Activated sludge bulking happens when sludge settles and compacts poorly in the secondary clarifier, most often because filamentous microorganisms have become excessive. Foaming is a related problem where stable foam or scum builds up on the aeration tank surface.
Both can interfere with normal treatment performance. Poor settling can lead to rising sludge blankets, solids carry-over and poorer effluent quality, while persistent foam can signal an imbalance in the activated sludge system.
This article explains what activated sludge bulking and foaming are, what causes them, the warning signs operators should watch for, and how to address the underlying conditions rather than rely on short-term fixes.
What is activated sludge?
Activated sludge is a biological wastewater treatment process that uses microorganisms to break down organic matter in an aeration tank. Air or oxygen is supplied to support the biology, creating a mixture of wastewater and microbial solids known as mixed liquor suspended solids (MLSS).
After aeration, the mixed liquor flows to a secondary clarifier at the treatment facility, where the biomass should settle and separate from the treated water. Part of the settled sludge is returned to the aeration tank as return activated sludge (RAS), while excess biomass is removed as waste activated sludge (WAS).
This basic principle is used across wastewater treatment systems, including conventional activated sludge systems, and in related processes such as sequencing batch reactors. Bulking affects the conventional activated sludge process and sequencing batch reactors alike.
What is activated sludge bulking, and how is it different from foaming?
Activated sludge bulking occurs when sludge does not settle and compact properly in the secondary clarifier. It is most commonly associated with excessive filamentous microorganisms, which can extend out from the sludge flocs and interfere with compact settling.
As settleability worsens, the sludge blanket may rise and solids can carry over into the treated effluent. Foaming is different: it appears as a persistent, often brown layer of stable foam or scum on the aeration tank surface.
The two problems can occur together and may share some of the same underlying conditions, but they are not the same failure mode. Bulking is primarily a settling problem, while foaming is a surface accumulation problem.
What causes activated sludge bulking and foaming?
Bulking and foaming usually develop when operating conditions favour filamentous or foam-forming microorganisms over the rest of the activated sludge population. The exact cause varies by plant, so identifying the pattern matters before deciding how to correct it.
| Cause | Why it can cause problems | Typical sign |
| Low dissolved oxygen | Some filamentous organisms compete well when oxygen availability is limited or unstable | Poor settling, rising sludge volume, filamentous growth under the microscope |
| Low food-to-microorganism (F/M) ratio or long sludge age | These conditions can favour certain slow-growing filamentous organisms | Bulking or persistent brown foam |
| Nutrient deficiency | Industrial wastewaters low in nitrogen or phosphorus can create an unbalanced microbial population | Weak floc formation and poor settling |
| Septic influent and sulphides | Septic wastewater can introduce reduced sulphur compounds that favour some filamentous organisms | Odour, dark influent and deteriorating settleability |
| Fats, oils, grease and surfactants | Hydrophobic materials can support stable foam and favour foam-forming organisms | Thick, persistent foam on the aeration tank |
Sludge residence time, or sludge age, is controlled by how much waste activated sludge is removed from the system. Hydraulic retention time is different. It refers to how long the wastewater remains in the treatment process.
Because several causes can produce similar symptoms, operators should look at dissolved oxygen, loading, nutrient balance, sludge age and influent conditions together rather than assume one factor is responsible.
What are the warning signs of activated sludge bulking?
The clearest warning signs are worsening settleability, a rising sludge blanket and increasing solids in the treated effluent. Operators may also notice persistent brown foam or changes in sludge behaviour before performance deteriorates significantly.
Common signs include:
- A sludge blanket that rises higher than normal in the secondary clarifier
- Cloudy effluent or visible solids carry-over
- Increasing settled sludge volume in the 30-minute settleability test
- Sludge volume index (SVI) trending above about 150 mL/g
- Thick brown foam that does not readily break up
- Long filamentous organisms visible under the microscope
How is sludge volume index calculated?
Sludge volume index is a simple way to track how well activated sludge settles. It is calculated by dividing the settled sludge volume after 30 minutes, measured in mL/L, by the mixed liquor suspended solids concentration in g/L.
SVI = settled sludge volume (mL/L) ÷ MLSS (g/L)
An SVI above roughly 150 mL/g can indicate bulking, but it should be treated as a guide rather than a universal cutoff. Trends over time are often more useful than a single reading.
How do you fix activated sludge bulking?
The right fix depends on the underlying cause, so the first step is to identify what is driving the loss of settleability. In a wastewater treatment plant, that means checking dissolved oxygen, sludge age, nutrient balance and influent conditions before changing the process.
Typical corrective actions include:
- Restore and stabilise dissolved oxygen in the aeration tank
- Adjust the waste activated sludge rate to correct sludge age and the food-to-microorganism ratio
- Correct nitrogen or phosphorus deficiencies where industrial wastewaters are nutrient-poor
- Reduce or prevent septic influent and sulphide loading
- Add or optimise a selector zone to favour better floc-forming organisms
Short-term measures such as RAS chlorination, defoamers or polymer can help control symptoms, but they do not remove the underlying cause. For long term control, operators need to correct the process conditions that allowed bulking to develop in the first place.
Why stable dissolved oxygen matters
Low or unstable dissolved oxygen is one of the operating conditions that can favour filamentous bulking, and one that operators can directly influence. Maintaining sufficient oxygen through the aeration tank helps support more stable biological treatment and reduces the advantage some filamentous organisms gain under oxygen-limited conditions.
This is especially important in nutrient removal plants, where dissolved oxygen may already be controlled within a tighter operating range.
Nanobubble oxygenation can improve oxygen transfer and help maintain more stable dissolved oxygen conditions through the tank. The main two recommended applications are:
- Use Nano bubbles and Ozone for improving the process in an existing digester by enhancing the process such that the same digester can receive higher volumes of sludge. This is a recommended economical solution and the preferred approach if you already have digester that is struggling to digest your increasing sludge volumes and the cost of upgrading or replacing it is evaluated to be too high.
- Use Nano bubbles and Ozone or oxygen for sludge reduction (i.e. receive higher % of solids in the treated sludge) – this is a recommended economical solution and the preferred approach if your main concern is the cost of Sludge disposal
a PMnB Technologies, a European nanobubble water treatment company backed by Palgey Maim’s engineering heritage, uses chemical-free nanobubble oxygenation to support activated sludge treatment systems where oxygen transfer and process stability are limiting factors.
Talk to an engineer about sludge settling.
