| Anammox (Anaerobic ammonium oxidation) is a sustainable nitrogen removal process, especially in the field of nitrogen removal from the ammonium-rich wastewater. However, some issues have restricted its wild application. For instance, the Anammox bacteria has an extremely slow growth rate with a doubling time of nearly2weeks, meanwhile Anammox sludge is easy to be washed out and difficult to be enriched in conventional reactors. Anaerobic membrane bioreactor (AnMBR), which have merits of compact structure, small footprint and good retention of sludge, is suitable for the cultivation of slow-growing Anammox bacteria, then adopting it to treat ammonium-rich wastewater. However, membrane fouling is the main bottleneck of its widely application in AnMBR. The aims of this study were to investigate Anammox-AnMBRs and operation conditions for cultivating Anammox bacteria and alleviating membrane fouling, provide theoretical and technical support for sustainable operation of autotrophic nitrogen removal process.Aiming at full retention of Anammox bacteria and relief of membrane fouling, growth rate, enrichment performance, granules formation of Anammox bacteria and effects of recycling biogas sparging on membrane fouling and nitrogen removal were investigated using a completely stirring AnMBR (CSTR-AnMBR). Moreover, a novel integrated air sparing CANON-EGSB-MBR was developed, and its membrane operation and nitrogen removal performance were investigated. The main research achievements of this study are as follows:(1) Anammox bacteria were enriched in conventional CSTR and CSTR-AnMBR respectively, and the growth kinetics was investigated. The results showed that, by linear fitting of the data with Lawrence-McCarty model, the real yielding rate(YT) and the decay coefficient (Kd) of Anammox bacteria in CSTR-AnMBR were obtained at0.133±0.017kg VSS/kg TN and0.012±0.001day-1, respectively. Because of even distribution of substrates and good retention of sludge by membrane, the maximum obvious yielding rate and maximum growth rate of Anammox bacteria in CSTR-AnMBR were both twice as much as those in conventional CSTR.(2) In a CSTR-AnMBR, effects of stirring speeds on Anammox granulation, and effects of Anammox granulation and particle size distribution on membrane fouling were investigated. The results showed that setting stirring speeds at150,120and100rpm respectively, Anammox bacteria were always in floc form in the reactor. While keeping stirring speed at50rpm, the particle size of the Anammox bacteria increased from287μm to896μm, and Anammox granules were formed gradually. During the granules formation, the distribution of small flocs/granules decreased by50%, and the EPS (Extracellular polymeric substances) contents of sludge decreased by18%, thus making membrane operation circle prolong by2times. The formed Anammox granules had a maximum specific Anammox activity (SAA) of2.14kg TN/kgVSS-d, a short doubling time of6.9days and a maximum endurable NO2-N limit of130mg/L. Using synthetic wastewater as the influent, the reactor reached a maximum nitrogen loading rate (NLR) at4.1kg TN/m3·d, with a TN removal of88%.(3) In a CSTR-AnMBR, using the produced N2from Anammox as inert gas source, effects of the recycling biogas sparging system for scouring the membrane on membrane were investigated. The results showed that the formation of cake layer composed of Anammox flocs/granules adhering to membrane surface and the secreted EPS (especially polysaccharides), was the main reason for membrane fouling. Keeping an aeration intensity at118m3/m2·h, the EPS increasing rate in cake layer decreased by92%of that without aeration, making the membrane operation circle prolong by20times. Using synthetic wastewater as the influent, the reactor reached a maximum NLR at1.7kg TN/m3-d, with a TN removal of83%.(4) In a SHARON reactor, by dissolved oxygen control, NOB (nitrite oxidizing bacteria) were limited while AOB (ammonium oxidizing bacteria) were enriched, then oxygen uptake kinetics of AOB was investigated. FISH analysis showed that AOB became the dominant bacteria in the reactor after acclimatization. By linear fitting of the data with Monod model, the maximum oxygen uptake rate (OURmax) and oxygen half-saturation coefficient (Ko) of the AOB were obtained at18.81±0.044mg O2/g VSS·h and0.45±0.05mg/L, respectively. In addition, in an EGSB-MBR, keeping an up-flow velocity at0.7m/h, Anammox granules were formed and the mean granule size reached at about300μm.(5) An integrated EGSB-MBR, inoculated with acclimated AOB and Anammox granules, was started up using air sparging and continuously recycling of mixed liquid. The membrane operation performance and nitrogen removal performance of the reactor were both investigated. The results showed that in the upper part of the reactor, keeping an aeration intensity at220m3/m2-h, the membrane operation circle could be prolonged by5times. Meanwhile, the mixture recycle from the upper part to the lower part of the reactor could not only provide dissolved oxygen to AOB, but also keep granules structure by promoting of sludge bed expansion. FISH analysis showed that the granules had a double layer structure, where Anammox bacteria located in the inner layer while AOB located in the outer layer (CANON granules). Using synthetic wastewater as the influent, under ambient temperature (20±3℃), the integrated air sparging CANON-EGSB-MBR reached a maximum NLR at0.55kg NH4+-N/m3-d, with a TN removal of81%. |
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