Recovery Of Phosphorus From Sludge By Phosphate Precipitation Method

Phosphorus in nature is an acylic resource. However, a large number of phosphorus in wastewater is discharged into rivers and lakes which cause eutrophication. Phosphorus removal by biological process in sewage treatment plant is widely used which generated a lot of phosphoric waste sludge. If the phosphorus in sludge can be recoverable, it will alleviate over-consumption of phosphorite and possible scarcity of phosphorus resource in future.This investigation focused on phosphorus recovery from sludge through phosphate precipitation method in laboratory. Sludge was collected from wastewater plant and the phosphorus contained was released to solution through ultrasonic treatment, heating, acidification and heating-acidification, respectively. CaCl2 and seawater were used as calcium and magnesium sources to precipitate dissolved phosphorus, and compositions of precipitation products were analyzed.Results showed that the concentration of released phosphate (Pi) and total phosphorus (TP) increased with ultrasonic treatment time. Under single running time of 4s and intermittent time of 1s, better result was got than any other pulse mode. In addition, phosphorus release at alkaline conditions (pH=11) was more efficient than at acidic conditions (pH=4).Results also indicated that both heating and acidification could enhance TP release and the optimal result was obtained under heating-acidification condition. Under 90℃for 2h, the TP release arrived at 22.5%, which was higher than that under 60℃. Under 10% HCl,10%H2SO4 and 10%HNO3 additions, the TP release reached 52.2%46.0% and 39.6% respectively, which had markedly improved compared to that of under heating condition. Heating-acidification brought with better result in compared with single treatments. Under 90℃with addition of 2%HCl for 2h,96.3% of TP was released. The treated sludge was collected and observed under microscope, the results showed that morphology of microorganisms was not destroyed under single heating treatments, while the microorganism cells were completely broken up under heating-acidification treatments.Both CaCl2 and sea water were used as precipitator for phosphate precipitation from digested sludge solution. When CaCl2 was used as precipitator, with mixing time of 5 minutes, temperature of 25℃, pH at 10.5, nCa:n P=1.55, Pi precipitation rate reached over 97%, TP precipitation rate reached 88%, sludge phosphorus recycle rate was 82%. The sediments contained CaO 52.3%, P2O541.7%. When seawater was used as precipitator, with mixing time of 15 minutes, temperature of 25℃, pH at 10, nMg2+:nP:nNH4+=1.8:1:1.2, Pi precipitation rate reached 92%, TP precipitation rate reached 80%, sludge phosphorus recycle rate was 75%. The sediments contained MgO 16.1%, P2O527.2%, N(NH4+-N)5.5%.These results suggested a feasible method for sludge phosphorus recovery. Heating-acidification effectively enhanced phosphorus release and dissolved phosphorus in the solution was efficiently precipitated by CaCl2 and seawater. Although seawater exhibited lower phosphorus precipitation rate, it saved reagent cost.