What are the commonly used chemical agents for phosphorus removal in wastewater treatment?

In the chemical phosphorus removal process for Wastewater Treatment, the core principle involves adding chemical reagents to the wastewater to react with phosphorus (primarily orthophosphate) and form insoluble phosphate precipitates. Subsequently, phosphorus is removed from the water through solid-liquid separation. Commonly used reagents are primarily categorized into two types: metal salts and calcium salts, as detailed below:

Ⅰ.Metal Salt Medications

This kind of chemical is the most commonly used chemical in sewage treatment, which has fast reaction speed and stable effect.

saline salts

Commonly used varieties: ferrous sulfate (green vitriol), ferric chloride, polyferric sulfate (Pfs), polyferric chloride (PFC).

Mechanism of action: Iron ions (Fe²⁺ or Fe³⁺) combine with PO₄³⁻ in wastewater to form iron phosphate precipitate. Simultaneously, the iron hydroxide colloid generated from iron salt hydrolysis exhibits adsorption properties, capable of capturing partial phosphorus and suspended solids in the wastewater.

Characteristics: The coagulation efficiency of polyferric salts is superior to that of conventional iron salts, with excellent sludge settling performance; ferrous sulfate is cost-effective but requires oxidation to Fe³⁺ to achieve optimal phosphorus removal, often necessitating aeration.

salts of aluminum

Commonly used varieties: aluminum sulfate, alum, polyaluminum chloride (PAC), polyaluminum sulfate (PAS).

Mechanism of action: Aluminum ions (Al³⁺) react with PO₄³⁻ to form aluminum phosphate precipitate; the hydrolyzed aluminum hydroxide colloid can also adsorb phosphorus and impurities.

Characteristics: High phosphorus removal efficiency, adaptable to a wide pH range (5.5~7.5); the corrosiveness of polyaluminum chloride is lower than that of ordinary aluminum chloride, requiring less dosage and resulting in relatively lower sludge production.

Ⅱ.Calcium Salt-Based Medications

This type of chemical primarily removes phosphorus by forming calcium phosphate precipitates, while also increasing wastewater alkalinity, making it commonly used in alkaline wastewater treatment.

Commonly used varieties: lime (calcium hydroxide), calcium chloride.

Mechanism of action: Calcium ions (Ca²⁺) react with phosphate ions (PO₄³⁻) in an alkaline environment (pH>10) to form calcium hydroxyphosphate precipitate Ca₅(PO₄)OH.

Characteristics: Lime is inexpensive and can neutralize acidic wastewater while removing phosphorus and certain heavy metals. However, it requires a large dosage, which may lead to an increase in effluent pH, potentially necessitating subsequent pH adjustment, and generates significant sludge.

Ⅲ.Auxiliary Phosphorus Removal Reagents

In certain cases, flocculants such as polyacrylamide (PAM) are used in combination to enhance sedimentation efficiency. PAM itself does not react with phosphorus but can adsorb bridging phosphate precipitate particles, thereby increasing the size of flocs, accelerating their settling velocity, and improving solid-liquid separation efficiency.