Stable Operation And Control Strategy Of Partial-nitrifying Granules

The’nitrite route’is referenced to the short-cut in the nitrogen cycle based on the oxidation of ammonia to nitrite and the further reduction of the nitrite to nitrogen gas. However, because of the low specific production or accumulation rates of nitrite, current partial nitrification unit, such as SHARON, has also been the key rate-limiting step in nitrogen removal processes.Partial-nitrifying granular sludge usually has compact structure, good setting ability, and high biomass retention.Currently, granular sludge are mainly cultured and maintain stability in SBR reactor which has big rate of height to diameter. However, continuous flow mode is generally the preferred process in practical engineering applications,Inoculum in conventional activated sludge generally last longer time.Therefore, achieve granular sludge in SBR firstly and seeded in a continuous stirred-tank reactor which is expected to solve the problem.In this thesis, the change process of partial-nitrifying granules in CSTR were investigated, the influence of HRT and temperature on partial nitrification granules in CSTR was also evaluated.Furthermore, the control strategy to improve the long-term stability of nitritation process was also explored.Main contents and results are as follows:(1) In order to investigate the effect of different types of reactors on the short-cut nitrification granular sludge, a continuous stirred-tank reactor (CSTR) was carried out, using mature short-cut nitrification granular sludge cultivated in sequencing batch reactor (SBR) as seed sludge. Results indicated that the change of reactor type and influent mode could induce part of granules loses stability with gradually decreasing in sludge settling ability during the initial period of operation. However, the flocs in CSTR achieved fast granulation in the following reactor operation. In spite of the changes of particle size distribution, e.g. the decreasing number of granules with diameter larger than2.5mm and the increasing number of granules with diameter smaller than0.3mm, granular sludge hold the absolute predominance of sludge morphology in CSTR during the entire experimental period. Moreover, results showed that the change of reactor type and influent mode didn’t affect the nitrite accumulation rate which still kept at about85%in effluent. Additionally, the average activity of sludge in CSTR was stronger than the seed sludge, because the newly generated small particles in CSTR had higher specific reactive activity than the larger granules.(2) Results showed that the system of short-cut nitrification still has good removal efficacy at lower HRT and temperature. The effect of different hydraulic retention time (HRT) on short-cut nitrification granular sludge and its performance at the low temperature (12～15℃) were investigated in a continuous stirred-tank reactor (CSTR) Under conditions of unchanged load.Results indicated that the mass fraction of granules with diameter smaller than0.3mm and larger than1.6mm were decreased,whereas the mass fraction of granules with diameter between0.3mm and0.8mm were increased during the operation of shorten HRT from4h to1h. The values of granules with diameter between0.3mm and0.8mm reached about50%in the finally reactor operation..The effluent SS maintained below10mg·L-1in spite of MLSS in CSTR increased from5.2g·L-1to14.16g·L-1during the entire experimental period.Results showed that the change of HRT didn’t affect the nitrite accumulation rate which still kept at about90%in effluent.Moreover,the activity of granules with diameter larger than0.8mm increased and granules with diameter smaller than0.8mm decreased with the shorting of HRT.However,the average activity of sludge in CSTR didn’t change.The activity of granules with diameter smaller than0.8mm is larger than granules with diameter larger than0.8mm under the same HRT.The activity of granules reduced and could induce part of granules loses stability after the temperature reduced to12～15℃, the part of granules with diameter smaller than0.3mm increased apparently.But the ammonia removal rate still kept at about95%and nitrite accumulation rate in effluent recovered to (85～90)%in a short time.The results indicated that the systerm of the short-cut nitrification granular sludgeCSTR still has excellent decontamination performance at the low temperature (12～15℃) and low HRT (1h).(3) A strategy to control the long-term stability of partial-nitrifying granular process was explored.The research demonstrated that controlling SRT is an effective strategy to improve the long-term stability of nitritation process.The reactor was operated for180days and divided into4phases. During phase one the solid retention time (SRT, extremely high) was controlled by the sludge which washed-out spontaneously with the effluent withdrawal, leading to deterioration of nitritation performance. The SRT was manually maintained at80days (phase two)、50days (phase three) and35days (phase four) by removing granular sludge.Results indicated that nitrite accumulation rate recovered to about90%when SRT shortened from uncontrolled to80d. With the shortening of the SRT, the MLSS reduced from28g·L-1to10g·L-1; On the contrary,the DO increased to more than1.2mg·L-especially when SRT is50days.Therefore, NOB had higher specific reactive activity and nitrite accumulation rate reduced to75%;When the SRT was35days,nitrite accumulation rate recoverd to90%under conditions of low DO.Results also indicated that it is difficult to form a granular sludge under lower SRT;The EPS increased to form granular sludge under lower SRT and the part of increased is mainly protein, Polysaccharide has been maintained at20～25mg·g-1. Additionally, the average activity of sludge in CSTR increased as shortening the SRT.