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Environmental engineering, Water pollution

Activated Sludge Process – Stages and Process Control

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The activated sludge process is a widely used and highly effective method in wastewater treatment, employing aerobic microorganisms to remove organic pollutants efficiently. This guide covers the activated sludge process definition, stages, and diagram, while explaining its role in secondary wastewater treatment for producing high-quality effluent. You will understand how aeration tanks, wastewater treatment microorganisms, mixed liquor suspended solids (MLSS), and return activated sludge (RAS) work together to maintain stable and efficient system performance. The activated sludge process in wastewater treatment relies on proper aeration, controlled sludge recycling, and effective process monitoring. Due to its reliability, adaptability, and high treatment efficiency, activated sludge wastewater treatment remains the preferred choice for many municipal and industrial wastewater treatment plants, ensuring consistent performance and environmental compliance.

Activated Sludge Process is a globally used wastewater treatment technique. In the previous blogs, I had shown you the various secondary wastewater treatment techniques. In this blog, we will dig deep into this widely used technique of activated sludge process, its configuration, process control and aeration methods.

Activated sludge Process – Definition

The activated sludge process is a type of wastewater treatment that uses aeration. It involves a biological floc made up of bacteria and protozoa to clean sewage or industrial waste waters. This process is biological and has various applications. It includes oxidising carbonaceous biological matter. Additionally, it deals with nitrogenous waste in the biological matter, mostly ammonium and nitrogen.

Before starting make sure that you take a quick glance through the blog, Wastewater Treatment- Stages and Process full details for better understanding

Aerial view of multiple circular wastewater treatment tanks with varying water clarity, surrounded by greenery.
Aerial view of multiple aeration tanks used in the activated sludge process for wastewater treatment.

The activated sludge process employs aerobic microorganisms that can digest organic substances in sewage. Also, they have the ability to cluster together via flocculation. The flocculated particles settle out as sludge. As a result, the liquid coming out is relatively free of suspended solids and organic matter.

Activated Sludge Process – Stages, Configuration, and Diagram

The activated sludge process is a widely used secondary wastewater treatment method. It removes carbonaceous organic pollution through aerobic biological activity. The configuration of an activated sludge wastewater treatment system consists of interconnected units that operate together to ensure efficient treatment.

Aeration Tank

The aeration tank is the heart of the activated sludge process in wastewater treatment. Primary treated sewage is mixed with microorganisms, forming mixed liquor suspended solids (MLSS). Air or oxygen is supplied using diffusers or mechanical aerators to support aerobic microbes. These microorganisms consume organic matter and convert it into stable end products.

Secondary Settling Tank (Secondary Clarifier)

The mixed liquor flows into the secondary settling tank, where biological flocs settle by gravity. Clarified effluent overflows for further treatment or discharge. Settled sludge collects at the bottom.

Sludge Recycling System

A portion of the settled sludge is returned as Return Activated Sludge (RAS) to maintain microbial concentration in the aeration tank. Excess sludge is removed as Waste Activated Sludge (WAS) to control sludge age and system stability.

Advanced control systems regulate aeration, sludge recycling, and flow rates, ensuring stable and efficient secondary wastewater treatment performance.

Now, how about looking deep into what happens inside an activated sludge process?

Diagram illustrating the activated sludge process, featuring an aeration tank, mixed liquor, secondary clarifier, return activated sludge (RAS), and waste activated sludge (WAS).
Diagram illustrating the activated sludge process, featuring the aeration tank, secondary clarifier, mixed liquor, return activated sludge (RAS), and waste activated sludge (WAS) pathways.

Activated Sludge Process Steps

The activated sludge process operates through a series of well-defined stages that enable effective secondary wastewater treatment. Each stage supports biological activity, sludge separation, and recycling to achieve high organic pollutant removal and regulatory compliance.

