| Biological nitrogen and phosphorous removal process are widely used to remove nutrients from municipal wastewater in China. The total phosphorous in the effluent in the most plants can not meet the national urban sewage treatment plant pollutant discharge standard GB18918-2002 the level of A standard, especially in summer and in the case of water quality variation, usually using chemical phosphorus removal process to enhance and supplement. The chemical precipitation of phosphorus removal effect is closely related to phosphorous forms in water. This study in Xi ’an third, fourth and fifth sewage treatment plant of biological nitrogen and phosphorous removal system as the object, the secondary processing out of the existing form of phosphorus in water in theoretical analysis and field measurement, based on this, in the third, fourth and fifth secondary treatment effluent of sewage treatment plant by using PAC and Al2(SO4)3 chemical precipitation and coagulation experiment was carried out, and explore the corresponding phosphorus removal effect. Basically have the following conclusion:(1) The secondary treatment effluent of municipal sewage treatment plant phosphorus exist in various forms, and the classification of the phosphorus in water quality control engineering method can not better express the composition of phosphorus, the test according to the chemical seen on qualitative classification, the city sewage treatment plant of total phosphorus in the secondary treatment effluent, divided into suspended phosphorus(particle diameter > 0.45 um), colloidal phosphorus(particle diameter 0.45 um ~ 1 nm) and dissolved phosphorus(particle diameter < 1 nm), which can more accurately reflect the comprehensive secondary treatment effluent of the existence form of phosphorus(2) Through to the Xi ’an third, fourth, fifth sewage treatment plant secondary treatment effluent in the form of various phosphorus determination results analysis, it is concluded that the form of phosphorus in Xi ’an fourth and fifth sewage treatment plant secondary treatment effluent mainly existed in dissolved phosphorus, suspended phosphorus, colloidal phosphorus and dissolved phosphorus respectively accounted for 5% ~ 15%, 15% ~ 25%, 60% ~ 70% and 12.5%~20%,12.5%~20%,50%~75%of the total phosphorus; The concentration of suspended phosphorus and colloidal phosphorus of Xi ’an third sewage treatment plant secondary treatment effluent closed to and respectively accounted for 20% ~ 27% and 15% ~ 30% of the total phosphorus. According to different forms of the phosphorus, the removal of countermeasures will also be different.(3) On the basis of phosphorus chemical, the morphology of phosphate is different under different p H conditions, mainly including H3PO4、H2PO4-、HPO42- and PO43- four existing forms. As the secondary treatment of urban sewage treatment plant effluent’ p H is between 6 and 9, through the theoretical calculation, the phosphate in the water mainly exist in the forms of H2PO4- and HPO42-, not the conventionally known form of PO43-; so in the process of chemical deposition dephosphorization, mainly H2PO4-and HPO42- react with Al3+ to form Al PO4 precipitation, so as to reduce the content of phosphorus.(4) Through beaker experiment, compared the phosphorus removal effect of polyaluminium chloride and aluminum sulfate. When the total phosphorus concentration was 0.94 mg'L-1, and the dosage concentrations of PAC and Al2(SO4) 3 were respectively 4 mg'L-1and 6 mg'L-1(Al), after treatment the total phosphorus concentration became less than 0.5 mg'L-1, which can meet the A level of national urban sewage treatment plant pollutant discharge standard(GB18918-2002). The PAC is better than Al2(SO4)3 in treatment effect... |
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