Phosphate Removal By Ferrous Sulfate From Wastewater

Phosphorus is an important pollutant to be controlled in environmental water-bodydue to that its over-discharge may result in eutrophication. Generally, phosphorus indomestic wastewater is removed by biological or chemical ways. However, it has beennoted that enhanced biological phosphorus removal （EBPR） is difficult to achieved insome wastewater treatment process, and its treatment efficiency is unstable, whichsignificantly affected by numerous operational parameters. Since that chemicalprecipitation method is capable of removing phosphorus with an excellent efficiencystably, it become increasedly attractive in recent years. Notwithstanding, the cost ofchemical precipitation is much higher than those of EBPR, and many efforts have to bedone in terms of operational optimization and chemical dosage saving. According toprevious reports, ferrous sulfate, one kind cheap chemicals, has high capability ofphosphorus removal, while the cost can be dramatically reduced. Therefore, in thepresent work, comprehensive and comparative study on the cost-effectiveness ofphosphorus removal by FeSO₄precipitation was conducted, in order to enhance TPremoval efficiency in BAF-based system, as well as to reduce the cost. The correlatedinfluential factors on TP removal were investigated, by which the process wasoptimized, even at a condition of low temperature and pH. Finally, the real domesticwastewater copper-containing industrial wastewater were selected to examine andverify the TP removal performance by means of FeSO₄precipitation.First of all, the TP removal effectiveness by FeSO₄and its determining affectingfactors were evaluated in a series of batch-tests treating synthetic wastewater. Theexperimental results shown that the dissolved oxygen （DO） concentration in wastewaterhas positive effect on TP removal efficiency, and one suitable TP removal way seems assimultaneous ferrous oxidation and phosphorus removal. The optimized pH for TPremoval is8.0. Increasing of temperature and initial ferrous concentration would befavorable for TP removal. Besides, TP removal efficiency could be enhance in a certainlevel by hydrogen peroxide addition.It is found that co-existing metallic ions in bulk solution, such as Cu²⁺, Mn²⁺, Zn²⁺and Fe（OH）3, displayed apparently positive effect on TP removal by means of FeSO₄precipitation, while NH4+-N and MnO2only have slightly influence. Moreover, someco-existing anions including NO₃^-, SO₄^2-and Cl-have definitely inhibiting function onTP removal efficiency. Humic acid would also affect TP removal in different level, asthat low humic acid concentration improve the TP removal, and continuously increaseof humic acid concentration become to constrain the TP removal. In addition, carbon source in wastewater, especially including glucose and starch, shown limited impactonto TP removal by FeSO₄precipitation.Considering the rather low TP removal efficiency in a low temperature （10℃） andlow pH （6.0） circumstance, enhanced methods for TP removal were carried out. It isrevealed that the phosphorus removal efficiency could be improved up to46.6%,46.2%and43.5%, respectively, by means of pH increasing, Cu²⁺addition and H₂O₂dosing, astreating synthetic wastewater below10℃. Moreover, by adding Cu²⁺and H₂O₂couldincrease the TP removal efficiency by34.4%and38.0%respectively, when treating thewastewater with a pH of6.0and temperature of10℃.Finally, the effectiveness of phosphorus removal by ferrous ion was evaluated andcorrelated enhanced method was proposed in the actual wastewater treatment. Whentreating wastewater with a low pH and low temperature, increasing dissolved oxygen,adjusting the pH and addition of H₂O₂were all proved as effective methods, resulting ina stable high TP removal. However, comprehensive considering the cost-effectivenessand removal stability, H₂O₂dosing seems as the best way for TP removal enhancement,as the TP concentration in the effluent can be stably kept as low as0.5mg/L. In the caseof TP removal from the copper-containing plating wastewater with a low pH, comparedwith the conventional treatment techniques, TP removal by means of Ca（OH）₂andFeSO₄can make best use of contained Cu²⁺to enhance ferrous oxidation, by which theTP and pH in the effluent can be well meet the wastewater discharge standards, andreduce the cost of phosphorus removal from wastewater to0.0618yuan/t.