Difference Between Activated Sludge And Trickling Filter

Activated sludge and trickling filter processes are two of the most commonly used wastewater treatment systems. The activated sludge process utilizes a combination of bacteria, oxygen, and chemicals to break down waste products in water. After the hazardous materials have been converted into harmless byproducts, the water is discharged back into its source. On the other hand, a trickling filter process works by passing wastewater through a bed of stones or gravel that bacteria have colonized. This method also relies on oxygen to break down dangerous materials.

What Is Activated Sludge?

Activated sludge is a wastewater treatment process used to remove pollutants from wastewater. In this process, bacteria are used to break down the organic matter in the wastewater. The bacteria consume the materials and produce a sludge-like material that can be removed from the water and disposed of properly. Activated sludge is often used as part of an overall wastewater treatment system.

What Is A Trickling Filter?

A trickling filter is a wastewater treatment process that uses a fixed bed of rocks, gravel, plastic media, or other material on which bacteria can grow to purify the water. In a trickling filter system, wastewater flows from the top down onto the media and passes through multiple stages before being collected at the bottom. The media provides a surface for bacteria and other microorganisms to grow, which then digest the organic matter in the wastewater. This process results in clean, purified water that is safe to be discharged into the environment. Trickling filters are most commonly used for secondary wastewater treatment because they can remove up to 95% of suspended solids and around 75-85% of organic material.

Difference Between The Activated Sludge And Trickling Filter

  • Definition: Activated sludge is a wastewater treatment process that uses aeration and activated sludge to degrade organic pollutants biologically. A trickling filter is a wastewater treatment process that uses different media types, such as gravel or synthetic materials, to provide a large surface area for bacterial growth and activity.
  • Components: Components of activated sludge are aeration basins and secondary clarifiers. In comparison, the components of a trickling filter are a circular tank, distributors, a clarifier, and an underdrain system.
  • Microorganisms: Activated sludge is suspended in the mixed liquor-suspended solids. The trickling filter is usually attached to the filter medium.
  • Type of system: Activated sludge is a suspended culture system. The trickling filter is attached culture system.
  • Efficiency level: Activated sludge is a more efficient way to convert organic pollutants into biomass, with the benefit of allowing some secondary wastewater treatment before discharge. It operates more efficiently than trickling filters and has less potential for odor problems. On the other hand, trickling filters are not as effective in removing specific types of organic matter. Still, they are better at removing ammonia, nitrites, and other inorganic compounds than activated sludge.
  • Space requirements: Activated sludge systems take up more space than trickling filters due to the larger tank volumes required for proper treatment. Trickling filters can be used in much smaller areas if the airflow is sufficient and the filter is designed correctly.
  • Maintenance requirements: Activated sludge systems require regular maintenance to ensure optimal efficiency and reduce the potential for odor problems. This includes regularly checking the aeration system, adding chemicals as necessary, and monitoring organic matter levels in the wastewater. Trickling filters need less frequent maintenance due to their more straightforward design but require more frequent backwashing to remove accumulated solids.
  • Cost: The initial cost of an activated sludge system is significantly higher than that of a trickling filter due to the more extensive equipment and space requirements. However, over time, these costs can be offset by the greater efficiency of the activated sludge system. On the other hand, trickling filters are a more affordable option regarding initial costs, but their lower efficiency means the total cost of ownership will be higher over time.

Similarities Between The Activated Sludge And Trickling Filter

  • Types of aerobic wastewater treatment processes: activated sludge and trickling filter systems use aerobic bacteria to break down organic matter. Both processes require oxygen for the microorganisms to metabolize or consume the pollutants in wastewater.
  • They are biological methods: Activated sludge and trickling filter are biological wastewater treatment methods.
  • They are also secondary treatment processes: Activated sludge breakdowns organic matter in wastewater, and trickling filter uses bacteria to do the same. Both processes remove up to 85% of organics from the water.


Is The Trickling Filter Primary Or Secondary?

Trickling filter is a primary treatment system that utilizes bacteria in a settling chamber to break down organic matter contained in wastewater. This process differs from secondary treatment processes like the activated sludge process, which uses microorganisms to treat wastewater aerobically and requires additional chemicals for nutrient removal.

What are the types of activated sludge?

The two most common types of activated sludge used in wastewater treatment are cascade aeration and contact stabilization. Cascade aeration is when wastewater passes through an aerated tank and into a settling basin, allowing microorganisms to break down pollutants. Contact stabilization involves adding activated sludge directly to the incoming waste stream, which allows for more efficient treatment by allowing for more contact with the microorganisms.

Are there any disadvantages of the trickling filter?

One of the main drawbacks of using a trickling filter for wastewater treatment is that it requires more space than other methods, such as activated sludge. Additionally, due to its reliance on oxygenation, trickling filters may clog during high organic matter loads. Trickling filters are unsuitable for treating highly toxic wastewater, as they cannot effectively break down more complex materials.


Activated sludge and trickling filter methods offer many advantages to wastewater treatment facilities. Both methods are proven systems that have been used successfully since modern wastewater treatment technology began. Activated sludge is the most commonly used system for treating wastewater, as it provides a higher quality of treated water than other methods. Trickling filters are more efficient at removing solids from wastewater and are often more cost-effective than activated sludge systems. Ultimately, the decision of which system to use should be based on the facility’s size, budget, and water quality requirements.

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