Tag Archives: Wastewater Treatment Plant

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.

water conservation

Wastewater Treatment- Stages and Process full details

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Wastewater treatment is an area where a lot of experts have carried out in-depth research. According to The United Nations World Water Development Report (WWDR), about 80% of wastewater is released into the water bodies without proper treatment on a global scale.

In the previous blog, I showed you some innovative ways to conserve water. Today we are going to dig deeper into the journey of wastewater through a wastewater treatment plant.

In the first section, let me introduce you to the wastewater treatment process.

Wastewater Treatment Process

The method of removing pollutants from wastewater or sewage and converting it into an effluent that can be added to the water cycle is known as wastewater treatment. In wastewater treatment plants, pollutants in wastewater are reduced to a degree that nature can accommodate.

Here’s a step-by-step guide to explain what happens at each point of the treatment process and how contaminants are neutralised or removed to help keep our rivers and streams safe. 

Also check out some rainwater harvesting methods.

Wastewater Treatment Stages

Wastewater Treatment Process
Wastewater Treatment Process

As shown in the diagram, the three main stages in the treatment of wastewater are:

  • Primary Treatment
  • Secondary Treatment
  • Tertiary Treatment

Let’s analyse what happens in each of the stages in detail.

Primary Treatment of Wastewater

Wastewater reaching a treatment plant through pipes first undergoes primary treatment irrespective of its source. About 60% of the suspended solids are removed from the wastewater during primary treatment along with aeration or stirring in the water to put oxygen back. It includes the following steps:

1. Screening

  • The majority of the floating materials are removed from the wastewater using screens of 10 mm openings and settling tanks.
  • The floating materials include stone, rocks, sticks and even dead animals. Solid materials make up about a third of the wastewater. 
  • As a result, eliminating solid waste at the initial stage makes subsequent treatment procedures easier. Also, they may cause problems later in the treatment process if they are not removed.
  • Solid waste is collected and disposed of in landfills after the screening process.

2. Sedimentation

  • The sewage then flows through the grit trap or grit chamber which collects sand, cinders, and small stones at the bottom.
  • The wastewater is then directed to sedimentation ponds, settling tanks, or clarifiers after the settled grit have been removed.
  • Organic and inorganic matter, as well as suspended solids, are removed in this process.
  • The suspended particles begin to fall to the bottom and form a solid mass known as sludge.

3. Sludge Removal

  • In the sedimentation tanks, sludge (the organic solid component of the sewage) settles out of the wastewater.
  • Mechanical scrapers in the tank’s base continuously move accumulated sludge to a hopper, where it is pumped to sludge treatment facilities.
  • The thickening step removes some of the water before the sludge is processed in digesters.

4. Scum Removal

  • Lighter materials rise to the surface as sludge settles to the bottom of the sedimentation tanks.
  • Grease, oils, plastics, and soap are among the ‘scum.’
  • Scum is skimmed off the surface of the wastewater by slow-moving rakes.
  • Scum is thickened before being poured into the digesters with the sludge.

Around 90% of suspended solids, 55% of faecal coliforms, and 50% of biological oxygen demand are removed during the primary treatment process. The waste must be subjected to secondary treatment in order to be completely free of toxic substances. Let’s see how to do that.

Wastewater Treatment Stages
Source: researchgate.net

Secondary Treatment of Wastewater

  • Secondary wastewater treatment is designed to significantly degrade the biological content of the waste by aerobic biological processes, and it operates at a deeper level than primary treatment.
  • During secondary wastewater treatment, about 85% of the organic matter in sewage is eliminated.
  • The method entails combining the wastewater with bacteria and oxygen at high pressures.
  • Bacteria digest organic matter with the aid of oxygen.

Secondary wastewater treatment reduces common biodegradable pollutants to acceptable levels, allowing for cleaner release into the nearby water bodies. It is done in one of three ways:

1. Biofiltration

Sand filters, contact filters, and trickling filters are used in biofiltration to ensure that any excess sediment is extracted from the wastewater.

  • A trickling filter consists of a bed of stones that is about 6 feet tall.
  • Sewage coming out of the sedimentation tank is permitted to pass through this stone sheet.
  •  The bacteria congregate on these stones and begin to multiply and evolve until all of the organic matter in the sewage has been consumed.
  • Following the process, the clean water is piped out and directed to another sedimentation tank.

2. Oxidation Ponds

  • Oxidation ponds, also known as lagoons or stabilisation ponds, are large, shallow ponds that use the interaction of sunlight, bacteria, and algae to treat wastewater.
  • Algae use the sun’s energy, as well as carbon dioxide and inorganic compounds released by bacteria in the water, to grow.
  • During photosynthesis, algae release oxygen, which is needed by aerobic bacteria. 
  • Mechanical aerators are often used to provide even more oxygen, reducing the size of the pond needed.
  • Dredging is needed to remove sludge deposits in the pond.
  • Filtration or a combination of chemical treatment and settling will kill any residual algae in the pond effluent.

3. Activated Sludge Process

  • The air and sludge are allowed to come into close contact with the bacteria during this process and then passed into the settling tank.
  •  The sewage from the settling tank is first sent to an aeration tank, where bacteria are added to the air and sludge.
  • The entire setup is left alone for several hours, during which time the bacteria decompose the organic matter into toxic by-products. 
  • The sludge, which has been activated by billions of bacteria, is returned to the aeration tank to handle fresh sewage.
  • The previously treated wastewater is pumped to the sedimentation tank, which filters out any bacteria.

We have seen the primary and secondary treatment of wastewater. Now it’s time to see what happens in the tertiary treatment of wastewater.

Tertiary Treatment of Wastewater

The aim of tertiary wastewater treatment is to improve the water’s quality to meet domestic and industrial standards, as well as to meet specific criteria for water discharge safety. It can be done by ion exchange, reverse osmosis, chemical precipitation, membrane filtration etc. In the case of municipally treated water, the tertiary treatment also includes the elimination of bacteria, ensuring that the water is safe to drink.

In some cases, quaternary treatment is also done. This stage deals with contamination levels of a few parts per million to billions of parts per billion, and it often includes oxidation or fine filtration.

Wastewater Treatment
Wastewater Treatment

Shall we wrap up?

Conclusion

Wastewater treatment refers to the purification of sewage from various sources and converting it into a reusable form. It involves mainly 3 stages namely primary, secondary and tertiary.

Primary treatment involves screening, sedimentation, sludge and scum removal. Secondary treatment deals with the removal of biological contaminants using oxidation ponds, trickling filters or activated sludge process. Finally, the Tertiary treatment converts water into a reusable form.

The management of wastewater is inextricably related to the availability of clean and adequate water sources. Far from being anything to discard or neglect, wastewater will play an important role in meeting the increasing water demand in rapidly expanding cities, improving energy production and industrial growth, and promoting sustainable agriculture.

Also see : Concepts of green building – 4 comprehensive concepts explained.