Enhanced Removal Of Nitrogen And Phosphorus By A2/O-BAF System

Eutrophication, the most urgent and troublesome problem in the world, hasgained significant attention worldwide. Once the eutrophication occurred, it isdifficult to control and handle. Simultaneous biological nutrients (particularlynitrogen and phosphorus) removal from wastewater is essential to preventeutrophication and achieve sustainable development.The anaerobic/anoxic/oxic (A~2/O) process is most commonly used in existingwastewater treatment plants (WWTPs) in the world. However, there are three mainoperational problems existing in A~2/O process, which are summarized as follows:Firstly, the confliction problem between sludge retention time (SRT) ofnitrifiers (long SRT) and polyphosphate accumulating organisms (PAO)(short SRT)can not be solved. So the SRT in the A~2/O process must maintain in a narrow rangefor removal of nitrogen (N) and phosphorus (P).Secondly, shortage of organic carbon sources in low strength wastewater is oftena rate-limiting factor to simultaneous N and P removal. Sufficient carbon source isnecessary for denitrification, in contrst, synthesis of poly-hydroxy-alkanoate (PHA)and glycogen degradation are the foundation of P removal. In A~2/O process, COD wasprimarily utilized by PAO in the anaerobic zone in which approximately70%of CODwas consumed, and around10%COD was consumed by denitrifiers in the followinganoxic zones. This leads to deterioration of N removal due to insufficient externalcarbon source.Finally, NO3–-N in return sludge is an inhibiting factor to the P release inanaerobic zone where denitrifiers will compete with PAO for external carbon source,and net P release won't occur until denitrification is completed. This leads todeterioration of P removal in anoxic and/or aerobic zone. Therefore, how to resolvethe contradictions between removal N and P in the traditional processes is a crucialissue.The novel system integrating A~2/O-biological aerated filter (BAF) had substantialadvantages to solve the above mentioned problems. In this system, the A~2/O wasmainly used for P removal and denitrification, and the BAF was used for nitrification.Short SRT was applied in A~2/O and relatively longer SRT was applied in BAF toenrich nitrifiers, which not only benefited PAO, but also nitrifiers. In the same time,NO3–-N was recycled from BAF to the A~2/O's anoxic zone rather than its anaerobiczone to provide an extremely strict anaerobic environment for phosphate release. Inaddition, influent with lower C/N ratio also stimulated the growth of denitrifyingphosphorus removing organisms (DPAO), which were capable to use NO3–-N as electron acceptors in simultaneous removal of N and P from wastewater. Furthermore,large amount of COD was consumed in the anaerobic zone of A~2/O and reduced theC/N ratio of supernatant that flowing into the BAF, which was favorable to the growthof nitrifiers in the biofilm and enhanced nitrification. Furthermore, the sufficientnitrate from BAF was recycled into the anoxic zones of A~2/O which benefits thedenitrifying phosphorus removal.A laboratory-scaled A~2/O-BAF system, for treating domestic wastewater, wasconsisted of an influent tank, an A~2/O reactor, a secondary settler and a BAF reactor.The transparent Plexiglas A~2/O reactor had nine compartments with a working volumeof30L. The mixed liquor from A~2/O reactor was settled in a cylindrical settler with aworking volume of20L. The media's depth of up-flow BAF was167cm and theworking volume of the BAF reactor was13L.After stable operation of the A~2/O-BAF system, the effect of volume ratio in theA~2/O on the N and P removal was discussed. The volume ratio ofanaerobic/anoxic/aerobic zone was1:6:2and the major operating parameters were:SRT10d, sludge recycle ratio (r)100%, nitrate recycling ratio (R)300%, TSS3.0g/L.The results showed that the A~2/O-BAF system has potential for the domesticwastewater treatment and67.4%of TN and98.6%of TP were removed. This canguarantee the advantage of PAO growth and effectively restrain nitrifiers, and thesystem's N and P removal efficiency is the highest.Cell assimilation was estimated to remove about30%of the TN concentrationpresent in the influent domestic wastewater, and the nitrification-denitrificationcontributed significant amount of the overall TN removal efficiency. High nitraterecycling stream would bring more nitrates back to the anoxic zone for denitrificationand prevent the nitrates from escaping out with the effluent. On the other hand, themajor factor influencing the occurrence of DAPO and associated anoxic P-uptake wasthe nitrate load in the anoxic zones, only if the nitrate load was high enough orexceeding the denitrification potential of ordinary heterotrophic organisms (OHO), i.e.non-PAO organisms in the anoxic zones, could it be possible to stimulate DPAO inthe system because the specific denitrification rate of OHO was significantly largerthan that of DPAO. There was no distinct relationship between the nitrate recyclingratio and the removal efficiencies of COD, TP and ammonia nitrogen. However, theremoval efficiencies of TN increased with the increasing of the nitrate recycling ratio,the rising rate was descending. Both the capacity of denitrifying and phosphorusremoval in anoxic zone increased simultaneously with the increasing of the nitraterecycling ratio.Also, the performance of A~2/O-BAF system was investigated when treatingdomestic wastewater with low C/N ratio at low temperature. Under the conditions of average temperature of14.2℃and C/N ratio of4.81, enhanced N and P removal wasachieved. Average effluent TN and TP concentrations were13.21mg/L and0.23mg/L, respectively,which could satisfy the class A discharge standards (Integratedwastewater discharge standard, GB18918-2002, China).Denitrifiers are facultative bacteria that energetically prefer oxygen to nitrate asthe terminal electron acceptor. A high nitrate recycling stream, typically required foreffective denitrification operation, will also bring a large amount of DO from the BAFto the anoxic zone of the A~2/O that, in turn, would inevitably deteriorate the TNremoval efficiency. This is especially crucial when the organic matters present in theinfluent wastewater are insufficient to deplete the high DO concentration present inthe nitrate recycling stream (C/N ratio of3.0). When the C/N ratio was4.0, TNremoval efficiency declined when R was above400%due to insufficient carbonsource and oxygen intrusion in the anoxic zone of the A~2/O. Highest C/N ratio of5.5improved the TN removal efficiencies when increasing R. The P removal efficiencyexhibited an upward trend with increasing of R. The lower C/N ratios of3.0and4.0had little influence on P removal. In contrast, the highest C/N ratio of5.5resulted in Prelease in settler due to the observed residual carbon source present, led to a decreasein P removal efficiency, although the higher R can relieve this conflicting influence.There is a critical ammonia concentration in the BAF. The removal ofphosphorus in BAF is due to the closure and adsorption by the suspended materials.The phosphorus release rate of PAO was not related to the phosphate concentration inmain solution but was positively related with the concentration to volatile fatic acid(VFA). The carbon source not only affected the rate of phosphorus release of PAO,but also the rate phosphorus uptake in oxic/anoxic conditions. Oxygen, nitrate andnitrite could be used as the electron acceptor for phosphorus uptake. However, therewas a nitrite threshold concentration because it would cause phosphorus release due todetriment to anoxic phosphate uptake.