Filtration

The barrier that catches what settling missed

Sedimentation takes out most of the floc; filtration catches the rest. It is the last place particles are removed before disinfection, and it is the step the rules watch most closely, because filtered-water turbidity is the daily proof that the physical barrier is working. The exam tests the media, the rates, and the backwash.

Filters and their media

The workhorse is the rapid gravity filter, a bed of sand, or sand under anthracite (dual media), or sand-anthracite-garnet (mixed media). Coarse media on top and fine below lets the bed hold more floc before it clogs, which is why dual and mixed media run longer than plain sand. Water passes down through the bed, the floc lodges in the media, and clean water collects in the underdrain. Slow sand filters and diatomaceous earth filters are the alternatives, used on smaller or cleaner sources.

The run, the rate, and breakthrough

The filtration rate is the flow divided by the filter surface area, in gallons per minute per square foot, and it is the number the exam asks for. As a filter runs, it loads with floc, head loss climbs, and eventually either head loss gets too high or turbidity starts to break through the bed. Either one ends the run and calls for a backwash. The regulated target is the combined filter effluent: turbidity at or below 0.3 NTU in ninety-five percent of the monthly readings, never above 1 NTU, for conventional or direct filtration. Filter-to-waste, sending the first water after a backwash to waste, handles the turbidity spike that comes when a clean filter restarts.

Backwash

Backwash reverses clean water up through the bed to lift and scrub the trapped floc out. The backwash rate has to fluidize and expand the bed enough to clean it without washing the media out the top, often with an air scour first to break the floc loose. Operators track the backwash rate, the rate of rise (how fast the water level climbs, which checks the wash rate), and the unit filter run volume, the gallons produced per square foot in a run, as a measure of how well the filter is performing.

Where operators lose points

• mixing gallons per minute with MGD, or square feet with cubic feet, in the filtration-rate math
• forgetting filter-to-waste, so the post-backwash turbidity spike reaches the clearwell
• reading the operational filter turbidity as the same thing as the regulated combined-filter-effluent limit
• washing media out by setting the backwash rate too high

Quick reference

• Rapid filters use sand, dual, or mixed media; coarse-to-fine holds more floc and runs longer.
• Filtration rate is flow in gallons per minute over filter area in square feet.
• A run ends on head loss or turbidity breakthrough; backwash cleans the bed; filter-to-waste handles the restart spike.
• Combined filter effluent: 0.3 NTU in nearly all monthly samples, never past 1 NTU.

Where this fits

The clean water still needs the final barrier, disinfection and CT. The filtration and backwash math is in the free practice tool, the turbidity rule in the regulations guide, and your state's program on your state's page.