Every activated sludge plant has some foam. A thin layer of light tan foam that breaks up easily is normal and even a sign of healthy sludge. But when the aeration basin grows a thick, stable, chocolate brown blanket that does not break down under water sprays, you have a problem that will not go away on its own.
This article explains how to read foam by color and texture, what causes stable brown foam, and what actually works to get rid of it.
Reading foam by color and texture
Light tan, thin, breaks easily. Normal for a stable plant at a moderate SRT.
Crisp white, billowing. Usually young sludge: startup, low MLSS, very short SRT, or recovery after washout or a toxic event. Can also come from detergents. See the white foam guide.
Dark brown to chocolate, thick, greasy, stable. Most often caused by filamentous foam formers, especially Nocardioforms (Gordonia and related actinomycetes) or Microthrix parvicella. This is the subject of this article.
Very dark brown to black. Often old or septic sludge, very long SRT, or anaerobic conditions. Check DO and blanket depth.
Gray or black scum on clarifiers. Rising septic sludge, not aeration foam.
Why stable brown foam forms
Foam forms when gas bubbles rise through the mixed liquor and something stabilizes them at the surface. In stable brown foam, the stabilizers are hydrophobic (water repelling) filamentous bacteria. Their cell surfaces are waxy and attach to air bubbles. The filaments form a mesh that traps bubbles, grease and floc particles into a thick scum that can be several inches to feet deep.
The two most common foam formers:
Nocardioforms
- Short, branched, Gram positive filaments, often seen inside floc and in the foam.
- Favored by long SRT, grease, oil and fats, warm water, and surface trapping.
- Common in plants that run long SRT for nitrification or have high grease loads.
Microthrix parvicella
- Thin, coiled, unbranched, Gram positive filaments.
- Favored by low F/M, long SRT, grease and long chain fatty acids, and cold water.
- Also causes bulking; often a winter problem.
Other filaments (for example Type 0092 and some Type 1863) occasionally foam, but Nocardioforms and Microthrix cause most stable brown foam.
Confirm with the microscope
Collect a sample of the foam itself, not just mixed liquor. Smear it thinly, let it dry, and Gram stain. At 1,000x under oil:
- Gram positive, branched, short filaments: Nocardioforms.
- Gram positive, thin, coiled, unbranched: Microthrix parvicella.
Compare abundance in the foam with abundance in the mixed liquor. Foam formers concentrate in the foam, often at many times the mixed liquor level. Our microscopy library has line sketches of both.
Why foam is hard to get rid of
Foam traps itself. Foam formers concentrate in the foam, so if foam stays in the basin or is returned to the process, those organisms effectively have a much longer SRT than the rest of the sludge. You may think you run an 8 day SRT, but the foam formers live for weeks.
Scum recycle. Many plants send clarifier scum and basin foam to the headworks or primary clarifiers, which sends the organisms right back to the aeration basin. Digesters are a common destination too, and Nocardioforms can cause digester foaming.
Baffles and walls. Underflow baffles at the end of the basin or between zones trap foam on the surface.
What works
1. Remove foam from the system
This is the most important step. Get the foam out of the process and send it to a place where it cannot return:
- Spray foam toward a removal point (selective surface wasting).
- Use foam hoppers or surface wasting boxes that skim the surface into the WAS line.
- Send scum to dewatering or a separate handling route instead of the headworks.
- Remove baffles or create overflow points so foam is not trapped.
2. Lower the SRT, if you can
Shortening SRT reduces Nocardioforms and Microthrix, which grow slowly. But it can also cost you nitrification, especially in cold water. Reduce gradually and watch ammonia closely. Removing foam from the system (step 1) effectively shortens the foam formers' SRT without changing the plant SRT, which is why it comes first.
3. Control grease
Work with pretreatment staff on grease interceptor programs, restaurant inspections, and hauled grease receiving. Hauled fats, oils and grease (FOG) should go to digesters through a controlled feed, not to the headworks.
4. Targeted chlorination
Spraying a dilute chlorine solution directly onto the foam can kill the filaments in it. Some plants also chlorinate RAS. Both need careful dosing because chlorine also harms floc formers and nitrifiers. Typical RAS chlorination starting doses are 2 to 3 lb chlorine per 1,000 lb MLSS inventory per day, adjusted with daily microscope checks. Use the chlorine dose calculator to work out pounds and feed rate.
5. Selectors
An anoxic or aerobic selector (a small, high F/M zone at the head of the basin) favors floc formers over many filaments. Selectors work well against some foam formers, especially Microthrix, but less reliably against Nocardioforms.
6. Other tools
Some plants use polyaluminum chloride (PAC) for Microthrix control, antifoam chemicals for short term relief, or classifying selectors that preferentially remove floating solids. Discuss these with experienced help before trying them.
What does not work well
- Water sprays alone. They knock foam down temporarily but leave the organisms in the system.
- Antifoam alone. Expensive and treats the symptom.
- Large SRT cuts. May lose nitrification and still leave foam trapped.
Example: White River WRRF
The fictional White River WRRF receives hauled FOG, which is fed to the digesters. In September, a 5,000 gpd FOG slug on two days upset Digester 2. In the aeration basins, foam has stayed light because surface scum from the clarifiers goes to the digesters rather than the headworks and the aeration SRT is moderate (10 to 14 days). If brown foam appeared in winter at a longer SRT, the plant's first steps would be to sample the foam for Microthrix, confirm scum is not recycled to the headworks, and review FOG receiving.
Track foam like any other process variable
Foam problems are easier to solve when you have numbers instead of impressions. A simple daily log makes trends obvious:
- Coverage: estimate the percent of each basin surface covered (0, 25, 50, 75, 100%).
- Depth: measure foam depth with a marked rod at the same spot each day.
- Color and texture: light tan, white, brown, dark brown; crisp, greasy, or stable.
- Microscope: Gram stain the foam weekly and rate foam former abundance on a 0 to 6 scale.
- Related data: SRT, MLSS, temperature, grease loads, and where scum was sent.
Take a photo from the same spot each day. A month of photos next to an SRT chart often shows the cause more clearly than any single test.
Also watch the digesters. Nocardioforms in WAS can cause digester foaming, which is harder to deal with than aeration foam because it can plug gas lines and damage covers. If digester foam appears a few weeks after aeration foam, the two are likely linked.
Summary
- Stable brown foam usually means Nocardioforms or Microthrix parvicella.
- Confirm by Gram staining the foam.
- Remove foam from the system and stop recycling scum.
- Lower SRT gradually if nitrification allows.
- Control grease at the source.
- Use chlorination and selectors as supporting tools.