  • After primary treatment, wastewater enters into an aeration tank. A portion of sludge from the secondary settling tank also enters.
  • Organic matter comes into close contact with sludge from the secondary settling tank. Sludge is densely populated with microorganisms that are actively growing.
  • Diffusers or surface aerators inject air in the form of bubbles into the sewage-sludge mixture.
  • Microorganisms break down organic matter into stable chemicals like NO3, SO4, and CO2 while also producing new bacterial cells.
  • The effluent along with the actively growing microbial population passes to the secondary settling tank.
  • The secondary settling tank separates the aeration tank’s effluent, which contains flocculent microbial matter into supernatant and sludge. The treated supernatant undergoes further treatment before discharge.
  • This sludge from the settled waste returns to the aeration system’s inlet to re-seed the new wastewater reaching the tank. Return activated sludge (R.A.S.) is the fraction of the floc that returns to aeration tank.
  • The remaining sludge goes to sludge digesters for further treatment and safe disposal.

“Mixed liquor” refers to the combination of the liquid and microorganisms in the aeration tank. The suspended solids are called “Mixed Liquor Suspended Solids” (MLSS).

In the next section, we will find out the basic process control parameters in an activated sludge process.

Close-up of foam and sediment in a wastewater treatment process, illustrating the activated sludge process used for organic pollutant removal.
Foamy surface of wastewater during the activated sludge process, illustrating the aeration and microbial activity involved in organic matter treatment.

Process Control in Activated Sludge Process

The general process control method monitors the following variables:

  • Sludge Volume Index (SVI)
  • Mean Cell Residence Time (MCRT)
  • Food to Microorganism Ratio (F/M)
  • Dissolved oxygen (DO)
  • Biochemical oxygen demand (BOD)
  • Chemical oxygen demand (COD)

Let me explain these parameters in detail.

Sludge Volume Index

Sludge Volume Index measures the volume of settled sludge in milliliters. This volume is occupied by 1g of dry sludge solids after 30 minutes of settling in a 1000 milliliter graduated cylinder. It gives a measure of the settling ability of the sludge. SVI ranges from 40 to 100 for a good sludge which settles down easily. Bulking Sludge is a biomass consisting of filamentous organisms with very poor settling characteristics. For a bulking sludge, SVI value can exceed 200. Sufficient pH control, adequate aeration and addition of hydrogen peroxide to the aeration tank prevents bulking.

Mean Cell Residence Time

Mean Cell Residence Time is the ratio of total mass (lbs) of mixed liquor suspended solids in the aerator and clarifier to the mass flow rate (lbs/day) of mixed liquor suspended solids leaving as final effluent.

Food to Microorganism Ratio

The Food to Microorganism Ratio indicates how much organic matter is fed to the microorganisms each day. It is relative to the mass of microorganisms under aeration. In other words, it is the ratio of the amount of BOD fed to the aerator (lbs/day). It is compared with the amount (lbs) of Mixed Liquor Volatile Suspended Solids (MLVSS) under aeration. 

Main Control Parameters

The mean cell residence time and F/M Ratio are the main control parameters used industrially. Both are directly related to the effluent quality. However, it is tedious to control the plant on the basis of the F/M ratio since it necessitates a lot of laboratory work to find the BOD and MLSS in the system. Therefore, the mean cell residence time is the best choice for controlling an activated sludge system.

Now you got an idea about the entire process and its important parameters. Next, we move on to the various aeration methods.

Aeration Methods in Activated Sludge Process

The decomposition of organic waste requires a very high concentration of oxygen at the initial stages of contact between microorganisms and the organic matter. The conventional systems usually maintain a plug flow hydraulic regime and keeps aeration and a mixing at an uniform rate along the entire tank. As a result, the oxygen concentration drops rapidly in the inlet and this can harm the microbes.

At the outlet, there is a surplus of oxygen which is not necessary and leads to economical losses. In order to match the oxygen supply and demand along the entire journey of wastewater from inlet to outlet, the mode of aeration needs some modifications. Let’s have a look at the different aeration methods in an activated sludge process.

Diffused Aeration

Sewage liquor is pumped into large tanks with floor-mounted diffuser grid aeration devices. Passing air creates a curtain of bubbles that oxygenates the liquor while also mixing it. An air blower usually creates the air. Oxygen replaces air for unusually strong and difficult to treat sewage.

Aeration tank in activated sludge process featuring air supply and diffuser grid for oxygenation, promoting biological growth with visible bubbles.
Diffused aeration system in activated sludge process, promoting oxygenation and biological growth.

