Secondary clarification and settling troubleshooting

# Secondary clarification and settling troubleshooting

> A clarifier is where the whole activated sludge process gets graded. Good biology behind a clarifier that can't keep up still shows up as bad effluent.

Why the exam tests this

Treatment Process Evaluation and Adjustment, the same 38 percent duty as the rest of the wastewater treatment process, is where secondary clarification falls. Most small-plant operators watch one clarifier every day and get a feel for its blanket and its weir. The exam asks you to back that feel with two loading checks and a short list of symptoms, each with its own fix.

Grades I and II mainly test the loading-rate math and the basic symptom list. Grades III and IV expect you to sort a clarifier problem into hydraulic or biological before you touch anything, and to tell apart two symptoms that look alike on the surface.

What you need to know

A secondary clarifier is a settling tank, but its job differs from a primary clarifier's. It settles biological floc out of the mixed liquor, returns most of that floc to the aeration tank, where it's called RAS (return activated sludge), and lets the clear water on top move on to disinfection. Two things can defeat it: too much flow for its surface area, or too many solids for its surface area.

Check the flow side first with surface overflow rate (SOR): peak flow in gallons per day, divided by clarifier surface area in square feet. Small package-plant clarifiers handle roughly 600 to 800 gallons per day per square foot.

Large clarifiers with mechanical scrapers can run up to about 1,000. Push flow past the small-plant limit and water moves up through the tank faster than floc can settle down. Solids wash over the weir no matter how well the biology is running.

Check the solids side with solids loading rate (SLR): MLSS times the combined influent-plus-RAS flow in million gallons per day, times 8.34, divided by surface area. Small package plants are limited to about 25 pounds of solids per day per square foot.

Large clarifiers with scrapers can take about 35. Run past the small-plant limit and the clarifier cannot compact the blanket fast enough, even at a flow that passes the SOR check.

Running both checks first matters, because SOR and SLR fail independently. A clarifier can pass one and still fail the other, and the fix for each is different: an SOR failure needs less flow through the tank, while an SLR failure needs less MLSS in the tank. Only once both check out should you go looking for a biological cause.

Settling quality itself gets read with two daily checks. Sludge blanket depth, read with a sludge judge, should stay under 30 percent of clarifier depth. A blanket climbing toward that line, even with good SVI, is an early warning.

RAS concentration should run about 1.5 to 2 times the aeration tank's MLSS. A ratio thinner than 1.5 usually means the RAS pump rate is set too high for the blanket actually in the tank.

The sludge volume index (SVI) rounds out the settling picture: the percent of a sample that settles in 30 minutes, times 10,000, divided by MLSS, in milliliters per gram. A preferable range is 50 to 150 mL/g, with lower meaning denser, faster-settling sludge. The full SVI calculation and its link to MLSS and MCRT are worked in the activated sludge process control guide.

Here, treat a rising SVI as a lead indicator you check before the blanket itself starts to climb.

Two symptoms get confused constantly, and telling them apart is the single most tested skill in this duty. Bulking looks like poor settling everywhere: a slow 30-minute settle, a high SVI, a blanket that keeps rising, and solids escaping the weir in a steady haze. It usually traces back to filaments and too little wasting.

Rising sludge looks different up close. Floc that settled fine in a jar test floats back to the surface later, in clumps, with fine gas bubbles visible underneath. That is denitrification, nitrate turning to nitrogen gas in the clarifier, not a settling failure at all.

The fixes run in opposite directions. Bulking calls for more MCRT, more DO, and less RAS pulled at once, giving the floc time to compact.

Rising sludge calls for more RAS and a faster collector, moving solids out before the gas has time to form, or a small aeration trim so denitrification happens earlier, in the tank, where you want it. Chlorinating the RAS line at a low dose is a short-term emergency move for a true filament bulking event. It does nothing for rising sludge and only wastes chemical.

Foam and washout round out the symptom set. A pale, light foam layer points to a young population that hasn't wasted enough yet. Thick, dark, greasy foam usually means old sludge, most often Nocardia, and calls for the opposite move, more wasting and less air at low flow.

Solids washing over the weir with no obvious foam or blanket problem often traces to a straightforward SOR failure, a hydraulic overload the biology cannot fix.

