| At present, excess sludge produced by has been a restriction of development formunicipal wastewater treatment plants (WWTPs) in the world. The way to solve theproblem from the root, is one of the urgent problems for the environmentalists. Andexcess sludge reduction in-Situ, a important way to solve the problem of excess sludge,has been gained great attentions. Our previous research showed that the A+OSA system,a modification of the anoxic/oxic (A/O) process via insertion of a sludge holdingtank into a sludge return line, has been demonstrated to be capable of achievingsimultaneous nutrient removal and sludge reduction. Also, the lower sludge yields of theA+OSA system resulted from both the sludge holding tank and the main reaction units.In order to identify the primary mechanisms involved, the research presented here wasundertaken to:1) the sludge characteristic and microbial community structure in the twoprocess;2) the characteristics of cell synthesis under alternate oxic/anoxic and itsinfluence on the sludge reduction；3) the physiological characteristics of the sludge inthe two process;4) carbon and energy transport and transformation in the A+OSAtechnology. The main results are as follows:Compared to the reference system, the A+OSA system can reduce excess sludgeby13%~27%. Moreover, the insertion of a sludge holding tank into the A/O process,had no obvious effect on the removal of nutrients. The hydraulic retention time (HRT) isan important parameters which affected the sludge reduction, and the effect of sludgereduction in whole syetem appeared an peak near the HRT5.56h. The contributionfactors to slude reduction differed under different HRT. When HRT is about less than5.56h, the excess sludge reduction, resulted from the lower yields of the main reactionunits, but when HRT is more than5.56h, the amount of sludge compensativeproliferation in the main reaction unit helped by the backflowed increasing organicloading offsets the amount of sludge reduced by decay. Also, The longer the HRT in thesludge holding tank, the greater contribution to the excess sludge reduction the sludgedecay, the less contribution the bacterial of lower yields in the sludge holding tank.The microscopic observation test showed that the void structure was moredeveloped and less filamentous bacteria was observed outside of the sludge floc. So,the A+OSA process showed a better sedimentation performance of activated sludge.DGGE profiles showed that the insertion of a sludge holding tank had a distinct effect on the microbial communities. By comparing the DGGE profiles, it was found that mostof the major bands were common for each HRT, while some of specific bands ofbacterial species were shifted at different HRTs, and the diversity index and the richnessindex declined. Also, cluster analysis further confirmed that the microbial communitiesfor the reference and the A+OSA system were in distinctly different clusters, while thebacterial communities of the sludge holding tanks and the oxic tank were statisticallysimilar for the A+OSA. The most abundant phylotypes in the A+OSA are those relatedto β-proteobacteria; followed by Bacteroidetes and of these microbes, the PAOs,anaerobic fermentative bacteria and slower grower were another reason for sludgereduction for its lower yields.By taking universal existence of B. subtilis, isolated from the A+OSA process, asthe research object, the phenomenon of B. subtilis after fasting/feasting treatment onstrain growth. Five pure strain cultures cultivated using synthetic wastewater composedof glucose medium:1) anaerobic batch culture,2) an intermittently aerated batch culture,3) an anoxic batch culture,4) a continuous aerobic culture, and5) an OSA culture, wereemployed. The results showed that the oxic/anoxic treatment has an obvious effect on B.subtilis. In batch culture, the yields was0.283g.dry weight/g. glucose underintermittently aerated condition, and lower by20.7%than it under oxic culture (0.354g.dry weight/g. glucose). In continuous culture, the yields of B. subtilis under OSAculture (0.439g. dry weight/g. glucose) decreased by11.7percentage point, whencompared with it under oxic condition. The content of ATP showed the same trend, andthe rate of glucose degradation was faster under ntermittently aerated culture. Also,energy utilization efficiency of B. subtilis degraded when exposed to the oxic/anoxiccondition. The change of metabolic pathway can resulted in lower sludge yields and thefactors, affected the sludge yields, differed under different anoxic time.The utilization of extracellular polymer substances (EPS) by microbes and thereorganization of EPS resulted from the environmental change may be an otherimportant way for excess sludge reduction of the A+OSA process. The results showedthat the proteins, polysaccharides, and metal ions in the supernatant of the sludgeholding tank are higher than it in other reaction units. The EPS dates indicated thatthe generation of EPS accelerated to maintain their survival in the sludge holding tank,due to the lack of matrix and lower ORP. When the sludge mixed liquor was backflowedto the main raction units, the organic compounds was reused. Moreover, the static testdemonstrated that the partial EPS can provide organic carbon for microorganism, the metal ions declined by the oxic time prolonged. The results indicated that the variousEPS dissociation occurred. After exposure to oxic environment, the metal ions in EPSincreased, which indicated that a new polymerization processes of EPS was traced. Theenergy loss in the process of EPS polymerization favored for the sludge reduction. So,the cryptic growth and energy loss caused by the dispersion and polymerization of EPScan reduce the sludge growth, but its contribution to the A+OSA process is hard tomeasure.The element analysis showed that the compozation are different between the twoprocess, and the compozation between the different reaction units also differd. Based onbalances of carbon element, we found that the content of carbon element was lowest.And this result showed the decayed in the sludge holding tank. The slude decayedincreased sharply when the HRT prolonged. Compared with the reference, the carbonoutput in the main reaction units of the A+OSA system was less, and the carbon used fororganism synthesis was less either. But the rate of cell synthesis in main reaction unitsof the A+OSA process was higher than it in the reference, while the carbon output in gasform was just oppost. The energy balace indicated that the less energy was used tosynthetize organism in the A+OSA process. The loss of energy in form of power, heat,and spill in the reference was less than it in the reduction process, but wether the energyspill or the energy uncoupling need further study. |
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