Design a settling tank for the coldest month, because cold water is more viscous and particles settle more slowly. A lecture on primary tanks says 10 °C water needs about 1.38 times the detention time of warm water, which cuts the flow a fixed tank can treat by about 28 percent. Check influent temperature against tank temperature as well.

Start With Stokes and Viscosity

A TU Delft chapter on sedimentation says the settling velocity of a particle depends on its size, its density and the viscosity of the water, and that viscosity depends on temperature. Colder water is thicker, so the same particle falls more slowly. The critical velocity for removal is set by flow and area, so it does not change with the season.

A particle that just reached the floor in summer therefore escapes in winter. Fewer of the slow ones are caught, and effluent solids rise even though flow has not changed.

Quantify the Cold Penalty

If a tank needs 2 hours of detention in summer, the 1.38 factor makes it 2.76 hours at 10 °C. A fixed tank then treats only 72 percent of the summer flow. The same tank at peak winter flow is overloaded long before it looks full.

Ask the designer which temperature the detention time assumes. If the answer is 20 °C, apply the 1.38 factor for 10 °C water yourself and compare the result with your winter flow and your regulator’s limit.

Size the Settling Tank on Winter Overflow Rate

Overflow rate is flow over area, so temperature does not change it, but it changes what the rate can remove. Apply a temperature factor to the design overflow rate as well as to the detention time. At 40 m³/m²·d in summer, dividing by 1.38 gives about 29 in winter.

That value sits inside the 20 to 40 m³/m²·d band one design article lists for primary tanks. I’d size the tank for the colder number wherever winter water drops near 10 °C.

Watch Influent Temperature Differences

A computational study of a drinking-water sedimentation tank found that a 1 degree Celsius gap separating influent from tank contents sufficed to start a density current. When the influent warmed, modeled settling efficiency fell from 99 percent to 76 percent. The direction of the current followed the sign of the difference.

Hot process water and cold stormwater both create a mismatch. I’d put a temperature probe on the inlet channel and trend the difference against effluent solids.

Expect Winter Gradients in Open Tanks

A conference paper on an uncovered rectangular sedimentation tank used computational fluid dynamics to study temperature effects on settling velocity at current and projected loadings. It reports temperature gradients in winter and notes that density currents and inlet and outlet geometry both shape performance. All its tanks had a perforated inlet baffle and a V-notch effluent weir.

Even with that hardware in place, temperature still mattered. A cover or windbreak may reduce surface cooling, though I’d test the effect before paying for one.

Equalize or Temper Hot Industrial Streams

Hot industrial discharges arrive in bursts. I’d put an equalization tank ahead of the settling tank to even out flow and temperature together, since a smaller temperature gap means a weaker density current. Where a stream is steadily warm, a heat exchanger may be cheaper than a larger tank.

Price the heat exchanger and the larger tank on the same sheet before you choose. The cheaper answer depends on how many degrees you must remove and for how long.

Account for Temperature in Plate Settlers

An Ecologix article says the upward Hazen velocity between plates must not exceed the particle settling velocity for effective removal. Cold water lowers that settling velocity, so a plate pack sized in July may be undersized in January. The same 1.38 factor is a reasonable first check.

If you are considering the lamella clarifier from Wuxi Yosun, give the vendor your coldest water temperature along with flow and solids. A design data sheet without a temperature is incomplete.

Log Temperature Beside Effluent Solids

Record inlet temperature, tank temperature, flow and effluent TSS on one sheet every shift for a winter. After a month you will see how many points of removal each degree costs. That figure beats any textbook factor for your plant.

Use it to set winter flow limits, polymer doses and sludge pumping. A settling tank that is tuned in January will rarely surprise you in July.

Apply the Cold Factor Now

Pull your coldest recorded water temperature, apply the 1.38 factor to the design detention time and overflow rate, and compare the result with your winter flow. If the tank falls short, price an equalization step, a larger tank or a plate pack from Wuxi Yosun’s lamella clarifier page, and design the settling tank for the cold month.

Frequently Asked Questions

Q: Why does cold water slow settling?
A: Settling velocity depends on viscosity, and viscosity rises as water cools. The same particle falls more slowly in cold water.

Q: How much more detention time does cold water need?
A: A lecture on primary tanks says 10 °C water needs about 1.38 times the detention time. That cuts the flow a fixed tank can treat by about 28 percent.

Q: Does a small temperature difference between inflow and tank matter?
A: Yes. A computational study found a 1 degree Celsius gap was enough to start a density current.

Q: Does the same logic apply to plate settlers?
A: Yes. Cold water lowers settling velocity, so check plate packs against your coldest water, not the average.