Study On Airlift Loop Reactor And Denitrifying Dephosphatation Process

It is significant to control the emission concentration of phosphorus to prevent eutrophication in close water systems. It is also important to combine phosphorus and nitrogen removal in one system. Denitrifying dephosphatation process is a promising alternative to traditional process. It utilizes a special kind of bacteria named Denitrifying Polyphosphate Accumulating Organisms (DNPAOs). DNPAOs are Polyphosphate Accumulating Organisms (PAOs) capable of denitrification. Because denitrifying dephosphatation process could uptake phosphorus anoxically, nitrification is needed for supplying nitrite or nitrate as electron acceptors for DNPAOs.In conventional biological nitrogen and phosphorus removal processes, bacteria and wastewater flow together through different stages within continuous flow systems or sequencing batch systems. These processes consume more energy. In order to solve these problems, an airlift loop reactor was designed to apply denitrifying dephosphatation process. Fibrous carriers were packed in the reactor. The reactor was divided into several zones and different bacteria grew dominantly in different zones. Wastewater, driven by aeration, flowed through different zones of the reactor at different stages to be treated. As a result, organic pollutants, nitrogen and phosphorus were removed.The paper included five parts as follows.1. The bioreactor is airlift loop sequencing batch biofilm reactor which belongs to fixed film reactor. It is different from other airlift loop reactors which are usually fluidized bed reactors and operated in continuous flow mode. It is also different from other SBBRs which usually do not utilize airlift loop. So, it is essential to study fluid velocity field. Particle image velocimetry (PIV) was used to study the flow field. Results show that when a small quantity of air is injected, the fluid cycle can occur between aeration zone and reverse flow zone. So it suggests that this character is suitable for the integration of nitrification and denitrification (or denitrifying dephosphatation).2. On the basis of original design, a large reactor of 700 L was built up to run in continuous flow mode. The continuous flow experiment show that the organic pollutants and nitrogen can be removed, but phosphorus removal can not be achieved when hydraulic retention time (HRT) is kept at 48 h. The results are as follows: (l)when the influent COD is 280 mg/1, the effluent COD is 48 mg/1 and COD removal efficiency is 82%, (2)when the influent Total Nitrogen(TN) is 85mg/L, the effluent TN is 25 mg/1 and TN removal efficiencyis 71% and (3)when the influent PO43'-P is 8 mg/1, the effluent PO43'-P is 8 mg/1 and no PO43'-P can be removed.3. On the basis of the experiment above, a little reactor of 18 L was built up to run in batch mode. The batch experiment show that the reactor operated with cycle duration of 24 h can remove organic pollutants, nitrogen and phosphorus. The results are as follows: (l)when the influent COD is 300mg/l, the effluent COD is 13 mg/1 and COD removal efficiency is 95%, (2)when the influent NH3-N is 118.7mg/L, the effluent NH3-N is 4-30 mg/1 and NH3-N removal efficiency is 90% and (3)when the influent PO43"-P is 10.5 mg/1, the effluent PO43'-P is 2.7 mg/1 and the PO43'-P removal efficiency is 73.8%.4. In the last chapter, the factors on nitrogen and phosphorus removal of the reactor in batch mode were discussed. General comments and conclusions are as follows: (1) the reactor can tolerant large variety of organic loading;(2) through the analysis of the effect of COD/P ratio on P removal, it can be concluded that the more COD consumed, the more P removed;(3) through the analysis of the effect of COD/N ratio on N removal, it can be concluded that there is little relationship between COD removal and Nitrogen removal. The reason may be that extra cellular substrate have been transferred to intercellular substrate, avoiding the negative influence of COD loading on nitrogen removal;(4) through the analysis of the relationship between TN removal and TP removal, it can be concluded that the reactor can simultaneously remove nitrogen and phosphorus;(5) the experiment of aerobic phosphorus release show that biological phosphorus removal is the mechanism of phosphorus removal in the reactor, and the experiment of anoxic phosphorus uptake show that phosphorus removal bacteria in this reactor can use three kind of electrons acceptor (O2, NO2", and NO3").