| Compared with traditional nitrification/denitrification nitrogen-removalprocess, completely autotrophic nitrogen-removal over nitrite (CANON) process hasmany distinct advantages. However, most of the research achievements and practicalexperiences are concerning wastewater that with low loading, high ammoniaconcentration and high temperature; and there are still some limitaions andchallenges of CANON to treat domestic sewage with low ammonia concentrationand ambient temperature, one of which is how to start up CANON quickly andobtain stable and effective nitrogen-removal performance at room temperature andlow ammonia concentration. Therefore, concerning of the two most importantfactors of temperature and influent ammonia concentration, this study combined themacroscopical process performance and microscopical biological characteristics forthe first time and investigated the physiology and biochemistry, morphologicalcharacteristics, spatial distribution, population, community structure and geneticsfeature of the functional microorganisms in CANON system based on a number oflab-scale CANON reactors. Except for the microbial characteristics, the start-upstrategy and nitrogen-removal performance were also analyzed, both of which mightstrengthen the awareness of the mechanism of CANON process and microbialcharacteristics, and provide theoretical guidance and technical support forCANON’s application to treat domestic sewage.The function microorganisms in CANON system are AOB and anammoxbacteria, and the removal of nitrogen relies on the harmonious balance between thetwo kinds of function microorganisms. The microbiological characteristics havebeen investigated in a number of CANON reactors operated in different conditions.The results showed some common features in the biofilm morphology, spatialdistribution of function microorganisms and species identification in differentCANON reactors. First, most microorganisms on the volcanic filter surface werespherical or ellipsoidal with a diameter of0.2~1.0μm, which showed clusteredappearance. However, the biofilm might be broken as a result of the strong hydraulicshearing when the hydraulic load increased, which might affect nitrogen removalcapacity. Second, AOB and anammox bacteria were side by side with each other,which was not consistant with previous reports that AOB distributed in the outerlayer while anaerobic anammox bacteria was in the inner part of the biofilm. Third,the main functional bacteria contributing to short-cut nitrification process wasNitrosomonas while Candidatus Brocadia contributed to anammox process. Thebiodiversity of AOB was better than that of anammox bacteria, which enhanced the anti-impact capactiy of CANON. Fourth, except for AOB and anammox bacteria,some other kinds of bacteria like Shewanella, Pseudomonas, Ignatzschineria andDechloromonas existed in CANON system. Finally, four AOB strains were screenedsuccessfully from the CANON reactor by using specific selective medium for AOB,which provided theoretical guides for future microbial enrichment, immobilizationand application of genetic engineering.The study of the effect of different temperature on CANON indicated theaverage TN removal loading was2.21kg N/(m3·d) and1.00kg N/(m3·d) respectivelyat high temperature (30℃) and ambient temperature (16~23℃). Microbiologicalexperiments showed that the population of AOB and anammox bacteria decreasedwhile NOB increased at ambient temperature. Besides, the community diversity oftotal bacteria and AOB also lowered slightly and the volume and spacing ofzoogloea increased. All the changes of microbial features mentioned above might bethe main reason for the decrease of nitrogen-removal capacity of CANON atambient temperature. It was also worth mentioning that the change of temperaturehad little effect on the spatial distribution of the functional microorganisms or thecommunity structure of anammox bacteria. Based on the microbial experimentalresults, such strategies should be in consideration so as to improve nitrogen-removalcapacity at ambient temperature: to add some CANON sludge, to set sludgeintercept instrument or to aerating intermittently.Except for temperature, influent ammonia concentration is the other mostimportant factor to effect CANON. The study indicated that CANON showed abetter performance when the influent ammonia concentration was higher than200mg/L. When influent ammonia concentration reduced to100mg/L, thenitrogen-removal capacity still existed but the biofilm destroyed and some biomasslost, and the SEM pictures showed the increased zoogloea spacing and loosezoogloea structure. During the whole decreasing of influent ammonia concentration,the community structure of AOB changed obviously, which was not the same withanammox bacteria. Besides, the population of total bacteria and AOB decreasedwhile Nitrospira increased. The population of anammox bacteria decreased slightlywhiel little Nitrobacter could be detected. Based on the microbial experimentsresults, using soft or half-soft filter, adding backwash, reducing aeration or adding acertain amount of nitrite might be some feasible approaches to improve theperformance of CANON at ambient temperature and low substrate concentration.In this study, a lab-scale CANON reactor was started up successfully within avery few period (180days) by inoculating with activated sludge obtained fromaeration tank of WWTP (A2/O process) at ambient temperature and low substrateconcentration for the first time based on the results above. After three phases ofaeration/intermittent aeration/limited aeration, stable CANON process was achieved with the TN removal loading of1.10kg N/(m3·d). Compared to completely stopaerating, intermittent aeration was conducive to maintaining the balance betweenAOB and anammox bacteria, leading to a much shorter start-up period. Nitrobacterwas hard to eliminated completely through the whole process and the communitystructure of it changed sharply, which was not the same wtih AOB and anammoxbacteria.In order to improve nitrogen removal capacity of CANON, optimal microbialcontrol was investigated and the results showed that the population was much biggerin the lower part of the filter, which trended to cluster. While in the above layer, itshowed less the number of microorganisms, more loosely, larger spacing and lessdiversity of AOB. It was suggested to change the inflow direction, to aerate equallyalong the filter or to rearrange filter every once in a while so as to optimizemicrobial distribution along the filter. Besides, the biofilm thickness at ambienttemperature was far from the optimal thickness value. Therefore, it was suggested toincrease the biofilm thickness or reduce DO concentration to some degree so as toimprove nitrogen removal capacity of CANON at ambient temperature. |
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