Nutrient Removal Using Agricultural Waste As Carbon Source:Performance And Microbial Communities

In wastewater treatment plants,biological nitrogen and phosphorus removal is the most economical and effective way to remove nitrogen and phosphorus pollutants in the wastewater.Carbon source is an essential electron donor for this process.Researches about the carbon source for biological nitrogen and phosphorus removal have been mainly focussed on the single carbon source,such as acetate,propionate and glycerol.However,the long-term dosing of single carbon source could lead to the deterioration of the performance of activated sludge,the weakening of its shock-resistant capability and the increase of operation cost.Consequently,alternative carbon source,such as mixed carbon source and waste carbon source,have attracted the attention of researchers.It has been reported that effective and efficient nitrogen and phosphorus removal could still be achieved with waste carbon source,which also leads to a more diversed functional mircrobial community.In this research project,an agricultural waste（AW）with the main ingredient of glycerol was selected as an alternative carbon source.Two sequencing batch reactors（SBRs）were set up in this project.One SBR used sodium acetate as the carbon source（SA-SBR）,and the other used the selected AW as the carbon source（AW-SBR）.By comparing the results in the two reactors,the composition of AW carbon source was analyzed,and the effect of AW carbon source on biological nitrogen and phosphorus removal capacity,sludge performance and the microbial community structure were comprehensively evaluated.The results of this study could provide a new insight for the selection of carbon source for biological nitrogen and phosphorus removal and a new perspective for the disposal of agricultural wastes.The main conclusions are as follows:（1）The composition of the AW carbon source was analyzed,and its effect on biological nitrogen and phosphorus removal were studied.The AW carbon source is weakly acidic,with more than 40%glycerol.Its COD equivalent and BOD₅/COD is 1.06 million mg/L and 47%.After the activated sludge was acclimated by the two carbon source（sodium acetate and the AW）for 9 days,the TN concentration in the effluent of both reactors were less than 10 mg/L.While the average TN removal efficiency of SA-SBR was 83.29%,the TN removal efficiency of AW-SBR was also more than 80%.After 15 days of acclimation,the denitrification potential in SA-SBR and AW-SBR increased to 20.80 and 30.28 mg NO₃^--N/（g VSS·h）,respectively.With the extension of acclimation time,the denitrification potential of activated sludge in the sodium acetate reactor showed a decreasing trend,while the AW reactor showed an increasing trend.The TP concentration in the effluent of AW-SBR was lower than 0.2 mg/L,and the average TP removal efficiency was 96.67%,which was slightly better than that of SA-SBR.However,the phosphorus release potential of AW-SBR was lower than that of SA-SBR,and the phosphorus release potential of both reactors showed a decreasing trend.（2）The particle size,settleability,morphology,extracellular polymeric substance（EPS）and zeta potential of the activated sludge in different acclimation stages in both reactors were studied.Compared with SA-SBR,the long-term acclimation by the AW carbon source resulted in larger particle size and lower abundance of filamentous bacteria in AW-SBR,while the diversity of microfauna was higher.In addition,the activated sludge in AW-SBR showed better settleability,stronger shock-resistant capability and less proneness to sludge bulking.Moreover,the EPS content and the absolute value of zeta potential were lower in AW-SBR,which enables better sludge dewaterability.（3）The abundance of functional genes and microbial community structure in both reactors were also analyzed.The abundance of napA gene in AW-SBR was higher than that in SA-SBR.The napA gene is responsible for the reduction of NO₃^--N to NO₂^--N in the denitrification process,which is the decision step of denitrification rate.The relative abundance of the functional genes associated with denitrifying metabolism in AW-SBR was higher than that in SA-SBR,which was consistent with the results of denitrification potential analysis.However,the abundance of functional genes of phosphorus accumulating bacteria in AW-SBR was lower than that in SA-SBR,which was also consistent with the results of phosphorus release potential analysis.High-throughput sequencing analysis showed that the microbial community structure in the two reactors was significant different.At the phylum level,the dominant bacteria in SA-SBR were Proteobacteria,Bacteroidetes,Chloroflexi,Actinobacteria and Nitrospirae,while the dominant bacteria in AW-SBR were Actinobacteria,Proteobacteria,Patescibacteria,Bacteroidetes and Chloroflexi.At the genus level,AW-SBR was inhabited with more dominant bacteria that those in SA-SBR,which were Micropruina,Kineosphaera,Saccharimonadales,Nakamurella,Defluviicoccus,Propioniciclava and Propionicicella.These genera are mainly related to the decomposition and fermentation of organic matters,among which Propionicicella is associated with the decomposition and fermentation of glycerol.The results of microbial metabolic pathway prediction analysis showed that the relative abundances of the functional genes for the metabolism of C,N,P were significantly different between the two reactors.The abundance of the genes related to the metabolism of aromatic compounds,fats and carbohydrates were higher in AW-SBR.The results of molecular ecological network analysis showed that the type of the carbon source had no significant effect on the topological structure of the co-occurrence network.Instead,it could change the species that played a key role in the network.In AW-SBR,Propionicella had the most betweenness in the co-occurrence network,suggesting that Propionicella might play a role in the nutrient transit.