At higher grades, expect a scenario with two symptoms reported together, for example a rising blanket alongside a mild ammonia excursion. Settle the clarifier problem first, because pushing a nitrification change on top of unresolved hydraulics tends to make both worse.

Worked examples

Example 1: Surface overflow rate. A clarifier has 615 square feet of surface area and a peak flow of 450,000 gallons per day.

SOR = peak flow / surface area = 450,000 / 615 = about 732 gallons per day per square foot.

That sits inside the 600 to 800 range for a small package-plant clarifier, so hydraulics check out at this flow. Rework the division yourself at /tools/practice before moving on.

Example 2: Solids loading rate. The same clarifier runs an MLSS of 2,800 mg/L, an influent flow of 0.5 MGD, and a RAS flow of 0.4 MGD, for a combined flow of 0.9 MGD.

SLR = MLSS x combined flow x 8.34 / surface area = 2,800 x 0.9 x 8.34 / 615 = about 34 pounds per day per square foot.

That is well past the small-plant limit of 25, so the clarifier is overloaded on solids even though the SOR check passed. The fix is not more air. It is less MLSS in the tank, by wasting more or trimming the RAS rate.

Rework this one at /tools/practice until you can spot which loading rate is the problem.

Example 3: Sludge blanket depth. A clarifier has a side water depth of 14 feet, and the sludge judge reads a blanket at 3.5 feet.

Percent of depth = blanket depth / side water depth = 3.5 / 14 = 25 percent.

That sits under the 30 percent target, so blanket depth is not the driver behind any solids you might be seeing at the weir. Look upstream at SOR and SLR instead.

Common traps

• Treating SOR and SLR as one check instead of two. A clarifier can pass one and fail the other, so run both before you decide what to fix.
• Calling a rising, gas-flecked blanket a bulking problem. It is denitrification, not filaments, and the fix runs the opposite direction: more RAS and a faster collector, not more wasting and not chlorine on the RAS line.
• Jumping to a biological fix before you have checked SOR and SLR. A hydraulic overload looks like a settling failure at the weir even when the sludge itself is settling fine.
• Reading blanket depth as a fixed number instead of a percent of clarifier depth. A 4-foot blanket is fine in a 16-foot clarifier and a warning sign in an 8-foot one.
• Mixing gallons per day with million gallons per day inside the SLR formula. That slip throws the answer off by a factor of a thousand.

Practice

Clarifier loading-rate problems, SOR and SLR, get paired with settling symptom questions on the wwt-1 practice test at /tools/practice, since the exam mixes both in the same block.

Work each loading-rate problem by hand first, checking your units before you calculate, then confirm with the practice tool's calculator. On a symptom question, decide hydraulic or biological before you pick an answer that sounds like a fix.

Quick reference

• SOR = peak flow (gpd) / clarifier surface area (ft²). Small package plants: 600 to 800 gpd/ft² max. Large scraper-equipped clarifiers: up to about 1,000.
• SLR = MLSS x (influent + RAS flow, MGD) x 8.34 / surface area (ft²). Small package plants: 25 lbs/day/ft² max. Large scraper-equipped clarifiers: about 35.
• Check SOR and SLR before you look for a biological cause; a clarifier can pass one and fail the other.
• Sludge blanket target: under 30 percent of clarifier side water depth, read with a sludge judge.
• RAS concentration should run about 1.5 to 2 times aeration-tank MLSS; below 1.5 means RAS is pumping too fast.
• SVI preferable range: 50 to 150 mL/g. Lower means faster settling; full calculation in the activated sludge process control guide.
• Bulking: slow settle, high SVI, rising blanket, steady solids haze at the weir. Usually filaments and too little wasting. Fix: raise MCRT, hold DO 2 to 4 mg/L, cut RAS to lengthen detention, or chlorinate the RAS line as a short-term move.
• Rising sludge: good floc floats later in clumps on fine gas bubbles. Denitrification in the clarifier, not a settling failure. Fix: raise RAS, speed the collector, or trim end-of-aeration DO.
• Foam: light and white points to young sludge and low MLSS. Thick, dark, and greasy points to old sludge, usually Nocardia.
• Washout with no foam or blanket problem often points to a straightforward SOR failure rather than biology.