To maintain stable operation of a reverse osmosis (RO) system, auxiliary chemicals are indispensable alongside well-designed membranes and process layouts. These include coagulants, scale inhibitors, biocides, reducing agents, pH adjusters, each with a designated dosing location and unique function. Improper dosing concentrations will trigger new operational issues. Below is a detailed summary of the functions and key dosing parameters for the five core chemical types.

1. Coagulants and Coagulant Aids

Coagulants

Function: Destabilize colloidal particles in raw water and aggregate them into large flocs for easy removal via subsequent filtration. Common varieties: Cationic inorganic agents, including polyaluminum chloride (PAC), polyferric sulfate, ferrous sulfate, aluminum sulfate, ferric trichloride, etc. Typical dosing concentration: 3–5 mg/L.

Coagulant Aids

When coagulants alone deliver unsatisfactory flocculation performance, coagulant aids are supplemented to accelerate floc formation, increase floc density and improve sedimentation efficiency.

• Organic coagulant aids: Polyacrylamide (PAM), bone glue, etc.
• Inorganic coagulant aids: Calcium oxide, calcium hydroxide, sodium carbonate, etc. Dosing location for both coagulants and coagulant aids: Upstream of sand filters.

2. Residual Chlorine and Reducing Agents

Oxidative biocides such as sodium hypochlorite leave free residual chlorine after disinfection of bacteria and algae. Sodium bisulfite (SBS) is dosed ahead of cartridge filters to eliminate residual chlorine before water enters the RO membrane. Empirical dosing ratio: 1.8–3.0 mg/L sodium bisulfite per 1.0 mg/L free residual chlorine in RO feedwater.

3. Scale Inhibitors & Dispersants

Two mainstream methods are adopted to prevent inorganic scaling on RO membranes: ion exchange softening or scale inhibitor dosing. Large-scale RO plants prefer scale inhibitors due to the high operating cost of softening, which only targets water hardness. Functions of scale inhibitors: Suppress precipitation of sparingly soluble salts including calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate, calcium fluoride and silica. Different scale inhibitors exhibit varying inhibition efficiency against different mineral scales. Typical dosing concentration: 2–6 mg/L.

• Insufficient dosage: Fails to deliver anti-scaling performance.
• Overdosing: Causes chemical fouling of membranes and accelerates fungal proliferation with no extra anti-scaling benefit.

4. Acid and Alkali Dosing (pH Regulators)

Acid Dosing (Primary RO Feedwater)

Calcium carbonate readily precipitates at high pH. Acid is injected into the feed stream of the first-stage RO to lower pH and prevent calcium carbonate scaling.

Alkali Dosing (Secondary RO Feedwater)

RO membranes barely retain dissolved carbon dioxide gas. The permeate from primary RO contains high CO₂ concentrations. While CO₂ does not raise total dissolved solids (TDS), it elevates conductivity and distorts the calculated salt rejection rate of the secondary RO based on conductivity readings. Alkaline agents are added to secondary RO feedwater to raise pH, converting dissolved CO₂ into ionic carbonate species that can be intercepted by RO membranes, thereby reducing carbon dioxide content in final permeate.

Core Principle of RO Chemical Dosing: Precise Concentration & Correct Dosing Point

Under-dosing leads to loss of treatment efficacy, while over-dosing results in membrane fouling. The dosing location and dosage of each chemical must be calibrated according to raw water quality and real-time system operating conditions.

Has your reverse osmosis system suffered from scaling or membrane oxidation caused by inaccurate chemical dosing? Feel free to share your operational experience in the comments section.