Lab 4:

Turbidity



Reading Assignment

Read Chapter 29 in Simplified Procedures for Water Examination.


Introduction

Turbidity of water is caused by suspended particles, primarily of clay, silt, organic matter, and microorganisms.  In natural waters, turbidity can range from a few NTU to about 1,500 NTU. 

While turbidity is not harmful by itself, excess turbidity hinders the efficiency of disinfection which can result in unsafe drinking water.  As a result, the EPA Drinking Water Standards limit turbidity to 1 NTU in filtered water.  In practice, coagulation, flocculation, and sedimentation followed by effective filtration should result in effluent water containing less than 0.2 NTU. 

Diagram of a turbidimeter.
In a nephelometer, light reflects off turbidity particles
in the water and is measured by a photomultiplier tube. 
The amount of light reaching the photomultiplier tube is
proportional to the amount of turbidity in the sample water. 

The test for turbidity is a physical test rather than a chemical test.  An instrument called a nephelometer is used to measure the amount of light which is scattered by hitting turbidity particles in the water. 

In the treatment plant, operators usually test the turbidity of the finished water at intervals throughout the day.  This gives the operator an indication of how well the water is being treated.  If the effluent water has an unexpectedly high turbidity, then the chemical feed rates may need to be adjusted or the filters may need to be backwashed. 


Equipment

 

The procedure given in this lab is tailored to the HACH Model 2100A Turbidimeter shown above.  Any approved model of turbidimeter can be used to test turbidity. 

You will also need cells to hold the samples and standards, a lint-free cloth, and silicone oil.


Reagents

Ether of the following standards can be used:
  • Stock primary standard formazin suspension.  The procedure used to create this standard is outlined in Standard Methods
  • Secondary standard.


Laboratory Procedure

1. Remove the dust cover from the instrument. 

The dust cover is used to keep the instrument clean while it is being stored, but it should not be used when the instrument is turned on since it can cause significant heat build up within the enclosure. 


2. Turn on the instrument and allow it to warm up for at least 30 minutes. 

This warm-up time is necessary to stabilize the photomultiplier detector.  If the instrument is going to be used at regular intervals, it may be left on to allow for quick, convenient, accurate measurements. 


3. Inspect and clean sample cells.

Before use, each sample cell must be carefully inspected for fingerprints, smudges, lint, dirt, or scratches which can interfere with the measurement by disturbing the light beam.  This step is especially important when measuring samples with low turbidities.

If any of the above imperfections are found on the sample cell, it should be cleaned with silicone oil.  Apply a light coating of oil to the outside of the cell using a lint-free cloth, then wipe the sample cell clean.  After oiling, the cell surface should have a thin but noticeable film of oil which will fill in the tiny scratches and imperfections. 


4. Rinse the sample cell three times with the sample water. 

This step guards against sample contamination by dust or residue in the cell. 


5. Fill the sample cell with approximately 25 mL of the test sample.  Wipe the outside of the cell dry with a clean, lint-free cloth or tissue.  Be particularly careful to make sure that the bottom of the cell is clean and dry. 


6. Hold the cell up to the light to check for small air bubbles on the walls.  Air bubbles in the sample are problematic since they will reflect and scatter light, resulting in a false high turbidity reading.  If air bubbles are present, they can be removed using one of the methods outlined below. 

Holding the cell by its lid and gently tapping the bottom with a finger will often remove the air bubbles. 

An aspirator or a vacuum pump can be used to remove gas bubbles rapidly.  This method is ideal since it will not alter the solids in the sample. 

Another method is to briefly immerse the end of the sample cell in an ultrasonic bath.  Be sure to minimize the exposure of the sample to the ultrasonic energy since exposure can alter the suspended particles and affect sample turbidity. 

In some cases, bubbles form on the side of the sample cell due to supersaturation of gases.  If so, the bubbles can be eliminated by adding a small drop of a surfactant solution such as Triton X-100 to the sample cell before filling. 

Finally, bubbles can sometimes be eliminated by letting the sample stand in the instrument compartment for several minutes before taking a reading.  Heat from the lamp will cause bubbles to rise out of suspension in the sample. 


7. Select the range.

The instrument will have several turbidity ranges, such as 2, 20, and 200.  In most situations, you will have a general idea of the expected turbidity of the sample.  In this case, you should select the smallest range which is greater than the expected turbidity reading. 

If you do not know the turbidity range of the sample, then choose the highest range and attempt to take a reading.  If the turbidity of the sample is too low, the instrument will not measure the turbidity.  In this case, you should try the next lower turbidity range.  Continue choosing lower ranges until a measurement is taken. 


8. Insert the cell riser, if necessary.

A cell riser is provided with the accessories of the instrument.  The cell riser is used to reduce the light patch length between the bottom of the sample cell and the detector window when measuring in the 100 and 1000 NTU ranges.  Make sure the cell riser is installed in the bottom of the sample compartment when the turbidimeter is operated in these ranges. 

If the cell riser is in place and you plan to read turbidity in a lower range than 100, you can remove the cell riser using the foam-tipped cell riser retriever.  Simply insert the foam end into the sample compartment and lift out the riser. 

 
9. Standardize the instrument each time the measurement range is changed to ensure accurate results. 

To standardize your instrument, you should either prepare and use a fresh stock of primary standard formazin suspension or should use a secondary standard.  Secondary standards are ready-made standards which often come with the instrument or may be purchased separately.  In most cases, secondary standards may be used, but when the secondary standards are first purchased a formazine primary standard should be used to calibrate the secondary standards.  If the turbidity values of the secondary standards are found to be different than labelled, the actual turbidity values for each secondary standard should be marked on the vial and used in subsequent standardizations. 

To standardize the instrument, place a primary or secondary standard in the cell holder with the index mark toward the front.  Cover the standard with the light shield and adjust the standardization control until the meter reading equals the value of the standard.  The standard used should have a turbidity value approximately equal to 90% of the upper limit of the measurement range. 

10. Place the sample cell into the sample compartment and cover with the light shield.  Position your eye above the meter scale where the reflection of the meter needle in the mirrored strip is directly under the needle and cannot be seen.  Read the turbidity of the sample from the scale corresponding to the selected range. 

The light shield is used to exclude external light and to act as a trap for light passing through the sample. 


11.  The instrument can be left on when readings are performed periodically.  Between uses, always remove any sample cell from the sample compartment and close the sample compartment door.  Set the range selection switch to the 100 or 1000 position to keep the instrument ready for use with the lamp and photomultiplier tube at peak operating condition.



Procedural Notes

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

Variations 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 cells.

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.


Large Particles

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

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.


Dilution

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.


Stray Light

Stray 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 2100A 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.


 

Data

Sample Source:_________________________________________

Turbidity:__________________________________


Virtual Lab

For more information on turbidity reading, you should view the virtual lab. There is an assignment that needs to be completed concerning the virtual lab, so please print the assignment first and then answer the questions as you perform the lab. Once you have the assignment completed, log in and complete the assignment online to be entered directly into the database.


Sources

American Public Health Association, American Water Works Association, and Water Environment Federation.  1998.  Standard Methods for the Examination of Water and Wastewater.  American Public Health Association, Washington, D.C.

HACH Model 2100A Turbidimeter Operation Manual.