Difference Between Activated Sludge and Trickling Filter

The key difference between activated sludge and trickling filter is that activated sludge is a suspended culture system in which biomass is mixed with the sewage while trickling filter is an attached culture system in which biomass is grown on media and the sewage is passed over its surface.

Wastewater treatment is a critical process in order to prevent waterborne diseases and maintain healthy environments for everyone. Microorganisms, especially bacteria, are utilized in wastewater treatment processes. In addition to bacteria, nematodes and other small organisms also participate in biological wastewater treatment.

CONTENTS

1. Overview and Key Difference
2. What is Activated Sludge 
3. What is Trickling Filter
4. Similarities Between Activated Sludge and Trickling Filter
5. Side by Side Comparison – Activated Sludge vs Trickling Filter in Tabular Form
6. Summary

Microorganisms break down complex organic matter in the wastewater and aid in purification. Aerobic wastewater treatment and anaerobic wastewater treatment are two types of biological wastewater treatment. Aerobic microorganisms carry out aerobic wastewater treatment under a constant supply of oxygen. Attached culture systems or fixed film reactors and suspended culture systems are two types of aerobic wastewater treatment. In the attached culture system, biomass is grown on solid surfaces or media and wastewater is passed over the microbial surfaces. Trickling filter and rotating biological contactor are two attached culture systems. In suspended culture systems, biomass is mixed with wastewater. Activated sludge system and oxidation ditch are two popular suspended culture systems.

What is Activated Sludge?

Activated sludge treatment is a suspended growth system in which dissolved and colloidal organics are oxidized in the presence of microorganisms. It is a type of secondary treatment in which the remaining suspended solids are decomposed by microorganisms, and the number of pathogens is reduced. Moreover, 90 -95% of BOD reduction can be achieved in the activated sludge system.

Activated sludge systems have aeration basins and clarifiers. A series of aeration basins are designed to promote the growth and metabolism of aerobic microorganisms. They form aggregates. They are the organisms that break down organic load in the wastewater. Microorganisms use oxygen to break down organic matter. Wastewater is retained in aeration basins for few hours, receives extensive aeration and goes to the clarifier. In the clarifier, activated sludge solids settle out from the suspension by flocculation and gravity sedimentation. The separated solids sink to the base of the clarifier, leaving a clear supernatant above.

There are several issues in the activated sludge process. Some of these include dispersed growth, bulking, sludge rising, foaming, scum overflow and infections.

What is Trickling Filter?

Trickling filter is an aerobic wastewater treatment procedure in which the biomass is grown on media and the sewage is passed over its surface. It is a type of an attached culture system. It is also known as a  percolating filter. There are four main components in the trickling filter system. They are a circular tank, distributors, an underdrain system and a clarifier.

Circulating tank contains the filter medium made up of different materials including stones, ceramic materials, treated wood, hard coal or plastics, etc. The filter medium provides a large surface area and sufficient void space for air diffusion. Moreover, the filter medium should not be toxic to microbes and should be mechanically stable.

Distributers or rotating arms sprinkle water and the water containing the organic load percolates through the filter material. A biofilm containing different types of aerobic microorganisms (bacteria, fungi, algae, protozoa, and other life forms) is formed on the surface of the filter medium. Biofilm breaks down the organic matter in the wastewater. An underdrain system is used for the collection of filtered liquid and the introduction of air. Clarifier separates solids from liquids.

There are several advantages to a trickling filter system. It is attractive to small communities due to ease of operation, low maintenance costs, and reliability. Moreover, it is also used to treat toxic industrial effluents and is able to withstand shock loads of toxic inputs. Furthermore, the sloughed-off biofilm can be removed easily during sedimentation.

What are the Similarities Between Activated Sludge and Trickling Filter?

• Activated sludge and trickling filter are two types of aerobic wastewater treatment processes.
• They are biological methods.
• They are also secondary treatment processes.

What is the Difference Between Activated Sludge and Trickling Filter?

Activated sludge is a suspended culture system in which the biomass is mixed with the sewage. In contrast, trickling filter is an attached culture system in which the biomass is grown on media, and the sewage passes over its surface. This is the key difference between activated sludge and trickling filter. Activated sludge process consists of a series of aeration basins and a secondary clarifier while trickling filter process consists if a circular tank, distributors, an underdrain system and a clarifier. So, this is the difference between activated sludge and trickling filter in terms of their composition.

Moreover, microorganisms in the activated sludge process are suspended in the mixed liquor suspended solids, while microorganisms in trickling filter are attached to the filter medium. Therefore, this is also an important difference between activated sludge and trickling filter.

Summary – Activated Sludge vs Trickling Filter

Wastewater treatment is an essential process that should be properly maintained to safeguard human health. Activated sludge and trickling filter are two aerobic wastewater treatment methods. The key difference between activated sludge and trickling filter is that activated sludge is a suspended culture system while trickling filter is an attached culture system. Moreover, activated sludge process has two main components: a series of aerobic basins and a secondary clarifier. In contrast, trickling filter has several components: a circular tank, distributors, an underdrain system and a clarifier.

Reference:

1. Rinkesh. “What Is Wastewater Treatment and Process of Wastewater Treatment.” Conserve Energy Future, 25 Dec. 2016, Available here.
2. “Activated Sludge.” Activated Sludge – an Overview | ScienceDirect Topics, Available here.

Image Courtesy:

1. “Activated Sludge diagram” By Eawag: Swiss Federal Institute of Aquatic Science and Technology Technical drawings: designport, Paolo Monaco, Zurich  eCompendium of sanitation systems and technologies: (CC BY 3.0) via Commons Wikimedia
2. “Trickling Filter diagram” By Eawag: Swiss Federal Institute of Aquatic Science and TechnologyTechnical drawings: designport, Paolo Monaco, ZurichSee – eCompendium of sanitation systems and technologies: (CC BY 3.0) via Commons Wikimedia