Turbidity is regularly measured at water treatment plants on both compliance and process control samples using hand-held or benchtop turbidimeters. It is also measured on reclaimed water samples at wastewater treatment plants.

Turbidity in drinking water and wastewater effluent can adversely affect water quality in several ways that include supporting bacterial growth, reducing the efficacy of chemical disinfection, and harming aesthetics.

Although often considered a relatively simple test, turbidity analysis can result in a number of problems that adversely affect sample results. This is especially true with low-level turbidity samples, which can be exceedingly problematic to analyze accurately. As a result, turbidity troubleshooting analyses can be challenging.

Turbidimeters

The first essential step is to be sure to follow an approved method and that the turbidimeter meets the minimum requirements detailed in Standard Methods (SM 2130 B) and EPA method #180.2. This includes:

• Detector at 90 degree to incident light
• Sensitivity that allows detection of turbidity differences of 0.02 NTU or less in samples with turbidities of less than 1.0 NTU
• No more than 10 cm distance traveled by light within the sample tube

If your turbidimeter does not meet all required criteria, you should acquire one that does.

Care and maintenance of turbidimeters is crucial to proper performance and reliable sample results. It is imperative to follow manufacturer-recommended service schedules. Furthermore, excessive humidity or dust is detrimental to the operation of these instruments; it is important to keep the inside of the cell compartment as clean and dry as possible.

For good sample results, make sure that any turbidimeter is warmed up adequately before sample analysis. Consider leaving the instrument turned on, if the manufacturer approves.

Sample cells

When troubleshooting turbidity analyses, it is extremely important to inspect the sample cells frequently. These cells must be made of clear, colorless plastic or glass and should be kept scrupulously clean, inside and out. Cleaning entails washing with hot, soapy water and rinsing with reagent water (distilled or deionized), then capping the cells to prevent contamination.

Acid-washing or use of an ultrasonic bath may be necessary to clean excessively dirty cells.

Immediately discard any damaged cells (such as those with deep scratches or cracks) or cells that cannot be cleaned. Minor imperfections in sample cells can be masked using a thin layer of silicone oil, which should have the same refractive index as glass. Follow the manufacturer’s procedure for applying the oil.

Unreliable or questionable sample results can also be caused by the use of multiple sample cells for turbidity analyses. To alleviate this, use one sample cell or a matched pair of cells to ensure the most reliable results.

Finally, for optimal results be sure to index sample cells (containing samples or standards) according to the manufacturer’s procedure, if applicable. Indexing is the process of slowly rotating a sample cell 360 degrees, noting the position that yields the lowest value, and then placing a mark on the top of the sample cell neck. Then, the cell can be oriented to this position for subsequent sample measurements until it is indexed again.

Calibration

To ensure reliable and defensible sample results, turbidimeters must be properly calibrated, following the manufacturer’s procedure. They should be calibrated at least quarterly using primary turbidity standards. Over time, some turbidimeters may need to be calibrated more often, as evidenced by drift, reading instability, or failed check standards. This is usually more common in instruments that are older or have been heavily used.

Only primary standards can be used to calibrate turbidimeters. The calibration standards you use should be dilutions of a stock formazin standard, manufacturer-prepared formazin standards, or styrene divinylbenzene standards, whichever the instrument maker recommends. Whether you use bulk standards or sealed standard cells, be sure they are not expired.

Daily or more frequent verification of turbidimeters will also help ensure accurate results. Check standards used for instrument verification can be primary or secondary, the latter typically being gel standards, such as Hach Gelex standards. Some manufacturers may provide solid check standards, such as HF Scientific ProCheck Solid NTU Standard, for performing verifications.

It is imperative to follow the manufacturer’s recommendations for using secondary standards, as some require assigning new values to these standards immediately after calibrating the instrument. Failure to do so can mean inaccurate and questionable verifications and sample results. Manufacturers typically provide acceptance criteria for secondary standards, and you should follow these closely to optimize results. When acceptance criteria fail, you will most likely have to recalibrate the turbidimeter and then perform another verification.

Samples

Even with a properly calibrated and verified turbidimeter and clean sample cells, you may from time to time obtain sample results that seem questionable. When this occurs, you can take several steps to troubleshoot the problem:

• Keep sample bottles scrupulously clean using soap and water, and rinse them with distilled or deionized water.
• Collect a representative sample.
• Mix samples thoroughly but gently before analysis.
• Slowly pour the sample down the side of the cell to minimize formation of small air bubbles.
• Allow samples to stand for a few minutes before analysis to allow small bubbles to dissipate. (It can be challenging to find a balance between allowing enough time for bubbles to dissipate but not enough for particles to settle out.)
• If necessary, degas samples with excessive bubbles, such as by drawing a vacuum or using an ultrasonic bath.
• Avoid interference or unstable readings from settling of particles or condensation. To avoid condensation, allow samples to warm to room temperature, then invert to mix.
• Avoid diluting samples.

Federal regulations for drinking water and wastewater allow a holding time of 48 hours for turbidity samples, provided they are refrigerated at less than 6 degrees C during that time. While this practice is acceptable, it is best to analyze turbidity samples as soon as possible after collection.

Regulatory requirements

In addition to federal regulations and method requirements, some state regulatory agencies have procedures or guidelines for laboratory analyses of turbidity. For more information about turbidity analysis methods, consult your manufacturer’s user manual. Other recommended resources are:

• Standard Methods for the Examination of Water and Wastewater Method 2130 B (Nephelometric Method), latest method revision: 2001.
• EPA Method 180.1 (Determination of Turbidity by Nephelometry), Rev. 2, August 1993.

About the author

Peter Strimple is a transition compliance specialist with Severn Trent. He can be reached at 321/229-7747 or peter.strimple@stservices.com.