Recovery Of Phosphorus From Surplus Sludge By Two-Stage Process

Phosphorus is one of the indispensable nutrients in life.The open cycle of phosphorus makes phosphorus recovery inevitable.The surplus sludge is rich in phosphorus and has high recycling value in the sewage treatment plant.Struvite crystallization has many advantages and is widely used in the method of recovering sludge phosphorous.The key factor which recover phosphorus from sludge is to release as much phosphorus from the mud as possible into the liquid phase.The thermal hydrolysis process of sludge combined with anaerobic digestion process can increase the production of biogas,reduce the digester capacity and reduce the amount of biogas slag.It is currently the mainstream process of domestic and international sludge treatment,but there is a high energy consumption during the thermal hydrolysis stage.Slurry of struvite is easily generated on the mud conveying pipe and the inner wall of the digester,and the phosphorus recovery rate is not high.This paper proposes two-stage struvite phosphorus recovery process,the first stage uses low temperature thermal hydrolysis to pretreat the sludge and recover phosphorus from the treated supernatant;the second stage takes the mud and water mixture after recovering phosphorus.The anaerobic digestion method further recovers the phosphorus in the digestive juice.Thus obtained satisfactory results.The main research results obtained in the paper are as follows:(1)The optimum conditions for phosphorus release from pyrolytic sludge were obtained.Through single-factor rotation experiments,it was determined that the heating temperature of 75?,heating time of lh,and pH=3 were the optimal conditions for thermal hydrolysis of phosphorus.At this time,the release amounts of TP and IP were 311.9 mg/L and 293.8 mg/L,respectively.The released phosphorus was IP-based,accounting for 94.2%of the supernatant TP;the sludge mixture was at 35?.After standing for 24h,the concentration of NH4+-N in the liquid phase of the sludge increased from 96.0 mg/L to 318.7 mg/L,which was 2.3 times the IP(molar ratio)in the liquid phase and was accompanied by a significant decrease in the SCOD concentration.It is beneficial to recover phosphorus by struvite method.(2)Determine the optimum process conditions for phosphorus recovery.Using single-factor rotation experiments to determine the pH,Mg/P and reaction time were the main factors affecting phosphorus recovery.The response surface methodology was used to obtain the optimum process conditions for phosphorus recovery from struvite:pH=9,Mg/P was 1.3 and the reaction time was 10 min.Under this condition,the recovery of phosphorus and the removal rate of NH4+-N were 96.7%and 20.9%,respectively.(3)After the anaerobic digestion(SRT=15d)of the mixture of the supernatant and mud after phosphorus recovery in one stage,the IP concentration in the digestion solution was 123.4 mg/L.After two stages of struvite recycling process,the recovery efficiency of phosphorus can reach 59.0%,which is 29%higher than that of a method.At this point,the dehydration effect of the mixed liquor is better,and the phosphorus in the digestive liquid is also more easily recovered as struvite.After cost accounting,two stages of phosphorus recovery are used,and a profit of approximately 1.44 yuan per ton of sludge(including 5%solids)can be obtained.(4)Determine the key factors affecting the purity of struvite,and propose a process scheme that uses cation exchange resin to increase product purity.The effect of Ca2+concentration in the supernatant on the purity of struvite was significant,especially pH and Ca/P had important effects on the purity of struvite.When the pH was 7,the purity of the struvite was 31.2%,but the yield was the lowest.When the pH was 9,the yield was 3.26 times that of the pH=7,but the product purity was only 6.3%;that is,the Ca/P ratio.The larger the phosphorus recovery,the lower the purity.When Ca/P=0.1,the recovery was 82.8%and the purity of struvite was 90.2%.When Ca/P=1.1,the recovery rate was 99.1%;the product purity was reduced to 35.1%.At pH=9,the sludge supernatant was treated with cation exchange resin,the Ca/P ratio decreased from 1.2 to 0.4,and the struvite product purity increased from 6.3%to 55.1%.