Comparison of Adsorption and Absorption Methods in Wastewater Treatment Systems

Wastewater treatment is one of the urgent issues in the context of increasing environmental pollution. To ensure that the treated water meets discharge standards and minimizes negative impacts on the ecosystem, selecting the appropriate treatment technology is crucial. Among the applied methods, adsorption and absorption are two common solutions, particularly effective in removing trace compounds, toxic gases, or heavy metals. The following article will help you clearly understand the differences between adsorption and absorption, thereby making suitable choices for each specific type of wastewater.

1. General introduction to wastewater treatment methods

Currently, wastewater treatment is not only a legal requirement but also a social responsibility of businesses and production facilities. To achieve optimal treatment efficiency, selecting technology that matches the characteristics of the wastewater source is a key factor. Each type of wastewater – from domestic, industrial to agricultural – contains different pollutants such as organic matter, heavy metals, microorganisms, or toxic gases, requiring specialized treatment methods that may combine physical, chemical, and biological processes.

Among the available technologies, adsorption and absorption play important roles in the intermediate or final treatment stage, especially when removing trace pollutants, odors, or persistent compounds. Adsorption is often applied to remove dissolved solids, heavy metals, and hard-to-treat organic matter, while absorption is mainly used to remove toxic gases or secondary pollutants in water or associated air. Both methods are often integrated into wastewater treatment systems to improve efficiency, minimize environmental impact, and ensure compliance with current discharge standards.

Wastewater treatment methods will remove trace pollutants, odors, or persistent compounds.

2. Concept and operating principle of the adsorption method

Adsorption is one of the important and common processes in wastewater treatment, used to remove pollutants by retaining them on the surface of a solid material called the adsorbent. Essentially, this phenomenon occurs when pollutant molecules in water are attracted and adhere to the adsorbent’s surface through interaction forces such as Van der Waals forces, weak chemical bonds, ionic bonds, or hydrogen bonds. Notably, the molecules do not penetrate into the adsorbent but remain concentrated on the surface, making it easy to control and separate them from the water stream.

In practice, the adsorption process takes place in three main stages:

• Contact: Wastewater carrying pollutants is passed through a system containing the adsorbent material.
• Adsorption: Pollutant molecules quickly adhere to the surface of this material under the influence of bonding forces.
• Separation: Once the adsorbent reaches saturation, it is removed from the system for treatment or replacement.
• This method is particularly effective in treating wastewater containing organic compounds, colorants, odors, and heavy metals, significantly improving water quality before discharge.
• Mechanism: Van der Waals forces, chemical bonds, ionic bonds
• Examples: activated carbon, zeolite, silica gel

The adsorption method removes pollutants by retaining them on the surface of solid materials

3. Concept and operating principle of the absorption method

Absorption is one of the important processes in wastewater treatment technology, used to remove pollutants by introducing them into another phase – usually a liquid or solid material. In this mechanism, pollutant molecules in water penetrate and disperse evenly within the structure of the absorbent material through physical or chemical reactions, thereby reducing the concentration of pollutants in the water.

Typically, the absorption process takes place in three main stages:

• Contact stage: Wastewater carrying pollutants is brought into direct contact with the absorbent.
• Absorption stage: Pollutants dissolve or diffuse into the volume of the absorbent, forming a homogeneous or heterogeneous phase system.
• Removal stage: Once the absorbent has retained enough pollutants, it is removed from the system to recover the treated water.

This method is particularly effective for dissolved gases in water, organic compounds, and some heavy metal ions. Thanks to its flexible treatment capacity, absorption is often applied in industrial wastewater treatment systems to ensure that the output water meets environmental standards.

4. Advantages and disadvantages of adsorption and absorption

4.1 Adsorption method

Advantages:

• High efficiency for trace pollutants: Particularly effective in removing persistent organic compounds, dyes, heavy metals, and color-causing agents.
• Diverse, readily available materials: Adsorbents such as activated carbon, zeolite, bentonite, silica gel… are widely used and customizable.
• Easy operation, simple equipment structure: Adsorption columns or tanks do not require high technology.
• Ability to regenerate materials: Many adsorbents can be regenerated, saving long-term costs.

Disadvantages:

• Easily saturated: Adsorbents quickly lose effectiveness if not regenerated in time.
• Replacement/regeneration costs: Improper regeneration may reduce efficiency or incur high costs.
• Dependent on environmental conditions: Adsorption efficiency is greatly affected by temperature, pH, and initial pollutant concentration.
• May generate sludge containing pollutants: Causes difficulties in final disposal.

4.2 Absorption method

Advantages:

• High efficiency for gases or volatile substances: Particularly suitable for treating ammonia, H₂S, CO₂, or odors causing secondary pollution.
• Fast reaction, easy to control: High diffusion rate, suitable for continuous flow processes.
• Low initial investment cost: Simple equipment, can utilize absorption towers or contact tanks.
• Capable of handling large volumes of gas/water containing dissolved substances: Suitable for large-scale, industrial use.

Disadvantages:

• Difficult to reuse absorbent solution: Often needs complete replacement after full absorption.
• Creates additional wastewater or chemical sludge: Post-treatment solution contains impurities that need further treatment.
• Requires strict chemical control: Can be hazardous if the wrong type or dosage of solution is used.
• Not suitable for poorly soluble pollutants: Limited when treating complex organics or heavy metals.

5. Comparison of adsorption and absorption in wastewater treatment

Both adsorption and absorption play important roles in modern wastewater treatment systems, especially in intermediate and final treatment stages. Adsorption is suitable for organic compounds, heavy metals, and colorants due to its ability to retain pollutants on the surface of solid materials, while absorption has an advantage in treating dissolved gases, odors, and ammonia through diffusion into liquids or soft solids. The choice of the appropriate method depends on the characteristics of the wastewater source, treatment goals, investment and operating costs. In many cases, combining both technologies can deliver higher efficiency, meeting technical and environmental requirements sustainably for businesses and investors.