Tapered Aeration

The organic waste needs more oxygen at the inlet. As it degrades progressively its oxygen demand decreases. Tapered aeration works on this principle. Aeration is intense at the inlet and decreases progressively along the length of the aeration tank. As this method involves the more efficient use of air, it results in savings in the pumping costs too.

Step Aeration

This method aims to equalize the oxygen supply and its demand. It introduces fresh feed at several points in the aeration tank, while keeping the rate of oxygen supply constant. This ensures a more even oxygen distribution over the entire tank and throughout the aeration stage. Baffles divide the aeration tank into several channels with each channel representing one step of the process.

Complete Mix Activated Sludge Process

In complete mix process, the aeration tank receives a mixture of fresh feed and recycled sludge at several locations within the tank. This ensures a constant supply and demand of oxygen along the length of the tank.

Contact Stabilisation

The microbial mass comes in contact with wastewater for short durations of time, approximately 0.5 to 1 hour in the biosorption unit. An anaerobic digestion unit stabilizes the resulting sludge after a retention period of about 2-3 hours. In the digestion unit, microbes consume the organic wastes removed in the biosorption unit. Since we stabilize the return sludge with higher solid concentrations, this reduces the volume of the aeration tank.

Pure Oxygen Activated Sludge Process

This type of activated sludge process supplies pure oxygen instead of air. It recirculates this oxygen into well mixed and converted chambers. Conventional processes utilize only 5-10% of oxygen. In contrast, the pure oxygen activated sludge process ensures about 90% utilization of oxygen. Further, it results in higher bacterial activity, lower sludge volume and sludge with better settling characteristics.

That’s it about activated sludge process. Let us know in the comments if you wish to know more.

Key Takeaways

  • The activated sludge process effectively treats wastewater using aerobic microorganisms to remove organic pollutants.
  • Key stages include aeration tanks, secondary settling tanks, and sludge recycling to maintain treatment efficiency.
  • Proper aeration and control parameters like SVI and F/M Ratio are essential for optimal performance.
  • Various aeration methods, such as diffused and tapered aeration, improve oxygen supply and microbial activity.
  • This process provides a reliable solution for secondary wastewater treatment, ensuring cleaner effluent.

Conclusion

The activated sludge process is a proven method for secondary wastewater treatment. It is highly efficient and used worldwide. This process removes organic pollutants from sewage and industrial wastewater. This process promotes aerobic microbial activity in aeration tanks. It achieves effective BOD reduction, suspended solids removal, and stable effluent quality. Proper control of operational parameters, such as dissolved oxygen, MLSS, and sludge age, ensures consistent system performance. It also prevents common issues like sludge bulking. With multiple activated sludge process configurations and aeration methods, the system can adapt to varying wastewater loads and treatment requirements. Due to its flexibility, reliability, and high treatment efficiency, the activated sludge process remains a cornerstone of modern biological wastewater treatment systems and sustainable water management.

Environmental engineering, Water pollution

Secondary Treatment for Wastewater – Methods and Process

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Secondary wastewater treatment uses biological processes to remove dissolved and suspended organic pollutants from sewage. It follows primary treatment and greatly lowers BOD levels. Microorganisms break down waste and improve water quality. This process creates cleaner effluent for discharge or reuse. Secondary treatment of sewage relies on activated sludge systems and trickling filters. These biological wastewater treatment methods work with constant aeration. The wastewater aeration process keeps microorganisms active and effective. Plants use aeration tanks and clarifiers for stable operation. Types of secondary wastewater treatment include activated sludge, oxidation ditches, SBR systems, and trickling filters. Each system supports reliable pollutant removal in municipal and industrial plants. Operators use the secondary treatment process in sewage treatment plants to reduce pathogens and organic matter. Advantages of secondary wastewater treatment include higher efficiency and better environmental protection. Secondary treatment vs tertiary treatment differs by the degree of purification offered.

n this blog, I will walk you through various biological methods used in the secondary treatment of wastewater. Before diving deep into the biological treatment processes, make sure you understand the entire wastewater treatment process. So please go through our blog,

 Wastewater Treatment- Stages and Process full details.

