| Total pollutant emissions from domestic wastewater and industrial wastewater gradually increased with the accelerating modernization process, In order to improve the ugly water environment, our government began to implement much stricter standards for the wastewater discharge, especially for the phosphate discharge. Based on the growth of the total phosphate emissions and the new discharge standard, it is important to find some methods to enhance phosphate removal with high efficiency and low cost.In this study, ferrous iron was chosen to be the chemical agent for the phosphate removal due to its obvious cost advantage. Effect of Fe(II) oxidation on the phosphate removal was firstly investigated, and the advantages as well as mechanisms of phosphate removal during Fe(II) oxidation was deeply explored. Based on the mechanisms, effects of oxidant, such as H2O2, DO and NO3-, on enhancing phosphate removal were assessed with comparing the performance of phosphate removal by using different oxidant and analyzing their advantages as well as disadvantages of each oxidant. DO and NO3- exhibited relative poor performance on enhancing phosphate removal during Fe(II) oxidation, hence methods of using metal ions as catalyst and using anoxic activated sludge as inducible factor were proposed to improve the process of phosphate removal. For the proposed methods, reaction conditions were optimized and the pathway for each method was also analyzed after the mechanism deeply being explored through characterizing the reaction products, which would provide the theoretical and technical foundation for the practical application of phosphate removal using Fe(II).In this study, the relationship between the Fe(II) oxidation process and phosphate removal efficiency was firstly assessed with investigating the effects of Fe(II) oxidation, oxidation order and oxidation rate on the phosphate removal. The obtained results indicated that simultaneously oxidizing Fe(II) sharply increased the phosphate removal rate from 4.7 to 44.3%. Based on changes of the precipitation characteristics and the ratio of phosphate ferrous, phosphate ferric and the ferric oxyhydroxide under varied p H, the pathway of the phosphate removal during Fe(II) oxidation was analyzed, and the mechanism of phosphate removal using in-situ Fe(III) was proposed.Based on the theory of the phosphate removal during Fe(II) oxidation, Effects of kinds of oxidants, such as H2O2, DO and NO3-, on the phosphate removal by Fe(II) were investigated. Results obtained from the experiments showed that the oxidants accelerating the phosphate removal by Fe(II) followed the sequence, H2O2> DO> NO3-. Dosing H2O2 showed better performance on enhancing phosphate removal, but the dosage was too expensive to widely application; though DO and NO3- showed insufficient performance on promoting phosphate removal, exhibited obvious cost advantages; therefore methods used to strengthen the process of phosphate removal using Fe(II)/ DO and Fe(II)/ NO3- should be explored. Considering the characteristics of phosphate removal using Fe(II) oxidized by DO, metal ions were selected to catalyze the process through accelerating the Fe(II) to be in-situ Fe(III), and then enhance phosphate removal. Results obtained demonstrated that Cu(II) was the optimal catalyst in all the selected ions, when the [PO43-]0 was 50 mg/L, [Cu2+]0 was 10 mg/L and Fe/P molar ratio was 2.4, removal rate of phosphate and Cu(II) achieved to 90.6 and 70.6%, respectively. Precipitation obtained from the process was analyzed and the results showed that the main products were Fe PO4 precipitation and Cu3Fe4(PO4)6 predecessor, in which Cu(II) not merely enhanced the phosphate removal by Fe(II)/DO but also be removed through co-precipitation. Combined the performance of phosphate removal and analysis result of phosphate products, the pathway of phosphate removal using Fe(II)/DO enhanced by Cu(II) was assessed, and the results showed that phosphate removal route mainly included chemical precipitation by single Fe(II) or Cu(II), adsorption by iron carbonyl, and precipitation using in-situ Fe(II) induced by Cu(II) which was the most important way enhancing the phosphate removal. The mechanism of phosphate removal using Fe(II) induced by Cu(II) was taking advantage of poor iron hydrolization and strength combination of in-situ Fe(III) with phosphate.Considering the characteristics of phosphate removal using Fe(II) oxidized by NO3-, anoxic activated sludge was selected to enhance the process through Fe(II) oxidation induction and synergy biological flocculation. Dosing Fe(II) ranging from 5 to 25 mg/L, the averaged removal rate of SP increased 29-82 % due to the presence of anoxic activated sludge. Under the condition with anoxic activated sludge, the systems of Fe(II)/NO3- and Fe(III)/NO3- were compared and the results showed that the former one exhibited better performance on nitrogen and phosphate removal, and also better sludge settleability and dewaterability. The mechanism of the enhcance phospahte removal by using Fe(II)/NO3- in the anoxic active sludge environment was that, compared with the additional Fe(III), the in-situ Fe(III) generated by the Fe(II) oxidation was poor in hydrolization and better in conbining with phosphate, thus resulting in higher iron utilization and then the enhanced phosphate removal.Based on the advantages of Fe(II)/NO3- with anoxic activated sludge system, a modified process employing simultaneous pre-anoxic activated sludge system and post-BAF was proposed. A series experiments was carried out to optimize the system by adjusting the Fe(II) dosage. Long-term dynamic operation results obtained demonstrated that when the Fe(II)/P molar ratio ranged from 2.0-2.2:1, the phosphate removal was sharply intensified and the residual TP and SP in effluent decreased to 0.37 and 0.31 mg/L, respectively; wilst the biological denitrification was also be enhanced and the nitrate removal rate increased from 70.6 to 91.5%; furthermore, the anoxic activated sludge settleability was significantly improved and the SVI value sharply decreased from 494 to 47 m L/g. Based on the optimization performance of the modified process, the pathway of phosphate removal using Fe(II)/NO3- enhanced by anoxic activated sludge was analyzed in detail and the results indicated that the phosphate removal ro ute mainly included the adsorption by single anoxic activated sludge or iron carbonyl, chemical precipitation using Fe(II) cooperating with biological flocculation, and chemical precipitation using in-situ Fe(III) generated from Fe(II) oxidation induced by anoxic activated sludge. |
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