Study On Phosphorus Recovery From Excess Sludge By Ferric Salt Modified Oyster Shell

This study was designed to recover phosphorus from excess sludge. At first, phosphorus and some metal cations were released to solution from excess sludge by heating and acidification methods, then oyster shell and ferric salt was used to prepare modified oyster shell(MOS) as adsorbent for phosphorus recovery from sludge digestion solution. The release conditions of phosphorus in excess sludge and adsorption-desorption characteristics of MOS were investigated.The content of phosphorus in the excess sludge is closely related to water quality and wastewater treatment process. Research showed recovering phosphorus from surplus sludge is favorable for the excess sludge containing a lot of phosphorus. Research also showed that both heating and acidification could enhance the release rate of phosphate(PO43--P) and total phosphorus(TP). Whereas, the optimal results was obtained when heating and acidification simultaneously used for excess sludge. Under80℃for2h, the release rates of PO43--P and TP were18.9%and25.3%, respectively; under2%HCl for2h, the release rates of PO43--P and TP were58.1%and68.7%, respectively. However, under2%HCl and80℃for2h, the release rates of PO43--P and TP reached84.1%and98.0%, respectively, which was higher than heating and acidification alone. That because phosphorus in intracellular and extracellular are released together into solution due to the cell was broken by acid. But when heating together with acidification used for excess sludge digestion, the dissolution rate of some metal cations is high.The results show that the modified oyster shell had a higher phosphorus adsorption capability, with surface area of16.955m2/g, total pore rate of0.01969cm3/g, and bore diameter of3.87nm. The adsorption isotherm of phosphorus fitted the Langmuir equation well, Phosphate adsorption capacity is influenced by the Fe3+concentration, pH and temperature, and the increase in temperature could enhance phosphorus adsorption capacity. The phosphorus adsorption capacity of modified oyster shell was18.55mg/g at40℃. By kinetic study, phosphate adsorption well fitted the pseudo-second-order equation with a correlation coefficient higher than0.99. The adsorbent regeneration tests showed that the phosphorus desorption occurred in HC1(0.025%-2.5%) solution, and the desorption rate reached99.97%when spent modified oyster shells desorbed in HCl solution (2.5%) for1h. After8times regeneration, the desorption rate could still reach85.0%and the phosphorus concentration in the solution was37.3g/L. The activation ratio reduced to76.8%after four cycles of adsorption-desorption-regeneration.CaCl2was used as precipitator for phosphate precipitation from desorption solution containing large amount of phosphorus. When CaCl2was used as precipitator, with mixing time of30minutes, temperature of303K, nCa:nP=1.55, PO43--P precipitation rate reached over99%, sludge phosphorus recycle rate was more than90%. These results suggested recovering phosphorus from excess sludge by ferric salt modified oyster shell is feasible.