It’s All Connected

A problem with slime bulking at a wastewater treatment plant turned out to have its origins on the potable water utility side
It’s All Connected
The lab detective ran a reverse India ink stain test on a wet-mount slide of the MLSS. The test proved that the MLSS had excessive slime, as the India ink could not penetrate the floc on the slide.

Wastewater operators encounter a wide range of conditions that affect their plants on a weekly or even daily basis. Flow surges from rain events, power outages, severe storms, and illegal dumping of oil, grease and other nasty fluids are just a few of the problems.

Personally, I have found it’s these varying conditions that make wastewater treatment so challenging and, in some ways, enjoyable. Don’t get me wrong, I like the days when the plant is running smoothly. But I also enjoy the days when I get to put my troubleshooting skills to work. This is a story of one of those days.


Handling smaller flows

Running package treatment plants allows an operator to use many skills learned over time. A small package plant serving a highway rest area, campground or small mobile home community can quickly humble even the most seasoned operator.

These small units might treat flows less than 100,000 gpd but can be very difficult to control. In fact, many veteran operators will admit that running a small package-type plant is more challenging than operating a plant with more than 1 mgd flow. Things happen much faster at a small plant, and when things go wrong, they usually do so quickly, without warning.

The lab detective responded to a call from a fellow professional who operates and maintains a 24,000 gpd extended aeration activated sludge plant that serves a small mobile home community. The operator met the detective at the gate and described the situation: the mixed liquor suspended solids (MLSS) had a very light, yellowish brown color.

It settled very poorly in the clarifier and in the settleability test container, but the most interesting symptom was the consistency of the MLSS: It appeared to be slimy, almost gel-like. When the operator used a core sampler to determine the sludge blanket depth, the MLSS would cling to the sampler and slowly ooze off the clear plastic tube.


Seeking clues

The MLSS acted similarly in the settleability container, settling slowly with an appearance like cotton candy. The lab detective was perplexed to say the least. He left the treatment plant with many notes, photos, and a small MLSS sample for microscopic diagnosis.

Back at his office, he collected his favorite reference manuals (note references at the end of this article), got a cup of coffee, and went to work.

He suspected a condition called slime bulking, but that condition is uncommon at plants that treat domestic wastewater. Slime bulking occurs frequently at industrial treatment plants where influent nutrient loadings are unbalanced or pH values are low. So what could be causing a slime bulking condition at this small package plant?

The lab detective ran a reverse India ink stain test on a wet-mount slide of the MLSS. The test proved that the MLSS had excessive slime, as the India ink could not penetrate the floc on the slide (see photo). The references offered several clues to the causes of slime bulking. Nutrient-deficient wastewater can be a cause of excessive slime coating on bacterial cell walls.

Just like human beings, bacteria need nutrients for proper cell growth. When bacteria receive proper amounts of vital nutrients like carbon, nitrogen, phosphorus and sulfur, they develop a thin slime coating around their cells that aids in floc formation.

But if one or more of these elements are lacking, the slime layer does not form properly, resulting in a much thicker slime coating. The floc-forming bacteria then cannot clump tightly together, and instead form a loose mass with a jelly-like slime that resists the water surrounding it. In the settleometer, it has a fluffy, cotton-candy appearance and consistency.


Into the records

Armed with this information, the detective reviewed the treatment plant’s daily logs, lab results and state discharge reports. One thing stood out: the plant was receiving just under 19,000 gpd of flow, but had a very low influent BOD, at 80 to 85 mg/L. The state did not require influent nutrient testing but did require effluent nitrate nitrogen reporting; the limit was 12 mg/L.

The facility seemed to meet that limit well, with an annual average nitrate nitrogen of 4 mg/L. But, the detective thought, does that mean everything is all right? Perhaps the facility is not nitrifying at all? He needed more information to confirm his hypothesis.

Back at the treatment plant, the detective and the operator collected additional samples from the influent lift station, aeration tanks and clarifier. The detective’s hunch proved correct: the influent wastewater contained very little nutrient and was much diluted.

After the detective shared the diagnosis with the plant operator, who also ran the small water system for the community, the operator described the water system operation and water usage. The residents pay a flat monthly rate for their water, rather than pay for the water they actually use, which would be measured by a water meter.


Too much water

The residential services had no water meters — the only meter was at the water plant, measuring daily usage. The local water management district was concerned about the high usage at the utility and required the utility owner to begin a water audit to measure the usage.

With this new information, the lab detective had several 24-hour composite samples of the influent wastewater tested for total Kjeldahl nitrogen (TKN), orthophosphate, ferrous iron, and CBOD. For wastewater to be considered nutrient-balanced, the ratio of carbon:nitrogen:phosphorus:iron should be roughly 100:10:1:0.5 mg/L.

The wastewater entering this treatment plant certainly appeared unbalanced, with readings of 74 mg/L CBOD; 30 mg/L TKN; 2 mg/L PO4; and 10 mg/L Fe. Why so much TKN and iron? The detective learned that people who cleaned the community recreation building used large amounts of ammonia, and that the drinking water supply contained high soluble iron.

The water system was planning to build an actual water plant to lessen the iron from the water supply, using greensand filters. In addition, water meters were to be purchased for residential metering. The current water plant simply disinfected the source groundwater.

Water systems that do not meter individual residences and use flat-rate monthly billing seem to promote excessive water consumption: The residents do not see the need to conserve water when they do not pay by the gallon.

When residents don’t mind their water usage, leaking toilet flapper valves cause toilets to run continuously, and leaking sink fixtures and excessive yard irrigation contribute to excessive usage. All this water running down the drains dilutes the wastewater, creating a high influent flow with low nutrient content.


Toward a solution

The lab detective and the plant operator worked with the utility owner and engineer to educate the residents on the value of water and ways to conserve, especially since the water would soon be billed based on consumption.

The new water plant went into operation shortly thereafter, much to the residents’ delight. The water was much more palatable and contained less hardness, and the chlorine demand and dosage decreased, as well.

The effects of the water system upgrades affected the wastewater greatly, too. With the influent flow decreasing, the dilution factor decreased. Nutrient levels approached a near-perfect ratio, and the slime bulking became a thing of the past.

This story is just one example of how connected water and wastewater utilities can be. The condition of the drinking water distribution system and the operation and maintenance of the water utility greatly affect the wastewater collection system and the treatment plant. By communicating more effectively, operators on both sides of a utility can learn from each other and make their jobs easier at the end of the day.


About the author

Ron Trygar is senior training specialist in water and waste-water at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at


Wastewater Microbiology, A Handbook for Operators, Toni Glymph. AWWA, 2005.

Manual on the Causes and Control of Activated Sludge Bulking, Foaming and Other Solids Separation Problems, Third Edition, D. Jenkins, M. Richard, G. Daigger. Lewis Publishers, 2004.


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