Drinking water treatment chemicals refer to chemical agents added during the purification process of potable water. They are used to remove contaminants, eliminate pathogens, and adjust water quality. The objective is to ensure treated water meets national drinking water hygiene standards.
The ultimate goal of drinking water treatment is to transform raw water sourced from surface or groundwater into tap water that meets health, safety, and sensory requirements (harmless, odorless, clear, palatable, and stable). This process must eliminate visible suspended solids and turbidity while eradicating or controlling microorganisms and removing or reducing heavy metals and organic pollutants. It also adjusts water chemistry to prevent corrosion and secondary contamination while meeting national or international drinking water standards. To achieve these objectives, treatment systems are typically organized according to the process flow: pretreatment—coagulation/sedimentation—filtration—advanced treatment—disinfection—distribution water quality control. Different treatment units perform distinct functions while coordinating with each other.
Process combinations and operational parameters vary depending on water source characteristics, pollution profiles, and scale, often requiring trade-offs between removal efficiency, cost, energy consumption, and byproduct generation. The following sections will introduce the principles, common methods, and key considerations for the primary functional modules: removing suspended solids and turbidity, disinfection, pH adjustment, removing heavy metals and organic pollutants, and softening and stabilizing water quality.
Functions of Drinking Water Treatment Chemicals
1. Removal of suspended solids and turbidity
By adding coagulants and flocculants, fine particles in water aggregate into larger flocs, facilitating sedimentation and filtration.
Coagulation is the first treatment step, primarily targeting invisible colloidal particles in water (such as clay, bacteria, and organic matter). These colloids carry negative charges, causing mutual repulsion that keeps them suspended indefinitely without natural settling.
The mechanism of coagulants involves two main actions:
- Charge neutralization: Most coagulants are positively charged inorganic salts (e.g., aluminum sulfate, polyaluminum chloride) that bind to negatively charged colloidal particles, canceling their negative charge and eliminating repulsive forces between particles.
- Double layer compression: The positive ions released when coagulants dissolve compress the negatively charged “double layer” surrounding colloidal particles. This further weakens the repulsive forces between particles, facilitating collisions.
Operationally, rapid agitation is required after adding coagulants to ensure thorough mixing with water, preparing for subsequent flocculation.
Flocculation is the second step in treatment, performed after coagulation. At this stage, colloidal particles in the water have lost stability but remain too small to settle. They require further aggregation to form larger, easily precipitable “flocs” (commonly known as “alum flowers”).
The mechanism of flocculants primarily involves two key actions:
- Bridging: Most flocculants are high-molecular-weight polymers (such as polyacrylamide, PAM) with long molecular chains. These chains act like “bridges,” adsorbing one small particle at one end and another at the other end, connecting multiple small particles into larger flocs.
- Sweeping action: When coagulants are added in larger quantities, they form substantial hydroxide precipitates. These precipitates act like a “net,” sweeping up remaining minute particles in the water to form larger flocs.
Operationally, slow agitation is required after flocculant addition. This allows flocs to grow sufficiently while preventing excessive agitation from breaking them apart.
3. Sterilization and disinfection
Using disinfectants (such as chlorine, sodium hypochlorite, chlorine dioxide, and ozone) to kill bacteria, viruses, and other pathogenic microorganisms in water ensures the biological safety of drinking water.
The fundamental principle of all disinfectants is to disrupt the “vital structures” of microorganisms through physical or chemical means, thereby eliminating their ability to reproduce and cause disease. There are three primary core mechanisms:
- Destruction of Cell Membranes/Cell Walls: Disinfectants penetrate the cell membranes of microorganisms, disrupting their structural integrity. This leads to leakage of cellular contents, ultimately causing the microorganism’s death.
- Disruption of enzyme systems: Microbial metabolic activities rely on enzyme catalysis. Disinfectants bind to the active sites of enzymes, rendering them inactive and unable to sustain normal metabolism, leading to “starvation” or “death by metabolic disruption.”
- Damage to genetic material: Certain strong oxidizing disinfectants directly act upon microbial DNA or RNA, disrupting their molecular structure. This prevents replication and genetic transmission, completely eliminating the microorganism’s ability to reproduce.
4. Adjusting pH Levels
Adding pH adjusters optimizes water acidity and alkalinity, facilitating subsequent treatment processes while preventing pipeline corrosion or scaling. In drinking water treatment, excessively high or low pH levels pose issues. The core objectives of pH adjustment include three key points:
- Optimize subsequent treatment efficiency: For instance, during coagulation, pH affects the hydrolysis efficiency of coagulants (e.g., polyaluminum chloride). Only within the appropriate pH range (typically 6.0–7.5) can dense flocs form, enhancing impurity removal rates.
- Control pipeline corrosion: Low pH (<6.5) accelerates corrosion in metallic pipelines (e.g., iron or steel pipes), increasing heavy metal levels like iron and lead in water. Conversely, high pH (>8.5) may cause scale formation (calcium carbonate precipitation), leading to pipeline blockages.
- Reduce disinfection byproducts: For example, during chlorine-based disinfection, an optimal pH level (around 7.0) minimizes the formation of harmful byproducts like trihalomethanes.
4. Removal of Heavy Metals and Organic Pollutants
Certain oxidizing agents (e.g., potassium permanganate, ozone) or coagulants (e.g., iron salts, aluminum salts) can convert dissolved harmful substances into insoluble forms for removal.
Common heavy metals in drinking water include lead, mercury, cadmium, arsenic, and chromium(VI), which predominantly exist as dissolved ions. These must be converted into precipitates or adsorbed through treatment processes before being separated by filtration.
Chemical agents are added to react with heavy metal ions, forming salts that are poorly soluble in water (e.g., hydroxides, sulfides). These precipitates are then removed through sedimentation tanks or filter beds.
The core of organic pollutant removal lies in decomposing them into harmless substances or separating and trapping them. The principle involves introducing strong oxidizing agents to break the chemical bonds of organic pollutants, oxidizing and decomposing them into smaller organic molecules (such as carboxylic acids), or even completely oxidizing them into CO₂ and H₂O (harmless substances).
5. Water Softening and Stabilization
Water softeners (such as phosphates and lime) reduce water hardness to prevent scaling. Simultaneously, corrosion inhibitors and stabilizers protect pipeline systems and extend equipment lifespan. Scale inhibitors prevent Ca²⁺ and Mg²⁺ crystallization and scaling through chelation and dispersion. Chelating calcium and magnesium ions to form soluble complexes, suitable for medium to low hardness water while providing some corrosion inhibition. Organic phosphonates offer high scale inhibition efficiency and high-temperature resistance. Suitable for high hardness or circulating water systems; dosage must be controlled in potable water (compliant with national standards). Corrosion inhibitors form protective films on metal surfaces, isolating water from metal contact.
Summary
Water treatment chemicals play an irreplaceable role in ensuring water supply safety and enhancing treatment efficiency. Properly selected and proportioned coagulants, coagulation aids, disinfectants, pH adjusters, adsorbents, and softeners not only efficiently remove suspended solids, reduce turbidity, inactivate pathogenic microorganisms, and eliminate heavy metals and organic pollutants, but also suppress corrosion and secondary contamination by adjusting water chemistry. This ensures treated water meets sensory and sanitary standards upon leaving the plant.
Simultaneously, chemical usage must be grounded in scientific process design and rigorous operational management: precise dosing and online monitoring, control of disinfection byproducts and residues, assessment of compatibility and side effects between different chemicals, and safe disposal of sludge and waste liquids are all critical steps in safeguarding public health and environmental safety.