  1. What is Secondary or Biological treatment for wastewater?
    1. Anaerobic Wastewater Treatment
    2. Aerobic Wastewater Treatment
      1. Biochemical Oxygen Demand
  2. Types of Secondary or Biological wastewater Treatment Methods
    1. Trickling Filter in Secondary wastewater treatment
    2. Oxidation Ponds
    3. Oxidation Pond Configurations
      1. Anaerobic Pond
      2. Facultative Pond
      3. Maturation Ponds
    4. Activated Sludge wastewater treatment Process
    5. Anaerobic sludge blanket reactors
  3. Conclusion

What is Secondary or Biological treatment for wastewater?

Secondary or biological treatment for wastewater uses bacteria to remove dissolved and suspended organic pollutants. These microorganisms consume organic matter as food and convert it into carbon dioxide, water, and energy for growth. This process reduces BOD and improves water quality in the secondary treatment of sewage. It also protects the dissolved oxygen balance in rivers, lakes, and receiving streams. Biological wastewater treatment methods work in controlled environments inside treatment tanks. Operators maintain proper aeration, mixing, and retention time to keep microorganisms active. The wastewater aeration process plays a major role in activated sludge systems and other types of secondary wastewater treatment.

A diagram illustrating the stages of secondary wastewater treatment, including chemical treatment, sedimentation, neutralization and filtration, and discharge. It shows various tanks and processes involved in each stage.
Illustration of the secondary wastewater treatment stages, including chemical treatment, sedimentation, neutralization and filtration, and discharge of treated water.

The Biological treatment or decomposition of organic matter takes place in two ways as shown below:

  • Anaerobic Wastewater treatment
  • Aerobic Wastewater treatment

Anaerobic wastewater treatment works without oxygen and produces biogas. Aerobic wastewater treatment uses oxygen and supports faster decomposition. Both methods enhance the secondary treatment process in sewage treatment plants, and moreover, they ensure efficient pollutant removal.

Anaerobic Wastewater Treatment

Anaerobic wastewater treatment uses anaerobic microorganisms to break down and remove organic pollutants from wastewater. The anaerobic wastewater treatment process consists of two major stages:

  • Acidification
  • Methane generation
Concrete anaerobic digesters at a wastewater treatment facility, with pipes and pumps visible for managing sewage processing.
Anaerobic treatment tanks used in secondary wastewater treatment to break down organic matter.

In the initial acid-forming or acidification phase, anaerobes convert complex organic molecules into simpler, short-chain volatile organic acids. The second stage, known as the methane-production phase, includes two key steps:

  • Acetogenesis
  • Methanogenesis

During acetogenesis, anaerobes transform organic acids into acetate, hydrogen gas, and carbon dioxide. In the methanogenesis step, microbes react with these products to generate methane gas and carbon dioxide, completing the anaerobic treatment process.

Anaerobic systems are widely used for treating wastewater streams with high organic loads and warm temperatures. They offer several advantages over aerobic or biological wastewater treatment methods, including lower sludge production and the generation of valuable energy-rich by-products.

Aerobic Wastewater Treatment

Aerobic wastewater treatment uses oxygen-dependent microorganisms to break down organic pollutants efficiently. Furthermore, this biological treatment process involves aeration tanks where microbes oxidize waste, producing cleaner effluent. As a result, aerobic wastewater systems offer fast decomposition, stable operation, and high treatment efficiency, making them ideal for municipal and industrial aerobic wastewater treatment processes.

The aerobic wastewater treatment systems use oxygen-feeding microorganisms to clean water. These systems take advantage of the natural microbial decomposition process to break down industrial wastewater pollutants and remove them.

Biochemical Oxygen Demand (BOD) is directly related to aerobic wastewater treatment. It measures the amount of oxygen microorganisms need. This oxygen is necessary to break down organic pollutants.

Biochemical Oxygen Demand

The biochemical oxygen demand (BOD) provides a clear measure of the organic pollutants decomposed by bacteria during wastewater treatment. BOD refers to the amount of dissolved oxygen required by aerobic organisms to break down organic matter into smaller molecules. Additionally, high BOD values indicate a greater concentration of biodegradable material in the wastewater, signaling increased organic pollution levels.

