What are you checking?

All checksEvery clarifier number at onceDetentionDetention time, any tankSurfaceSurface overflow rateWeirWeir overflow rateSolidsSolids loading rate

Your numbers

ClarifierPrimarySecondary

Flow basisAverage dayPeak

ShapeCircularRectangular

Units in servicetanks

Diameterft

Water depthftSide water depth for a clarifier.

FlowMGDgpmgal/dayTotal to all units in service.

Weir length, each optionalftBlank on a circular tank means one weir around the rim. Count both sides of a double launder.

RAS flowMGDgpmgal/dayTotal to all units in service.

MLSSmg/L

Detention time

3.8

hoursWithin typical. At average flow: 2–4 hours.

Surface overflow

566

gpd/ft²Within typical. At average flow: 400–800 gpd/ft².

Weir overflow

8,493

gpd/ftWithin typical. At average flow: up to 10,000 gpd/ft.

Solids loading

21.2

lb/day/ft²Within typical. At average flow: 20–30 lb/day/ft².

Your numbers, step by step

1. Flow to each of 2 units3.2 MGD ÷ 2 = 1.6 MGD
2. Flow in gal/day1.6 × 1,000,000 = 1,600,000 gal/day
3. Surface area0.785 × 60 ft × 60 ft = 2,826 ft²
4. Volume2,826 ft² × 12 ft = 33,912 ft³ × 7.48 = 253,662 gal
5. Flow per hour1,600,000 gal/day ÷ 24 = 66,667 gal/hr
6. Detention time253,662 gal ÷ 66,667 gal/hr = 3.8 hours
7. Surface overflow rate1,600,000 gal/day ÷ 2,826 ft² = 566 gpd/ft²
8. Weir length, one peripheral weir3.14 × 60 ft = 188.4 ft
9. Weir overflow rate1,600,000 gal/day ÷ 188.4 ft = 8,493 gpd/ft
10. RAS to each of 2 units1.6 MGD ÷ 2 = 0.8 MGD
11. Solids applied(1.6 + 0.8) MGD × 3,000 mg/L × 8.34 = 60,048 lb/day
12. Solids loading rate60,048 lb/day ÷ 2,826 ft² = 21.2 lb/day/ft²

Why it works

A clarifier works when solids sink faster than the water carries them up and out. Each of these numbers measures one way the water can win that race. They all start with the flow reaching one tank, so total flow is split evenly across the units in service.

• Surface overflow rate is the gallons per day rising through each square foot of surface. It is really an upward speed: if the water climbs faster than a floc settles, the floc goes over the weir. That's why it's the number to watch in a storm.
• Weir overflow rate is the flow per foot of weir. Too much flow over too little weir speeds the water up near the launder and pulls light solids along with it.
• Solids loading rate is the pounds of solids each square foot has to settle and thicken each day. It uses plant flow plus RAS, because return sludge passes through the clarifier too. That's also why turning the RAS up raises it.
• Detention time is how long the average gallon stays in the tank: volume divided by flow. Too short and solids don't have time to settle. Too long in a primary and the sludge can go septic and float. The same math works for any tank, so the Detention tab covers aeration, chlorine contact and digesters too.

All four formulas are on the formula sheet.

Example: a storm with two clarifiers online

Two 60 ft secondary clarifiers, 12 ft deep, handle an average 3.2 MGD with 1.6 MGD of RAS at 3,000 mg/L MLSS. A storm pushes flow to 7.5 MGD and RAS stays where it is.

• 0.785 × 60 × 60 = 2,826 ft² of surface per clarifier
• 7.5 ÷ 2 = 3.75 MGD to each clarifier
• 3,750,000 ÷ 2,826 = 1,327 gpd/ft² surface overflow, above the usual ~1,200 peak limit
• 3,750,000 ÷ (3.14 × 60) = 19,904 gpd/ft over the weir
• (3.75 + 0.8) × 3,000 × 8.34 ÷ 2,826 = 40.3 lb/day/ft² of solids
• 253,662 gal ÷ 156,250 gal/hr = 1.62 hours detention

On an average day the same clarifiers run at 566 gpd/ft². At peak the surface overflow rate is the number out of line, so a third clarifier would help the most: 7.5 ÷ 3 = 2.5 MGD each, or 885 gpd/ft².

Put this example in the calculator

Do the numbers look reasonable?

These are broad textbook ranges for catching a typo or a units slip. Your plant's design basis and state standards are what count. The results above are checked against the same values.

Number	Average flow	Peak flow	If it is far off
Surface overflow, primary	About 800–1,200 gpd/ft²	Up to about 3,000	A radius entered as the diameter makes the area four times too small.
Surface overflow, secondary	About 400–800 gpd/ft²	Up to about 1,200	Check the number of units. Total flow over one tank's area reads double with two online.
Weir overflow	Up to about 10,000 gpd/ft	Up to about 30,000	A double-sided launder has twice the weir length.
Solids loading, secondary	About 20–30 lb/day/ft²	Up to about 50	Leaving out the RAS flow makes it read low.
Detention, clarifiers	Primary about 1.5–2.5 hours, secondary about 2–4	Shorter. Watch the rates instead	Gallons need the 7.48 conversion from cubic feet.
Detention, aeration	About 4–8 hours conventional, 18–36 extended	—	Use influent flow, not influent plus RAS, unless your SOP says otherwise.
Detention, chlorine contact	At least about 30 minutes	At least about 15 minutes	Your permit's contact time rules. Short-circuiting means the real time is shorter than the math.
Detention, anaerobic digester	About 15–30 days	—	Use the sludge feed rate in gal/day, and the active volume, not the full tank.

Before you change anything

• Count only what's online. A clarifier drained for maintenance doesn't share the load. Neither does one with a broken drive.
• Check the split. The math assumes flow divides evenly. If one unit gets more, its rates are higher than these.
• Match the flow to the question. Use the day's average for routine checks and the peak hour when you're asking whether solids will wash out.
• Look at the blanket. A rising sludge blanket tells you more than any single rate. Measure it before and after a change.
• Level weirs. An uneven weir sends most of the flow over the low side, so the real weir rate there is higher than the average.

This is a study and planning aid. Your plant's SOPs, permit and process engineer take priority over any number on this page.