In water treatment processes, chemical dosing is a key step to ensure qualified water quality and stable system operation. Whether for municipal sewage, industrial wastewater or reclaimed water reuse systems, accurate chemical selection and standard dosing are essential skills for water‑treatment operation‑maintenance and process commissioning.
Many water‑treatment practitioners and new‑comers tend to mix up the functions of different chemicals and misuse them under improper working conditions. A large number of misleading tips about water‑treatment chemicals can also be found online. Today we bring you the fully‑checked and revised guide covering 12 commonly‑used water‑treatment chemicals. Concise and to‑the‑point, this material covers core roles, main applications, applicable working conditions and common misunderstandings. It is ideal for daily reference, on‑the‑job training and document filing.
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01 Poly‑Aluminum Chloride (PAC)
Core Role: Inorganic Coagulant
Suitable for domestic sewage, municipal sewage and various industrial wastewater treatment. It rapidly destabilizes suspended particles, colloidal organics, algae and colloidal heavy metals in water, agglomerates them into dense alum flocs for sedimentation, and effectively reduces water turbidity and COD.
???? Important Note: It only works on suspended and colloidal pollutants, and has very poor removal performance for dissolved pollutants.
02 Polyacrylamide (PAM)
Core Role: High‑molecular Flocculant
Available in anionic, cationic and non‑ionic types. It shall not be dosed alone and must be used together with PAC. It quickly adsorbs tiny flocs to form large floc clusters, speeds up solid‑liquid separation and greatly improves water clarity.
???? Key Applications: Raw water purification and wastewater clarification. Cationic PAM is specially used for sludge dewatering. It can greatly lower sludge moisture content and improve the dryness of sludge cake.
03 Lime (Quicklime / Slaked Lime)
Core Role: Alkalinity Regulator, Coagulant Aid, Heavy‑metal Treatment Agent
A multi‑purpose basic chemical for water treatment. It adjusts water pH, assists coagulation‑sedimentation, breaks complex compounds, demulsifies and removes oil, precipitates heavy‑metal ions and reduces water hardness. It also provides auxiliary bacteriostatic effect.
???? Important Note: Large volume of sludge will be generated after dosing. It easily causes scaling on equipment and pipelines. Regular sludge disposal and anti‑scaling maintenance are required.
04 Caustic Soda Liquid (NaOH / Sodium Hydroxide)
Core Role: Strong‑alkali pH Regulator, Neutralizing Agent
Mainly used for acid‑base neutralization of wastewater and precise pH adjustment. It removes heavy‑metal ions via alkaline precipitation reaction. It is also applied for ion‑exchange resin regeneration and water hardness reduction.
???? Important Note: It is only for water quality adjustment and precipitation assistance. It has no flocculation or oxidation function.
05 Hydrochloric Acid (HCl)
Core Role: Strong‑acid pH Regulator, Cleaning Agent, Regenerant
Commonly used to neutralize alkaline wastewater and lower water pH. It cleans pipelines, membrane modules and water‑treatment equipment to dissolve scale and rust impurities. It is also a common regenerant for cation exchange resin.
???? Important Note: Highly corrosive. It is only for acidification, cleaning and resin regeneration and cannot remove heavy metals.
06 Sodium Hypochlorite (NaClO)
Core Role: Broad‑spectrum Oxidizing Disinfectant
A common disinfectant and oxidant for sewage systems. It powerfully kills bacteria and algae. Through breakpoint chlorination process, it oxidizes and decomposes toxic pollutants such as ammonia nitrogen and cyanide. It is widely used for disinfection of final sewage effluent, water decolorization and impurity removal.
???? Important Note: Its disinfection performance is heavily affected by pH and disinfection by‑products may form. Strictly control dosing dosage to prevent residual chlorine from oxidizing and damaging RO membrane elements.
07 Ferrous Sulfate
Core Role: Flocculant, Reducing Agent, Biochemical Nutrient
A multi‑functional ferrous salt chemical. It works as an inorganic flocculant for water purification and reduces high‑valent heavy metals. Meanwhile, it supplies iron for biochemical systems to meet nutritional demands for microorganism growth.
???? Important Note: It will lower water pH after dosing. Over‑dosing tends to cause yellowish effluent and excessive turbidity. Dosing dosage must be precisely controlled.
08 Sodium Metabisulfite / Sodium Bisulfite
Core Role: Special Reducing Agent for Chromium‑containing Wastewater
Standard chemical for industrial chromium‑containing wastewater treatment. Under acidic working conditions, it reduces toxic hexavalent chromium to low‑toxic trivalent chromium. Chromium ions can be fully removed with subsequent alkali‑adding precipitation. It also eliminates residual chlorine in water.
???? Important Note: Reduction effect only exists in acidic environment. It will completely lose chromium‑removal capacity when pH value is high.
09 Hydrogen Peroxide (H₂O₂)
Core Role: Strong Oxidant, Disinfection & Purification Agent
It has multiple functions including oxidative degradation, decolorization, deodorization and sterilization. It decomposes refractory organics, removes water color and odor, and supplements dissolved oxygen to support stable operation of biochemical systems.
???? Important Note: Its oxidation capacity is limited when used alone. For complex industrial wastewater, it is mostly compounded with ferrous salt to form Fenton advanced oxidation system.
10 Citric Acid
Core Role: Industrial Cleaning Agent, Weak‑acid Regulator
⚠️ Misconception Correction: Citric acid is NOT used for overall sterilization and disinfection in sewage systems!
Main applications: clean membrane modules, heat‑exchange equipment and pipelines; dissolve scale and metal oxides. It can only slightly adjust pH with weak bacteriostatic property and has no system disinfection function.
11 Glucose
Core Role: External Carbon Source for Biochemical Process
⚠️ Misconception Correction: Glucose has no anti‑scaling or flocculation effect!
Special carbon‑supplement chemical for biochemistry. It provides nutrition for microorganism growth, strengthens pollutant degradation capacity and improves sludge settling performance. It is the preferred emergency carbon source for low‑temperature sewage conditions.
12 Methanol
Core Role: High‑efficiency External Carbon Source for Nitrogen Removal
High‑performance biochemical carbon source. Its core function is biological denitrification to accurately remove total nitrogen from water. It features high carbon utilization rate and low sludge output, suitable for high‑standard water treatment projects with upgraded effluent requirements.
???? Important Note: It is flammable hazardous chemical with certain toxicity. Microorganisms need long‑term acclimation. It is not fit for quick emergency dosing.
???? Core Summary of the Full Article
✅ Coagulation & Clarification: Classic combination of PAC + PAM, suitable for solid‑liquid separation in most sewage cases.
✅ pH Adjustment: Lime, liquid caustic soda and hydrochloric acid, select according to different acid‑alkali working conditions.
✅ Disinfection & Oxidation: Sodium hypochlorite, hydrogen peroxide; Special chemicals for chromium‑containing wastewater: sodium metabisulfite / sodium bisulfite.
✅ Biochemical Carbon Supplement: Glucose for emergency and low‑temperature conditions; methanol for advanced denitrification and upgraded effluent standards.
✅ Key Points to Avoid Mistakes: Citric acid has no sterilization function for sewage systems; glucose has no anti‑scaling or flocculation effect.
This accurately‑reviewed practical guide for water‑treatment chemicals covers daily chemical dosing scenarios for municipal and industrial wastewater. Newcomers can get started quickly, while experienced practitioners can check and fill knowledge gaps. Highly practical! Share and save it to improve your water‑treatment process skills!
