Effect Of Suspended Carrier On The Enhancement Of Nitrification In A~2/O Process For Urban Sewage Treatment

Recent years, with the increasingly strict of wastewater discharge standard, many of the early investment sewage treatment plants must be upgraded. But most of the sewage treatment plant in the process of upgrading faced with the problem of landless,therefore adopt the method of adding suspended carrier has become the first choice.This method is mainly used the suspended carrier to provide more attached growth substrate for microorganisms, such as nitrifying bacteria. There are two advantages of this method, one is do not need to increase the volume, the other is increase the concentration of microorganisms and solve the contradictions of the sludge age in nitrifying bacteria and phosphorus accumulating organisms. Whereas in the past, the home and abroad research of this system most focused on the middle and small scale,and the attached and suspended phase microbial were seemed as a whole, this limitation makes the nitrification contribution of suspended biofilms for actual engineering’s is difficult to measure. In order to solve this situation, this study takes xi’an fifth WWTP as an example, focus on the nitrification contribution of biofilm for a composite system. The purpose is to provide theoretical basis for engineering application of suspended carrier.Firstly, this study makes an investigation and analysis about xi ’an fifth WWTP,monitoring the water quality, biomass, nitrification rate and so on. Secondly,according to the volume of packing area to aerobic area to calculate the nitrification contribution of biofilm for a composite system. Finally, using SBBR to simulate urban sewage, discussed nitrification character of biofilm at C/N rations are 2 and 5. The main research results obtained are as follows:The investigation and analysis about xi ’an fifth WWTP:(1)The average MLSS and MLVSS of activated sludge was 4780 mg/L and 3207mg/L, VSS/SS = 0.67. The average MLSS and MLVSS of biofilm was 4520 mg/L and2359 mg/L, VSS/SS = 0.52.The thickness of Biofilm was range from120 to 200 um, the average thickness was 140 um. When the dosage of the carrier accounted for 3.8% of the total volume of aerobic zone, the biomass conversion to the aerobic zone was 89.6mg/L. Therefore, the biomass of biofilm contribution to the system was very small.(2) The AUR and NUR of activated sludge was 4.22mgNH4+-N/gVSS·h and3.92mgNO2--N/ gVSS·h, the AUR and NUR of biofilm was 1.98mgNH4+-N/gVSS·h and 5.14mgNO2--N /gVSS·h, respectively. Using nitrification rate analogy method, the nitrification contribution of biofilm for a composite system is only 1.3%.The biofilm hardly plays any nitrification.(3) The amount of AOB in activated sludge was(2.53+0.34)%, dominant genera was Nitrosomonas, and also detected Nitrosospira. The amount of NOB was(2.87+0.41)%, dominant genera was Nitrospira. The amount of AOB and NOB in biofilm were(0.68+0.2)% and(7.8+0.2)%,dominant genera were Nitrosomonas and Nitrospira, respectively.(4) The proportion of ammonia nitrogen concentration which was less than 5mg/L was 96%.So one of the reason for the small contribution of biofilm to system is low concentration makes nitrifying bacteria chronically hungry, the activity is very poor. At the same time, It also suggested that the of aerobic is too big. It’s appropriate to reduce the aerobic volume, increase anaerobic volume, making carrier can have the effect of enrichment of nitrifying bacteria, the system could more efficient.The sequencing batch biofilm reactor(SBBR):(1) With the increasing of C/N ratio from 2 to 5, the nitrification rate of sludge and biofilm were reduced. The ammonium utilized rate(AUR) of sludge reduced from17.634 mgNH4+-N/gVSS·h to12.78 mgNH4+-N/gVSS·h, dropped about 27%. And the ammonium utilized rate(AUR) of biofilm reduced from 4.57 mgNH4+-N/gVSS·h to2.88 mgNH4+-N/gVSS·h, dropped about 36.9%.The proportion of AOB in biofilm decreased from 1.9% from 1.3%.NOB decreased from 18.9% from 14.7%.(2)For the hybrid system, sludge and biofilm play the role of nitrification together.As C/N rations were 2 and 5, the contribution of the biofilm to the overall nitrification were 18.7% and 21.6% respectively.(3)There has a close relationship between the spatial distribution of nitrifying bacteria and C/N ratio. As C/N ratio of influent increased, the thickness of biofilm also increased. The heterotrophic bacteria migrated to the outer layer of biofilm and nitrifiers began to colonized the inner layer of biofilm, so the C/N ratio determined the spatial distribution of microbial community.