The Research Of The Use Of Zero-valent Iron To Enhance Nitrogen And Phosphorus Removal From Urban Wastewater With Low C/N Ratios

The present situation of water environment is very serious in our country, therefore the control of point source pollution in urban sewage treatment plant is very important. There is a sewage treatment plant in Guangdong province which has very low carbon-to-nitrogen wastewater but very high standard of effluent, because the treated effluent will be discharged into the upper reaches of DongJiang River. The plant is using Food-Chain Reactor technology with chemical phosphorus removal process, to ensure COD, TP, TN, Ammonia Nitrogen in the effluent constantly in accordance with Grade III of “Water Quality Standards for Surface Waters GB 3838-2002”。 The traditional biological nutrient removal process requires external carbon source, and even with that it still can hardly meet the high standard of the effluent. So usually it must be accompanied by chemical phosphorus removal process, which significantly increases the OPEX of the plant.The theoretical basis of this research is biochemistry nitrogen and phosphorus removal, and on this basis, especially for wastewater with low carbon source, we discussed and studied the feasibility of zero-valent iron coupling methanol to enhance the removal of nitrogen and phosphorus, influencing factors, the optimal conditions and reaction mechanism. In order to optimize the removal of nitrogen and phosphorus and reduce the cost of chemical phosphorus removal process, this research presents the idea of using Zero-valent iron coupling methanol to enhance the removal of nitrogen and phosphorus and reduce the use of external carbon source and the dosing agent for chemical phosphorus removal process. The main conclusions are as follows:(1) Compared to divalent iron and trivalent iron, ZVI has a more obvious role in promoting denitrification, the experiments show zero valent iron coupling methanol to enhance the removal of nitrogen and phosphorus is feasible;(2) 40 mg / L of zero-valent iron is beneficial to zero-valent iron coupling methanol biological denitrification process. With the zero-valent iron concentration increases but then Inhibition occurs. The efficiency of phosphorus removal increases with the rise of zero-valent iron concentration; 125 mg / L of methanol is beneficial to zero-valent iron coupling methanol biological denitrification process. After that, the microorganism is unable to utilize higher concentrations of methanol; Methanol plays a very limited role in phosphorus removal process;(3)The use of ZVI to enhance the removal of nitrogen and phosphorus is acid-catalyzed reaction. When pH equals to 6, it is more conducive to the removal of nitrogen and phosphorus; A small accumulation of NH4 + and NO2- was found after completion of the reaction;(4)According to the results of orthogonal experiments and single factor experiments, it is suggested to set the optimal conditions for ZIV coupling methanol to remove nitrogen and phosphorus process in zero valent iron concentration 30 mg / L, methanol 125 mg / L, pH =7;(5)The use of ZVI coupling methanol to enhance denitrification, On the one hand, zero-valent iron redox reactions can occur with nitrate in water, which produces NO2-, NH4+, NH3+, N2, etc. and achieves the removal of Nitrogen. On the other hand, zero-valent iron can act as the electron donor for autotrophic denitrifying bacteria, NO3--N as a direct recipient. The electron for nitrate reduction is from electronic ZVI, o corrosion products from a zero-valent iron, ferrous iron and hydrogen, which enables autotrophic denitrification and increases the efficiency of nitrogen removal;(6)The use of ZVI coupling methanol to remove phosphorus, on one hand zero valent iron is oxidized to divalent and trivalent iron salts. Then it reacts with phosphate ions to have chemical precipitation reaction, and Iron salt hydrolysis reaction can occur at the same time, and more polymerization reaction occurs during hydrolysis. Polyhydroxy polymer can be produced by charge neutralization, bridging adsorption, flocculation, then precipitation separation, thereby removing phosphorus in water. On the other hand, with addition of zero-valent iron, it enhances the metabolism of activated sludge microorganisms, which increases the efficiency of carbon source utilization and reduces the competition between polyphosphate and denitrifying bacteria microbial of carbon source nutrients, so phosphorus removal efficiency is improved.