Turbidity is a measurement of the cloudiness of water caused by suspended particles. These can include fine sediment, clay, organic matter, iron deposits and other materials that scatter light as it passes through the water.
Although cloudy water is often associated with poor water quality, even water that looks clear can contain measurable turbidity.
For borehole and private water supplies, turbidity is particularly important because excessive suspended particles can interfere with filtration equipment and reduce the effectiveness of UV disinfection.
Understanding the source of turbidity is the first step towards selecting suitable treatment.
What Causes Turbidity in Water?
Common causes include:
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Sand, silt and clay: Fine particles entering from boreholes, wells or surface water.
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Iron and manganese: Oxidised metals forming suspended brown, orange or black particles.
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Organic matter: Decomposing vegetation, algae and other natural materials.
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Surface water contamination: Rainfall and agricultural runoff carrying sediment into a water source.
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Disturbed deposits: Sediment released from tanks, pipework and distribution systems.
Turbidity can vary considerably throughout the year, particularly in shallow boreholes and surface-fed supplies.
A water analysis taken during dry weather may not represent conditions following heavy rainfall.
How Is Turbidity Measured?
Turbidity is normally measured in Nephelometric Turbidity Units (NTU) using a turbidimeter.
The instrument measures light scattered by particles suspended in the water.
Higher NTU readings generally indicate greater turbidity.
UK Drinking Water Standards
The Drinking Water Inspectorate identifies two important values:
| Turbidity | Significance |
|---|---|
| Below 1 NTU | Recommended pretreatment target before disinfection |
| 1–4 NTU | May be within the tap standard but requires further reduction before UV disinfection |
| Above 4 NTU | Exceeds the UK drinking-water standard at consumers' taps |
The 4 NTU standard should not be interpreted as an acceptable target for water entering a UV system.
For UV applications, the objective should be to maintain turbidity consistently below 1 NTU, including during periods when source-water quality deteriorates.
Source: Drinking Water Inspectorate – Protecting Your Private Water Supply.
Why Does Turbidity Matter for UV Disinfection?
UV disinfection works by exposing microorganisms to ultraviolet light at a sufficient dose to inactivate them.
However, suspended particles can shield microorganisms from UV exposure.
Iron deposits and other contaminants can also accumulate on the quartz sleeve, reducing the amount of UV light reaching the water.
This means that installing a UV system on heavily turbid water may not provide the intended level of disinfection.
UV disinfection does not remove turbidity or make cloudy water clear. Suitable filtration must take place beforehand.
Turbidity and UVT Are Not the Same
This is an important distinction when assessing water for UV treatment.
Turbidity (NTU) measures light scattering caused by suspended particles.
UV Transmittance (UVT) measures how much ultraviolet light passes through the water, normally at a wavelength of 254 nm.
Water can have very low turbidity but still have poor UVT if it contains dissolved organic matter, colour or other UV-absorbing substances.
For example, water from a peat-influenced supply may look relatively clear but contain dissolved organic compounds that absorb UV light.
A turbidity test alone therefore cannot confirm that water is suitable for UV disinfection.
UV equipment manufacturers specify the UVT and maximum flow conditions under which their systems achieve the intended dose.
Practical Guide: What Treatment Is Needed Before UV?
The following guide provides a starting point for selecting pretreatment.
| Water condition | Suggested approach |
|---|---|
| Consistently below 1 NTU | Check UVT, iron, manganese and other relevant parameters before selecting UV |
| Between 1 and 4 NTU | Trial suitable sediment filtration and retest |
| Above 4 NTU | Investigate the source and consider higher-capacity or backwashing filtration |
| High or fluctuating sediment loading | Consider automatic backwashing filtration rather than relying entirely on cartridges |
| Low turbidity but high colour | Test UVT and investigate dissolved organic matter |
| Turbidity caused by iron or manganese | Consider oxidation and appropriate metal-removal filtration |
| Persistent fine or colloidal particles | More advanced treatment, such as coagulation or membrane filtration, may be necessary |
These are general selection principles rather than guaranteed performance limits.
The correct system depends on the source-water analysis, particle characteristics, flow rate and required treated-water quality.
Methods of Removing Turbidity
1. Sediment Cartridge Filtration
Sediment cartridges are often the simplest solution for relatively low levels of suspended solids.
A common arrangement is:
20-micron sediment filter → 5-micron sediment filter → UV disinfection
The first cartridge captures larger particles, reducing the loading on the finer filter.
