What is backwash and when should it be done

Maintaining filtration systems is a process that requires precision and an understanding of fluid dynamics. One of the most critical technical procedures in this field is backwashing. Although often seen simply as a cleaning task, the physics behind this process is fundamental to maintaining water clarity and safety.

What exactly is backwash?

To fully grasp the concept of backwashing, one must first understand how a sand or diatomaceous earth (DE) filter operates under normal conditions. Water is pushed by the circulation pump and enters the filter from the top. As it descends through the filter bed (the sand), microscopic particles, oils, and debris are trapped between the grains. The clean water exits through the bottom and returns to the system.

Backwashing is the deliberate reversal of this flow.

By changing the position of the multiport valve, the system redirects the water so it enters through the base of the filter and rises toward the top. This upward flow physically lifts the sand bed (a process known as fluidization), separating the grains and releasing all the accumulated dirt. That dirty water does not return to the system; it is expelled directly to the drain or waste line.

What is rarely mentioned: Technical facts about backwashing

There are several phenomena invisible to the naked eye that occur inside the filtration tank. Knowing them completely changes how this maintenance should be approached:

• The paradox of efficiency: A freshly backwashed sand filter is not an efficient filter. Clean sand can retain particles down to 20 to 40 microns. However, as the filter begins to accumulate a light layer of dirt, this very dirt acts as an additional mesh, allowing it to trap much smaller particles (down to 10 microns). Therefore, a slightly dirty filter actually filters better than a pristine one.
• The danger of channeling: Water always seeks the path of least resistance. If a backwash is not performed when the filter is obstructed by extreme dirt, the water pressure will carve tunnels or direct “channels” right through the sand bed. Once this channeling occurs, the water travels through these tunnels without being filtered at all, returning cloudy water straight back to the system.
• Microscopic sand erosion: Silica filter media works because sand grains have sharp, irregular microscopic edges that “catch” the dirt. The continuous process of filtering and backwashing over the years acts like a river rushing over stones: it rounds the edges of the sand grains. When the sand becomes smooth (typically between 3 and 5 years), backwashing is no longer effective, and the entire filter bed must be replaced.

When should backwashing be done?

Performing the procedure too frequently ruins filtration efficiency (due to the previously mentioned paradox), while doing it too rarely causes channeling and dangerous internal pressures for the equipment. The exact metric for doing it is not based on time (such as “once a week”), but on hydraulic pressure.

The absolute indicator is the filter’s pressure gauge.

1. Establish the baseline: When the filter has new sand or has just received a deep backwash, the system must be turned on and the normal operating pressure recorded (for example, 12 PSI). This is the “clean pressure”.
2. The action threshold: As the filter traps debris, the pressure inside the tank rises because the water has to work harder to push through the dirty bed. Backwashing must be performed strictly when the pressure rises between 8 and 10 PSI above the baseline. If the baseline is 12 PSI, the exact time to backwash is when the dial reads 20-22 PSI.

Exceptional situations

Although the pressure rule is the primary guideline, there are scenarios where a corrective backwash is immediately necessary, regardless of what the gauge reads:

• After a severe storm or high winds that have introduced a large amount of fine dirt and debris.
• Following the eradication of a severe algae bloom (dead algae saturate and clog the filter media almost instantly).
• After an event involving an unusually high number of people using the system, which introduces massive levels of sunscreens, body oils, and cosmetics.

Precise knowledge of these variables ensures that the equipment is not overworked and that water quality is maintained at optimal parameters, maximizing the lifespan of all internal components.