Filtration in Water Treatment

Introduction

The water in your tap probably began in a river, a reservoir or underground rock. That raw water contains leaves, mud, fine clay and microorganisms. Before it reaches you, it passes through a treatment works where one key step is filtration — not through paper, but through great beds of sand and gravel. This topic shows how the simple separation methods you have learnt are scaled up to supply millions of people with potable water every day.

Core explanation

Potable is not pure. Drinking water must be safe, with low levels of harmful substances and microorganisms, but it still contains dissolved minerals such as calcium and magnesium salts. These are harmless and give water its taste. Chemically pure water would contain only H₂O.

The main stages. A typical works treating river or reservoir water uses:

1. Screening — the water passes through metal screens (large sieves) that remove twigs, leaves, fish and litter. 2. Sedimentation — the water rests in large tanks. A flocculant such as aluminium sulfate is often added so tiny clay particles clump into flocs, which settle to the bottom. 3. Filtration through sand and gravel — the partly cleared water flows through deep filter beds. 4. Sterilisation — chlorine (or ozone, or ultraviolet light) kills remaining microorganisms.

How a sand filter bed works. A filter bed is a large concrete tank filled with layers: fine sand on top, coarser sand below, and gravel at the bottom resting on pipes that collect the water. Water trickles down through the gaps between the grains. Fine particles and flocs that did not settle are trapped between the sand grains, much as solids are trapped by filter paper. The gravel supports the sand and lets clean water drain away freely. In slow sand filters a thin living layer forms on the surface, and its microorganisms help break down some organic matter and trap bacteria.

Keeping the filter working. As trapped dirt builds up, water flows more slowly. In rapid filters, the bed is cleaned by backwashing : clean water is pumped upwards through the bed, lifting the sand and washing the dirt out to waste.

Why filtration is not enough. A sand bed removes suspended solids, but dissolved substances pass through, and some bacteria and viruses are too small to be reliably trapped. That is why chlorination follows. A small amount of chlorine is left in the water so it stays protected on its journey through the pipes.

Step-by-step reasoning

To explain what each stage removes:

1. Large objects are removed by screening, because they cannot pass the mesh. 2. Heavy and flocculated particles are removed by sedimentation. 3. Fine suspended particles are trapped by the sand filter. 4. Microorganisms are killed by chlorination. 5. Dissolved minerals remain throughout, which is acceptable.

Visual explanation

Picture a cutaway of a filter bed: murky water entering at the top, a thick layer of pale fine sand, a thinner layer of coarse sand, then rounded gravel over perforated pipes. Clear water flows out along the pipes at the bottom towards a chlorine contact tank.

Real-world analogy

A sand filter bed is like a crowded stadium exit. Water molecules slip easily through the gaps between people, but anyone carrying a large bag gets stuck in the crowd. The deeper the crowd, the more bags are caught.

Real-world example

Portable camping filters and household jug filters use the same ideas on a small scale: layers of fine material trap particles, while activated carbon removes some tastes and odours. Where treatment works are unavailable, communities use simple biosand filters made from layered sand and gravel in a container.

Why?

Why is sand used rather than filter paper? Paper would clog within minutes and cannot be cleaned. A deep sand bed traps particles throughout its depth, handles enormous volumes of water, is cheap, and can be cleaned by backwashing and reused for years.

Common misconception

"Filtered water is safe to drink." Filtration removes suspended solids but not dissolved chemicals, and it cannot be relied upon to remove every microorganism. Water should be sterilised as well, and in emergencies it may need boiling.

Worked example

Question: A treatment works misses out the chlorination stage. Which harmful component could remain in the water, and why?

Reasoning: Screening, sedimentation and filtration remove solids. Some bacteria and viruses are small enough to pass through the sand bed.

Answer: Harmful microorganisms could remain, because filtration does not remove all of them; chlorination is needed to kill them.

Quick check

1. What is the purpose of the sand bed in water treatment? Answer: To trap fine suspended particles that did not settle out.

Exam focus

Learn the sequence screening → sedimentation → filtration → chlorination and the purpose of each stage. Use the term potable correctly: safe to drink, not pure. Explain that filtration removes insoluble particles and chlorination kills microorganisms.

Advanced insight

Some modern plants replace or supplement sand beds with membrane filtration, using membranes with pores small enough to remove bacteria and many viruses physically. Activated carbon filters are added where water contains dissolved organic compounds, which stick to the carbon's huge internal surface area.

Summary

Fresh water is made potable by screening, sedimentation, filtration through sand and gravel beds, and sterilisation with chlorine. The filter bed traps fine suspended particles between sand grains and is cleaned by backwashing. Filtration cannot remove dissolved substances or reliably all microorganisms, so sterilisation is essential. Potable water still contains dissolved minerals.

Practice questions

1. Put these stages in order: chlorination, filtration, screening, sedimentation. Answer: Screening, sedimentation, filtration, chlorination. 2. Explain the difference between potable water and pure water. Answer: Potable water is safe to drink but contains dissolved substances; pure water contains only water molecules. 3. Why is gravel placed below the sand in a filter bed? Answer: It supports the sand and lets filtered water drain freely into the collecting pipes. 4. What is backwashing and why is it needed? Answer: Pumping clean water upwards through the bed to flush out trapped dirt, because the dirt slows the flow of water.