You walk out to the secondary clarifiers on a warm afternoon and see rafts of brown sludge floating on the surface. Some patches are the size of a dinner plate; some cover half the clarifier. Small gas bubbles cling to the clumps. The settleometer looked fine this morning. What is going on?
In most cases this is rising sludge caused by denitrification in the clarifier. It is one of the most common clarifier problems at plants that nitrify, and it is very fixable once you understand the mechanism.
How denitrification floats sludge
When a plant nitrifies, ammonia is converted to nitrate in the aeration basin. The mixed liquor leaving the basin carries that nitrate into the clarifier. Once the sludge settles to the bottom, it runs out of dissolved oxygen quickly, because the bacteria keep respiring. With no oxygen available, many heterotrophic bacteria switch to using nitrate instead. That process, denitrification, converts nitrate to nitrogen gas.
Nitrogen gas is not very soluble. Tiny bubbles form inside the sludge blanket and attach to floc particles. When enough gas collects, the floc becomes buoyant and rises to the surface in clumps.
Three things have to be present at the same time:
- Nitrate in the mixed liquor, usually more than about 5 to 6 mg/L nitrate-N.
- Time in an anoxic blanket long enough for the bacteria to produce gas, typically more than one to two hours.
- Active biomass with some available carbon, which is always the case in mixed liquor.
Warm water speeds up all of this, which is why rising sludge often shows up in summer.
Confirming denitrification
Several simple checks separate denitrification from other causes of floating solids.
The long settleometer. Run the settleometer for two hours or longer instead of 30 minutes. Sludge that settles well and then rises as a mat after an hour or two is denitrifying. Note the time it takes to rise; that is roughly the time limit for sludge in your clarifier blanket.
The stir test. Gently stir a floating clump with a rod. If small bubbles release and the clump sinks, gas is the cause.
Effluent nitrate. If effluent nitrate is low (below about 3 mg/L), denitrification is unlikely to float sludge. Look elsewhere.
Appearance. Denitrification sludge is typically light brown, in clumps or patches, with fine bubbles. Foam from Nocardioforms is darker, greasy and stable. Grease balls and scum are different again.
Other causes of floating solids
Not every floating clump is denitrification. Rule out:
- Septic sludge from a very deep blanket or a dead spot. It is usually dark gray or black, smells of sulfide, and rises from anaerobic gas (methane, hydrogen sulfide, carbon dioxide). The fix is similar (move the sludge faster), but the cause is more severe holding time.
- Collector problems. A broken flight, failed drive, or plugged suction header leaves part of the floor unscraped. Sludge builds up in that zone and rises. Look for rising in one area only.
- Foam carryover from the aeration basin, brown and stable, often greasy.
- Bulking, where the whole blanket rises evenly but does not float in clumps.
Why the blanket is too deep
In almost every case of denitrification rising sludge, the root issue is that sludge stays in the clarifier too long. Common reasons:
RAS rate too low. If the RAS rate does not match the solids coming in, the blanket grows. A deep blanket holds sludge for hours.
A RAS pump or line problem. One clarifier with a weak pump or plugged suction will have a deeper blanket than the others.
Rising SVI. Slower settling and compaction means more blanket volume for the same mass. See Why Is My SVI Increasing?.
High MLSS. More mass sent to the clarifier means more blanket.
Uneven flow split. One clarifier taking more flow and solids than the others.
Estimating sludge holding time
A rough but useful number is the solids holding time in the blanket:
Blanket holding time (hr) = blanket volume (gal) / RAS flow (gpm) / 60
For a 90 ft diameter clarifier (6,362 ft²) with a 3 ft blanket, the blanket volume is about 6,362 x 3 x 7.48 = 142,800 gallons. At a RAS flow of 1.0 MGD per clarifier (694 gpm), the holding time is about 142,800 / 694 / 60 = 3.4 hours. Since the sludge is not evenly mixed this is approximate, but if your settleometer showed rising at 90 minutes, a 3.4 hour holding time explains the problem.
Raising the RAS to 1.3 MGD per clarifier and pulling the blanket down to 2 ft reduces the holding time to about 1.8 hours. That is a big improvement.
Fixing rising sludge
Short term: move the sludge
- Increase RAS. This is usually the fastest fix. Raise RAS in steps and watch the blanket. Use the state point tool to make sure you do not overload the clarifier hydraulically.
- Check every clarifier. Balance RAS so each clarifier holds a similar, shallow blanket (often 1 to 3 ft).
- Fix collectors and suction lines. Clear plugs, repair drives, and check that the RAS draw-off is even.
- Remove floating sludge. Use surface sprays, scum skimmers or a hose to break mats and send them to the scum system. Do not let mats sit in the sun.
Long term: reduce nitrate going to the clarifier
If your plant has an ammonia limit, turning down air to stop nitrification is not a fix. Instead, denitrify before the clarifier:
- Anoxic zone. An unaerated zone at the head of the basin, fed with nitrate through an internal mixed liquor recycle (MLE process), removes much of the nitrate. See Denitrification.
- Internal recycle rate. Typical internal recycle ratios are 2 to 4 times influent flow. Check that the pumps run and that the anoxic zone stays low in DO.
- Cycled aeration. Some plants switch zones on and off to denitrify.
- Supplemental carbon such as methanol or MicroC, only when the influent carbon is not enough and the permit demands low total nitrogen.
If your plant does not have a nitrogen limit and was not designed to nitrify, partial nitrification may be happening by accident in warm weather. In that case, discuss with your supervisor whether a shorter SRT or lower DO is acceptable. It is a judgment call that depends on your permit.
Prevention routine
- Run a long (2 hour) settleometer weekly in warm months.
- Measure blankets in every clarifier every shift.
- Trend effluent nitrate.
- Keep RAS pumps and collectors in good repair.
- Have a summer RAS plan written down before the weather turns.
Case example: White River WRRF
At the fictional White River WRRF, effluent nitrate typically runs 6 to 9 mg/L because the MLE process removes only part of the nitrate. In late summer, when mixed liquor temperatures reach the low 20s C (about 72 F), operators begin to see rising sludge patches in the afternoon whenever blankets go above about 3 ft. The fix they use is simple: raise RAS from about 4.0 MGD toward 4.8 MGD on hot afternoons, keep the internal recycle pumps at full speed, and check that the anoxic zones are not getting DO carried in from the aeration grid. The patches disappear within a day.
Rising sludge is a clear message from the clarifier: sludge is sitting too long in the dark with too much nitrate. Move it faster and denitrify upstream, and the clarifier surface stays clean.