Where should a coagulant such as alum be added for the best results?
- At the outlet of the sedimentation basin, ahead of the filters
- In the clearwell, together with the disinfectant
- At the rapid (flash) mix, where it is dispersed within seconds
- At the last stage of the flocculation basin
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C. At the rapid (flash) mix, where it is dispersed within seconds
Coagulant reactions are complete in seconds, so the chemical must be spread through the water instantly by intense mixing. Added later, where mixing is gentle or absent, it would not contact the particles evenly.
Reference: Conventional treatment: rapid (flash) mixing
What is a jar test used for?
- Finding the best coagulant dose for the raw water
- Measuring the chlorine demand of the filtered water
- Checking a filter for mudballs after backwashing
- Counting coliform bacteria in a distribution sample
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A. Finding the best coagulant dose for the raw water
A jar test treats several beakers of raw water with different coagulant doses under timed rapid mix, slow mix and settling, so the operator can pick the dose giving the best floc and clearest settled water.
Reference: Process control: jar testing
Which condition tells an operator that a gravity filter is due for backwashing?
- The chlorine residual in the clearwell has increased
- Raw water alkalinity has dropped below normal
- The flocculator paddle speed has been reduced
- Head loss across the filter has reached its limit
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D. Head loss across the filter has reached its limit
As a filter collects solids, resistance to flow (head loss) rises. Filters are backwashed on high head loss, turbidity breakthrough, or a set run time. The other conditions are not filter performance indicators.
Reference: Filtration: backwash criteria (head loss, effluent turbidity, run time)