Study On Magnesium Ammonium Phosphate Production From The Alkaline Hydrolysis Supernatant Of Excess Sludge From The Second Level Biochemical Sewage Treatment Plant

In a second level biochemical sewage treatment system, more than90%phosphorus andparts of nitrogen in influent will be transferred into the sludge，and thus phosphorus andnitrogen usually accounted for up to4%and9%of the dry sludge. If the sludge was directlydisposed in sea or landfills, the leaching of phosphorus and nitrogen would lead to serioussecondary pollution. Besides, in the sewage treatment system,20～50%of phosphorus andpart of nitrogen in effluent sludge would be returned back to the wastewater treatment system.If phosphorus and nitrogen of the excess sludge can be recycled through magnesiumammonium phosphate (MAP) method, it can not only recover the limited phosphorus resource,but also reduce the phosphorus and nitrogen loading of the sewage treatment system. Besides,the MAP hexahydrate (MgNH4PO4.6H2O), commonly known as struvite, can also be utilizedas an effective slow-release fertilizer in agriculture.In this study, magnesium ammonium phosphate (MAP) was used to recoverorthophosphate (PO43--P) and ammonium nitrogen (NH4+-N) from the alkaline hydrolysissupernatant of excess sludge from the second level biochemical sewage treatment plant. Inorder to reduce alkali consumption and decrease the pH value of the alkaline hydrolysissupernatant, two-stage alkaline hydrolysis process was designed. According to the data of theeffects of pH, sludge concentration and reaction time on the release of phosphorus andnitrogen for the single-stage alkaline hydrolysis process, which were investigated for the firsttime, the experiments for two-stage alkaline hydrolysis process were carried out, and therunning conditions of two-stage alkaline hydrolysis process were determined. Then theinfluences of Mg/P (molar ratio of Mg2+/NH4+-N), pH and reaction time on the recovery ofPO43--P, TP and NH4+-N by MAP method were studied. Finally, the order of parametersinfluencing MAP reaction was determined and the optimal conditions of MAP reaction werepredicted through the response surface methodology (RSM). Besieds, the dried precipitateproduced under the optimum conditions was analyzed by energy dispersive X-rayspectroscopy (EDS) and the chemical dissolution method. The main results were as follows:(1) For the release of phosphorus and nitrogen through alkaline hydrolysis of excess sludge,high pH range of12.5-13.0was beneficial to the release of TP and PO43--P. The concentrations of TN, TP, NH4+-N and PO43--P increased with the increase of the sludgeconcentration, while the release efficiencies of PO43--P and NH4+-N decreased with theincrease of the sludge concentration. The suitable sludge concentration was20-30g/L. Thereaction time had little impact on the release of phosphorus and nitrogen, and a good PO43--Pand NH4+-N release effect could be achieved in0.5-1h of alkaline hydrolysis sludge.（2）During the two-stage alkaline hydrolysis process, the release efficiencies of PO43--P andNH4+-N were41.96%and7.78%, respectively, and the pH of the supernatant was below10.5under the running conditions of pH13，volume ratio (sludge dosage/water dosage) of1.75insecond-stage alkaline hydrolysis reactor,20g/L of fresh sludge concentration in first-stagealkaline hydrolysis reactor. Besides, it can save more than15%of the alkali consumption thanthat of the single-stage alkaline hydrolysis.（3）During the recovery of phosphorus and nitrogen from the effluent supernatant of thetwo-stage alkaline hydrolysis of excess sludge through MAP reaction, the recoveryefficiencies of TP, PO43--P and NH4+-N increased with the increase of Mg/P molar ratio. Andthe recovery efficiency of PO43--P would stay constant at the molar ratio of Mg/P1.4. Inaddition, the PO43--P recovery efficiency would also remain constant after pH increased to9.5.While the reaction time had little impact on the recovery of phosphorus and nitrogen, andMAP reaction could be completed within10min.（4）The parameters influencing MAP reaction were ranked as Mg/P> pH> reaction time.The recovery rates of PO43--P and NH4+-N were46.88%and16.54%, respectively under theoptimized conditions of Mg/P of1.8, pH9.7and reaction time of15min.（5）The existence of rich aluminum in excess sludge would affect MAP formation as well asits purity. Therefore, in order to recover phosphorus and nitrogen from excess sludge by MAP,an addition of aluminium sulfate to strengthen the biological phosphorus removal should notbe advocated.