Pool filter sand replacement: when, how, how many kilograms?

The first is pressure: the manometer reading taken right after a backwash with clean media is the system's reference, and a rise of about 0.5 bar above this value is accepted as the wash threshold in fast pressure sand filters. If pressure after backwashing no longer returns to its old reference, settling 0.1 to 0.2 bar higher with every wash, the bed has clogged permanently. The second is backwash interval: if a system washed once a week has started needing washing every two or three days, the media is exhausted. The third is clarity: if pool water with correctly balanced chemistry stays cloudy despite chemical intervention, and fine particle passage is seen at the filter outlet, the bed is no longer holding.

A fourth symptom is seen when the tank is opened. If pressing the bed surface by hand moves it as a single rigid piece, and hardened lumps come away in the hand, lime scale and oil have glued the grains together. In this case the media is replaced regardless of remaining lifespan.

Backwashing is passing water upward through the bed so the dirt it holds is flushed to the drain. During this, the bed expands, the grains scrub against each other and clean themselves, and flow rises to roughly three times the normal filtration rate; bed expansion is on the order of 30 percent. Once the process finishes, the media stays in place; only the dirt on it has gone.

In sand replacement, the media itself is exhausted. A grain of silica sand is angular in its early years, and the gaps between the edges mechanically trap fine particles. Years of backwash friction wear these edges down, the grain becomes rounded, and the pore structure becomes uniform; water now passes through the bed with little resistance. Backwashing cannot reverse this wear. Likewise, a bed glued together by lime scale cannot be loosened by washing either, because expansion cannot separate a mass that has become one solid piece.

Opening the tank under pressure can end with the cover bolts flying off; for this reason the first three items are never skipped.

The first backwash of new media is the step most often skipped. Sand from the bag, even when sieved, carries fine dust on it. If this dust enters the system unwashed, the pool water turns milky within the first hours, and the cloudiness is chased with chemicals. Running the first wash until the drain clears removes this problem from the start.

The table below gives typical fill quantities for common tank diameters. The exact figure is always on the filter label or in the manual; the table is only for rough planning before you order.

The amount decision is checked against flow rate as well as volume. Dividing the flow passing through the filter in pool water circulation by the tank's cross-sectional area gives the surface velocity; this is the value that sets tank diameter in pool filter selection. If the existing filter is already running at its limit, increasing bed height during sand replacement does not fix it; since surface velocity stays the same, retention performance stays the same too. Increasing bed height only eats into the expansion space above and leads to media loss during backwash.

When the bed surface crusts over, or distribution at the base is disrupted, water opens a narrow channel where resistance is lowest and passes through it. The filter appears to be working, pressure may even read normal; but because most of the bed is left out of the flow, retention surface drops by several factors. The only reliable sign of channeling is cloudiness that does not clear despite low pressure. Adding an annual tank opening and surface check to the regular pool maintenance plan makes this picture visible before replacement time actually arrives.

Slow sand filters work with fine sand at 0.15 to 0.30 mm effective grain size and a biological layer; rapid sand filters use coarser sand in the 0.5 to 1.2 mm range, with a bed depth of 0.6 to 1.2 m, and a filtration rate on the order of 3,000 to 6,000 litres per square metre per hour. The pressure sand filter in pool plumbing is the closed-tank form of this second family, operating in the 2 to 5 bar range, and in pressurised systems the wash threshold is a pressure drop of about 0.5 bar.

Backwashing comes from the same source. In water treatment, the wash decision is made by one of three criteria: a set duration, the outlet water turbidity exceeding a limit, or the pressure differential across the bed crossing a threshold. In pool applications, two of these three criteria can be used directly; a visual clarity check replaces turbidity measurement. Water passing evenly through the bed, with no preferential flow paths forming, is defined as the basic requirement of sand filter design. Pool filter sand replacement is the maintenance work that re-establishes this requirement years later; the media itself is a consumable and does not last a lifetime the way a pump or valve does.