Backwashing in sand filters and its importance in water

Sand filters are among the most critical units in industrial water treatment. They retain turbidity, total suspended solids (TSS), and colloidal particles, playing a key role in protecting downstream processes such as UF and RO.

However, filtration efficiency is not determined only by the filter itself. It directly depends on the quality of the backwashing process.

Inefficient backwashing leads to:

• channeling;
• mudball formation;
• increased ΔP, or pressure loss;
• reduced flow rate;
• shorter filter bed lifetime.

So-called blower-assisted backwashing offers significantly higher efficiency compared to conventional water-only backwashing.

What Is Blower-Assisted Backwashing?

Backwashing is a process in which accumulated contaminants in the filter bed are removed through reverse flow.

With standard water-only backwashing, the following problems are often observed:

• bed compaction;
• channel formation;
• reduction of the effective filtration area.

That is why two-phase backwashing, using air and water, is applied in industrial systems.

Principle of Two-Phase Backwashing

Air Phase

The blower supplies air to the filter bed at:

60–70 m³/m²·h flow rate

0.3–0.5 bar pressure

Main effects:

Shear force — air bubbles create microturbulence that separates contaminants from the surface of the media.

Intensive mixing — a 3D movement of the grains is created, which is impossible with water alone.

Mudball destruction — especially important for Fe/Mn-loaded waters.

Opening of “dead zones” — air reaches areas that water cannot access.

This is the most critical phase of the process.

Water Phase

After the contaminants are loosened, water is supplied at:

15–20 m³/m²·h flow rate

This has the following effect:

• carries away the released particles;
• expands the bed by 20–30%;
• fully cleans the surface of the media;
• reduces ΔP to values close to the initial ones.

Combined Effect: Air + Water

The combination creates a strong mechanical effect:

• abrasion between particles;
• destruction of deposits;
• breakdown of colloidal structures;
• complete elimination of compaction.

This cannot be achieved with water alone.

As a result, blower-assisted backwashing leads to shorter washing time, lower water consumption, stable filtration quality, and preservation of the filter bed structure over time.

Main Advantages of the Blower

Prevents Channeling

Air creates:

• microturbulence;
• strong mixing;
• uniform cleaning.

All areas of the filter remain active.

Reduces Water and Chemical Costs

Water consumption can be reduced by up to 30–40%, with shorter cycles, for example from 12 minutes down to 7–8 minutes, and less frequent chemical regeneration.

Extends Media Lifetime

It prevents cementation, reduces local wear, and ensures uniform loading. This can provide up to +2–3 years of additional media lifetime.

Maintains Low ΔP and Stable Operation

This results in:

• lower energy consumption;
• stable flow rate;
• better quality of treated water.

Blower-assisted backwashing is not just an optimization. It is a key engineering standard for industrial sand filters. This makes it a critical component for the sustainable and efficient operation of filtration systems.