Phosphorus Recovery And Nitrogen Removal From Reject Water By Different Combined Processes

The supernatant of reject water in wastewater treatments plants(WWTPs) has the characteristics of high concentrations of phosphorus(P) and nitrogen(N). In present, this reject water is usually returned to the headworks of the wastewater treatment system, which will increase the loading of N and P in WWTPs and can`t recycle P effectively. This paper studied the recovery and removal of phosphorus as well as nitrogen from reject water in No.5 WWTP of Xi’an by three kinds of combination process. Firstly, combination processes were used to recover phosphorus and nitrogen in reject water by struvite(MgNH4PO4·6H2O, MAP). Then, another biological reactor/ reactors was/were adopted to remove the residual nitrogen further. Moreover, the activity of microorganisms were measured, and the diversity as well as the abundance of bacteria were determined by fluorescence in situ hybridization(FISH) technology and High-Throughput Sequencing(NGS) technology.(1) The following conclusions were gained in Draft-Tube Crystallisation Reactor(DTCR) + Moving Bed Biofilm Reactor(MBBR) combined process. The average removal rates of phosphorus and ammonia nitrogen in DTCR were 58.4% and 29.71%, respectively. The yield of crystallization was about 232 g per m3 of reject water and the purity of struvite(MgNH4PO4·6H2O) was 92% in crystallization. At the stage of short-cut nitrification in MBBR, approximately 50% ammonia nitrogen was transformed to nitrite, which caused the concentration ration of ammonia and nitrite in effluent close to 1:1. The average removal rate of phosphorus was 63.1% in this combined process, and the average removal rate of ammonia nitrogen could reach up to 98.8% at the stage of complete nitrification in MBBR. In addition, the average of phosphorus removal rate was 48.8% in this combined process.(2) The following conclusions were gained in Mixed Crystallisation Reactor(MCR) + MBBR combined process. The average removal rates of phosphorus and ammonia nitrogen in MCR were 82.6% and 14.3%, respectively. The yield of crystallization was about 230 g per m3 of reject water and the purity of struvite(MgNH4PO4·6H2O) was 93.8% in crystallization. At the stage of MBBR, the highest removal rate of the residual nitrogen could reach up to 96%. The average removal rate of TN and phosphorus was about 73.17% and 81.11% respectively in this combined process. FISH results confirmed that AAOB(anammox bacterium) existed in biofilm and activated sludge of the MBBR. NGS analysis revealed that both AOB and AAOB were present in MBBR. Large numbers of AAOB were linked together in biofilm and AOB were abundant in activated sludge. The results showed that the number of AAOB in biofilm were more than that in activated sludge, which held 4.31% and 0.71% respectively. On the contrary, AOB account for 21.46% in activated sludge and 10.46% in biofilm. Furthermore, the major species of AAOB was Candidatus Kuenenia.(3) The following conclusions were gained in MCR+SBR+Biological Filtration Column(BFC) combined process. The average removal rates of phosphorus and inorganic nitrogen in this combined process were 81.07% and 86.01%, respectively. In addtion, the result showed that AAOB can survived for a long time(147d) in the environment without ammonia and nitrite, but AAOB can be reactivated in just one day when ammonia and nitrite were supplied again. FISH results confirmed that AAOB existed in BFC. NGS analysis revealed that AAOB were present in BFC, the major species of AAOB was Candidatus Brocadia, which accounted for 5.06% in all bacterium.