Sedimentation and clarification

Letting the floc fall out

After coagulation and flocculation, the water carries floc heavy enough to settle. Sedimentation gives it the time and the still water to drop to the bottom before the water moves on to the filters. The exam asks two things here: how the basin is supposed to work, and the loading numbers that tell you whether it is working.

How a settling basin works

A conventional basin has four zones the exam likes to name: the inlet, where flow is spread evenly and slowed, the settling zone, where the floc falls, the outlet, where clarified water leaves over weirs, and the sludge zone, where settled solids collect to be removed. The whole job is to hold the water still long enough, at a low enough velocity, that the floc reaches the bottom. Short-circuiting, where water races from inlet to outlet without settling, is the enemy, and it shows up as floc carrying over to the filters.

The loading numbers that govern it

Two rates decide whether a basin keeps up with flow. The surface overflow rate is the flow divided by the surface area of the basin, in gallons per day per square foot, and it is the headline number: push too much flow over too little area and floc carries over. The weir overflow rate is the flow divided by the length of the outlet weirs, in gallons per day per foot, and a too-high weir rate pulls floc up and out. Detention time, the basin volume divided by the flow, rounds out the picture. These are the calculations the exam pairs with the process.

The clarifier types

Not every plant uses a long rectangular basin. Tube and plate settlers add settling surface in a smaller footprint by giving floc a short distance to fall. Solids-contact or upflow clarifiers combine mixing, flocculation, and settling in one unit and recycle settled sludge to seed new floc. Dissolved air flotation floats light floc and algae to the top instead of settling them, which suits cold or algae-laden water. The settled or floated solids become a residual the plant has to handle.

Where operators lose points

• reading detention time when the question wants the surface overflow rate, or the reverse
• ignoring short-circuiting, which lets floc carry over even when the math looks fine
• forgetting the weir overflow rate as a cause of carryover
• treating a solids-contact clarifier like a plain rectangular basin

Quick reference

• Four zones: inlet, settling, outlet, sludge. The job is still water and time.
• Surface overflow rate is flow over basin area; weir overflow rate is flow over weir length. Both cause carryover when too high.
• Tube and plate settlers add surface in less space; solids-contact units combine the steps; dissolved air flotation floats light floc.
• Settled solids are a residual to handle.

Where this fits

What the basin cannot settle goes to filtration, the final particle barrier. The loading and detention math is in the free practice tool, the whole-exam shape on the water treatment exam page, and your state's specifics on your state's page.