4.2 Filter Operation, Backwash & Troubleshooting
Key Takeaways
- A filter run ends on turbidity breakthrough, terminal headloss, or maximum run time, and the operator backwashes on whichever arrives first.
- Backwash must expand the bed roughly 20 to 50 percent so grains scour against one another without washing media to waste.
- Filter-to-waste during the ripening period keeps the initial turbidity spike out of the clearwell.
- Mudballs, media loss, and filter cracking all trace back to inadequate backwash energy or excessive polymer carryover.
- Negative head develops when headloss through the media exceeds the water depth above it, releasing dissolved gas and causing air binding.
4.2 Filter Operation, Backwash & Troubleshooting
A filter is the last physical barrier between raw-water pathogens and the clearwell, and turbidity is the surrogate the regulations use to prove it is working. Filtration questions on the standardized exam are heavily weighted toward operation — when to backwash, how to backwash, and what a specific symptom is telling you.
1. The Filter Run
A filter run begins after backwash and ends when the operator takes the filter off line. Three things can end it, and the operator responds to whichever comes first:
| Termination trigger | What it means | Typical setpoint |
|---|---|---|
| Turbidity breakthrough | Floc is passing through the bed; the barrier has failed | Individual filter effluent rising toward or above 0.3 NTU |
| Terminal headloss | The bed has clogged; driving head is exhausted | Commonly 6–10 ft of headloss |
| Maximum run time | Biological growth and floc compaction over time | Commonly ≤ 96 hours |
Never run a filter to breakthrough to "get the run time." Turbidity is the compliance parameter, and breakthrough puts unfiltered particles — including Cryptosporidium oocysts, which chlorine will not inactivate — directly into finished water.
Turbidity compliance targets
For conventional and direct filtration, combined filter effluent turbidity must be ≤ 0.3 NTU in at least 95% of measurements each month, and must never exceed 1 NTU. Individual filter effluent turbidimeters are monitored continuously so a single failing filter can be identified rather than hidden in the combined number.
2. The Filter Cycle in Four Phases
- Ripening. Immediately after backwash the media is clean, and clean media filters poorly — there are no captured particles to help capture more. Effluent turbidity spikes for the first several minutes.
- Effective filtration. The bed reaches steady state and produces its best water for most of the run.
- Breakthrough. Accumulated solids begin shearing off and passing through; effluent turbidity climbs.
- Backwash. The bed is cleaned and the cycle restarts.
Filter-to-waste
Filter-to-waste (rewash) routes the ripening-period water to waste instead of to the clearwell, typically for the first 5 to 30 minutes. It is the single most effective way to keep the post-backwash turbidity spike out of finished water. Where filter-to-waste piping does not exist, operators bring the filter back on line slowly to blunt the spike.
Rate control
Bring filters on line and take them off line gradually. A sudden rate increase on the remaining filters — for example when one of four filters is removed for backwash — shears accumulated floc out of the beds and causes turbidity spikes across the plant. This is why slow-start and slow-rate-change controls exist.
3. Backwashing
Backwash reverses flow up through the bed, fluidizing the media so grains abrade against one another and release captured solids.
| Parameter | Typical value | Why it matters |
|---|---|---|
| Bed expansion | 20–50% | Too little and the grains never scour; too much and they separate so far they stop colliding, and fine media washes out |
| Backwash rate | 15–20 gpm/ft² (temperature dependent) | Cold water is denser and more viscous, so it fluidizes the bed at a lower rate; summer water needs a higher rate for the same expansion |
| Duration | 8–15 minutes, or until wash water clears | Watch the waste trough, not just the clock |
| Surface wash / air scour | Precedes or accompanies the main backwash | Breaks up the dirt-laden top layer where most solids collect |
Backwash water usage typically runs 1–5% of plant production. A sudden rise in that percentage means filters are running short — investigate coagulation before blaming the filters.
Temperature trap: a fixed backwash rate that works in July will over-expand the bed in January, because cold water fluidizes media more readily. Plants that never adjust seasonally lose media every winter.
4. Troubleshooting Table
| Symptom | What is happening | Cause | Correction |
|---|---|---|---|
| Mudballs | Compacted media/floc agglomerations sinking into the bed | Inadequate backwash energy; no surface wash or air scour; excess polymer | Increase expansion, restore surface wash, reduce polymer, physically remove and possibly replace media |
| Filter cracking / shrinkage | Vertical cracks, bed pulling from the walls | Mudballs and excessive solids compacting the bed | Same as above; severe cases require media replacement |
| Media loss | Falling bed depth, media in the wash troughs | Backwash rate too high for water temperature; excessive expansion; underdrain or gravel upset | Reduce rate seasonally, verify expansion, inspect underdrain |
| Air binding | Rapid headloss rise, air bubbling up during backwash, reduced output | Negative head — headloss exceeds the water depth over the media, so dissolved gases come out of solution in the bed | Backwash sooner; maintain adequate water depth above the media; avoid excessive terminal headloss |
| Short filter runs | Runs terminating far earlier than normal | Upstream failure — poor coagulation, high raw turbidity, algae, sedimentation carryover | Fix chemistry with jar tests; do not simply increase backwash frequency |
| Turbidity spike at start of run | Ripening spike reaching the clearwell | No filter-to-waste, or filter returned to service too fast | Use filter-to-waste; use slow-start rate control |
| Gravel mounding / boils | Uneven backwash flow, visible boiling at the surface | Underdrain damage or displaced support gravel | Take out of service, excavate, repair the underdrain |
5. Media Basics Worth Remembering
- Dual media (anthracite over sand) works because coarse, light anthracite sits on top and captures larger floc, while finer, denser sand polishes below — giving depth filtration rather than surface straining, and therefore longer runs than sand alone.
- Effective size (ES) is the sieve opening passing 10% of the media by weight; uniformity coefficient (UC) is the 60%-passing size divided by the 10%-passing size. A lower UC means more uniform media, which backwashes and stratifies more predictably.
- After backwash, media restratifies by density and size — fine on top, coarse at the bottom — which is why anthracite returns above sand every time despite being the coarser material.
Which condition should cause an operator to take a filter off line for backwash?
An operator uses the same backwash rate year-round and finds anthracite in the wash troughs each winter. What is the explanation?
A filter shows rapidly rising headloss, reduced output, and air bubbling up through the bed during backwash. What is the most likely cause?
Why does a filter produce its worst-quality water immediately after backwash?