Study On The Performance Of Nitrogen And Phosphorus Removal In UniFed SBR Process And Its Automatic Control

Today, eutrophication of water body becomes significantly one of worldwide environmental problems. The wastewater techniques developed gradually from the period of pure COD removal to the period of simultaneous COD, nitrogen and phosphorus removal. The nitrogen and phosphorus removal technology aimed for eutrophication control has been the study focus in the wastewater treatment area. UniFed SBR was a novel nutrient removel process developed by Australia in recent years. Compared with the complex continuous flow processes for biological nitrogen and phosphorus removal, the UniFed SBR process employs a single tank operation without any physical zone separation or sludge recycling to obtain a high nitrogen and phosphorus removal efficiency. The construction cost is low.UniFed SBR process is different from traditional SBR process in the method of influent supply and mode of operation. One UniFed SBR cycle includes three periods: feed/decant,aeration and settle. The influent is introduced slowly and evenly into the sludge blanket at the bottom of the SBR tank. The disposal water in previous cycle in the sludge blanket at the lower part of the tank is gradually replaced by incoming fresh sewage. The upper clear effluent is pushed upwards by the influent and drained by a decanter or overflow from a decant weir. The design of feed distributor at the bottom of the UniFed SBR reactor is very important. When the wastewater passed through the distributor, the sludge and wastewater is not mixed greatly. Good separation of influent and effluent can be achieved, which is the basis and guarantee of successful operation of UniFed SBR process. After feed/decant period is over, aeration period and settling period followed. During feed/decant period, denitrification and anaerobic phosphate release take place. In the following aeration period, nitrification and aerobic phosphate absorption also take place. Therefore, nitrogen and phosphorus removal from the wastewater could be achieved through a UniFed SBR cycle.In order to accelerate the investigation development and future application in practical engineering of novel UniFed SBR process, a lab-scale UniFed SBR process apparatus was established, based on overseas basic theory and research of UniFed SBR process. A new drainage pattern—air weir drainage was tried and key technology which could achieve normal operation of the process were exploited. Computer automatic control system of UniFed SBR process with air weir drainage was developed and operational automation of the process has been achieved. The nitrogen and phosphorus removal performance were investigated in a 40L UniFed SBR reactor treating domestic wastewater. The effect of influent C/N ratio, C/P ratio and volumetric exchange ratio on nitrogen,phosphorus and organic matter removal of the process were studied systematically. Changing law of DO,pH and ORP during aeration period was investigated and the feasibility of taking them as real time control parameters was discussed.A novel drainage pattern with air weir instead of decanter in UniFed SBR process was developed in this study, which avoided effluent quality polluted by overflowed mixed liquor during aeration period when using normal overflow weir. The operation of the process was more complex after adopting air weir. It is impossible to operate the process manually. An automatic control system must be established.The personal computer auto-control system of the UniFed SBR process incorporating the air weir drainage was developed successfully using Visual C++ language,personal computer and input/output card. The sequential control of the process was realized.TN removal in UniFed SBR process occurs during both feed/decant period and aeration period. TN removal during aeration period is attributed to simultaneous nitrification and denitrification. The mechanism of phosphorus removal in UniFed SBR process is enhanced biolgocial phosphorus removal mechanism.Good performance of organic matter and ammonium removal could be achieved in UniFed SBR process treating domestic wastewater. Not more thn 30mg/L COD concentration and no ammonium could be detected in the effluent. The removal efficiencies of total nitrogen and phosphorus mainly depend on influent C/N ratio. When the volumetric exchange ratio was 25% and the C/N ratio was lower than 5.7, TN removal efficiency increased sharply as C/N ratio increased, from 43.6% of 2.75 to 80.84% of 5.7. When C/N ratio was 5.7, TN removal efficiency during feed/decant period had reached the maximum value. Afterwards, TN removal efficiency increased very slowly as C/N ratio increased, because only TN removal efficiency caused by SND during aeration period could be enhanced. P removal efficiency increased as C/N ratio increased, because PAOs could get more organic carbon source from the influent to produce PHB and release PO43-, avoiding the adverse influence of NOx- on PO43- release. The higher the C/N ratio, the more the quantities of P release and uptake, the quicker P uptake rate, the better P removal performance was. Regardless of the C/N ratio, excellent COD (average 93.15%) removal efficiencies were maintained throughout the experiment. When Influent C/N ratio was bigger than 6.97 and organic load exceeded 0.38 kg/(kg?d), SVI rose quickly as C/N ratio increased. Non-silk bacteria sludge bulge appeared because of low DO and high organic load.Influent C/P ratio was an important factor that affects phosphorus removal in UniFed SBR process. When the volumetric exchange ratio was 33% and the C/N ratio was 6, complete phosphorus removal could be achieved as C/P ratio was greater than 33. When C/P ratio was lower than 33, phosphorus removal efficiency increased as C/P ratio increased linearly. However, TN and COD removal were hardly affected by feed C/P ratio. The average removal efficiencies of TN and COD were 84% and 94.5% respectively.TN removal efficiencyηin a whole UniFed SBR cycle are the sum ofη1 caused by denitrification in the sludge blanket at the bottome the reactor duringfeed/decant period andη2 caused by SND during aeration period. Changing the volumetric exchange ratio has adverse effects onη1 andη2. When the volumetric exchange ratio increases,η1 decreases ,butη2 increases under the same influent quality. Because TN removal during feed/decant period contributes more in UniFed SBR process, the more the volumetric exchange ratio, the worse total nitrogen removal. The more the volumetric exchange ratio, the better phosphorus removal is under the same influent quality in UniFed SBR process.In the changing curves of pH,DO and ORP there is no turning point during feed/decant period in UniFed SBR process, which can not show the course of biochemical reactions such as denitrification and phosphate release. Therefore, it is not feasible taking pH,DO and ORP as real time control parameters during feed/decant period.During aeration period in UniFed SBR process, pH,DO and ORP curves can show the biochemical reactions. When nitrification ends, turning points appear in 3 curves. pH turns up, both DO and ORP going up quickly. Therefore, it is feasible taking pH,DO and ORP as real time control parameters during aeration period. Normal operation of UniFed SBR process can be achieved by on-line detecting pH,DO and ORP values combined with practical operational experience. Aeration should be extended when nitrification is over so as to blow out of air from the system, avoiding sludge float when feeding in the following cycle, which is the optimal real time control strategy during aeration period in UniFed SBR process.