Stable Operation Of Phosphorus Recovery Processes For Wastewater With Low Phosphorus Concentration Based On Induced Crystallization

Phosphorus is an important non-renewable resource and one of the main pollutants in water environment.Phosphorus recovery from sewage can not only improve water environment quality,but also alleviate the global phosphorus resource crisis.The tail water of urban sewage treatment plant has a significant characteristic of low phosphorus.In the early stage,the recovery efficiency of tail water of urban sewage treatment plant is more than 80%by using fluid-bed calcium hydroxyphosphate(HAP)induced crystallization,but the recovery rate decreases rapidly with the extension of operation time.Therefore,two types of crystallization reactor,fully mixed and circulating fluidized bed,were proposed to study the improvement effect of the two types on the stable operation of HAP-induced crystalline phosphorus recovery.In this paper,the single factor pilot scale experiment of phosphorus recovery induced by fluidized bed was carried out to explore the factors leading to the reduction of phosphorus recovery efficiency.On this basis,combined with the beaker test results,the fully mixed and circulating fluidized bed reactors were optimized,and the pilot study of phosphorus recovery was carried out respectively.Finally,the mechanism of phosphorus recovery was preliminarily discussed based on the particle size distribution and characterization results of crystallization products.The main conclusions are as follows:(1)Conventional fluidized bed induced crystallization recovery of phosphorus from waste water is poor in stable operation,and the phosphorus recovery rate is reduced to less than 30%after 8 h.The reasons for the rapid reduction of phosphorus recovery include seed loss,settling,insufficient crystallization reaction time,and difficulty in ensuring the stirring intensity.Conventional fluidized bed operations cannot solve these problems simultaneously.(2)By adding mechanical stirring unit and optimizing crystallization reaction time,the stable operation effect of induced crystallization phosphorus recovery was effectively improved in the completely mixed reactor.Under experimental conditions,the phosphorus recovery rate was still about 90%after 72 h operation of the completely mixed reactor.(3)The circulating fluidized bed reactor can overcome the problem that conventional fluidized bed agitation intensity and hydraulic residence time cannot be guaranteed at the same time by setting up bypass reflux pipeline,and it is more energy saving than complete mixed reactor.Under the conditions of hydraulic retention time of 30 min,rising flow rate of 40 m/h and seed dosage of 30 g/L,the crystallization reaction time and stirring intensity were guaranteed,and the stable operation of the device was realized.The phosphorus recovery rate remained above 97%after 72 h operation.(4)SEM,EDS,XRD,FTIR and TG analysis results showed that HAP was the main component in the original seed and crystallization products,and there were also formation of CaCO3,ACP,OCP and other HAP precursors in the crystallization products.Carbonate in raw water can inhibit HAP-induced low phosphorus crystallization,which is one of the reasons for the decrease of phosphorus recovery efficiency.The results of this paper can provide theoretical basis and technical support for the depth dephosphorization and upgrading of secondary effluent.