Study On Ammonia Nitrogen Removal From Fgd Wastewater With The Struvite Precipitation Method

The quality of desulfurization wastewater produced by limestone wet operation is poor,and some constituents in it are strictly controlled by the environmental standards as the first class of pollutants.Meanwhile,the ammonia nitrogen of desulfurization wastewater is usually high,and the traditional "triple box process" technology has few effects on removing the ammonia nitrogen,which makes the wastewater discharge blocked.The struvite precipitation method can treat high concentration of ammonia nitrogen,which has a good practical value by removing and recovering nitrogen and phosphorus as slow release fertilizer.Therefore,this thesis used the struvite precipitation method to study the remove ammonia nitrogen from desulfurization wastewater in thermal power plant.The impacts of pH,phosphorus : nitrogen ratio,magnesium : magnesium ratio and temperature on the removal rate of ammonia nitrogen were determined by orthogonal experiment.The effect of single factor test according to the orthogonal experiment is good,and the removal of ammonia nitrogen could be further improved if the combination of sodium hypochlorite added so that the effluent water reach discharge standards.On this basis,the actual desulfurization wastewater of 1 # and 2 # power plants was experimentally studied.The main resluts were showed as follows:1)The optimum conditions made by single factor experiment were showed as follows: pH= 8.5,Mg: N = 5.0: 1,P: N = 2.0: 1,stirred at 150 r / min for 20 min,room temperature,sediment time=20min.Under this optimum conditions,the residual ammonia concentration was 41.59 mg / L while the removal rate of ammonia nitrogen in desulfurization wastewater was 91.19%.The concentration could meet the "Integrated Wastewater Discharge Standard"(GB 8978-1996)that was less than 15 mg / L after adding 8g / L sodium hypochlorite(10%)2)The removal rate of 1 # thermal power plant whose ammonia nitrogen concentration is 1267 mg / L can reach 99%.The concentration is 12.72 mg / L after treatment.Meanwhile,The removal rate of 2 # thermal power plant is 88.03%.3)Scanning electron microscopy(SEM)showed that most of the precipitated products of 1 # and 2 # power plants are orthorhombic crystals.The main components of the precipitated product are MgNH4PO4 · 6H2 O by X-ray diffraction(XRD).Sediment is not easy to absorb water,and can be maintained for a long time.The composition is similar to that of the struvite and can be used as a slow release fertilizer.4)The removal of ammonia nitrogen from desulphurization wastewater has a certain environmental and economic benefit,which can make up part of the cost.Moreover,the cost can be further decreased if the phosphorus wastewater can be mixed or the cheap magnesium source can be used.The struvite precipitation method can remove ammonia nitrogen from desulfurization wastewater efficiently,and is simple to operate while the product struvite has certain economic value.It is necessary to study mechanism process before application,and the combination of other processes(broken point chlorination,biochemical method,blowing method)may be also considered to improve the removal of ammonia effect and reduce the cost of treatment.