Enhancing Biological Nitrification Performance Of Nitrobacteria-enriched Reject Water In WWTPs

The flow of reject water in the Municipal Wastewater Treatment Plant （WWTP）was known as small amount, but with high pollutant concentrations. In most cases inChina, the reject water was returned to the beginning of the treatment process ofWWTPs, thus leading to the increased NH₄⁺_-N、TP etc. loadings in the real treatmentsystem. This causes the N, P loadings being higher than the designed values andtherefore affects the treatment efficiency of N and P. This study aims to investigate thereject water treatment （when it is returned to a pilot plant O/A reactor）, via enhancedbiological nitrification performance by bio-augmentation in the pilot plant A2/O reactor.The main findings showed:（1）The reject water was a nitrobacteria-enriched flow and can be treated in thepilot plant O/A reactor. The result showed that average removal rate of TCOD，TP，NH₄⁺_-N and SS were92.6%，52.5%，99.9%and88.8%, respectively. However, averageremoval rate of TN was as low as9.72%, which was due to the reasons of the low C/Nin reject water. Average ammonia utilization rate （AUR） and nitrogen utilization rate（NUR） were7.84mg/（gMLSS×h） and12.17mg/（gMLSS×h）, respectively.（2）The analysis of the pilot plant A²/O reactor system showed that, in period ofstable operation and without bio-augmentation, the average removal rate of NH₄⁺_-N andTP were, respectively,88.7%and72.5%with SRT of10.1d¹4.4d. While in theoperation period with the bio-augmentation, the rates increased to92.9%and81.9%,respectively, with SRT of about8.5d. When operation with the bio-augmentation,average AUR and NUR in A²/O reactor were5.08mg/（gMLSS×h） and4.92mg/（gMLSS×h）, respectively. In comparison, the average AUR and NUR in the No.4WWTP of Xi’an City were2.64mg/（gMLSS×h） and2.95mg/（gMLSS×h）, respectively, which showed that biochemical reaction rate could be improved by bio-augmentation.（3）Phosphorus removals of the pilot plant A²/O reactor and the influent andeffluent under the special operating conditions in No.4WWTP of Xi’an City wereinvestigated. The reasons of poor phosphorus removal efficiency were:1) the actual TPconcentration of influent exceeded the designed capacity caused by the reject water;2)the short HRT of anaerobic zone; and3) the high nitrate concentration in the returnsludge. Measures for optimizing the HRT of anaerobic zone, enhancing the treatment ofreject water in the sludge treatment system, and making use of chemical phosphorusremoval process were proposed which could be adopted in similar WWTPs.（4）The SV₃₀and Microscopic photos of the activated sludge in the pilot A²/Oreactor were investigated. The SV₃₀was low when the water temperature was highwhile the SV₃₀was high when the water temperature was low, according to the changeof seasons and water temperature. The longer the SRT was, the higher the SV₃₀could be.It was expected that the conclusions derived from this study may be valuable foroptimizing purpose in design and management in operation of the similar WWTPs.