Backwash water treatment by coagulation in the presence of phosphates at underground water iron removal stations

Introduction. The analysis of the present-day condition of water resources has proven the relevance and expediency of developing highly effective backwash water treatment methods to be used at iron removal stations designated for groundwater treatment. In accordance with effective technical regulations, backwash water must be reused. The operation of iron removal stations has proven the inefficiency of backwash water treatment facilities. Water and wastewater treatment companies stop using backwash water treatment facilities and refrain from reusing backwash water. Highly concentrated iron-bearing backwash water is discharged into sewage networks, nearby water bodies or onto the terrain, which means irrational use of high-quality groundwater and environmental pollution with iron compounds. Materials and methods. The results of experimental research efforts and statistical processing of data on the qualitative and quantitative composition of backwash water at iron removal stations are presented. The chemical nature of the components and the principle underlying the formation of the backwash water composition in the process of groundwater deferrization have been studied. It’s been identified that if backwash water supplied by iron removal stations is treated by sodium phosphate reagent Na3PO4 and aluminum sulphate Al2(SO4)3 as a coagulant, precipitation of iron compounds intensifies, as colloidal particles FePO4 are formed. They have very low solubility, and they are effectively removed by coagulation. It has been theoretically proven and experimentally confirmed that anions H2 PO4– and PO4 3– fformed in the process of hydrolysis of sodium phosphate Na3PO4 help to reduce the electrokinetic charge of the colloidal particle of iron hydroxide Fe(OH)3, and high purification efficiency reaching 99.0–99.9 % is attained by attaching iron compounds to the surface of the colloidal particle of aluminum hydroxide Al(OH)3. Conclusions. The co-authors have developed a math-and-stats model simulating the backwash water treatment process that employs coagulation in the presence of phosphates. It describes the dependence between the concentration of residual iron, doses of sodium phosphate Na3PO4, aluminum sulphate Al2(SO4)3 and the settling time. A backwash water treatment technology has been developed. It employs coagulation in the presence of phosphates, and it is designated for use at iron removal stations. This technology comprises a chemical plant for sodium phosphate and aluminum sulphate used as a coagulant, a post-treatment filter, and sludge dewatering facilities. The application of this technology enables to reduce iron concentration to 0.05–0.20 mg/l, to reuse backwash water for drinking and other household purposes, or to have this water reused by iron removal stations, this, preventing pollution of water sources with iron compounds.