Clear Improvement

Clarifier evaluations help operators understand solids profiles and flow patterns and make physical or operation adjustments to improve capacity.
Clear Improvement
John Esler, president, Clarifier Performance Evaluations.

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Clarifiers are often the limiting factor in a wastewater treatment plant's capacity, says John Esler, president of Clarifier Performance Evaluations in Enfield, N.H.

Esler and colleagues perform tests on clarifiers using tools from advanced measurement technologies to garage-built devices to create solids profiles and assess flow patterns and currents. Then they give plant operators recommendations on how they can improve clarifier performance by making structural or operational changes.

Esler talked about assessing and improving clarifier performance in an interview with Treatment Plant Operator. He was accompanied by Ron Wood, central regional manager with Xylem's analytics businesses, which include Royce Technologies, maker of a suspended solids analyzer used in clarifier evaluations to measure the sludge blanket and create complete clarifier solids profiles.

What's the basic concept behind clarifier evaluations? What is the need in the market for such a service?

Esler: A car has a carburetor that you can tune up to make the car go faster. That's what we do with clarifiers. Clarifiers are most often the limiting factor in a treatment plant. We evaluate those clarifiers. We do hydraulic evaluations, and we evaluate the solids to see how the clarifiers are working under different loadings.

When we find out how they're working, we can address the weakest part of it, strengthen it, and get more gallons through. We look at how they're performing, and make whatever adjustments we can.

What exactly makes clarifiers a limiting factor?

Esler: The major issue is hydraulics, and that's true in both circular and rectangular clarifiers. In either case, there's a current, like a current in a stream. It carries solids that you would like to see settle, but instead they get transported to the effluent. There's a density current from the weight of the mixed liquor settling, but there's also current that's created by the way the inlet is configured.

For example, the way the flow comes in the inlet of a circular clarifier, it can blow out the center of the clarifier. Operators never look there. They're trained to look with their core samplers at the mid-radius of the clarifier. Because of the way we sample with the Royce technology, we can see that the blanket is actually going down in the center.

What exactly is the Royce technology and what are its benefits?

Esler: It's a Royce Model 711 handheld, portable suspended solids/interface level analyzer. Some operators use it just to measure the blanket level. We use it to assess at every foot of depth at all sections across the clarifier. We create what we call vertical solids profiles. We read the milligrams per liter of TSS at every foot of depth all the way down to the bottom, so we can see how concentrated the solids are. But the main thing we can see is how the solids move as we stress the clarifier.

We do vertical solids profiles inside the center well and all across the clarifier. We start at seven o'clock in the morning, we do it again at nine, and we do it again at 11, while cranking flow to the clarifier. We stress the clarifier, pushing it to its limits, so we can see what its limiting factors are.

From a technology perspective, how does this instrument work?

Wood: The technology is based on light absorption. You have a sensor with a gap in it. Liquid flows through the gap, which has a light emitter on one side and a light detector on the other. A light beam is passed across the gap, and any solids present in the water will absorb light.

The sensor is on a cable marked in one-foot increments. You lower the sensor into the clarifier, and once you find the heaviest concentration of solids, you know that's where the main blanket is. Then you pull the sensor up and count the cable markings.

What kind of information can you get with this method that operators couldn't get with a core sampler?

Esler: With this technology, we can actually quantify the solids in the clarifier. We can tell how many pounds are in there at 7 a.m., at 9 a.m., at 11 a.m. We can tell how much is in clarifiers 1, 2 and 3, and how the solids have shifted.

Why would an operator have an evaluation done if the plant seems to be working fine and is meeting its permit?

Esler: Maybe they want to try something different. They may want to compare Clarifier 1 to Clarifier 2. They may want to get more flow through the plant. In that event, how is it running now? What's the weak link? Nowadays, the question often is how to increase plant capacity because of storm flows and I&I. They own the concrete shape, and they want to get as many gallons as possible through it. That's where these evaluations come in. Once they've run an analysis, they know which direction to go.

Is there more to an evaluation than solids profiling?

Esler: Yes. We do dye studies so they can see how the flow picture is moving through the clarifier. We also do current measurements using a simple device called a drogue. Once we see how the solids are moving, how the currents are moving, and how the overall hydraulics are working, we write our conclusions and recommendations.

How do the dye studies work?

Esler: We put a slug of fluorescent dye into the mixed liquor channel and test effluent samples every five minutes with a fluorometer to see how the dye has progressed through the clarifiers. We use the data to create a flow curve showing the dye concentrations versus time.

What is a drogue and what is its function?

Esler: You can make a drogue by getting a 2- by 3-foot sheet of aluminum from a hardware store, cutting it into four pieces, and putting two pieces together to make an X-vane. Then you go to a party store and buy a Styrofoam float and attach the two items together with a line with one-foot links in it. If you want to see what the flow is five feet down in the clarifier, you put five feet of links on it and monitor the flow as the device moves through the clarifier.

What would be an example of an operational change a treatment plant might make as a result of your recommendations?

Esler: Take draft tube clarifiers for example. After measuring the solids concentration coming out of the draft tubes, we may be able to tell the operators: On this arm, shut off tubes 1, 3 and 5; on the opposite arm, shut off tubes 2 and 4. In some cases we've actually reduced the volume of return activated sludge flow by 40 percent while increasing the pounds of solids returned by 40 percent, just by running thick tubes and not running thin tubes.

Can you describe a specific case where a clarifier evaluation helped lead to a major improvement in plant capacity?

Esler: The Hyperion treatment plant in Los Angeles had 150 mgd worth of secondary clarifiers and 450 mgd worth of primary clarifiers. The 450 mgd would go through the primaries, and 150 mgd would go through the secondaries. The balance would go around secondaries, and then they'd blend the flow and send it to the ocean.

The EPA then ordered them to provide secondary treatment for the entire flow. The upgrade was to include 36 round clarifiers. They built the first 16, and they weren't working as well as expected under wet-weather conditions — they were not meeting their effluent requirements. We were called in by an engineering firm to evaluate the clarifiers.

We tested a battery of four clarifiers. We were able to identify the mode of failure: As the flow increased, the clarifier's sludge blanket left the center and migrated to the outside, causing the return sludge concentration to decrease and the effluent solids concentration to increase to the point of failure. Without the Royce analyzer, we would not have been able to conduct as many tests and acquire the data necessary to identify the conditions inside the clarifier at the point of failure.

Once we classified the cause, we helped design a novel energy-dissipating inlet that drastically improved the clarifiers' performance and helped increase capacity by 10 mgd per clarifier. They implemented that solution on all 36 new clarifiers, delaying the need for an expansion of the secondary treatment system for the foreseeable future.

Can clarifier improvements help reduce usage of treatment chemicals?

Esler: We try to eliminate a "drug habit" if we can. Particularly these days in a biological nutrient removal plant, they have to reduce phosphorus loading, and that really means reducing TSS. You can crank a lot of "drugs" through to get the solids to settle, or you can get the clarifier to settle better and use less "drugs."

At the most basic level, what would you say are the key benefits of these clarifier evaluations?

Wood: I would say the key benefit is process improvement, and probably energy savings. In the studies John does, plants typically end up pumping heavier sludge and less water, and therefore pump less volume.

Esler: We show them a way to look at their clarifiers so that they can operate them better. They come away with a much better understanding of their clarifiers, and given that understanding they should be able to make improvements to their operations to get more gallons through the clarifiers. We would rather see a plant improve its four clarifiers to get 25 percent more capacity out of them than build a new one for $2 million.


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