Ferrous Sulphate in Water Treatment: What It Does and When It Makes Sense

The Dual Role of Iron(II) in Water Chemistry

Unlike ferric salts (such as ferric chloride or ferric sulphate) which arrive already in the trivalent oxidation state, ferrous sulphate introduces divalent iron (Fe²⁺). In raw water clarification, Fe²⁺ alone does not form immediate flocs at neutral pH. It must first undergo oxidation to Fe³⁺, which then hydrolyzes rapidly into dense, gelatinous amorphous ferric hydroxide flocs (Fe(OH)₃):

4Fe²⁺ + O₂ + 10H₂O → 4Fe(OH)₃↓ + 8H⁺

When properly controlled, dosing Ferrous Sulphate Heptahydrate 98% Min creates heavy, fast-settling flocs that settle up to 25% faster than conventional aluminium sulfate flocs.

Alkalinity Consumption and pH Control

As demonstrated in the hydrolysis equation, the oxidation and precipitation of ferrous iron releases hydrogen ions (H⁺), consuming natural bicarbonate alkalinity in the raw water. For every 1 mg/L of FeSO₄·7H₂O dosed, approximately 0.36 mg/L of alkalinity (as CaCO₃) is consumed. In soft, low-alkalinity surface waters (such as highland streams in the Aberdares or Mount Kenya catchments), co-dosing with hydrated lime (calcium hydroxide) or soda ash is essential to keep the flocculation pH between 7.5 and 8.5.

Phosphorus Removal via Ferrous Precipitation

In municipal wastewater lagoons and effluent polishing basins across East Africa, phosphorus is a strictly regulated parameter under NEMA guidelines to prevent eutrophication in Lake Victoria, Lake Naivasha, and the Athi River basin. Divalent iron reacts with orthophosphate to form vivianite-type insoluble salts (Fe₃(PO₄)₂·8H₂O), which settle readily into primary or secondary sludge.

The Golden Rule: Dosing Must Be Established by Jar Testing

There is no single universal dosing rate for ferrous sulphate. Over-dosing leaves unoxidized dissolved Fe²⁺ in the clarifier overflow, which oxidizes downstream in distribution pipes to produce reddish-brown tap water and customer complaints. Under-dosing leaves colloidal turbidity in suspension. Operators must execute standardized 6-paddle jar tests, varying coagulant dose, aeration rate, and pH buffer, before establishing automated dosing setpoints.

Learn about treating complex effluent streams: Ferrous Sulphate for Wastewater & Industrial Effluent.

Understand the reaction mechanisms of nutrient removal: Ferrous Sulphate and Phosphate Removal Chemistry.