What’s Growing on Your RBC? A Field Guide to Unusual Growth on RBC Units

For wastewater treatment plant operators, the RBC can tell you a lot before the laboratory results arrive. The color, texture, thickness, and distribution of the growth on the media are valuable clues to what is happening biologically inside the process.

A healthy RBC does not necessarily look “clean.” In fact, operators should expect to see a relatively thin, uniform biological film. In carbonaceous-treatment stages, a healthy growth is generally brown to gray and shaggy, while later stages designed for nitrification may develop a thin reddish-brown or bronze-colored growth. The trouble starts when the biomass becomes unusually thick, white, black, or otherwise abnormal.

The following conditions are among the most important problematic growth patterns operators should recognize.

1. Thick, Heavy, Shaggy Brown or Gray Growth

What it looks like

The first-stage media become covered with a thick, heavy, shaggy layer of brown-to-gray slime. Instead of a relatively thin and even film, the biomass looks furry or matted and may extend well out from the disc surface.

In severe cases, the growth becomes heavy enough to affect shaft rotation. Operators may notice uneven rotation, surging, increased motor load, or “loping” as the biomass becomes unevenly distributed.

What conditions favor it?

This is most commonly associated with excessive organic loading, particularly excessive BOD loading to the first stage. The organisms are receiving more food than the available RBC surface and oxygen-transfer capacity can efficiently handle.

As the biofilm gets thicker, oxygen has difficulty penetrating to the deeper portions of the film. The result can be a cycle of excessive growth, oxygen depletion, internal biomass death, and eventual sloughing. Excessive biomass can also create significant mechanical loading on shafts and drives.

What should the operator do?

• Check influent and primary-effluent BOD, soluble BOD, TSS, and flow.

• Look for an industrial or septage discharge that may have suddenly increased the organic load.

• Check DO in the RBC stages, particularly the first stage.

• Place additional RBC capacity into service if available.

• Adjust flow distribution or baffles to spread the load more effectively.

• Consider step-feed operation where the facility is designed for it.

• Increase rotational speed only within the manufacturer's operating limits; increased rotation can improve oxygen transfer and help control excessive biomass.

• Where provided, use supplemental aeration or an approved biomass-stripping procedure.

• Remove excessive biomass when necessary rather than allowing it to continue accumulating.

Do not ignore this condition simply because the effluent laboratory results still look acceptable. Excessive biomass can become an equipment problem before it becomes an effluent problem.

2. White or Gray, Stringy/Filamentous Growth

What it looks like

The media may develop a white, gray, cottony, stringy, or filamentous-looking growth. It may appear as long strands or as a pale coating over much of the disc.

This is one of the most important visual warning signs on an RBC.

What conditions favor it?

White filamentous growth is strongly associated with septic conditions, low dissolved oxygen, and elevated sulfide loading. Sulfur-associated organisms such as Beggiatoa and Thiothrix can become dominant under these conditions.

Sources of the problem may include:

• Septic influent

• Long collection-system detention times

• Hydrogen sulfide entering the plant

• Septage receiving

• Industrial sulfur compounds

• Excessive organic loading causing oxygen depletion in the first stage

These organisms can interfere with normal BOD removal and may be accompanied by the unmistakable rotten-egg odor of hydrogen sulfide.

What should the operator do?

Start by determining whether the problem is septic wastewater, excessive organic loading, or both.

Operators should:

• Check influent DO, sulfide/H₂S, BOD, pH, and temperature.

• Check for septic conditions in the collection system.

• Investigate long force-main or wet-well detention times.

• Evaluate septage receiving practices.

• Look for industrial sources of sulfide.

• Increase aeration where supplemental aeration is available and appropriate.

• Reduce excessive organic loading to the first stage.

• Improve flow distribution between RBC stages.

• Consider approved sulfide-control measures where appropriate.

The important point is to correct the cause rather than simply strip the white growth off the media. If septic wastewater continues to enter the RBC, the white growth will usually return.

3. Black Slime

What it looks like

Black or very dark biomass is not a normal RBC appearance. The media may develop a black coating or black, greasy-looking slime, often accompanied by an unpleasant or sulfur-like odor.

What conditions favor it?

Black growth is generally a sign that the biological environment has become excessively oxygen-limited. It is commonly associated with solids and/or BOD overloading, particularly when treatment performance is deteriorating and DO is low.

