6.6 Secondary Clarification: Overflow and Solids Loading Rates, RAS Rate Setting & Clarifier Diagnostics

The clarifier is where activated sludge succeeds or fails

The aeration basin grows the biomass. The secondary clarifier has to separate it, thicken it for return, and produce a clear effluent — all at once. A plant with a perfect biological process and a failing clarifier will violate its permit, because the permit is written on effluent TSS, and every pound of solids that leaves over the weir is both a violation and a loss of inventory.

Three simultaneous duties

1. Clarification — produce a low-solids overflow.
2. Thickening — concentrate the settled sludge so the return stream carries solids back at 4,000 to 12,000 mg/L rather than at mixed liquor concentration.
3. Sludge storage — hold a working blanket that buffers flow variation.

Design and loading parameters

Two of these deserve emphasis. Surface overflow rate is the upflow velocity in disguise: a particle settles out only if its settling velocity exceeds the rate at which water rises to the weir. Raise the flow and you raise the upflow velocity, and the slower-settling particles are carried out. Solids loading rate, not SOR, is usually what limits an activated sludge clarifier, because the unit must thicken as well as clarify. Note also that when you calculate SOR and SLR, SOR uses influent flow only, while SLR must include the RAS flow, because returned solids pass through the tank too.

Worked example. A circular secondary clarifier is 75 ft in diameter, receiving 2.4 MGD of plant flow plus 1.2 MGD of RAS, with MLSS of 2,800 mg/L.

• Area = 0.785 x 75² = 0.785 x 5,625 = 4,416 sq ft
• SOR = 2,400,000 gpd / 4,416 sq ft = 543 gpd/sq ft (RAS excluded)
• Solids load = (2.4 + 1.2) MGD x 2,800 mg/L x 8.34 = 3.6 x 2,800 x 8.34 = 84,067 lbs/day
• SLR = 84,067 / 4,416 = 19.0 lbs/day/sq ft

Both are within normal range, so a settling problem at this plant would point to sludge quality rather than to hydraulic overload.

Circular versus rectangular

Circular clarifiers feed through a center column into a flocculating center well, flow radially outward, and are swept by a rotating rake to a center hopper. Some use a suction (Tow-Bro or riser pipe) mechanism that withdraws sludge across the full radius rather than plowing it to the center, which reduces the time solids spend in the tank.

Rectangular clarifiers use chain-and-flight or traveling bridge collectors moving sludge to a hopper at one end. They share common walls, so they are efficient on land, and they are less prone to density currents at high side water depth.

The inlet baffle or flocculating center well is critical in either geometry: it dissipates inlet energy so the incoming mixed liquor does not shoot across the tank, and it gives floc particles a chance to collide and grow.

Density currents and short-circuiting

Mixed liquor entering a clarifier is denser than the clear water in the tank. It sinks, travels along the floor, hits the far wall, and rises — a density current. If it rises near the weir, it carries solids over. Countermeasures are deeper side water depth, inboard weir placement with a Stamford or peripheral baffle that deflects the rising current back toward the center, and adequate inlet energy dissipation.

Short-circuiting from wind, temperature differences, uneven weirs, or a poorly placed inlet reduces the effective detention time. An unlevel weir is one of the most common and most correctable causes: water preferentially runs over the low spot, creating a high local overflow rate. Weirs are checked with a level and adjusted, and they are kept clean of algae, which both plugs the notches and sloughs into the effluent.

Reading the clarifier

The surface of a secondary clarifier is the most informative diagnostic view in a wastewater plant.

Rising sludge from denitrification is the classic exam item and it is often confused with bulking. In a bulking event the sludge settles slowly but does not float; the SVI is high and the blanket rises gradually. In rising sludge, well-settled sludge sits on the bottom, and after some time in the anoxic blanket, denitrifying organisms convert nitrate to nitrogen gas which becomes trapped in the floc and floats discrete chunks to the surface. The distinguishing evidence is that the floating clumps still look like good floc and the settleometer shows a normal SVI. Remedies are to increase the RAS rate to shorten sludge residence time in the blanket, reduce sludge blanket depth, increase wasting to lower sludge age, or provide an intentional anoxic zone upstream so denitrification happens where it is wanted.

Return activated sludge rate

RAS is the operator's primary lever on the clarifier. Too low, and solids accumulate in the blanket until it reaches the weir. Too high, and the clarifier is hydraulically overloaded, the sludge is returned thin, and the aeration basin retention time falls.

Common approaches to setting RAS:

• Percentage of influent flow, typically 20 to 100 percent, adjusted as flow changes.
• Settleometer-based (SSV method): RAS percentage is estimated from the 30-minute settled sludge volume, since RAS % ≈ SSV30 / (1,000 − SSV30) x 100.
• Sludge blanket control: adjust RAS to hold a target blanket depth.
• Solids mass balance: return enough solids to hold the target MLSS.

Worked example. A settleometer shows SSV30 of 280 mL/L. The estimated RAS rate is 280 / (1,000 − 280) x 100 = 280 / 720 x 100 = 38.9 percent of influent flow.

Secondary clarifier maintenance

• Inspect and adjust weir levelness annually and after any structural work; clean weirs and troughs of algae routinely.
• Check rake or flight mechanism torque; a rising torque alarm means the blanket is too deep or debris is jammed.
• Inspect chain, flight, and sprocket wear on rectangular units and drive gear oil on circular units.
• Verify the scum skimmer, beach, and scum box are removing rather than recirculating floatables.
• Confirm the RAS and WAS pump flow meters are accurate — every process control calculation depends on them.
• Dewater and inspect the basin on a multi-year cycle for corrosion, concrete condition, and baffle integrity.