Turbidity is a principal physical characteristic of water and is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. It is caused by suspended matter or impurities that interfere with the clarity of the water. These impurities may include clay, silt, finely divided inorganic and organic matter, soluble colored organic compounds, and plankton and other microscopic organisms. Typical sources of turbidity in drinking water include the following:
Simply stated, turbidity is the measure of relative clarity of a liquid. Clarity is important when producing drinking water for human consumption and in many manufacturing uses. Once considered as a mostly aesthetic characteristic of drinking water, significant evidence exists that controlling turbidity is a competent safeguard against pathogens in drinking water.
Turbidity's Significance to Human Health
Excessive turbidity, or cloudiness, in drinking water is aesthetically unappealing, and may also represent a health concern. Turbidity can provide food and shelter for pathogens. If not removed, turbidity can promote regrowth of pathogens in the distribution system, leading to waterborne disease outbreaks, which have caused significant cases of gastroenteritis throughout the United States and the world. Although turbidity is not a direct indicator of health risk, numerous studies show a strong relationship between removal of turbidity and removal of protozoa.
The particles of turbidity provide “shelter” for microbes by reducing their exposure to attack by disinfectants. Microbial attachment to particulate material or inert substances in water systems has been documented by several investigators and has been considered to aid in microbe survival. Fortunately, traditional water treatment processes have the ability to effectively remove turbidity when operated properly.
Turbidity Removal and Pathogen Removal
Low filtered water turbidity can
be correlated with low bacterial counts and low incidences of viral disease.
Positive correlations between removal (the difference between raw and plant
effluent water samples) of pathogens and turbidity have also been observed in
several studies. In fact, in every study to date where pathogens and turbidity
occur in the source water, pathogen removal coincides with turbidity/particle
Nephelometric Measurement Procedure
The procedure above is tailored to the HACH Model 2100N
Turbidimeter shown above. Any approved model of turbidimeter can be used
to test turbidity.
Accurate and repeatable turbidity measurements depend on good, consistent measurement techniques. Measurements are more accurate and repeatable if close attention is paid to proper measurement techniques. Four important considerations are:
Measure samples immediately to prevent changes in sample characteristics due to temperature shifts and settling. Avoid dilution whenever possible; particles suspended in the original sample may dissolve or otherwise change characteristics when the temperature changes or the sample is diluted. Thus, the measurement may not be representative of the original sample.
Cleaning Sample Cells
Cells must be meticulously clean and free from significant scratches. Glass imperfections and superficial scratches from manufacturing are effectively masked by the silicone oiling procedure. Clean the inside and outside of the cells by washing thoroughly with a nonabrasive laboratory detergent. Then continue cleaning with a 1:1 HCl bath followed by multiple rinses with distilled or deionized water. Air dry the cells. Handle sample cells by the top only to minimize dirt and finger prints.
Silicone Oil Procedure
Treat the outside of the cells with a thin coating of silicone oil to mask minor imperfections and scratches that may contribute to light scattering.
Apply a thin bead of silicone oil from the top to bottom of the cell—just enough to coat the cell with a thin layer of oil. Using the oiling cloth provided, spread the oil uniformly. Then, wipe off the excess so that only a thin coat of oil is left. The cell should appear nearly dry with little or no visible oil. Applying excess oil may attract dirt and contaminate the sample compartment of the instrument.
The accuracy of turbidity testing depends on the care used in following the procedure. This section lists some additional information which will make turbidity testing as accurate as possible.
Sample Cells and Standards
Throughout the procedure, you should handle cells only by the top to avoid fingerprints on the optical surfaces. Fingerprint oils can etch the surface of the glass and must be removed as soon as possible.
Scratches, especially on the bottom, will refract and scatter light, adding a positive interference to the turbidity measurement. Avoid rough handling of the cells and do not use abrasive cleaning compounds. Caustic cleaning solutions also should be avoided because they can etch the glass with prolonged or repeated exposure. If a sample cell does become etched or scratched, discard and replace it with a new cell. Tiny scratches can be covered with oil before each use as explained in the procedure above.
Matching Sample Cells
in individual glass sample cells can result in the same sample producing
slightly different readings in different sample cells. These differences
can be minimized or eliminated by matching cells against each other. This
is done by arbitrarily selecting one cell as a master for comparison with other
Variations are noticeable when the cell is inserted with different rotational orientations. Clean and fill a cell with a low turbidity sample, insert it into the sample compartment and note the turbidity reading. Mark the cell at the front as a position reference. Rotate the cell 90° at a time and note the turbidity reading in each position. It is not unusual for the turbidity reading to vary + 5%.
Clean and fill a set of sample cells with the same sample. Mark one cell at the tip edge. (Fingernail polish works well as a permanent marker.) Insert this cell with the mark facing forward and note the turbidity reading. Remaining cells are matched to the first cell by taking a series of turbidity readings and rotating each cell a few degrees between each reading. If a cell reading matched that of the master cell, mark the front of the cell in that position so it can always be inserted in the same orientation.
A few larger particles in the sample can cause sudden, temporary increases in the turbidity reading as they drift past the photodetector window. If the meter needle is erratic or jumpy, you can verify the presence of larger particles by looking into the cell. In this situation, you can use the average reading or use the lowest stable indication as the background turbidity and discount the effect of the larger particles.
Condensation may form on the outside of the sample cell when cold samples are measured in a humid environment. Water droplets or fog on the cell can interfere with the measurement. It may be necessary to let the sample warm to room temperature to avoid condensation. Wiping the cell dry before measurement may not be effective because condensation can re-form quickly in certain conditions.
When highly turbid samples are measured, it may be necessary to dilute the sample to bring it within the range of the instrument. When dilution is required, the sample should be diluted with a finely filtered portion of the same sample. (Diluting with distilled or demineralized water may dissolve some of the suspended solids and alter the turbidity). Remember to multiply the turbidity of the diluted sample by the dilution factor to obtain the turbidity of the original sample.
light in a nephelometer is any light reaching the photodetector from a source
other than scattering by particulates in the sample. Sources of stray
light include reflections, refractions, and scattering from imperfections in
the lenses, sample cell, light shield, and sample compartment. Stray
light adds a positive turbidity measurement interference that can be noticeable
when measuring low turbidities.
The design of the 2100N Turbidimeter incorporates many features to minimize stray light. Stray light levels typically are less than 0.04 NTU. When measuring turbidities on the 0 - 0.2 range, subtract 0.04 NTU from the measured turbidity for a more exact turbidity value for the sample.