Where Have All the Solids Gone?

Tiny, short-lived flying insects created a serious process problem at a small package-type activated sludge treatment plant
Where Have All the Solids Gone?
FIGURE 1: Sludge clumps in the aeration tanks looked gray, like shaggy mop heads rising and sinking.

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Early one morning, the Lab Detective received a call from a local operator, who described an extended aeration activated sludge package-type wastewater treatment plant serving a small community, with a permitted capacity of about 50,000 gpd.

The operator said the plant had been running well, producing quality effluent with discharge values well below the state permitted levels. When the operator came in to service the facility after a recent weekend, the effluent looked cloudy, with a hazy brown coloration.

He described the aeration tank’s appearance: The mixed liquor suspended solids (MLSS) looked thin, as if most of the solids had disappeared. The return activated sludge (RAS) flow from the clarifier was also thin and watery. He noted that sludge clumps were seen in the aeration tanks that looked gray, like shaggy mop heads rising and sinking again (Figure 1).


Solids washout?

The Lab Detective arrived at the plant and found the operator’s description accurate. One theory discussed with the operator was the possibility of a hydraulic washout of the mixed liquor solids, but further investigation of the chlorine contact tank and the effluent discharge into a percolation pond found no evidence of such an event.

There were several sludge clumps in the corner of an aeration tank near the surface. The detective picked up a long-handled pool-cleaning brush and broke up the clump releasing a massive number of small red worms. Acting on a hunch, he assembled his core sampler, a clear plastic device for measuring the sludge blanket.

Using the sampler in various locations around the actively mixing aeration tanks, he located the missing mixed liquor solids. As the Lab Detective poked the sludge core sampler all around the bottoms of the tanks, large amounts of bubbles were released, and clumps of dark brown to gray sludge rose to the surface.

As the clumps surfaced, the detective stirred the clumps with the pool brush, again releasing large numbers of red worms that began wiggling throughout the tank. He identified the worms as midge (Chironomid) fly larvae. These larvae, commonly called bloodworms, range from a few fractions of an inch to as long as one inch, depending on where they are in their life cycle (Figure 2).


Short life cycle

Like most flying insects, Chironomids go through metamorphosis from egg to adult. Male midges gather in a mating swarm that can be readily seen near standing water, ponds, creeks and streams, and there may be 100 or more males flying around in the group. When a female enters the swarm, a male and the female will mate.

As they descend together to the water surface, the female deposits her clutch of fertilized eggs onto the still water surface (most likely a secondary clarifier in this case). The egg mass sinks into the settled sludge or clings to the algae that grow on the tank walls. As the eggs incubate, they become tiny larvae.

The larval midge has a sticky outer layer covering its body, and the very fine suspended solids surrounding the young midge begin to accumulate on its exterior, forming a kind of tube. The young midge also feeds on decaying organic matter, so it grows and grows as it collects the silt-like organic material that enters the tube.

As the midge larvae mature (anywhere from two to seven weeks), they transform into pupae. After several days, the pupae emerge from the tube and swim to the surface, where they become adults after a few hours. The adult midge flies up into the air, enters a mating swarm, and the whole cycle begins again.

Adult midges live only a few days. Each adult female can lay an enormous number of eggs, from 100 to 3,000, depending on the species of Chironomid. The adult midge looks very similar to a mosquito (some folks call them blind mosquitoes), but they do not bite, and male midge flies have distinct feather-like antennae (Figure 3).


Time to pump

The operator of this small wastewater treatment plant had experienced a midge fly invasion. As the midge eggs were laid in the clarifier, the return activated sludge (RAS) airlift pump sent the egg-filled sludge into the aeration tank. Since the solids were so thick with sticky egg masses, the end result was like adding a coagulant (polymer) to the basins.

Piles of settled solids were gathered in the aeration tanks and clarifiers. A condition known as coning had been occurring in the clarifier, where thick solids would not move toward the airlift return sludge pump suction. That allowed a thin sludge to flow over and around the thick sludge to the pump suction.

To remedy the situation, the operator called in a sludge hauler to pump out the thick, midge-infested solids from the aeration tank bottoms and clarifiers. He re-seeded the treatment plant with fresh, healthy return activated sludge from another nearby treatment plant. The facility recovered quickly and began producing a good-quality effluent within a few days.


Nuisance on the wing

Midge flies can be a significant nuisance. Operators can breathe them in or get them in their eyes or mouths while walking around treatment plant grounds. Swarms can affect residents of surrounding neighborhoods. Midges provide food to spiders and other insects, so equipment, handrails, pumps, motors and light fixtures can quickly become filthy. So how can midge flies be controlled? There are several methods:

• Minimize plant lighting at night. Operate outdoor lighting at treatment units, filters and tanks only when needed. This may save a few dollars on the electric bill, too.

• Keep treatment units clean. Control algae buildup and keep areas of standing water clean.

• Use a product that contains Bti (Bacillus thuringiensis israelensis), a group of bacteria that produce toxins that only affect the larva of flies. Bti is an active ingredient in a product called Mosquito Dunks, produced by Summit Chemical. This product can be found at most home-improvement stores or by contacting a local aquatic insect control agency near you. It has been used to control all kinds of mosquitoes and midges.

• Use a product containing methoprene, a chemical that mimics a juvenile growth hormone that inhibits midge larvae from becoming adult midges, breaking the life cycle. Methoprene is found in a product called Strike by Zoëcon. It has been used successfully in the wastewater treatment industry to control trickling filter flies and midges.

• Use a product that forms a monomolecular film (MMF). A thin film spray-applied to standing water, reducing the surface tension. Mosquito and midge larvae, pupae and adults cannot cling to the liquid surface (Figure 4) and essentially drown. Cognis Corporation produces a product called Agnique MMF that has been used for this purpose.

• Introduce a natural predator, the Gambusia, better known as the mosquitofish (Figure 5). They can be found in most shallow freshwater habitats, where they are safe from larger predators. Gambusia feed on mosquito and midge fly larvae, pupae and adults. They are resilient to harsh environments (such as low dissolved oxygen and temperature variations) and are considered by some to be the most widespread freshwater fish.

Mosquitofish can be harmful to native fish in streams and ponds, but for use in a wastewater secondary clarifier, that probably won’t be an issue. The Lab Detective has used this method at a small package-type treatment plant with some success. He released 20 mosquitofish into a midge- and mosquito-infested clarifier. Several days later, most all the larvae were gone, and the fish looked as if they had gained 10 pounds.

Whatever the method of Chironomid control you choose, it’s always a good idea to check with your local and state regulatory authorities to be sure it is approved and will not harm the environment, the treatment plant or the public in any way. Check with other users of the chosen products to get their input on dosage and efficacy. As always, use chemicals with care and practice safe handling.


About the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.



1. “Chironomid Midges,” Ernest C. Bay, Emeritus Urban Entomologist, Washington State University Cooperative Extension, Puyallup, 2003.

2. “Biology and Control of Non-Biting Midges,” Charles Apperson, Michael Waldvogel and Stephen Bambara, Extension Entomology, North Carolina State University, North Carolina Cooperative Extension, 2006.


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