Aerobic digestion is preferred for large quantities of dilute wastewater with BOD₅ < 500 mg/L. It is suitable for most aerobic wastewater treatment systems. For highly polluted wastewater streams with BOD₅ > 1000 mg/L, anaerobic digestion is recommended. It handles stronger organic loads more efficiently.

Types of Secondary or Biological wastewater Treatment Methods

The three most commonly used type of aerobic secondary wastewater treatment procedures for are listed below:

  • Trickling filter
  • Oxidation pond
  • Activated sludge wastewater treatment
  • Moving Bed Biofilm Reactor (MBBR)

Now, let’s dig deeper into their features and working. Off, we go.

Trickling Filter in Secondary wastewater treatment

A trickling filter is an aerobic secondary wastewater treatment system. It uses a microbial biofilm attached to filter media to break down and remove organic pollutants. This biological wastewater treatment method is called an attached-growth process. In contrast, suspended-growth systems keep microorganisms mixed within the effluent. Trickling filters offer stable performance, effective BOD reduction, and low operational complexity, making them a reliable option in modern secondary wastewater treatment processes.

A trickling filter system for wastewater treatment showing a circular tank filled with rocks, with a spray mechanism distributing water over the surface.
A trickling filter system used in secondary wastewater treatment, showcasing a microbial biofilm attached to filter media for effective organic pollutant removal.
  • A trickling filter consists of a fixed bed of rocks, coke, gravel, slag, polyurethane foam, sphagnum peat moss, ceramic, or plastic media.
  • As wastewater trickles down, bacteria attach to the media and form a microbial slime layer (biofilm).
  • The continuous flow of sewage over the biofilm allows microbes to consume dissolved organic pollutants.
  • They release carbon dioxide, water, and other oxidised end products as wastewater passes over the media.
  • This process lowers the sewage’s biochemical oxygen demand (BOD).
  • Air moving upward through the media crevices supplies oxygen required for aerobic microbial activity.
  • The biofilm absorbs and adsorbs organic compounds and inorganic ions such as nitrite and nitrate.
  • The biofilm layer needs dissolved oxygen for effective biological oxidation.
  • As the biofilm thickens, available oxygen depletes before reaching the lower layers.
  • Anaerobic conditions develop at the base of the slime layer.
  • Microbes enter a decay stage and lose their ability to attach.
  • The biofilm detaches and becomes part of the secondary sludge, a process known as sloughing.
  • Trickling filters are widely used in milk processing, paper mills, and pharmaceutical wastewater treatment.

Ever heard of a pond which treats wastewater? Let’s look at what’s happening inside such oxidation ponds.

Oxidation Ponds

Oxidation ponds are artificial ponds that treat wastewater through the combined action of sunlight, microbes, and oxygen to reduce organic content and pathogens. Moreover, this waste stabilization pond uses microbes to stabilise residential, commercial, and industrial wastes. Typically, it appears as a shallow treatment pond with a water depth of 2–6 feet.

Industrial or domestic wastewater enters the oxidation pond through the inlet system. Bacteria then convert biodegradable organics into inorganic molecules through microbial interaction, producing carbon dioxide. Common bacteria in these stabilisation ponds include Achromobacter, Proteus, Alcaligenes, Pseudomonas, Thiospirillum, and Rhodothecae.

A shallow oxidation pond for wastewater treatment, showcasing water with visible aquatic vegetation and a concrete inlet structure, surrounded by earth banks.
An oxidation pond used for biological wastewater treatment, combining sunlight, microbes, and oxygen to reduce organic content and pathogens.

Anaerobic bacteria first convert insoluble organic waste into soluble organic acids such as ethanol without oxygen. These acids are further decomposed, releasing H₂S, NH₃, CH₄, CO₂, and other gases. Non-biodegradable solids settling at the bottom form sludge.

Most ponds require both bacteria and algae to maximise the breakdown of organic matter and remove contaminants. Algae produce oxygen during photosynthesis and leave an excess amount. Aerobic bacteria use this oxygen for respiration and oxidation of organic matter.

Treated water exits through the pond’s outlet system. Dredging removes accumulated sludge from the pond. Filtration or a combination of chemical treatment and settling removes algal and bacterial biomass.

