What is Turbidity?
The definition of Turbidity is the cloudiness or haziness of a fluid caused by suspended solids that are usually invisible to the naked eye. The measurement of Turbidity is an important test when trying to determine the quality of water. It is an aggregate optical property of the water and does not identify individual substances; it just says something is there.
Water almost always contains suspended solids that consist of many different particles of varying sizes. Some of the particles are large enough and heavy enough to eventually settle to the bottom of a container if a sample is left standing (these are the settleable solids). The smaller particles will only settle slowly, if at all (these are the colloidal solids). It’s these particles that cause the water to look turbid.
The term Turbidity (also called haze) can also be applied to transparent solids like plastic and glass.
What Causes Turbidity?
Organisms like phytoplankton can contribute to turbidity in open water. Erosion and effluent from highly urbanized zones contribute to the turbidity of waters in those areas. Construction, mining and agriculture, disturb the soil and can lead to raised levels of sediment which run off into waterways during storms. Storm water from paved surfaces like roads, bridges and parking lots also contribute to turbidity.
In drinking water the higher the level of turbidity, the higher the chance that those using it could develop gastrointestinal diseases. Contaminants like viruses and pathogenic bacteria can attach themselves to the suspended solids. These solids then interfere with disinfection.
High turbidity levels can reduce the amount of light reaching lower depths in bodies of water like rivers, lakes and reservoirs, which inhibits growth of some forms of aquatic plants and can negatively affect species that are dependent on them, like fish and shellfish. High turbidity levels will also hinder a fish’s ability to absorb dissolved oxygen. This condition has been observed and documented throughout the Chesapeake Bay in the Mid-Atlantic region of the USA.
How is Turbidity Measured?
The most common measurement for turbidity in the United States are the Nephelometric Turbidity Units (NTU).
There are several ways you can check turbidity in water, the most direct being a measure of attenuation, or reduction in strength, of a light source as it passes through a water sample. An older system was called the Jackson Candle method, with units expressed as JTU or Jackson Turbidity Units. It used a candle flame viewed through a clear column filled with water. The length of water that the candle could be seen through related to the turbidity in the water sample. With the advent of electronic meter technology this method is no longer used.
The particles suspended in the water will scatter a light beam focused on them. The scattered light is then measured at various angles from the incident light path. This is now accepted as a more precise measure of turbidity. To measure turbidity this way use a nephelometer, such as the LaMotte 2020we. Nephele is the Greek word for "cloud"; metric means “measure." Nephelometric, therefore, means "measuring cloudiness." Most nephelometers measure the scattered light at 90°. If more light is able to reach the detector it means there are many small particles scattering the source beam, less light reaching the detector means fewer particles. Nephelometric Turbidity Units (NTU) are the units of measurement used by a nephelometer meeting EPA design criteria. The amount of light scattered is influenced by many aspects of the particles like color, shape, and reflectivity. Because of this, and the fact heavier particles may settle quickly and may not contribute to the turbidity reading, the relationship between turbidity and total suspended solids (TSS) can change depending on the location that the test sample was collected.
Measuring turbidity in environmental applications, such as the oceans, rivers and lakes, a Secchi disk can be used. This is a black and white disk that is lowered into the water until it can no longer be seen. At that depth (called Secchi depth) the correlating number is recorded as a measure of the clarity in the water. The advantage in using this device in open waters is the ability to measure turbidity at various depths where multiple turbidity layers are present. This device is also easy to use and relatively inexpensive.
Drinking Water Standards and Testing Methods
Many things can affect the quality of drinking water, so government regulations set the level of turbidity that is permissible. In the United States, public drinking water systems that use flocculation or direct filtration for turbidity control cannot exceed 1.0 nephelometric turbidity unit (NTU) leaving the treatment plant. In the samples collected for turbidity measurement the turbidity should remain less than or equal to 0.3 NTU for at least 95 percent of those collected in any month. If a public drinking water system uses any filtration other than flocculation or direct filtration then they are subject to their individual state limit, but even these must not exceed a turbidity level of 5 NTU. Usually utilities will try to maintain a turbidity level of about 0.1 NTU.
The published analytical test methods for turbidity include:
- US EPA Method No. 180.1, "Turbidity"
- ISO 7027 "Water Quality: Determination of Turbidity"
- "Standard Methods," No. 2130B.
- Numerous ASTM methods
Nephelometers and Turbidimeters
In this discussion we will be focusing on using nephelometers and turbidimeters to analyze turbidity in drinking water and in environmental and industrial applications. The difference in the two is a subtle one. If the light detector is at an angle of 90° to the light source the meter is considered a nephelometer, if it is at a 180° angle then it is a turbidimeter. As the light source or sources in most portable meters contain both types of detectors the meters are usually called turbidimeters.
