How the Activated Sludge Process Works

Introduction

Activated sludge is a biological secondary treatment process used to remove biodegradable organic matter from wastewater. It works by bringing wastewater into contact with a concentrated microbial population in an aeration tank, then separating the biomass from the treated water in a secondary settlement stage. The settled sludge is partly returned to maintain the active culture, while excess sludge is wasted from the system.

For operators, municipalities and consulting engineers, the key point is simple: the process depends on keeping the biology, oxygen supply and solids separation in balance. When one of these elements is out of step, effluent quality and process stability usually deteriorate quickly.

If you want a broader overview of the process family, see the related page on activated sludge.

What activated sludge is designed to do

Activated sludge is a form of secondary treatment. Under the EU urban wastewater treatment framework, secondary treatment is defined as treatment that generally involves biological treatment with a secondary settlement stage, or another process that removes biodegradable organic matter.

In practical terms, the process is designed to:

reduce biochemical oxygen demand (BOD5)
remove a substantial share of chemical oxygen demand (COD)
reduce total suspended solids after clarification
produce a stable effluent that can proceed to discharge or further treatment

For EU secondary-treatment effluent standards under Annex I of the recast directive, the verified reference values are:

BOD5: 25 mg/L, or 70–90% reduction
COD: 125 mg/L, or 75% reduction
TSS: 35 mg/L

Those values are regulatory references, but actual plant performance depends on influent variability, temperature, sludge age, oxygen transfer, clarifier performance and operator control.

The main process stages

1. Influent enters the aeration tank

Primary settled wastewater, or in some cases wastewater with little or no primary treatment, enters the aeration basin. Here it is mixed with return activated sludge from the secondary clarifier. This return sludge contains the biomass that performs most of the organic removal.

The mixed liquor in the aeration tank contains:

wastewater substrate, mainly biodegradable organics
active microorganisms, especially heterotrophic bacteria
dissolved oxygen supplied by diffused air or mechanical aeration
inert solids and biomass floc

The basic purpose of this stage is to keep the microorganisms in close contact with the food source under aerobic conditions.

2. Aeration supplies oxygen and mixing

Aeration has two functions:

it provides oxygen for the microorganisms
it keeps the mixed liquor suspended and well mixed

As the biomass consumes soluble and finely particulate organics, it grows and forms flocs. These flocs are important because they settle more readily in the clarifier than dispersed single cells would.

From an operating point of view, aeration is rarely just a matter of adding more air. Too little aeration can create low dissolved oxygen conditions, poor treatment and odour risk. Too much aeration can increase energy use and may worsen floc break-up or carryover in some configurations. The correct setting is site-specific and should be based on process monitoring.

3. Biomass converts organics into new cells and end products

The activated sludge system relies on aerobic microbial metabolism. The organisms consume biodegradable organic matter and convert it into:

new biomass
carbon dioxide
water
other end products of metabolism

Some of the organic load is captured as new cell mass rather than being fully oxidised. That is one reason why sludge production is an inherent part of the process. The excess biomass must be removed from the system and managed as waste sludge.

4. Secondary clarification separates solids from treated water

After aeration, the mixed liquor flows to the secondary clarifier. This is the settlement stage that makes activated sludge a true suspended-growth process with solids separation.

In the clarifier:

biomass floc settles by gravity
clarified effluent overflows to discharge or further treatment
settled sludge accumulates at the bottom
part of the settled sludge is returned as return activated sludge (RAS)
the remainder is wasted as waste activated sludge (WAS)

Good clarification is essential. If solids do not settle properly, suspended solids can escape in the effluent even when biological removal in the aeration tank is adequate.

Why return sludge matters

The return activated sludge stream maintains the concentration of microorganisms in the aeration tank. Without this recycle, the biomass would wash out of the system and treatment performance would collapse.

The return sludge flow influences:

mixed liquor solids concentration
sludge blanket depth in the clarifier
solids loading to the aeration basin
process stability during hydraulic peaks

The wasting stream is equally important because it controls the age of the biomass in the system. By wasting sludge, the operator removes older biomass and controls sludge retention time. In practice, sludge age is a key control lever, but the suitable value is plant-specific and depends on temperature, treatment objectives and sludge settleability.

