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The basic method of wastewater treatment
The principle of wastewater treatment involves separating or breaking down pollutants in the water into harmless substances, allowing clean water to be recovered. To effectively manage wastewater, it is essential to understand the nature, form, and physical-chemical properties of the pollutants present. Generally, pollutants can be categorized into three main types: suspended solids, colloidal particles, and dissolved substances. These categories are typically defined by particle size—suspended solids range from 1 to 100 micrometers, colloidal particles from 1 nanometer to 1 micrometer, and dissolved substances are smaller than 1 nanometer.
The size and type of pollutants significantly influence the choice of treatment method. Larger particles, such as suspended solids, are easier to remove through processes like sedimentation, filtration, or flotation. However, smaller particles, including colloids and dissolved materials, require more advanced techniques such as coagulation, chemical precipitation, adsorption, or membrane filtration to increase their size or remove them entirely.
Wastewater treatment aims to separate, remove, recycle, or convert contaminants into non-hazardous forms, ultimately purifying the water for reuse or safe discharge. Modern treatment technologies are generally classified into three categories: physical, chemical, and biological methods.
Physical treatment methods rely on physical forces to remove contaminants, such as screening, sedimentation, flotation, filtration, and reverse osmosis. Chemical treatment methods involve chemical reactions to neutralize, precipitate, or extract pollutants, using techniques like coagulation, oxidation-reduction, ion exchange, and adsorption. These two approaches are often combined under the term "physical-chemical treatment."
Biological treatment uses microorganisms to break down organic pollutants. This method is divided into aerobic and anaerobic processes. Aerobic treatment, commonly used for municipal and industrial wastewater, includes activated sludge and biofilm systems. Anaerobic treatment is suitable for high-strength organic waste and sludge, and is increasingly being applied to low-concentration wastewater as well.
In addition to these, natural biological treatments such as stabilization ponds and land treatment are also employed. Stabilization ponds use natural biological processes to treat wastewater, while land treatment involves applying wastewater to soil where it is purified through plant and microbial activity.
Depending on the type and quality of wastewater, a combination of treatment methods is often necessary to meet discharge standards. Urban wastewater treatment is typically divided into primary, secondary, and advanced stages. Primary treatment removes floating and suspended matter, secondary treatment uses biological processes to eliminate organic matter and nutrients, and advanced treatment targets residual contaminants that remain after secondary treatment.
When selecting a treatment method, it's crucial to match the process to the specific characteristics of the wastewater. For example, organic wastewater may require biological treatment if BOD and COD levels are high, while inorganic wastewater might need chemical precipitation or ion exchange. If standard methods fail, advanced techniques like adsorption or membrane filtration may be required.
In some cases, changing the production process itself may be necessary to reduce the difficulty of treating certain pollutants. By integrating process improvements with treatment strategies, wastewater management becomes more efficient and sustainable.