Secondary Clarifier Troubleshooting — A Field Diagnosis

Before you look at MLSS, SVI, or the daily TSS trend, walk the clarifier and look at what it's telling you. A well-running secondary clarifier has a clean, uniform effluent launder, a faint feathering of solids near the weir, and a settled blanket you can judge with a sludge judge to a predictable depth. A clarifier in trouble shows it visually long before lab results confirm it.

Floating solids, a thick blanket near the weir, pin floc washing over, a clouded effluent, or a scum mat that's grown past the skimmer — each of these points to a different root cause. Diagnosing by eye first saves the day of guessing that comes from chasing a high effluent TSS number without a visual context.

The most common symptom is just solids in the effluent. The cause is almost always one of three things: the clarifier is overloaded hydraulically, the sludge isn't settling because of a biology problem, or the sludge blanket has risen into the effluent zone because RAS rate hasn't kept up.

Check the sludge blanket depth first. If it's high, increase RAS rate to pull solids out faster — but watch the solids loading rate, because pulling too hard hydraulically can resuspend solids. If the blanket is normal but solids are still washing over, the issue is settling velocity, not blanket depth. That points to an SVI or floc-structure problem in the biology, not a clarifier mechanical issue.

Surface overflow rate (SOR = flow / clarifier area) is the key hydraulic check. Most secondary clarifiers are designed for 400-800 gpd/sq ft at average flow with peaks around 1,000-1,200. If you're exceeding peak SOR during a storm, the fix is more clarifier capacity (take another online), lower MLSS, or flow equalization — not more RAS.

If solids are floating to the surface in chunks, not washing over as pin floc, the cause is usually denitrification in the blanket. Nitrate is being reduced to nitrogen gas under the blanket, the gas lifts solids to the surface, and the skimmer can't keep up. This is a distinct problem from bulking and needs a different fix.

The telltale sign is solids that float, often with visible gas bubbles, and a clarifier where the blanket is actually thin. The fix is to reduce blanket detention time — increase RAS rate to pull the settled sludge out faster before it goes anoxic. Do not increase wasting to fix rising sludge; the SRT isn't too long, the blanket detention is too long.

Confirm the diagnosis by checking the effluent nitrate. If the plant nitrifies and the clarifier sits with a deep, old blanket, rising sludge is the expected outcome. A small blanket, promptly removed, prevents it.

Bulking sludge doesn't compact. The SVI climbs above 150, the settled volume in the settleometer stays high, and the clarifier struggles to concentrate the return. The cause is almost always filamentous organisms, and the fix depends entirely on which filament is dominant — which is the part you can't guess.

Run a wet mount microscope exam before changing anything. The filaments that matter most: Type 1701 and Sphaerotilus (low DO — fix by raising DO), Nocardia and Microthrix (low F/M or high grease — fix by shortening SRT), and Thiothrix/Beggiatoa (septicity/sulfide — fix upstream, not in the basin). The wrong correction makes the problem worse: raising DO on a low-F/M Nocardia problem does nothing, and shortening SRT on a low-DO filament removes the wrong organisms.

RAS chlorination is a legitimate short-term bulking control but it's non-selective and can damage floc-forming organisms if overdosed. Use it as a stopgap while the underlying cause (DO, F/M, septicity) is corrected, not as a permanent operating strategy.

The opposite failure from bulking is pin floc — small, weak, dispersed floc particles that don't settle and leave the effluent turbid even though SVI looks fine. Pin floc usually means the floc structure is being damaged: too much shear from over-aeration, a very low F/M starving the biology, or a toxic slug that broke the floc apart.

The fix is the opposite of bulking. Reduce aeration shear (lower DO setpoint toward the bottom of the range, reduce blower output if it's over-driving the diffusers), raise F/M if it's been starved, and check the microscope for a deflocculated, weak-floc appearance with almost no filaments. If you recently had a toxic event, recovery may just need time and seed sludge, not process changes.

A useful cross-check: if SVI is normal (80-120) but effluent TSS is high, you're in pin-floc territory, not bulking territory. The treatment for the two is nearly opposite.

If one part of the clarifier is carrying more solids than another, the cause is often hydraulic, not biological. An unlevel weir concentrates flow on one side, so solids carry over there even when the clarifier is otherwise fine. Influent well design and baffle condition can also distribute flow unevenly, sending solids down one wall and up the far side.

Check the weir level with a transit or a simple gauge across the launder. Even a small tilt concentrates flow — and solids carryover risk — on the low side. Check the effluent launder for biofilm or mineral buildup that's changed the hydraulics. Both are cheap fixes that solve otherwise mysterious, intermittent turbidity spikes.

Verify center-feed and peripheral-feed distribution. A poorly designed or partially blocked influent well is a common, hard-to-retrofit source of chronic short-circuiting, and it explains why a clarifier that 'should' work on paper underperforms in practice.

When the clarifier fails, work the diagnosis in order: visual walk-through, blanket depth, SVI and a microscope exam, then hydraulic checks (SOR, weir level, influent distribution). Each step rules out a category of cause. Changing variables before diagnosing is how operators spend a week chasing their tails — and how a small RAS adjustment becomes a week-long SVI crisis.

Keep a clarifier log: blanket depth, SVI, effluent TSS, weather, and a one-line visual note each shift. Patterns emerge within a few weeks that pinpoint recurring causes — the storm that always pushes SOR over the edge, the season that always brings a filament, the side of the clarifier that always carries first.