Phosphorus Recover From Sewage Sludge Based On Thermal Treatment

In order to reduce the influence of urban sewage on the water environment quality,a large number of new waste water treatment plants(WWTPs)has been built during the "twelfth five-year" and the "thirteenth five-year" in C hina,which has significantly improved the water quality,but also produced a large number of by-product(sewage sludge).In recent years,the sludge thermal treatment process has aroused the interests of scholars at home and abroad due to the richness of sludge in organic matter,the easiness of obtaining heat source and the obvious effect of thermal treatment process for sludge reduction.In this study,low-temperature heat treatment technology combined with chemical crystallization method was studied to recover phosphorus from the metal-salt-rich activated sludge.This method has lower energy consumption,less requirements of the reaction container and more ideal recovery of nitrogen and phosphorus,which has broad application prospects in small and medium-sized sewage plants.In this research,the thermal treatment of the activated sludge at the end of an aerobic tank in the urban sewage treatment plant was carried out,and the phosphorus was recovered in the form of magnesium ammonium phosphate.The effects of EDTA,and a metal complexing agent,on the release of phosphorus from sludge were studied by comparing the results with and without EDTA.The effects of different factors on the process of phosphorus release and recovery were investigated and the optimal conditions were also determined.The results indicated that the release of phosphorus from the sludge after merely thermal treatment was very limited,only 5% of the T-P.With 10 mmol /L EDTA addition,a large amount of phosphorus was released,and the release of Pi and T-P reached 110 mmol/ L and 140 mmol/L,which was nearly 10 times higher than that without EDTA.The optimal parameters for P release were 10 mmol/L EDTA addit ion,80?C and 60 min by taking these factors into account.Under the optimal condition,the release of T-P was 140 mg/L(65%),with Pi accounting for 78% and poly-P 20%.The effect of EDTA in increasing the release of phosphorus was mainly through affecting the release of metal in sludge and C a,Mg,Al and Fe was the most susceptible four metal.According to the formation constants(KMY)between metal cations and EDTA,the degree of influence was as follow: Al>Fe>Ca>Mg.Adding EDTA c ould also significantly improve the degree of sludge disintegration.The molar ratio of Mg: N : P and p H also had a significant and similar effect on the recovery of Pi and poly-P in the liquid supernatant.Under the condit ion of p H = 9,n(Mg2+): n(NH4+): n(P)= 1.5: 1: 1,magnesium ammonium phosphate precipitation method achieved the best efficiency of recovering phosphorus.With the above condition,105 mg/L Pi and 40 mg/L poly-P were recovered,and the precipitation rates were 88% and 80%,respective ly.Most of the phosphorus in the product was recovered in the form of magnesium ammonium phosphate,accounting for about 75% of the total recovery.In this experiment,the recovery of phosphorus reach more than 50%.This research drawn the following conclusions: the release of phosphorus would severely be hindered by the metal salts in the sludge during the process of thermal treatment,and could be improved by adding the metal complexing agent EDTA.Most of the Ca ions and a small amount of other ions combined with phosphorus would form insoluble compounds;and Al could adhere to the cell surface forming insoluble compounds and blocking the microbial cell membrane and cell wall aisle.With the addition of EDTA,sludge disintegrates more completely during thermal treatment which not only indicated organism release in the process,but also reduced the surplus sludge volume.It implements both sludge reduction and resource recycling.There are still other metals formed in the product of impurities.