antibiotic content in residual sludge of STWs decreased with the sludge stabilization time.And the removal rates of CIP,AZM and OTC were 56%?88%,65%?91%and 64%?89%,respectively.STW2 performed the best in terms of antibiotic removal and planting reed considerably improved the antibiotic removal performance of the STWs.Biodegradation and absorption resulted in removal of most of the antibiotics in the test STWs.Meanwhile,STWs obtained the highest removal effciencies of antibiotics in summer compared to other seasons.Besides,there is a linear positive correlation between antibiotic content and organic matter content and a linear negative correlation with humification ratio(degree of sludge stabilization).(3)The Illumina-Miseq sequencing technology was used to analyze the characteristics of bacterial a diversity index in three sludge treatment wetlands at different operation periods.The results illustrated that the richness and diversity index of microbial community in STWs in feeding period were significantly higher than those in resting period.This is mainly due to the cessation of sludge input and the continuous decrease of nutrient content available to microorganisms,resulting in a sharp decline in microbial activity.Meanwhile,the richness and diversity index of microbial community in STWs were positively correlated with the removal rate of pollutants.Therefore,the ability of STWs to remove pollutants during feeding period was significantly higher than that during resting period,and the removal rate of pollutants is the highest in the second year of operation.The STW2 with aeration tubes had higher bacterial diversity and richness and higher pollutants removal efficiency during whole sampling campaigns compared to those in STW1 and STW3,particularly in summer.(4)The principal coordinate analysis(PCoA)based on the weighted UniFrac distance showed that Proteobacteria,Bacteroidetes and Planctomycetes were dominant in the bacterial communities of STWs during feeding period.These bacteria were positively correlated with the removal rate of pollutants.Therefore,the higher pollutants removal efficiency of STWs in feeding period was mainly due to the high abundance of the above microbes.In contrast,the formation of hyperoxic and oligotrophic environment in STWs during resting period makes the relative abundance of Nitrospirae,Chloroflexi and Actinobacteria increased.Meanwhile,the dominant genera were Thiobacillus,Flavobacterium,Pseudomonas and Dechloromonas,and their relative abundance of STWs in feeding period was higher than in resting period.Therefore,the higher pollutants removal capacity of STWs during loading period was directly related to these dominant bacteria.The relative abundance of these bacteria in STW2 were higher than those in STW1 and STW3,resulting in the higher removal efficiency of pollutants in STW2.From the microbiological point of view,the simultaneous presence of aeration tubes and plants helps to improve the sludge stabilization performance of STWs.