3.7 Secondary Clarifier Operation & Sludge Blanket Control
Key Takeaways
- Secondary clarifiers are governed by two limits at once, the hydraulic surface overflow rate and the solids loading rate, and either one can cause failure independently.
- Solids loading rate in pounds per day per square foot equals the sum of influent and return flows times MLSS times 8.34 divided by clarifier surface area.
- Sludge blanket depth should generally be held in the range of one to three feet, deep enough to thicken and shallow enough to avoid denitrification and solids carryover.
- Increasing the return activated sludge rate lowers the blanket and reduces blanket detention time but thins the return concentration and raises the clarifier solids loading rate.
- Rising sludge from denitrification produces buoyant clumps with attached gas bubbles and is distinguished from bulking by a normal sludge volume index and a high effluent nitrate.
Secondary Clarifier Operation & Sludge Blanket Control
An activated sludge system is a two-part machine: the aeration basin grows the biomass, and the clarifier separates it. A plant with perfect biology and a failing clarifier violates its permit. The wastewater outline names "primary treatment equipment: clarifiers/sedimentation basins" and evaluation of clarification processes as core tasks.
1. The Four Zones
A secondary clarifier is not simply a quiet tank. Solids move downward through four regimes:
- Clarification (discrete settling) zone - near the surface, low solids, individual floc particles settling independently.
- Hindered (zone) settling - the interface where the whole mass of floc settles together as a blanket with a visible boundary. This is what the settleometer measures.
- Transition zone - settling velocity decreasing as concentration rises.
- Compression (thickening) zone - at the floor, where floc particles rest on one another and water is squeezed out by the weight above. Compression is what makes the RAS thick enough to be useful.
The blanket you measure with a sludge judge is the top of zone 2. The concentration you get in the RAS depends on how much time solids spend in zone 4.
2. The Two Governing Loading Rates
Surface overflow rate (SOR)
SOR (gpd/sq ft) = Plant flow (gpd) / Clarifier surface area (sq ft)
Note that plant flow, not plant flow plus RAS, is used for SOR - the RAS leaves through the bottom, not over the weir. Typical design for activated sludge secondary clarifiers is 400 to 800 gpd/sq ft at average flow, with peak-hour limits of 1,000 to 1,200.
Solids loading rate (SLR)
SLR (lb/day/sq ft) = [(Q + Qr) x MLSS x 8.34] / Surface area
Here flow plus RAS flow both count, because all of that mass must be settled. Typical design is 20 to 30 lb/day/sq ft at average flow, up to 50 at peak.
Worked example
A 60-ft diameter clarifier, plant flow 1.4 MGD, RAS 0.7 MGD, MLSS 2,800 mg/L.
- Area = 0.785 x 60^2 = 2,826 sq ft
- SOR = 1,400,000 / 2,826 = 495 gpd/sq ft - comfortable
- SLR = [(1.4 + 0.7) x 2,800 x 8.34] / 2,826 = 49,039 / 2,826 = 17.4 lb/day/sq ft - comfortable
Now double the flow in a wet-weather event, with RAS following at 50 percent:
- SOR = 2,800,000 / 2,826 = 991 gpd/sq ft - at the peak-hour ceiling
- SLR = [(2.8 + 1.4) x 2,800 x 8.34] / 2,826 = 34.7 lb/day/sq ft - above the average-flow design range
That is the arithmetic of a washout event, and it explains why the standard wet-weather response is to reduce MLSS in advance (by wasting ahead of a forecast storm) rather than to chase the blanket once it starts rising.
Weir overflow rate
WOR (gpd/ft) = Plant flow / Total weir length
Typical target is 10,000 to 20,000 gpd/ft for plants above 1 MGD. Excessive weir loading creates a localized upward velocity at the weir that draws floc over the edge.
3. The Sludge Blanket
Measuring it
A sludge judge (a clear graduated tube with a foot check valve) or a core sampler is lowered to the floor and withdrawn, showing the interface directly. Ultrasonic or optical blanket level detectors provide continuous SCADA input. Measure at the same points and same times each day - blanket depth varies across the radius and through the diurnal cycle, so a single unstandardized reading is not comparable to yesterday's.
Target depth
Generally 1 to 3 feet, with a shallower target during high flow.
| Blanket too shallow (under about 6 inches) | Blanket too deep (over about 3 to 4 ft) |
|---|---|
| RAS is thin and dilute | Solids carryover over the weirs |
| Excessive RAS flow raises SLR unnecessarily | Denitrification in the blanket, causing rising sludge |
| Poor thickening in the compression zone | Phosphorus release from PAOs held anaerobically, defeating EBPR |
| Wasting calculations become imprecise | Septicity and odor |
Blanket detention time
Blanket detention time (hours) = Blanket volume (gal) / RAS flow (gph)
Holding it under about 30 to 60 minutes at a nitrifying plant is the practical protection against denitrification-driven rising sludge.
