Phosphorus Recovery From Phosphorus Release Solution Of Excess Sludge And Acid Leachate Of Sludge Incineration Ash With Struvite

With the development of the global economy,the rate of urbanization in China has risen sharply in recent years,and the output of municipal sewage has also increased.Correspondingly,a large amount of excess phosphorus-containing sludge is produced through biological phosphorus removal process with the sewage entering the sewage treatment plant.The magnesium ammonium phosphorus precipitation undoubtedly provides an effective method to solve the problem of phosphorus pollution and phosphorus deficiency.In this dissertation,recovering phosphorus from sludge was invesgated in detail.The parameters of the synthesis of struvite through the simulated wastewater were optimized,and the effects of acid-alkali,low-temperature heating and ultrasonic pretreatment on the release of phosphorus from sludge,and changes in phosphorus form in the solid phase were investigated.Then,the obtained supernatant was subjected to optimization of conditions for struvite synthesis,The sewage sludge incineration ash obtained at different incineration temperatures was used as the object,and phosphorus was recovered by a two-step method of acid leaching and struvite synthesis.The key research findings are as follows:1.Optimization of struvite from simulated wastewaterSimulated phosphorus-containing wastewater prepared with pure reagents was used as the object,and the effects of different N/P molar ratios,different Mg/P molar ratios,and different pH on the phosphorus recovery and the properties of the precipitates were investigated.The results showed that:with N/P molar ratio and Mg/P molar ratio increasing,the time to start precipitation shortened,and the removal rate of phosphorus and the yield of precipitation increasing.The increase of pH favors the precipitation of struvite.However,the purity of struvite was reduced when pH increased to 10.0,with Mg₃（PO₄）₂ produced.The optimal conditions for the synthesis of struvite were N:Mg:P=5:1.6:1,pH=9.5,and the purity of struvite was 97.56%.2.Sludge pretreatment to release phosphorusThe results of acid-alkali,low-temperature heating,and ultrasonic pretreatment of phosphate release from the sludge showed that both acidic and alkaline conditions promoted the release of phosphorus from sludge,and the highest concentration of phosphorus in the supernatant was at pH 2.At pH 2.The phosphorus released from low temperature heat-treated sludge reached the highest at 70 ^oC for 2 h.The phosphorus released from ultrasonic treated sludge reached the highest at 2.13 W/mL for 0.5 h.These three methods of phosphorus releasing mainly released inorganic phosphorus.Taking into account the release of phosphorus and the treatment time through sludge pretreatment,the pretreatment method chosed was acid conditioning at pH 2 for 0.5 h.3.Optimization of phosphorus recovery from phosphorus-enriched liquidThe Ca/Mg molar ratio in the supernatant obtained by pretreating the sludge at pH 2 was as high as 2.51,which inhibited the synthesis of struvite.The contents of N、Mg and Ca in the supernatant after treating with cation exchange resin were all reduced to very low concentrations.The supernatant was used to synthesize struvite at the optimal conditions of N:Mg:P at 5:1.8:1,pH 9.5,and the phosphorus removal rate can reach 97.77%.The XRD results showed that the purity of struvite was 97.78%.4.The acid leaching of incineration ash and synthesis of struviteThe incinerated ash is burned from the dried sludge at different temperatures（600、700、800、850、and 900 ^oC）,sulfuric and hydrochloric acid were used to leach phosphorus.The results showed that the contents of P and heavy metals increased with the increase of the concentration of sulfuric acid and hydrochloric acid.The phosphorus leaching rate of incineration ash obtained at 800 and 850 ^oC was above 90%and the concentration of heavy metals in the leachate was significantly reduced.The obtained leachate of 850 ^oC incineration ash from 0.05 mol/L sulfuric acid was successfully used to synthesize struvite under the conditions of N:Mg:P at 5:1.8:1 and pH 9.5.