This article explains NTU turbidity and its role in assessing water quality. It describes what turbidity is, what causes it, why it matters, and how it is measured. The article also outlines UK water quality guidelines, common treatment methods to manage turbidity, and its link to suspended solids.
What is the Nephelometric Turbidity Unit (NTU)?
NTU stands for nephelometric turbidity unit. It measures how cloudy water is based on the amount of suspended particles. More particles mean higher turbidity and a cloudier appearance.
What is Water Turbidity?
Turbidity measures how clear or cloudy water is. The cloudier the water, the higher the turbidity. It is based on how much light bounces off particles in the water. More light bouncing off means higher turbidity. Turbidity can be caused by materials like silt, clay, algae, organic matter, and microscopic organisms.
The Difference between Turbidity Units NTU, FNU, FTU, JTU and FAU
NTU
NTU stands for Turbidimetric Turbidity Units. It measures how much light is scattered at a 90-degree angle from the incoming light.
FNU
FNU stands for Formazin Nephelometric Units. It means the instrument measures light scattered at a 90-degree angle from the incident light. FNU is commonly linked to the ISO 7027 turbidity method used in the UK.
FTU
FNU stands for Formazin Nephelometric Units. It measures scattered light at a 90-degree angle to the incoming light.
JTU
JTU, or Jackson turbidity units, were used in the past to measure water clarity. The measurement was done using a Jackson candle turbidimeter. Water was added to a test tube until the flame beneath it could no longer be seen.
FAU
FAU stands for Formazin Attenuation Units. It measures how much light is blocked as it passes through a sample at a 180-degree angle. This is done using a spectrophotometer or colourimeter. Most regulatory agencies do not accept it as a reliable method for measuring turbidity.
The Importance of Turbidity Level in Water Treatment?
Turbidity shows how clear water is by measuring particles that scatter light. These particles, like silt, clay, organic matter, and microorganisms, can be harmful if not removed. High turbidity can signal dangerous contaminants, shield pathogens from disinfectants, and make water treatment less effective. Controlling turbidity in drinking water is essential for safety and health.
- Public Health: Cloudy water often suggests potential health risks. Turbidity can shelter harmful microorganisms, allowing them to grow and lead to waterborne illnesses. Such outbreaks have caused intestinal sickness in the UK and globally.
- Treatment Efficiency: Turbidity can block disinfectants from reaching pathogens effectively. Lowering turbidity ensures the treatment process works properly to eliminate harmful organisms.
- Aesthetics: Cloudy water can taste or smell unpleasant and look unappealing. Clear water is more likely to satisfy consumers and reduce complaints.
- Environmental Impact: High turbidity can block sunlight, reduce oxygen levels in water, harm aquatic life, and disrupt ecosystems. It can also lead to silt build-up in water bodies, disturbing the natural balance.
What Does a Turbidity Measurement Mean in Water Treatment?
Baseline testing is essential because every body of water has unique factors that affect turbidity, and conditions can change over time. Testing in the same location regularly helps establish what is normal for a specific body of water.
Turbidity is often measured in Nephelometric Turbidity Units (NTU) using a nephelometer, though other methods are also available. Higher NTU values mean the water is cloudier, blocking more light. Lower NTU values indicate clearer water with fewer suspended particles.
- 1 NTU: Typical measurement for tap water.
- 1 NTU: Common regulatory limit for drinking water quality.
NTU Limits
- Drinking water:
The UK sets strict limits for turbidity in drinking water to ensure safety and quality. The maximum permitted level is 1 NTU. However, water companies aim for much lower levels to maintain clarity and reduce the risk of harmful contaminants. Regular testing ensures turbidity stays within acceptable limits.
- Wastewater:
The UK has guidelines for turbidity levels in wastewater to protect the environment. Turbidity should be controlled before wastewater is discharged into natural water sources. Treatment plants use methods like filtration to lower turbidity and meet regulatory standards. Regular monitoring ensures compliance and helps prevent water pollution.
Controlling Turbidity
Reducing water turbidity helps extend the lifespan of filtration systems and prevents contamination. Here are some methods to remove turbidity from water:
- Coagulation and Flocculation:
The coagulation and flocculation process removes turbidity by destabilising particles in the water, causing them to clump together so they can be filtered out. Coagulation binds smaller particles, while flocculation forms larger, visible clusters.
Water is placed in a container where a coagulant is added. This helps particles stick together and settle at the bottom. Common coagulants used in large-scale treatment include ferric sulfate, aluminium sulfate, and ferric chloride. The efficiency of this method depends on the water’s pH level. Once sedimentation occurs, the settled sludge is removed, and the water is then filtered.
- Filtration:
Filtration removes small particles and impurities from water. Water flows through layers of sand, gravel, or other materials that trap these particles. This process reduces cloudiness, making the water clear and safe. Maintaining the filter media is essential for it to work effectively. Filtration is a key step in treating water to ensure its quality.
- Sedimentation:
Sedimentation is used to reduce turbidity by allowing solids to settle at the bottom of a container or basin. This process separates heavier particles from the water using gravity. Clear water remains at the top, ready for further treatment. Sedimentation helps improve water clarity and is an essential step in water purification.
Instruments for NTU Measurement
Nephelometer
A nephelometric turbidity meter measures scattered light at a 90-degree angle from the main light beam. This is called the backscatter reading.
The backscatter reading is not affected much by the liquid’s colour, making these meters ideal for checking turbidity in water treatment processes.
Portable Turbidity Meters
Portable turbidity meters are handheld devices used to measure turbidity in the field. They are convenient and easy to carry, making them ideal for on-site testing. These meters are designed to provide quick and reliable results, even in remote locations. Their compact size and user-friendly interface make them practical tools for professionals monitoring water quality.
Laboratory Turbidity Meters
Laboratory turbidity meters are designed for precise and accurate measurements. These devices are commonly used in controlled environments, such as research facilities and water treatment plants. They offer advanced features to ensure reliable results, such as higher sensitivity and calibration options. Laboratory meters are ideal for detailed analysis and routine testing.
Turbidity Testing Solutions with Comfort Services Group
Comfort Services Group works with accredited labs to meet your turbidity testing requirements, ensuring compliance with local water authorities. With over 30 years of experience, we offer environmental commissioning and water treatment services to businesses in London and the South East. Our highly trained team supports your water quality monitoring needs with expertise and professionalism. Contact us to find out how we can help.
FAQs
Turbidity or Secchi depth? Which measurement should I use?
Turbidity or Secchi depth: which is better for measuring water clarity?
Both methods measure clarity, but each works best in specific conditions:
Use Secchi depth for:
- Clear water with low to moderate turbidity, where the bottom is visible.
- Still or slow-moving water such as lakes, ponds, or large rivers.
Use turbidity for:
- Water that looks cloudy or murky.
What is 1 unit NTU equal to?
Water with 1 milligram of Formazine per litre has a turbidity level of 1 NTU. This is determined using a colour-matching method. A nephelometer, an electronic device, is used to measure NTU.
What is a good NTU for water?
UK guidelines state that tap water should have a turbidity level below 1 NTU, which appears clear. If levels rise above 5 NTU, it is often noticeable to users. At 55 NTU, the water will start to look cloudy.
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