4.4 Filter Underdrains, Support Gravel, Surface Wash, Air Scour & Filter-to-Waste

Below the media: the hardware that makes a filter work

The filtration chapter covers media, run length, and turbidity limits. This section covers the equipment underneath and around the bed, because most filter problems an operator actually diagnoses are equipment problems.

Underdrains

The underdrain collects filtered water and distributes backwash water evenly across the entire bed. Uneven distribution is the root cause of most chronic filter trouble.

Support gravel is graded from coarse at the bottom to fine at the top, transitioning to the filter media. Its job is to prevent media loss into the underdrain and to spread backwash flow. If backwash rate is excessive or distribution is uneven, the gravel layers shift and intermix, a condition called gravel upset or boiling. The symptoms are a localized violent boil during backwash, a depression or a mound in the bed surface, sand appearing in the clearwell, and turbidity breakthrough. Repair means opening the filter, removing the media, and rebuilding the gravel layers.

Surface wash and air scour

Backwash water alone lifts and fluidizes the bed but does not scrub the grains. Without auxiliary scour, floc that has penetrated the media consolidates into mudballs — accretions of media, floc, and biological growth that grow, sink, and reduce effective bed volume.

• Surface wash uses fixed or rotating arms with nozzles just above the settled media, delivering water at roughly 40 to 100 psi. It runs for a period before and during the early part of backwash and shuts off before the bed expands over the arms.
• Air scour injects air through the underdrain to agitate the grains vigorously. Sequences vary; a common one is air alone, then air and low-rate water, then water alone to remove the loosened material. Air scour with too high a simultaneous water rate will float media out of the filter.

The two are alternatives, not complements; a filter is designed for one or the other.

Backwash appurtenances

• Backwash supply comes from an elevated washwater tank, dedicated backwash pumps, or the distribution system through a rate-of-flow controller.
• Rate-of-flow controllers hold backwash at the design rate. Rates are typically 15 to 20 gpm per square foot for a dual-media bed, adjusted for water temperature, since cold water is denser and more viscous and expands the bed more at a given rate.
• Bed expansion of roughly 20 to 50 percent is the target. Too little does not clean; too much washes media over the troughs.
• Wash troughs must be level and set at the correct elevation above the media. Troughs set too low lose media; set too high, dirty water is not carried away and settles back into the bed.
• Filter-to-waste (rewash) discards the first filtered water after a backwash. During the ripening period the freshly washed bed produces its highest turbidity, and this is when Cryptosporidium is most likely to pass. Filter-to-waste for a set time or until turbidity falls below the target is one of the most effective operational barriers available, and it is why the piping includes a waste connection with its own valve.

Filter control and instrumentation

• Rate control. A constant-rate filter uses an effluent control valve that gradually opens as the bed clogs, holding flow steady while headloss builds. A declining-rate filter has no controller: flow simply falls as the bed loads, which is gentler on floc but requires the operator to watch total plant output.
• Loss of head gauge shows accumulated headloss through the bed. Terminal headloss is a backwash trigger.
• Individual filter effluent (IFE) turbidimeters are required for regulatory compliance and are the single most useful filter instrument. Continuous turbidity per filter identifies which filter is failing, whereas combined filter effluent alone hides a single bad unit.
• Effluent flow meter and totalizer, plus run time, complete the record set.

Diagnosing filter problems

Air binding deserves explanation because it is commonly misunderstood. As a bed clogs, headloss increases. If the loss exceeds the depth of water above a point in the bed, the pressure at that point drops below atmospheric — negative head. Dissolved gases come out of solution and collect in the voids, further restricting flow and causing bubbles to erupt during backwash, disturbing the media. The corrective actions are to backwash before terminal headloss, maintain sufficient water depth above the media, and avoid drawing the filter down.

Media inspection

Filter beds are inspected on a scheduled basis with the filter drained:

1. Measure media depth at several points against the design depth; losses of more than about 10 percent need topping up.
2. Probe for mudballs with a rod, and take core samples to assess distribution.
3. Check for surface cracking, depressions, and mounding, which indicate underdrain or gravel problems.
4. Verify trough levelness with a level or by filling the filter slowly and watching where water first spills.
5. Observe a full backwash looking for uneven boils, media carryover, and expansion.
6. Sieve analysis of media samples to determine effective size (the size that 10 percent passes) and uniformity coefficient (d60 divided by d10) confirms the media still matches specification.

The filter as a barrier

For the exam, hold two ideas together. First, the filter is the primary physical barrier against Cryptosporidium, which chlorine does not inactivate, so a filter that is passing turbidity is a public health failure and not merely a performance issue. Second, filter performance is bounded by what happens upstream: you cannot filter your way out of bad coagulation. When run lengths shorten and effluent turbidity climbs, the first tool is a jar test and a coagulant dose adjustment, not a longer backwash.