Key Technologies Of Nitrogen And Phosphorus Recovery From Swine Wastewater

With the fast economic growth of our society and the gradual improvement of our living standards,the demand for livestock products is soaring,which driven rapid development of livestock industry.However,it makes people increasingly concern about food safety and environmental pollution problems.Livestock wastewater contains large amounts of nitrogen and phosphorus that cause serious environmental pollution like eutrophication when it is discharged arbitrarily.Meanwhile,human beings are facing phosphate resources shortage crisis.Therefore,it is of great significance in removal and recovery of nitrogen and phosphorus from livestock wastewater.In this paper,studies on nitrogen and phosphorus recovery as struvite from swine wastewater was carried out.Instruments of laser scanning confocal microscope,scanning electron microscope?SEM?,energy dispersive spectroscopy?EDS?,fourier transform infrared spectroscopy?FTIR?and X-ray diffraction?XRD?as well as mass balance were utilized to analyze the particles morphology and determine the species of precipitated minerals;Mastersizer and Zetasizer were used to identify the particles sizes and agglomerative property.The influences of various parameters on nitrogen and phosphorus removal efficiency,sediment composition and particles morphology under different conditions were investigated.The main results were as follows:?1?A relatively stable phosphorus removal efficiency at 94%from real swine wastewater was observed at pH 9.0-11.0.The minerals precipitated in the deposits were struvite?MgNH₄PO₄·6H₂O?,K-struvite?MgKPO₄·6H₂O?,amorphouscalciumphosphate?Ca₃?PO₄?₂·x H₂O,ACP?and Mg?OH?₂ except sulfate and carbonate.Struvite was the dominant species in the precipitates at pH 8.0-9.0.Enhancing pH from 9.0 to 10.0 resulted in struvite decline and gave rise to K-struvite and ACP steadily.With regard to highly alkaline conditions at pH above 10,drastic decrease of struvite and rapid increase of ACP and Mg?OH?₂ were observed.Maximum concentration of K-struvite was obtained at pH 11.0,following a sharp decline to pH12.0.The struvite purity was 77.2%at pH 8.0,increasing pH resulted in the decrease of purity.Whereas a drastic decline to 0 was detected when pH was up to 12.0.Controlling pH between8.0 and 9.0 could inhibit other minerals formation and obtain the highly pure struvite crystal product.?2?The effect of pH,Mg/P and Ca/P mass ratio were investigated in synthetic wastewater in the presence of Ca ion.A relatively stable phosphorus removal efficiency at above 94.9%was observed at pH 8.0-11.5.The amount of struvite was increasing and simultaneously generated ACP when pH at 8.0-10.0.Enhancing pH from 10.0-11.5,the product of struvite was declined,while ACP was improved and Mg was started to deposit in the form of Mg?OH?₂.Adjusting pH from 11.5 up to 12.0,struvite has a sharp decline.The removal of N was mainly in the form of NH₃.The product of ACP and Mg?OH?₂ improved continuously,and the sediments were the mixture of ACP and Mg?OH?₂ when pH up to 12.0.As to morphology,when pH came to 8.0-9.0,struvite showed a regular crystal form,short and thick.The shape became to longer and thinner when pH was 9.5-11.0.The amorphous sediments including ACP and Mg?OH?₂ were increasing at pH above 11.0.With the rise of Mg in the solution,the product of struvite was improved in the precipitate,whereas ACP was reduced under the condition of pH 8.5,Mg/P at 0.1-2.0.Excessive Mg dosage wasn't in favor of forming struvite when Mg/P>2.0.With the increase of Mg in the solution,the condition of pH 8.5 had the same history of sediment concentration as pH 9.5.When Ca/P was 0.1-1.0,the product of struvite increased at pH 8.5 that means low concentration of Ca in the solution was in favor of crystallizing struvite.A sharp decline in the amount of struvite was detected when Ca/P increased from 1.0 to 4.0 under the condition of pH 8.0 and 9.5,which means excessive Ca concentration inhibited struvite crystallization.?3?The influences of Ca on struvite crystallization and particles sizes were conducted in synthetic swine wastewater.Changes in Ca ions concentrations had great effect on particles sizes at different supersaturation.At metastable state,the particles were growing with the increase of time.Conversely,the particles sizes declined at the beginning and came to stable gradually at supersaturation.The sizer of d_0.5 achieved to 60.94mm and 61.39mm when 0.01 mmol/L Ca or0.4 g/L quartz was added,while 13.31mm and 15.85mm were detected at supersaturation,that was much smaller.At metastable state,when Ca<0.50 mmol/L,with the increase of Ca concentration,the particles sizes decreased,while the particles were detected bigger when Ca>0.50 mmol/L due to agglomeration.As to supersaturation state,the effect of agglomeration was weaker,leading to smaller particles.There has been a significant difference in the way of crystals growth between metastable state and supersaturation.Robust crystals with the form of single patten accompanied by clusters were observed at metastable state.Whereas at supersaturation,polycrystalline crystals shaped as X or branch grown separately were observed.As for crystal morphology,when reaction solution contained 0.01-0.50 mmol/L Ca,the struvite crystals had prismatic form with smooth surface.With the increase of Ca concentration,struvite crystals began to be attached tiny particles of ACP.As Ca concentration grown up to 3.00mmol/L,the crystal form couldn't be identified clearly because of gathering great amount of ACP.At supersaturation,needle-like struvites were observed.As to be attached with tiny particles on the struvite faces,the history was the same as metastable state.?4?A pilot-scale struvite crystallization reactor was operated.Results showed that controlling hydraulic retention time?HRT?with 1 h as well as metstable state,phosphorus and nitrogen removal efficiency in swine wastewater could be achieved to 95%and 24%respectively.The particles sizes observed at metastable state condition were much larger than supersaturation.Under the four operating conditions,equilateral triangle-like struvite crystals were observed.