4. RAS Rate Control
RAS returns settled solids to the aeration basin. The rate can be set several ways:
| Strategy | Method | Comment |
|---|---|---|
| Constant RAS flow | Fixed gpm regardless of influent | Simplest; blanket rises and falls with the diurnal cycle |
| Constant RAS percentage | RAS paced to influent flow, typically 30 to 100 percent | Most common; keeps SLR proportional |
| Settleometer (SVI) method | RAS % = (settled volume %) / (100 - settled volume %) | Ties RAS to actual settling behavior |
| Sludge blanket control | RAS trimmed automatically to hold a target blanket depth | Best where a reliable blanket sensor exists |
| Solids mass balance | RAS set so that RAS concentration and aeration MLSS meet a computed target | Most rigorous, requires reliable TSS data |
The settleometer method, worked. If mixed liquor settles to 280 mL in 1,000 mL (28 percent):
RAS % = 28 / (100 - 28) = 0.389 = 38.9 percent of influent flow
At 1.4 MGD influent that is 0.54 MGD of RAS.
The trade-off to remember: raising RAS lowers the blanket and shortens blanket detention time (good for rising sludge), but it thins the return and raises the solids loading rate on the clarifier (bad if the clarifier is already loaded). Lowering RAS thickens the return and reduces SLR, but deepens the blanket. There is no free move.
5. Collection Mechanisms and Hydraulics
- Scraper (plow) mechanisms push settled sludge along the floor to a central hopper. Simple and robust, but slow - solids can spend an hour or more traveling.
- Suction (rapid-withdrawal) mechanisms - riser pipes or a manifold that draw sludge from across the entire floor simultaneously. Much faster removal, which is the preferred design for nitrifying plants because it minimizes blanket detention. Their weakness is that individual risers can plug or can short-circuit and draw clear water.
- Flocculating center well - an enlarged inlet well that dissipates inlet energy and lets small floc collide and grow before entering the settling zone. Retrofitting one is among the most cost-effective clarifier improvements available.
- Inboard weirs and Stamford baffles - a peripheral baffle deflects the density current that runs up the outer wall so it does not sweep solids over the weir.
- Effluent launders must be level. A launder that is out of level by even a fraction of an inch concentrates flow at the low end and raises the local weir loading substantially. Level the weirs whenever a clarifier is drained.
6. Diagnosis Table
| Symptom | Likely cause | Confirming evidence | Response |
|---|---|---|---|
| Blanket rising steadily, effluent TSS climbing | Hydraulic or solids overload | SOR and SLR calculations above design; high MLSS | Reduce MLSS by wasting; add a clarifier; reduce RAS if SLR-limited |
| Buoyant clumps floating, gas bubbles attached | Rising sludge from denitrification in the blanket | Normal SVI, high effluent nitrate, low blanket DO | Increase RAS to shorten blanket detention; increase wasting; add anoxic capacity |
| Blanket high, sludge settles slowly and compacts poorly | Filamentous bulking | SVI above 150, filaments on microscopic exam | Address the cause: low DO, low F/M, nutrient deficiency, septic influent; selector; RAS chlorination as a last resort |
| Fine dispersed particles, cloudy effluent, low SVI | Pin floc from over-oxidation | SVI below 70, high MCRT, turbid supernatant in settleometer | Reduce MCRT by increasing wasting |
| Thick brown foam on clarifier surface | Nocardia or Microthrix | Branching filaments on microscopic exam | Reduce MCRT, physically remove foam, never recycle skimmings |
| Solids over the weir on only one side | Short-circuiting: wind, density current, uneven weirs, unequal splitting | Dye test, weir level survey, flow split measurement | Level weirs, install baffles, balance the splitter box |
| Clear water in the RAS line, blanket unchanged | Coning at the withdrawal point | RAS TSS far below expected | Reduce RAS rate, increase frequency, check for plugged risers |
A circular secondary clarifier is 70 feet in diameter and receives a plant flow of 2.2 MGD with a return activated sludge flow of 1.1 MGD at an MLSS of 3,000 mg/L. What is the solids loading rate?
An operator observes buoyant clumps of dark sludge rising to the clarifier surface with fine gas bubbles attached. The sludge volume index is 105 mL/g and effluent nitrate is 14 mg/L. What is happening and what is the correct first response?
A settleometer test shows mixed liquor settling to 320 mL in a 1,000 mL cylinder after 30 minutes. Using the settleometer method of setting the return activated sludge rate, what percentage of influent flow should the RAS rate be?