Filtration
The Filtration domain tests whether you can operate filters that consistently meet turbidity limits after sedimentation or, for direct filtration, after coagulation. Expect questions on filter types, media, backwash, head loss, filter-to-waste, and individual filter effluent monitoring.
Core rules and concepts
- Conventional filtration uses coagulation, flocculation, sedimentation, and filtration. Direct filtration omits sedimentation; slow sand and membranes are alternatives.
- For conventional and direct filtration, combined filter effluent turbidity must be ≤0.3 NTU in at least 95% of monthly measurements and must never exceed 1 NTU. Slow sand and diatomaceous earth filters have a 1 NTU/5 NTU framework.
- Individual filter effluent monitoring triggers assessment or corrective action when limits are exceeded.
- Backwash expands media, removes captured solids, and must be followed by a filter-to-waste or ripening period if initial turbidity is high.
- Media loss, mudballs, air binding, and short filter runs are operating warnings.
- Filter rate changes should be gradual to avoid turbidity spikes.
Worked example
A filter has 300 ft² of area and treats 1.5 MGD. Convert flow to gpm: 1.5 MGD × 694.4 gpm/MGD = 1,041.7 gpm. Filtration rate = 1,041.7 ÷ 300 = 3.47 gpm/ft². Compare this with the design rate and with the rate used during backwash; a rate that is too high can force particles through the media and shorten the filter run.
Traps
Treating 0.3 NTU as an average rather than a 95th-percentile limit; ignoring the 1 NTU absolute limit; recycling backwash water without accounting for solids; starting a filter at full rate after backwash; confusing individual filter effluent with combined filter effluent.