Chemical Phosphorus Precipitation Of Wastewater From Municipal Treatment Plant And Its Environmental Effects

With stringent discharge of urban sewage treatment plant, many wastewater treatment plants(WWTPs) faced problems that it cannot reach the discharging standards of total phosphorus. Chemical phosphorus removal usually is adopted at that case. However, there are many factors affecting chemical phosphorus precipitation such as water quality and operating conditon. But there are few studies on that by now. The batch experiments of chemical phosphorus removal o f different sewages(raw wastewater, effluent from the secondary treatment, the supernatant from anaerobic tank, the effluent from sludge concentration tank and sludge dewatering from municipal sewage treatment plant) were investigated. It is aimed at ascertaining the influences of precipitator, dissoved organic matter(DOM), e.g., humic acid and volatile fatty acid(VFA) in water and their interaction on the chemical phosphorus removal,knowing the precipitation characteristics of the different wastewater of urban sewage treatment plants, and laying foundations for chemical phosphorus removal.The main conclusions drawn from the research are as follows:(1)The batch chemical phosphorus removal experiments using precipitators of ferrous sulfate, calcium chloride, calcium oxide, ferric chloride, aluminum sulfate show that the removal rates are lower using calcium chloride and calcium oxide, compared with that of iron and aluminium salt. The removal rate of phosphate in effluent from the secondary treatment by PAC and ferric chloride could reach above 70%. Removal rate by using aluminum salt in the raw wastewater is higher and that of iron salts; the removal rate(about 75%) by PAC in the supernatant from anaerobic tank is higher; the phosphate in the effluent from sludge dewatering from municipal sewage treatment plant could be removed more than 72% using ferric chloride and PAC; the remove rate of phosphate in the effluent from sludge concentration tank is about 75% with ferric salts and PAC.(2)The optimum operation parameters are as the following, stirring at 200r/min for 1min and then 50 r/min for 10 min, and precipitation for 30 min in the batch experiments.(3)At p H of 6.5, with the increase of VFA concentration, the removal rate of phosphate decreased gradually. It had much influences on the removal when the VFA concentration was higher than 60mg/L, but brought less influence in concentration of 10mg/L~50mg/L. It was nearly no influence on phosphorus removal by adding humic acids less than 40mg/L.But the phosphorus removal rate decreased slowly after dosage of more than 60mg/L.(4)The main products are aluminum salt and iron salt precipitation of its hydrolysis product and with phosphate combined with precipitation product. The PAC precipitation products are more complicated. The main products are Al PO4, Al2O3, A l6P6O2 4·4 H2 O, A l( H2 P O4)3, A l8H1 2( P2O7)9, A l2P6O1 8. F e r r i c salt crystallization products mainly include Fe(OH) 3, Fe2 P, Fe4(PO4)3(OH) 3, Fe3 PO7.(5) The forming rate of ferric phosphate had reached top level under the condition of p H=4.5 and phosphorus removal effect was good. One of the causes is hydrolysis of ferric salt is relatively strong at this p H, the hydrolysate hydrolyzed to produce a strong adsorption of phosphorus. The second cause is due to part of iron and phosphate combined. While p H value of the sewage is about 7, it needs to adjust p H firstly before adding ferric salt. Precipitation reaction could research to balance within one hour, but the adsorption reaction could not for 5 hours. The supernatant of precipitation was alkaline, and products of Al(OH) 3(am) and Al(OH) 3(s) could adsorb the phosphate, so that the removal rate was higher.