Recovery Of Ammonia In Anaerobically Digested Food-waste Slurry By Struvite Crystallization Jointly Combined With Biological Coagulation Dewatering

Restaurant food wastes commonly known as hogwash,in a narrow sense,generally refers to the food residue generated from food processing and consumption.According to the statistics,about 99 million tons of restaurant food waste was produced in China in 201 7.Restaurant food waste is extremely perishable and smelly due to its high water content and high concentration of organic matter.If not treated timely or improperly,it will cause serious environmental pollution and threaten human health.At present,anaerobic digestion is the most efficient method for the treatment of restaurant food wastes and is relatively mature.However,anaerobically digested slurry from restaurant food wastes(hereinafter referred to as "anaerobically digested food-waste slurry")has high ammonia nitrogen,high chemical oxygen demand(COD),high suspended solid(SS)contents and low C/N ratio,those make it extremely difficult to meet the discharge standards by the subsequent conventional bioschemical treatment,especially ammonia nitrogen.Therefore,it is key step to remove or recycle ammonia nitrogen in anaerobically digested food-waste slurry in advance to easily meet the discharge standards.Struvite crystallization is the method that NH4+ and PO43-are removed by precipitating as struvite crystal(MgNH4PO4·6H2O).It has the characteristics of simple operation and rapid reaction,and is especially suitable for treating high ammonia nitrogen wastewater.Biological coagulation dewatering can improve the dewaterability of the digested slurry,and further treated by the slag remover to remove almost all of the SS.Therefore,this article takes the anaerobically digested food-waste slurry(ammonia nitrogen 3382 mg·L-1,COD 20153 mg·L-1,total phosphorus 476.7 mg·L-1,TS 32031 mg·L-1,SS 12250 mg·L-1)from the food-waste disposal center of Shijiazhuang after high temperature anaerobic digestion as the object,and the effect and mechanism of removal and recovery of ammonia nitrogen by struvite crystallization combined with biological coagulation dewatering was studied.The conclusions are as follows:(1)Under the optimal conditions including reaction temperature of 28℃,initial pH of 9.0,the reaction time of 90 min,and the mole ratio of magnesium,phosphorus and nitrogen with 1.43:1.3:1,the recovery rate of ammonia nitrogen removal could reach 98%,and the residual ammonia nitrogen and water soluble phosphorus contents were 48.74 and 35.35 mg·L-1,respectively.The COD/TN ratio of the slurry was enhanced by about 8 times that increased from 5.38 before treatment to 43.96 after treatment,which was beneficial to the subsequent biochemical treatment.The main component of precipitate was struvite with a purity of 73.82%.EDS data showed that the elements of the precipitate were carbon,magnesium,phosphorus,potassium and calcium.The precipitate had abundant inorganic salts,which could be used as high quality slow-release fertilizer.(2)It was found that there was a significant negative correlation between the content of SS and the recovery rate of ammonia nitrogen removal in simulated ammonia nitrogen wastewater with the addition of SS(extracted from anaerobically digested food-waste slurry):as the content of SS increased,the recovery rate of ammonia nitrogen removal in simulated ammonia nitrogen wastewater decreased gradually.PO43-was easily adsorbed by SS(the maximum adsorption amount can reach 35.79 mg·L-1).The decrease of available PO43-content from external sources is one of the reasons for the decrease of the removal rate of ammonia nitrogen.In addition,the dissolved organic matter(DOM)in anaerobically digested food-waste slurry also had a obvious negative impact on the treatment of ammonia nitrogen by struvite crystallization.In the supernatant of biogas slurry(without SS),ammonia nitrogen removal rate(84.70%)was 8.94%higher than that in the original biogas slurry containing both SS and DOM(75.76%),but it was still 4.95%lower than that in the simulated wastewater without SS and DOM(89.65%).Obviously,SS had a greater influence on the nitrogen removal efficiency of struvite crystallization than DOM.Therefore,the removal efficiency of ammonia nitrogen could be improved by removing SS in advance,when using struvite crystallization to treat ammonia nitrogen in anaerobically digested food-waste slurry.(3)Biological coagulation dewatering through improving the particle coagulating performance and degrading the protein and polysaccharide components in extracellular polymer(EPS),could improve the dewatering performance of anaerobically digested food-waste slurry.Then SS would be removed by the small slag remover in anaerobically digested food-waste slurry.It was found that the pH value and ammonia nitrogen content of pressure filtrate was significantly reduced by aeration treatment,but the residual ammonia nitrogen still reached 929.4 mg·L-1.Although the content of ammonia nitrogen in the pressure filtrate was reduced by struvite crystallization,the composition of the pressure filtrate was changed.More phosphate needed to be added to effectively improve the removal rate of ammonia nitrogen due to the addition of biological coagulant previously and the recovered struvite is very impure.Under the same treatment conditions(constant pH of 9.0 and the mole ratio of magnesium,phosphorus and nitrogn with 1:1:1),the purity of struvite obtained from the anaerobically digested food-waste slurry(77.74%)was significantly higher than that obtained from the pressure filtrate(58.26%).Therefore,it had a higher cost to remove and recover the nitrogen via struvite crystallization,when the ss removed by biological coagulation dewatering firstly in the anaerobically digested food-waste slurry.(4)The mixed liquid(contains struvite)of anaerobically digested food-waste slurry pretreated by struvite crystallization could be separated by gravity.The sedimentation rate could reach 70%at 30 min and 80%at 3 hours.After solid-liquid separation,the solid phase could be directly dehydrated by a slag remover,then the struvite could be recovered and used as a high-quality slow-release fertilizer.The liquid phase could be further treated by biological coagulation dewatering to obtain clear wastewater with low nitrogen and high C/N ratio(ammonia nitrogen 46.33 mg·L-1,COD/TN 27.62),and it is easy to meet the standard of subsequent biochemical treatment.In conclusion,struvite crystallization jointly combined with biological coagulation dewatering can not only effectively remove ammonia nitrogen from anaerobically digested food-waste slurry but also efficiently dewater and remove slag.Moreover,the generated struvite has abundant nutrients and thus can be used as high quality slow-release fertilizer after recycling.