The condition can be especially concerning when it occurs together with:

• High influent BOD

• High TSS

• Low RBC-stage DO

• Septic wastewater

• Hydrogen sulfide odors

• Heavy biomass accumulation

What should the operator do?

Treat black growth as a process warning, not merely a housekeeping issue.

• Check DO throughout the RBC train.

• Check influent and primary-effluent BOD/TSS.

• Determine whether the plant is experiencing an organic overload.

• Inspect pretreatment and primary clarification.

• Check for septic conditions and sulfide.

• Put additional RBC capacity into service if available.

• Increase supplemental aeration where appropriate.

• Correct hydraulic or organic distribution problems.

• Remove accumulated solids from the RBC basin if necessary.

4. Green Algae Growth

What it looks like

Algae are usually easy to recognize. The media may develop green, green-brown, or moss-like growth, particularly on portions of the discs exposed to sunlight.

What conditions favor it?

Algae require light, so the problem is most common on uncovered RBC units or portions of the media exposed to significant sunlight.

Algae are not necessarily an indication of an overloaded biological process. However, excessive algal growth can interfere with normal biofilm development and can contribute to operational and maintenance problems.

What should the operator do?

If algae are becoming excessive:

• Inspect covers and other means of limiting direct sunlight.

• Correct damaged or missing covers.

• Maintain the RBC enclosure as designed.

• Physically clean excessive growth when appropriate.

• Avoid making major biological process adjustments solely because algae are present; first determine whether sunlight exposure is the actual cause.

The key distinction is that green growth usually points toward light exposure, whereas white, gray, or black growth is more likely to indicate a biological/process problem.

5. Uneven or Patchy Growth

What it looks like

Instead of a fairly uniform biological film, operators may see:

• Heavy growth on one portion of the disc

• Bare patches

• Thick growth alternating with thin growth

• Growth concentrated near particular stages or portions of the shaft

Uneven growth can eventually produce an unbalanced shaft, resulting in irregular rotation or loping.

What conditions favor it?

Uneven growth can result from uneven organic loading, poor hydraulic distribution, variations in stage loading, excessive biomass, or mechanical/rotational problems.

What should the operator do?

• Inspect the baffles and flow distribution.

• Verify that each RBC train is receiving the intended flow.

• Check shaft rotational speed.

• Look for mechanical problems.

• Compare biomass thickness from stage to stage.

• Check for excessive organic loading.

• Clean or strip excessive biomass when necessary.

• Check for shaft deflection and abnormal motor loading.

Do not assume that uneven biomass is purely a biological problem. Mechanical problems can produce uneven growth, and uneven growth can then create mechanical problems.

A Simple Operator's Field Diagnosis

When making your daily RBC inspection, think in terms of color + texture + location + odor.

What you see	What it may indicate	First things to check
Thin brown/gray, fairly uniform film	Normal carbonaceous biomass	Routine DO and process monitoring
Thin reddish-brown/bronze film in later stages	Normal nitrifying biomass	Ammonia, DO, temperature, pH
Thick, shaggy brown/gray growth	Organic overload	BOD, flow, DO, loading distribution
White/gray filamentous growth	Septicity, sulfide, low DO	H₂S, sulfide, DO, septic sources
Black slime	Severe oxygen limitation/overload	DO, BOD, TSS, sulfide, loading
Green growth/algae	Sunlight exposure	Covers and light exposure
Uneven/patchy	Distribution or mechanical problem	Flow distribution, baffles, shaft/drive

The visual appearance is a diagnostic clue—not a laboratory diagnosis. When an unusual growth appears, confirm the suspected cause with process data. A good operator will correlate what is being seen on the discs with flow, BOD, TSS, DO, pH, temperature, sulfide, ammonia, and effluent performance.

The Most Important Rule: Don't Just Clean the RBC

When operators see an unusual growth, the natural reaction is often to ask, “How do we get this stuff off the discs?”

That is only half the question.

The better question is:

“Why did this organism have the opportunity to grow here in the first place?”

Stripping or cleaning excessive biomass may provide immediate relief, but if the underlying organic overload, septic condition, sulfide loading, poor flow distribution, or low-DO condition remains, the abnormal growth will return.

RBC operation is fundamentally a balancing act between organic loading, oxygen transfer, biomass growth, hydraulic distribution, and mechanical loading.

For operators, the RBC should therefore be treated as an early-warning system. Learn what normal biomass looks like at your plant, inspect the media consistently, and investigate changes before they become an effluent violation—or a broken shaft.