| The supernatant of reject water in wastewater treatments plants(WWTPs) of biological nutrient removal(BNR) has the characteristics of small water volume and high concentrations of phosphorus(P) and nitrogen(N). If this liquor is returned to the headworks, it would increase the loading of N and P, which always leads to unstable discharge of P and N of WWTPs. On the other side, P resource will be expected to deplete since P resource is non-renewable and the exploitation of reserved P rock is increasing. Thus removal and recovery of P and N from reject water is important for conservation of P resource and the discharge the stability of P and N in WWTPs.In this study, a crystallization reactor to recover phosphorus and nitrogen by struvite(MgNH4PO4·6H2O, MAP) was designed and operated firstly. Then a moving-bed biofilm reactor(MBBR) was run to remove the residual nitrogen further. Finally, the effect of struvie on mazie growth and soil properties was studied based on the pot experiment, where the soil properties and crop growth were measured. The conclusions were as the follows:(1) The removal ratios of phosphorus were 78.4% and 84.5% respectively when the ratio of reaction zone and settling zone were 1:4.2 and 1:3.2, both under a gas-water ratio of 30:1 and HRT of 5h during the start-up of the MAP crystallization reactor. The removal ratio of phosphorus were respectively 90.1% and 86.9% when the HRT were 12.5h and 5h, both under a gas-water ratio of 30:1 and volume ratio of 1:3.2. The removal ratio of phosphorus were respectively 73%, 82% and 87% respectively when the gas-water ratio were 15:1, 30:1 and 60:1, both under a volume ratio of 1:3.2 and HRT of 5h. In addition, there was a recognizable difference between the crystals formed at different parts of the reactor, and the discharge ratio of crystal has influence on the size of crystal.(2) The MBBR reactor was operated in the continuous-flow mode with an air bubble diffusers(Qair=18L/h) mounted at the bottom of the reactor to aerate and mix, HRT of MBBR was from 160 h to 33 h gradually to culture nitrobacteria with synthetic influent of 180 mg NH4+-N /L. After 28 days’ culture, this effluent was used to treat practical reject water. The removal percentage of NH4+-N reached to 61% and the concentration ratio of NH4+-N and NO2--N was 1:1.35, suitable to be the feedwater of an anaerobic ammonia oxidation.(3) A combined reactor, including MAP crystallization reactor and MBBR together, was used to remove and recover the nutrient of reject water. The oxygen supply of MBBR was only from the outlet of the MAP crystallization reactor. The recovery percentage of PO43--P was 83.8-89.2% and the removal percentage of NH4+-N reached 64.7%, among which 14.7% was recoveried as MgNH4PO4·6H2O in MAP crystallization reactor, and 50% was removed in MBBR. The concentration ratio of NH4+-N and NO2--N was 1:1, which was suitable to the feedwater conditions of anaerobic ammonia oxidation. Meanwhile, 25% COD and 77% SS were removed in the combined reactor.(4) 1000-grain weight of maize under struvite fertilizerd was slightly higher than that under commercial mineral fertilizer in the harvest time. Soil nutrients of available P, available N under struvite fertilizerd maintained a high level during the later stage of maize growth. The activity of urease, phosphatase, catalase and invertase activity reached the lowest value in early growth stage, and reached the highest value in mid stage of maize growth. |
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