Study On Degradation Of Residual Sludge And Separation And Recovery Of Phosphorus By Ultrasonic-Bioelectrochemical System

With the rapid expansion of the number and scale of urban sewage plants,the output of activated sludge,a by-product of the main sewage treatment process,is increasing day by day.Therefore,the treatment and disposal of residual sludge has always been the focus.Phosphorus is a non-renewable mineral resource,and the utilization process has a one-way flow.Therefore,the recycling and utilization of phosphorus is also the focus of the society.In this paper,the sludge was processed through the ultrasonic-biological electrochemical system,and then generated struvite to recycle nitrogen and phosphorus in the sludge.Firstly,the excess sludge was ultrasonically cracked to release the cellular contents,and then placed in the bioelectrochemical system,and the released organic matter was used for biodegradation to generate electricity,so as to provide the driving force for anion and cation in the sludge liquid to pass through the ion exchange membrane migration into the enrichment liquid.At the same time under the anaerobic environment of system anode,the sludge further released nitrogen and phosphorus,and finally the sodium chloride recovery solution enriched in nitrogen and phosphorus was immobilized crystal precipitation through magnesium ammonium phosphate（MAP）crystallization method.In this paper,ultrasonic,anaerobic hydrolysis,bioelectrochemical system and MAP method were combined organically.The parameters optimization of sludge ultrasonic pretreatment and sludge hydrolysis after ultrasonication,the nitrogen and phosphorus recovery efficiency of the bioelectrochemical system,and the influencing factors and efficacy of nitrogen and phosphorus enrichment by magnesium ammonium phosphate（MAP）crystallization method were studied.The results showed that the release amount of nitrogen and phosphorus in sludge was positively correlated with the solid content of sludge and ultrasonic conditions（time,power density）,etc.,the pulse ratio of ultrasonic had little influence on the release of nitrogen and phosphorus in sludge.The influence degree of ultrasonic condition on phosphorus release is ultrasonic time>sludge concentration>ultrasonic power density.Ultrasonic power density of 0.78 w/mL,ultrasonic time of 60 min and sludge concentration of 6 g/L are relatively optimal conditions for the release of nitrogen and phosphorus,and the release ratio of nitrogen and phosphorus after ultrasonic is far beyond the requirements of MAP.For the hydrolysis of the sludge after ultrasonication,under the same ultrasonic energy,the combination of low power and long time has better release effect on nitrogen and phosphorus than the combination of high power and short time.With the increase of ultrasonic energy,the release of nitrogen and phosphorus will increase.The successfully started bioelectrochemical system operated steadily for 120 h（120 h for one cycle）,and the effect of nitrogen and phosphorus enrichment was better than that of replacing the ultrasonic sludge centrifugal fluid every 12 h.In the four-chamber reaction system,5 g/L sodium chloride recovery solution has the best recovery effect of nitrogen and phosphorus.With different concentration,the phosphorus removal rate in the anode solution was above 65%,and the recovery rate of sodium chloride recovery solution was up to 91%.The removal rate of liquid nitrogen in the anode is between 50%and 60%,and the nitrogen recovery rate is 50%.In the five-chamber reaction system,whenr the sludge supernatant was used as the anode substrate for one cycle,the removal rate of phosphate and ammonia nitrogen in the anode was above 60%,and the concentration rate of sodium chloride recovery solution was 58.352 mg/L of phosphate and 63.943 mg/L of ammonia nitrogen,the recovery rate was above 70%.And the removal rate of total phosphorus and nitrogen was above 60%.When ultrasonic sludge was used as anodic fluid for experiments,it was found that after running for 240 h,there could concentrated 94.90 mg/L PO₄^3-,103.78 mg/L TP,69.86mg/L NH₄⁺,85.20 mg/L TN in the system.The optimal conditions for MAP:the initial concentration of phosphorus is 75mg/L,pH is 10,and mg:P is 1.8:1.Since the concentration of ammonia nitrogen in the system is much higher than the 4:1（nitrogen-phosphorus ratio）in the study,no additional nitrogen source is needed.Scanning electron microscope（SEM）and X-ray diffraction analysis showed that the product was struvite.In terms of drug wastage alone,it is profitable to produce struvite because there is no need to add a nitrogen source.In summary,the recovery of nitrogen and phosphorus in the sludge can make up for the limited phosphorus resources,improve the one-way utilization status,and bring economic,ecological and social benefits while achieving sustainable economic development.