The second cartridge provides additional filtration before UV.
However, the exact micron ratings should be selected according to the water analysis and filter performance.
A 5-micron cartridge does not automatically guarantee turbidity below 1 NTU, particularly where extremely fine or colloidal particles are present.
Cartridge replacement frequency depends on sediment loading and water consumption.
Explore our sediment filter cartridges.
2. Backwashing Turbidex Filtration
Where turbidity is higher or sediment loading is continuous, a backwashing filter may offer a more practical solution.
Turbidex sediment filtration systems use granular media to capture suspended solids.
During backwashing, accumulated material is flushed from the filter bed to drain.
Advantages include:
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Reduced dependence on disposable cartridges
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Suitable options for higher sediment loading
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Automatic backwashing
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Protection for downstream treatment equipment
Turbidex is commonly used before iron removal equipment, cartridge filtration, reverse osmosis and UV systems.
However, the vessel must be sized for both the required service flow rate and available backwash flow rate.
An undersized pump or drain arrangement may prevent effective backwashing.
3. AFM Filtration
Activated Filter Media (AFM) is another option for reducing suspended solids and turbidity.
AFM systems use specially processed glass-based filtration media within backwashing vessels.
They can be considered for domestic, commercial and industrial applications where appropriately sized filtration is required.
Explore our AFM filtration systems.
4. Bag Filtration
Bag filters are useful for many commercial and industrial applications.
They offer relatively large dirt-holding capacities and are available in different materials, sizes and micron ratings.
Bag filtration is particularly useful where larger volumes of water need sediment reduction before further treatment.
However, fine turbidity may require additional filtration, depending on the filter's retention rating and the characteristics of the particles.
Explore our bag filtration range.
5. Ultrafiltration
Where conventional filtration cannot consistently achieve the required water clarity, ultrafiltration may be considered.
Ultrafiltration membranes provide a much finer physical barrier than conventional sediment cartridges.
They can reduce very fine suspended particles and certain microorganisms, depending on the membrane and system design.
However, ultrafiltration requires suitable pretreatment and maintenance to control membrane fouling.
It should not be assumed to remove dissolved contaminants such as arsenic, hardness or dissolved salts.
Example: Treating Turbid Borehole Water Before UV
Consider a borehole supply with a turbidity reading of 5 NTU.
Installing a UV unit directly onto this supply would not be advisable without further treatment.
An initial approach might involve:
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Investigating whether the turbidity originates from sediment, iron or organic matter.
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Trialling sediment filtration to establish how readily the particles can be removed.
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Considering a backwashing Turbidex system if cartridge filtration becomes impractical.
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Installing a suitable fine polishing filter before UV, where required.
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Testing the treated water for turbidity and UVT before confirming the UV system specification.
If dissolved iron, manganese or organic colour is present, additional treatment stages may be necessary.
A successful filtration trial is useful evidence, but the system must also cope with variations in water quality and the maximum required flow.
Maintaining a Turbidity Filtration System
Regular maintenance is essential to prevent declining performance.
Important checks include:
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Monitoring pressure loss across cartridge filters
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Replacing cartridges when required
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Confirming backwashing filters operate correctly
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Checking that sufficient backwash flow is available
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Inspecting filter vessels, valves and drains
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Periodically testing turbidity after treatment
For systems supplying drinking water, monitoring should also reflect the microbiological and chemical risks associated with the source.
Can Clear Water Still Be Unsafe?
Yes.
Low turbidity does not prove that water is free from bacteria, viruses or dissolved chemical contaminants.
Similarly, achieving turbidity below 1 NTU is an important pretreatment objective, but it does not independently establish that UV disinfection will be effective.
UV performance also depends on:
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UV transmittance
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Delivered UV dose
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Maximum flow rate
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Lamp and quartz sleeve condition
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Correct system operation
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Suitable monitoring and fail-safe arrangements
Where a private water supply is used for drinking, appropriate water testing and a properly designed treatment system remain essential.
Need Help Reducing Turbidity?
At GAPS Water Treatment, we supply sediment cartridges, bag filters, Turbidex systems, AFM filtration and other equipment for domestic and commercial water treatment.
If you have a water analysis showing elevated turbidity, please email it to us with:
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The source of your water supply
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Turbidity reading in NTU
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Any iron, manganese or colour results
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Required service flow rate
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Available pressure and backwash flow
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Details of existing treatment equipment
We can review the information and suggest suitable filtration products and system options.
Email: sales@gapswater.co.uk
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