Understanding the Full Water Treatment Process in One Minute

Water treatment is a systematic project vital to public welfare and environmental protection. Its core objective is to remove impurities and harmful substances from water through physical, chemical, and biological methods, ensuring water quality meets standards for drinking, industrial use, or discharge. This article systematically outlines the complete process flows for three major segments: potable water treatment, wastewater treatment, and sludge treatment.

I. Water Supply Treatment (Conventional Drinking Water Process)

Transforming surface water or groundwater into safe drinking water typically employs the classic sequence of “coagulation-sedimentation-filtration-disinfection,” supplemented by necessary pretreatment and advanced purification steps.

1. Pretreatment Stage

Primary Functions: Remove larger suspended impurities, some organic pollutants, and substances causing odors and tastes, creating favorable conditions for subsequent treatment.

Common Methods:

Grating Interception: Removes large floating debris like branches and plastic.

Sedimentation Tank: Utilizes gravity to settle sand particles and other high-density inorganic solids.

Pre-Oxidation Treatment: Adds oxidizing agents such as chlorine, ozone, or potassium permanganate to inhibit algae growth and decompose some organic matter and odor sources.

2. Main Purification Process

(1) Coagulation

Add coagulants (e.g., polyaluminum chloride) to destabilize colloidal particles in water, causing them to aggregate. Then introduce flocculants (e.g., polyacrylamide) to promote the binding of fine particles into larger flocs (commonly known as “flocs”).

(2) Sedimentation

The coagulated water is directed into sedimentation tanks where flocs settle naturally under slow flow conditions, separating solids from water. (3) Filtration

The treated water flows through filter beds of quartz sand, anthracite coal, or similar media to further trap residual suspended particles and microorganisms. After a period of operation, backwashing is required to restore filtration efficiency.

(4) Disinfection

Eliminates pathogenic microorganisms in water to ensure sanitary and safe supply. Common methods include: Chlorine Disinfection: Low-cost with sustained disinfection effects in distribution networks; however, potential disinfection byproducts require attention. Chlorine Dioxide Disinfection: High disinfection efficiency with minimal trihalomethane formation, but higher operational costs.

Ultraviolet Irradiation: Physical disinfection with no chemical residues, often used in combination with other disinfection methods.

Ozone treatment: Strong oxidation capability improves water color and odor, but requires significant equipment investment.

3. Advanced Purification Technologies

For poor water quality or stringent requirements, the following can be added post-conventional treatment: Ozone-Bioactivated Carbon Combination: Ozone decomposes large organic molecules, which are then adsorbed by activated carbon and degraded by surface microorganisms.

Membrane Separation Technologies:

Ultrafiltration: Effectively removes bacteria, viruses, and large organic molecules.

Nanofiltration/Reverse Osmosis: Removes ions, heavy metals, etc., for high-quality drinking water production or seawater desalination. 📌 Typical Process Flow:

Raw Water → Bar Screen/Grit Chamber → (Pre-Oxidation) → Coagulation → Sedimentation → Filtration → (Advanced Treatment) → Disinfection → Clear Water Reservoir → Water Distribution Network

II. Wastewater Treatment Processes

Wastewater treatment aims to eliminate organic matter, nitrogen/phosphorus nutrients, and suspended solids from domestic sewage and industrial effluents, achieving discharge or reuse standards.

1. Primary Treatment (Stage 1): Sequentially passes through screens and grit chambers to remove coarse debris and sand, protecting downstream equipment.

2. Primary Enhanced Treatment: Utilizes primary sedimentation tanks to remove suspended solids via gravitational settling.

3. Biological Treatment (Secondary Stage) utilizes microbial metabolism to degrade pollutants. Common processes include:

• Activated Sludge Process Series:

◦ A/O Process: Anaerobic-aerobic combination, emphasizing nitrogen removal.

◦ A²/O Process: Anaerobic-anoxic-aerobic combination, simultaneously removing nitrogen and phosphorus, widely applied.

◦ Oxidation Ditch: Features simple structures and convenient operation/management.

◦ SBR Process: Sequentially completes multiple treatment stages within a single reactor.

• Biofilm Processes: Such as biofilters and rotating biological contactors, where microorganisms attach to carrier surfaces forming membrane-like colonies to purify wastewater.

Representative processes: Biofilters, rotating biological contactors, and the biological stage of MBR systems.

4. Advanced Treatment (Tertiary) To meet higher standards or reuse requirements, further removes pollutants difficult to eliminate in secondary treatment, such as nitrogen, phosphorus, trace organic compounds, and color.

Processes:

• Chemical Phosphorus Removal: Adds aluminum salts, iron salts, etc., to form insoluble phosphate precipitates.

• Media Filtration: Removes residual suspended solids from water.

• Membrane Technology Applications: ◦ MBR (Membrane Bioreactor): Replaces secondary sedimentation tanks with ultrafiltration/microfiltration membranes to enhance effluent quality.

• Advanced Oxidation Processes: Degrades recalcitrant organic compounds using hydroxyl radicals.

• Final Disinfection: Eliminates pathogenic microorganisms via UV, chlorine, or ozone treatment.

III. Sludge Treatment Pathways

Sludge generated during water treatment requires specialized processing:

• Thickening and Volume Reduction: Lowers sludge moisture content to reduce bulk volume.

• Stabilization Treatment: Decomposes organic matter through anaerobic or aerobic digestion, minimizes odors, and recovers biogas.

• Dewatering and drying: Further reduce moisture content through mechanical dewatering to form sludge cake.

• Final disposal: Includes sanitary landfilling, incineration for power generation, land application, and conversion into building materials.

IV. Summary

Water treatment operates like a precision purification assembly line, adhering to the principle of “tiered processing with progressively deeper treatment.” It begins with physical screening for coarse filtration, followed by chemical and biological methods for refined treatment, progressing from simpler to more complex steps. The entire process emphasizes the integration of prevention and treatment, upholding the principle that “water quality dictates the process.” This ensures the most economical and effective purification solutions are tailored for water sources of varying origins.