The control variables operators watch

Activated sludge is often described as a simple process, but it is actually highly dynamic. Common operating variables include:

dissolved oxygen in the aeration tank
mixed liquor suspended solids (MLSS)
sludge age or solids retention time (SRT)
food-to-microorganism ratio (F/M)
return sludge flow
sludge blanket level in the clarifier
settleability indicators such as sludge volume index, where used locally

These are typical process-control variables, not fixed universal targets. Common design or operating values may be used as indicative starting points, but they should always be confirmed against the plant’s own influent, tank configuration, aeration system and compliance needs.

In general:

low dissolved oxygen can reduce organic removal and worsen sludge behaviour
excessively high MLSS may overload aeration or clarification capacity
too low a sludge age can lead to biomass washout
too high a sludge age can increase oxygen demand and affect sludge characteristics
poor settleability can cause solids carryover even when the biology is working

Common configurations and what changes between them

Activated sludge is not one fixed arrangement. The biological principle is the same, but the hydraulic and process layout can differ.

Common variants include:

conventional plug-flow activated sludge
complete-mix systems
extended aeration
oxidation ditch arrangements
nutrient-removal configurations with anoxic and aerobic zones

The differences matter because they affect mixing pattern, oxygen demand, sludge age and response to load variation. For example, a stable low-load extended aeration plant behaves differently from a conventional high-rate system. Likewise, configurations intended for nitrogen removal require additional process zones and tighter control of recycle flows.

For the purpose of this article, the key point is that the basic mechanism remains the same: biomass consumes biodegradable organics in the aeration basin, then is separated in a clarifier and mostly returned to the process.

How activated sludge fits into EU treatment obligations

The recast Urban Wastewater Treatment Directive entered into force on 1 January 2025. It repeals the old Directive 91/271/EEC with effect from 1 August 2027, and Member States must transpose it into national law by 31 July 2027.

For secondary treatment, the verified EU framework is:

mandatory secondary biological treatment for agglomerations of 2,000 p.e. and above
extension of the obligation to agglomerations of 1,000 p.e. and above by 31 December 2035

A population equivalent is defined as the organic biodegradable load having a five-day biochemical oxygen demand of 60 g of oxygen per day.

The directive also sets longer-term requirements for nutrient removal, quaternary treatment and energy neutrality, but those are beyond the core scope of activated sludge as secondary treatment. In practice, however, many activated sludge plants are already being assessed as part of broader upgrade pathways.

Typical operating issues and their meaning

When a plant is not performing well, the symptom is often seen first in the clarifier or effluent, but the cause may lie in aeration, sludge age or upstream load variation.

Common issues include:

High effluent suspended solids: often linked to poor settling, sludge bulking, filamentous growth or hydraulic overload
Rising BOD5 or COD in effluent: may indicate insufficient aeration, short-circuiting, overload or process shock
Poor sludge settling: can result from biomass imbalance, low dissolved oxygen, toxic influent or inappropriate sludge age
Foaming or scum: may be associated with grease, surfactants, filamentous organisms or load changes
Loss of nitrification in systems designed for nutrient removal: often related to insufficient sludge age, temperature effects or aeration limitations

The correct response is usually not to change one variable in isolation. A better approach is to check the full process chain:

influent characteristics and hydraulic loading
primary treatment performance, if present
aeration capacity and distribution
mixed liquor condition and sludge settleability
clarifier loading and sludge return
wasting practice and sludge age

How this applies to your plant

If you are operating, upgrading or diagnosing an activated sludge plant, start with the process balance rather than the symptom alone. The system only works well when oxygen transfer, biomass concentration, sludge return and clarification are aligned with the actual influent load.

That is why a structured review is useful before deciding on capital works or control changes. Use an engineering assessment to check whether the current process configuration is suitable for present and future loads, effluent expectations and regulatory context.

You can begin here: Start the Engineering Assessment.

A good assessment should examine:

current hydraulic and organic loading
aeration system condition and capacity
clarifier performance and solids separation
sludge age and wasting practice
scope for process optimisation or staged upgrading

For many plants, the most effective improvements come from better control and diagnostics rather than immediate process replacement. When activated sludge is understood as a coupled biological and separation system, its performance becomes much easier to manage.