Water treatment is a basic step in safeguarding drinking and industrial water quality. From raw water to finished water, different chemicals are dosed according to the treatment objective. Their functions differ, and so does their safety profile. This article examines the question in three steps: which chemicals are used in water treatment, which of them are comparatively safe, and whether the "unsafe" label is accurate.
1. What chemicals are used in water treatment
By function, the chemicals commonly used in water treatment fall into four main groups:
|
Group |
Main role |
Common examples |
|
Coagulants / Flocculants |
Aggregate suspended solids and colloids so they settle |
Aluminum salts: polyaluminium chloride (PAC), aluminum sulfate (Al₂(SO₄)₃), alum (potassium alum, KAl(SO₄)₂·12H₂O); iron salts: polyferric chloride (PFC), ferric chloride; organic polymers: polyacrylamide (PAM) |
|
Disinfectants |
Inactivate pathogenic microorganisms |
Chlorine, chlorine dioxide; UV (physical method, no chemical added) |
|
Scale inhibitors |
Prevent scaling inside pipes and heat exchangers |
Organophosphonates (e.g., ATMP, HEDP), polycarboxylates |
|
Auxiliary chemicals |
Added as the process requires |
Coagulant aids, pH adjusters, corrosion inhibitors, etc. |
Note: In Chinese industry practice, PAC, PFC, aluminum sulfate and similar products are usually classed as "coagulants," while polymeric products are classed as "flocculants"; the two are often used together.
2. Which of these chemicals are safe
"Safe" is a relative term. It has to be judged on acute toxicity, residual levels, disinfection by-products and environmental impact combined. Among the common products, the following are widely regarded as safer choices:
Organic polymer flocculants - their degradation products have a low environmental impact and residual levels are low. For drinking water applications, PAM-type products should meet the limit on residual acrylamide monomer.
Modified ferric chloride and composite coagulants - compared with conventional aluminum salts, they leave less residual aluminum and give more stable coagulation.
Chlorine dioxide - strong, broad-spectrum disinfection with far fewer trihalomethane (THM)-type by-products than chlorine; chlorite and chlorate residuals must, however, be controlled.
UV disinfection - a physical method that adds no chemical and leaves no chemical residual; its effectiveness depends on water turbidity and light conditions, so an adequate irradiation dose is required.
Polycarboxylate scale inhibitors - non-toxic, non-corrosive and with a low ecological impact, widely regarded as a safer option.
3. Are the "unsafe" chemicals really dangerous?
A more accurate way to put it is that these products are not "dangerous substances"; they are chemicals that must be used to specification. Their risks come mainly from improper use, not from inherent high toxicity.
Residual risk - aluminum-based coagulants (aluminum sulfate, alum, PAC, etc.) can leave residual aluminum, and poor-grade raw materials can introduce heavy metals. With raw materials meeting the relevant standards and dosing within the recommended range, residuals stay within safe limits. The risk comes mainly from substandard raw materials or overdosing.
By-product risk - chlorination forms disinfection by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs); chlorine dioxide forms chlorite and chlorate. These can be kept low by controlling dose, contact time and precursor organics in the water.
Ecological risk - some organophosphonate scale inhibitors are toxic to aquatic organisms at certain concentrations. This is concentration-related aquatic toxicity; it can be managed by controlling discharge levels, and polycarboxylates are a lower-toxicity alternative.
Operational risk - ferric chloride, for example, is strongly corrosive. This is a handling issue that is avoided by wearing proper protection and dosing correctly.
