Coagulation, flocculation and clarification are common process steps at drinking water treatment plants. The general purpose of this sequence is to remove solids and turbidity from the water to produce clear product water and meet turbidity standards, expressed as NTUs (nephelometric turbidity units). Coagulation, flocculation and clarification can also remove bacteria and color from the water.

First, coagulating chemicals are mixed with water in a rapid-mix step for even distribution. The chemicals cause particles in the water to clump together.  

In the flocculation step, the water and chemicals are gently mixed, causing the coagulated particles to come together and form even larger clumps, called floc, which then settles out of the water in sedimentation basins, sometimes called clarifiers.

The basins can be rectangular or circular and generally consist of four zones: an inlet zone, settling zone, sludge collection zone and outlet zone. The clarified water passes on to downstream processes such as filtration and disinfection.

Like most water treatment processes, coagulation, flocculation and clarification sound simple but are subject to many variables that affect the efficiency and success of the operation.

Travis Zurcher of WesTech in Salt Lake City, Utah, is a specialist in the design and operation of the process for his company. He offers these tips for improving results:

1. Use the correct polymer or coagulating chemicals
 “Since no two source waters are the same, they are all going to need something a little bit different to help attract the particles to each other,” he says. “If you’re having trouble forming good floc, then this might be a key place to look for improvement. With jar testing you can tell how effective different chemicals are and find the ones that work best for your water.”

2. Keep injection point close
Also, make sure the injection point for the chemicals being placed into the mixing chamber is close to the mixer.

“The further away the mixing is from the injection point, the less (effective) mixing you’re going to see,” Zurcher says.

3. Slow mixing is good mixing
“If you are overloading your flocculation basins with too much flow, they won’t have the ample time necessary to form those nice big flocs,” he says.

He recommends 10 to 15 minutes per flocculation stage with a total of 30 to 45 minutes.

4. Velocity gradient is important
“Too fast and you’re simply mixing, breaking apart any floc that might form,” Zurcher says. “Too slow and you don’t allow the particles the chance to bump into each other to conglomerate.”

The ideal situation, according to Zurcher, is “tapered flocculation,” where the velocity gradient slowly decreases through the basin. 

“As larger flocs form, they break apart more easily and are more likely to run into each other,” he says, “so less mixing becomes helpful.” 

5. Keep an eye on temperature
Temperature also affects mixing because water becomes more viscous and harder to mix as it gets colder.

“As the seasons change,” Zurcher says, “you might find that you need to adjust the speed of your mixer slightly to get the same velocity gradient, compensating for the seasonal temperature variation.”

6. Balance the velocity
For clarification, or settling, to be effective, the upward velocity of water has to be less than the downward velocity of the particles.

“This is a key point,” Zurcher says. “You can either slow down the rate at which the water is rising by decreasing the flow or increase the rate at which the solids are settling by adjusting your coagulation and flocculation to increase the particle size.”