Clarifier Surface Overflow & Weir Loading Calculator
Determine surface overflow rate (SOR), peripheral V-notch weir loading rate (WLR), hydraulic detention time, and solids loading rate (SLR) for sedimentation clarifiers.
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Clarifier Hydraulics: Surface Overflow, Weir Loading & Detention
Sedimentation clarifiers separate solids from liquid by gravity settling. The design of a clarifier is governed primarily by hydraulic surface area rather than tank volume. For a solid particle or biological floc to settle to the bottom hopper, its downward gravity settling velocity must exceed the upward hydraulic fluid velocity of the water rising toward the effluent overflow weirs.
Key Clarifier Governing Equations
- Surface Overflow Rate (SOR, GPD / sq ft):
SOR = Daily Flow (Gallons / Day) / Surface Area (sq ft)
Ten States Standards recommend:- Primary Clarifiers: 800 - 1,200 GPD/sq ft
- Secondary Clarifiers (Conventional): 400 - 800 GPD/sq ft (Peak ≤ 1,200)
- Extended Aeration Clarifiers: 200 - 400 GPD/sq ft
- Weir Loading Rate (WLR, GPD / linear ft):
To prevent localized high-velocity "waterfall currents" that scour settled sludge up over the effluent troughs:WLR = Daily Flow (GPD) / Total Overflow Weir Length (ft)
Design limit is typically ≤ 10,000 to 15,000 GPD per foot. - Hydraulic Detention Time (HDT, Hours):
HDT = [ Tank Volume (Gallons) / Daily Flow (GPD) ] × 24
Typical requirement: 2.0 to 4.0 hours. - Solids Loading Rate (SLR, lbs / sq ft · day):
For secondary clarifiers receiving mixed liquor plus Return Activated Sludge (RAS):SLR = [ (Flow + RAS_flow) × MLSS × 8.34 ] / Surface Area
Safe design limit: ≤ 24 to 30 lbs/sq ft·day.
Frequently Asked Questions
Why does surface overflow rate matter more than water depth in clarifiers?
Per Hazen's classic sedimentation theory, a settling particle must reach the tank floor before the water parcel reaches the overflow weir. As tank depth increases, the distance to settle increases proportionally with detention time, exactly canceling out depth. Settleability is strictly dictated by flow divided by horizontal surface area.
What are V-notch weir baffles and why are they notched?
90-degree V-notch weirs accommodate fluctuating hydraulic flow rates without allowing slight millimeter variations in concrete construction levelness to cause water to short-circuit through only one side of the tank.
What causes sludge blanket washout during storm events?
During wet weather inflow and infiltration (I&I), influent flow spikes. If SOR exceeds 1,200 GPD/sq ft or SLR exceeds 35 lbs/sq ft·day, the upward fluid velocity sweeps the top of the sludge blanket right over the effluent weirs into the receiving stream.
What is typical Return Activated Sludge (RAS) flow rate?
RAS pumps recycle settled bacterial solids from the clarifier bottom back to the head of the aeration tank, typically at a rate between 50% and 100% of average daily influent flow.