Purification of Water: Filtration Theory Explained | How Filter Media Work

Introduction: Why Filtration Is the Heart of Water Purification

Water purification is not just about removing visible dirt it is about eliminating suspended solids, colloidal impurities, organic matter, and harmful microorganisms to make water safe for use. Among all treatment processes, filtration is the most critical and universally used step in water treatment systems worldwide.

Whether it is drinking water treatment, wastewater reuse, industrial process water, swimming pools, or desalination pretreatment, filtration plays a central role. Yet, many people assume filtration simply means “water passing through sand.”

In reality, filtration works through four powerful natural mechanisms that operate simultaneously inside a filter bed. These mechanisms together form what is known as the Purification of Water – Filtration Theory.

This blog explains the complete filtration theory in a clear, structured, and practical way connecting classical principles with modern filter media used today.

What Is Filtration in Water Treatment?

Filtration is the process of allowing water to pass through porous media such as sand, gravel, anthracite, garnet, or zeolite, where impurities are removed by physical, chemical, biological, and electrical actions.

During filtration:

• Suspended and colloidal impurities are removed

• Turbidity is reduced

• Organic matter is broken down

• Bacteria levels decrease significantly

• Water chemistry remains mostly unchanged

Filtration does not rely on a single action. Instead, it works through four fundamental mechanisms explained below.

1. Mechanical Straining – Physical Removal of Particles

What Is Mechanical Straining?

Mechanical straining is the most basic and visible filtration mechanism. It works on a simple principle: particles larger than the gaps between filter grains cannot pass through.

Sand grains form a network of tiny voids. When water flows through these voids:

• Large suspended particles are physically blocked

• Dirt gets trapped between sand grains

• Clean water flows downward

In simple terms, sand acts like a sieve.

Why Mechanical Straining Is Important

Mechanical straining removes:

• Silt

• Suspended solids

• Flocs formed after coagulation

• Larger organic debris

This mechanism is especially effective in rapid sand filters, multimedia filters (MGF), and pressure sand filters.

However, mechanical straining alone is not sufficient to remove very fine or colloidal particles, which is why additional mechanisms are essential.

2. Sedimentation & Adsorption – Sticking and Settling of Fine Impurities

Understanding Sedimentation Inside Filters

Inside a filter bed, water velocity slows down significantly. The spaces between sand grains act like mini sedimentation tanks.

As water slows:

• Fine particles settle

• Turbulence reduces

• Impurities come into contact with filter media surfaces

This process allows particles that are too small for straining to still be removed.

What Is Adsorption in Filtration?

Adsorption occurs when impurities stick to the surface of filter media grains instead of passing through.

This happens because:

• Sand surfaces are not perfectly smooth

• Organic matter forms sticky, gelatinous layers

• Previously trapped impurities enhance surface adhesion

Over time, a thin layer of sticky material develops around sand grains, improving filtration efficiency.

Why Filters Improve with Time

Many operators notice that filters perform better after initial operation. This is due to sedimentation and adsorption building an effective capture layer inside the filter bed.

This mechanism explains why filter ripening is a real phenomenon in water treatment systems.

3. Biological Metabolism – Natural Purification by Microorganisms

What Is Biological Filtration?

Biological metabolism refers to the activity of beneficial microorganisms that grow naturally within the filter bed.

These microorganisms include:

• Bacteria

• Algae

• Micro-flora

• Micro-fauna

They attach themselves to sand grains and form a thin biological layer known as the zooglial film.

How Biological Metabolism Works

When water passes through the filter:

• Organic impurities serve as food for microorganisms

• Microbes consume and break down organic matter

• Harmful substances are converted into harmless compounds

• Pathogenic bacteria levels reduce

Most biological activity occurs in the top layers of the filter bed, especially in slow sand filters and biologically active filters.

Why Biological Filtration Is Powerful

Biological metabolism:

• Removes dissolved organic matter

• Reduces taste and odor

• Improves microbial safety

• Enhances overall filtration efficiency

This natural purification process is one of the reasons why traditional sand filtration remains relevant even today.

4. Electrochemical Action – Attraction by Electrical Charges

Electrical Charges in Filtration

Both sand particles and impurities carry electrical charges on their surfaces.

• Sand grains often carry a positive charge

• Colloidal impurities usually carry a negative charge

Because opposite charges attract, impurities are drawn toward sand particles and become attached.

How Electrochemical Action Helps Filtration

This attraction causes:

• Neutralization of charges

• Agglomeration of fine particles

• Improved removal of colloidal impurities

This mechanism explains how filters can remove particles much smaller than visible pore sizes.

Why Backwashing Is Necessary

Over time:

• Electrical charge capacity of sand becomes exhausted

• Impurity layers build up

• Filter efficiency decreases

That is why periodic backwashing or media replacement is essential to restore filtration performance.

Why Coagulation Is Needed Before Filtration

Sedimentation and filtration alone cannot remove very fine colloidal particles, because:

• They are extremely small

• They remain suspended

• They carry similar electrical charges

Role of Coagulation

Coagulation involves adding chemicals (like alum or ferric salts) that:

• Neutralize electrical charges

• Allow particles to combine

• Form larger flocs

These flocs can then be easily removed by sedimentation and filtration.

Without coagulation, fine turbidity cannot be effectively treated.

How Filtration Theory Applies to Modern Filter Media

Modern filtration systems use different media to enhance each filtration mechanism:

• Silica Sand – Mechanical straining and sedimentation

• Anthracite – High dirt-holding capacity and adsorption

• Garnet – Fine filtration in multimedia beds

• Zeolite – Adsorption and ion-exchange

• Activated Carbon – Advanced adsorption

At Starke Filter Media, filtration media are engineered and graded to ensure:

• Optimal void structure

• Controlled uniformity coefficient

• High mechanical strength

• Long service life

This ensures all four filtration mechanisms work together efficiently.

Summary: Filtration Is More Than Just Sand

Understanding filtration theory helps engineers, operators, and decision-makers choose the right filter media, design better systems, and achieve consistent water quality.

Why Choose High-Quality Filter Media for Effective Filtration

Filtration efficiency depends not just on theory, but on media quality:

• Correct particle size distribution

• Proper effective size (ES)

• Controlled uniformity coefficient (UC)

• Clean, washed, and graded media

Poor-quality media can disrupt filtration mechanisms, leading to poor performance and frequent maintenance.

Conclusion

The purification of water through filtration is a multi-layered, science-driven process, not a simple physical barrier. Mechanical, biological, chemical, and electrical forces work together inside every filter bed.

Understanding filtration theory allows water treatment professionals to:

• Design efficient systems

• Select the right filter media

• Improve water quality

• Reduce operating costs

At Starke, filtration is not just a product it is a science we engineer every day.