As stated before, turbidity measurement in drinking water is important because of the possibility that bacteria can use the suspended particles to “hide” from the chemicals that utilities use for disinfection. The particles themselves also interact with disinfectants making it difficult to maintain a high enough residual to effectively neutralize the pathogens present.
Most portable turbidity meters differ by the type of light source they employ. The two types usually found are incandescent tungsten bulbs (white light) and infrared-LED bulbs.
The Turbidity of a sample will increase with the amount of undissolved solids present. Measuring the light as it scatters off the sample at a 90° angle is a better and more accurate method when measuring in the lower ranges, <40 NTU. At higher ranges the 180° angle is more accurate. Between 500 and 1000 NTU most meters will switch from measuring at the 90° angle in NTUs to the 180° angle and attenuation units, or AU’s.These two units are directly comparable.
A turbidity meter with an ISO specified design uses an infrared LED (IR-LED) with a wavelength of 860nm and collimated light path required for methods: ISO 7027/DIN EN 27027 (EN ISO 7027).
Turbidity meters with EPA specified designs use an incandescent tungsten type lamp and are required for compliance sampling under the EPA 180.1 method for determination of turbidity by nephelometry, which states:
“Differences in physical design of turbidimeters will cause differences in measured values for turbidity, even though the same suspension is used for calibration. To minimize such differences, the following design criteria should be observed:
- Light source: Tungsten lamp operated at a color temperature between 2200-3000°K.
- Distance traversed by incident light and scattered light within the sample tube: Total not to exceed 10 cm.
- Detector: Centered at 90° to the incident light path and not to exceed ±30° from 90°. The detector, and filter system if used, shall have a spectral peak response between 400 nm and 600 nm.
The sensitivity of the instrument should permit detection of a turbidity difference of 0.02 NTU or less in waters having turbidities less than 1 unit. The instrument should measure from 0-40 units turbidity. Several ranges may be necessary to obtain both adequate coverage and sufficient sensitivity for low turbidities.”
It is important to determine which type of meter to use before purchasing. A drinking water utility that has to comply with the EPA 180.1 method should use a tungsten type nephelometer. For most other applications the IR-LED type ISO turbidimeter should be used.
The reason for this is that an infrared light source will minimize or possibly eliminate the influence of coloration in a sample. These may lose some sensitivity of smaller particles at this 860nm wavelength because the smaller particles tend to scatter less light at 860nm than at visible wavelengths. The tungsten type “white light” meter will have a higher sensitivity for those small particles, but will lose accuracy when any color is in the sample.
It is important to remember that with both types of portable turbidity meters, floating and moving particles may cause slight measurement deviations. In order for these meters to provide the best possible results one should always measure the sample immediately, as particles will settle over time. It is best to maintain a constant lamp temperature by not turning the meter on and off frequently between analyzing samples. Also the position on the sample cells should be marked when placed in the sample chamber to eliminate variances in the glass vials.
Sampling, Calibration and Analysis
For the purposes of this discussion the LaMotte 2020we/wi Turbidity Meter will be used as an example. Most portable turbidity meters will follow similar procedures for calibration and testing. It is important to always follow the manufacturer’s recommendations for the use, care and storage of the meter.
The meter should come with a set of turbidity standards. If it does not, then purchase the standards that the manufacturer recommends for that unit. The 2020we/wi meters come with a blank, or 0 NTU standard, a 1 NTU standard, and a 10 NTU standard. Additional NTU level standards can be purchased separately. Always choose standards close to the range of turbidity samples to be tested. For the most accurate results select standards over the smallest range possible. The meter should be calibrated at least on a monthly basis, but the calibration should be checked daily to ensure it is still accurate. A check can be performed by scanning a sample of one of the standards to see that the meter is still reading true.
Testing for turbidity in regulated water systems is a critical step in assuring compliance and treatment efficacy. The best results are obtained by careful attention to procedure and technique. Maintaining equipment, including the meter, tubes and sample chamber as well as careful sample handling will minimize interferences and provide the most accurate results. Periodically check the sample chamber in the meter to determine if any scratching has occurred. If it has, have the chamber replaced as soon as possible. The same applies to sample tubes if they become scratched. The application of silicone oil to scratched glass surfaces is not recommended as this can produce an uneven surface of oil on the tube and alter final readings.
No matter how well a meter is designed, it can only work properly if attention to these details and proper calibrations are followed.