| Sludge bulking refers to a state of activated sludge with slow-rate of settleability and bad-compression of flocs.Common types of sludge bulking are filamentous bulking caused by over-growth of filaments and non-filamentous bulking resulted from over-secretion of extracellular polymeric substances.Sludge bulking can be caused by many external inducements including operation parameters,sewage quality and environmental factors.Hiemal sludge bulking is such a problem which is easy to happen in winter operation of municipal sewage treatment plants(MSTPs).Although many studies on sludge bulking have been conducted,little study on hiemal sludge bulking can be found currently.This thesis selected Matougang MSTP as a typical object to research sludge bulking taken place in winters.We analyzed the external inducements of hiemal sludge bulking in Matougang MSTP comprehensively by a one-year investigation.Meanwhile,for further survey of biological causes of hiemal sludge bulking,this paper intensively sampled five MSTPs and analyzed the microbial composition and diversity of sludge samples by high-throughput sequencing.Based on the analyses of external inducements and biological causes,a control scheme was proposed in terms of theory.A laboratory-scale experiment was designed and conducted further to confirm the assumption and feasibility of the control scheme.Results indicated that the operational parameters of Matougang MSTP,with an average sludge load(0.096 kgBODs/kgMLSS-d)and sludge retention time(14.28d)presented,was at a stable level all the year round.Hence the changes of operational parameters can be excluded as the external inducements.Similarly,influent quality also stayed stable yearly with proper pH,nitrogen and phosphorous.So influent quality can be confidently excluded as well.Sludge volume index(SVI)and water temperature demonstrated negative correlation,and changes of water temperature is the main inducement of hiemal sludge bulking.Among the yearly operation,sludge bulking lasted for about 4 months,and 16℃ and 22 ℃ were treated as the start and end of sludge bulking respectively.Sludge bulking induced the increase of suspended solids in effluent of secondary settlement tank,a 15%decrease of sewage amount treated,and a 5.4%-15%reduction of pollutant removals.At phylum level,microbial communities of different MSTPs in winter shared the same kinds and abundances of main bacteria.At genus level,Haliscomenobacter and Trichococcus were the principal bulking bacteria detected in MSTPs,and the abundances of them were higher in bulking samples.The regional differences of microbial communities were very obvious.Microbial communities of one MSTP in different seasons were also consistent in the kinds and abundances of main bacteria at the phylum level.Haliscomenobacter and Trichococcus were detected in all seasons of the year with a highest abundance in spring.These communities also presented a high level of seasonal discrepancies.Over-reproduction of bulking and foaming bacteria(BFB)aroused the hiemal sludge bulking,and Microthrix parvicella was the dominant BFB in sludge.Based on the properties of M.parvicella,the sludge bulking can be controlled by maintenance of proper temperature,improvement of sludge load,adopting hydraulic pattern of plug flow and increase of dissolved oxygen.Under the continuous operation in low temperature,sludge bulking would happen in bio-reactors with flow patterns of plug flow and completed mixing.Compared to completed mixing,plug flow possessed a robust ability to resist sludge bulking.Improving sludge load from 0.1 kg/kg·d to 0.2 kg/kg-d can distinctly decrease the SVI of activated sludge,but the SVI would increase again when the sludge load was improved to 0.4 kg/kg·d.the SVI would go down slightly when the ratio of gas and water was increase from 3:1 to 6:1.However,the settleability of sludge would become deteriorative when the ratio was turned up to 9:1.In conclusion,low temperature was the primary external inducement of hiemal sludge bulking.Over-growth of bulking and foaming bacteria(BFB)was the main biological cause of the sludge bulking.Among the BFB detected,Microthrix parvicella played an important role in sludge bulking arisen.According to the property of M.parvicella and results of lab-scale experiment,plug flow,rising sludge load in limited range and increasing aeration in proper extent can control the sludge bulking effectively. |
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