Now, let’s move on to the various configurations of oxidation ponds.

Oxidation Pond Configurations

Waste stabilization ponds are artificial basins designed for biological wastewater treatment, consisting of single units or multiple series of anaerobic, facultative, or maturation ponds. Moreover, the main oxidation pond configurations used in wastewater treatment are:

  • A single facultative pond.
  • An anaerobic pond followed by a facultative pond.
  • A facultative pond followed by maturation ponds in series.
  • A series of maturation ponds preceded by an anaerobic pond and a facultative pond.

These configurations help optimise performance in waste stabilization pond systems.

Anaerobic Pond

Anaerobic ponds are deep ponds (usually 3.0 to 5.0 m) that receive raw wastewater. Most of the solid matter in the wastewater settle to the bottom as sludge. Due to the depth of the pond, oxygen can’t penetrate to the bottom of the pond. Thus the sludge digestion takes place under anaerobic conditions.

Facultative Pond

After coming out of an anaerobic pond, the remaining solid particles in the wastewater settles into a larger but shallow pond called a facultative pond. Air and sunlight kill the harmful germs in the wastewater and makes it less dangerous to the aquatic flora and fauna.

Maturation Ponds

Maturation ponds are two or three ponds in series wherein sunlight and oxygen destroy more harmful germs and make the liquid fit enough to be released for irrigation or into a river. Moreover, the higher the number of maturation ponds, the cleaner the effluent becomes.

Activated Sludge wastewater treatment Process

The Activated Sludge wastewater treatment method employs aerobic microorganisms that can digest organic substances in sewage. Also, they have the ability to cluster together via flocculation. The flocculated particles settle out as sludge. As a result, the liquid coming out is relatively free of suspended solids and organic matter.

Aerial view of a wastewater treatment facility with several circular and rectangular tanks, featuring flowing water and foam, used for secondary treatment processes.
Overview of an activated sludge wastewater treatment plant, showcasing aeration tanks and clarifiers for effective pollutant removal.

The sludge blanket becomes Return Activated Sludge (RAS) once it has settled. Then, the RAS returns to the primary clarifying tanks, where the bacteria in it aid in the breakdown of organic waste in the entering sewage.

Anaerobic sludge blanket reactors

  • A popular method used in the anaerobic secondary treatment for water.
  • The wastewater is carried across a free-floating “blanket” of suspended sludge particles in sludge blanket reactors, which are a type of anaerobic treatment.
  • Anaerobes in the sludge multiply and accumulate into larger granules that settle to the bottom of the reactor tank and can be recycled for future cycles as they decompose the organic contents in the wastewater.
  • The treated effluent rises and exits the unit.
  • Throughout the treatment cycle, collection hoods collect biogases produced by the degradation process.

Shall we wrap up?

Key Takeaways

  • Secondary wastewater treatment uses biological processes to remove organic pollutants through methods like activated sludge and trickling filters.
  • Anaerobic and aerobic treatments both improve water quality, with anaerobic processes operating without oxygen and producing biogas.
  • Trickling filters employ a biofilm of microbes on media to reduce biochemical oxygen demand (BOD) and purify wastewater.
  • Oxidation ponds rely on sunlight, microbes, and oxygen to treat wastewater, showcasing various configurations for effectiveness.
  • Activated sludge processes involve aerobic microorganisms that cluster together, aiding in the removal of suspended solids and organic matter.

Conclusion

We explored the key secondary wastewater treatment methods, including the trickling filter system, oxidation pond process, activated sludge process, and anaerobic sludge blanket reactors. Each of these biological wastewater treatment methods plays a crucial role in reducing organic pollutants, lowering BOD levels, and improving overall effluent quality. The sludge produced from these secondary treatment units undergoes dewatering and digestion to reduce volume and enhance stability. Afterward, the dried sludge is commonly used in landfills or converted into nutrient-rich fertilizer, supporting sustainable wastewater treatment and reuse practices. Ultimately, these methods strengthen modern secondary wastewater treatment systems and ensure reliable environmental protection.

That’s it about secondary treatment methods for wastewater. Hope you found it informative.