There are a multitude of available test procedures and methods for water and wastewater treatment operators. They range from colorimetric, titrimetric, electrometric (meter & probe), turbimetric, nephlometric, and demonstrative methods. Normally, at least of these methods can be used to measure a single unknown (parameter). For example, chlorine residue can be measured through colorimetric, titrimetric, or electrometric means. What is the best method for your application? We first need to define these methods, shows examples of each, determine the limitations of each method, and then make a decision on which method (test kit) (procedure) is best suited for your needs.
The key issue here is to decide upon the test procedure that best meets your requirements for:
- accuracy
- cost (initial & cost per test)
- skill level
- repeatability
- portability
- decision-making information obtained
- safety and reagent disposal
- are the results reportable?
Remember, the more complete, reliable, and accurate the information you obtain from your testing protocol will give you with the correct decision-making tools to monitor your water or wastewater treatment systems, make operational changes and meet permit requirements and state mandates.
Colorimetric Methods
Colorimetric methods possess a measurement parameter, where the concentration is directly proportional to color development and intensity after addition of a known volume of reagent(s). Where there is an immediate reaction, as with chlorine residues, the results can be extrapolated straight away. Other tests such as nitrates and phosphates can require up to 10 minutes before the full color development is obtained, as a result of the chemistry involved.
There are some interesting colorimetric tests that react in reverse. That is - the lower the concentration of a particular parameter, the greater the color development. Examples include Fluoride, and some Ozone test methods. The color developed in the sample is compared visually with manufacturer supplied standards (color comparator) to deduce the concentration; or it is inserted into a photometer, colorimeter, or spectrophotometer to give direct results on a meter scale, or digitally through a discreet readout. Results obtained are expressed as parts per million (ppm), milligrams per liter (mg/L), grains per gallon (gpg), etc.
Visual Comparator Limitations
Everyones ability to distinguish color intensities are different and background lighting of the waterwaste can affect this. Most manufacturers create their color standards using natural daylight. It is unacceptable to use Incandescent, fluorescent and direct sunlight as they may produce errors. Color blindness is a definite issue with visual color comparison methods. It is notably difficult to differentiate between some color variations. Examples include: yellows, and some shades of blue. Even with the above limitations, visual color comparison methods are an inexpensive tool that are generally easy to use, are conveniently packaged and are simply designed. Some visual test results are reportable for the purpose of permits and you should check with your local inspector. Calorimetric methods using a photometer, colorimeter, or spectro- photometer offer a distinct advantage. Meters are often battery powered and conveniently packaged for portability. To quickly describe their operation, a beam of light is passed through the sample. The light is transmitted through the water sample and detected by a photodiode with the amount of light detected being dependent upon the color. With aid of electronics, the results are displayed on a meter, either as a function of concentration or as a percentage of light transmitted. Advantages of the instrumentation are:
- eliminates need for visual interpretation by operator
- eliminates concern for background lighting
- ultimately greater accuracy
Obviously, using a meter to "read" color development can incur a high initial expense, but colorimeters offer a chance to provide on-the-spot results for a variety of parameters. This includes testing for Chlorine, Iron, Manganese, Copper, Zinc, Aluminum, Fluoride, Ozone, Nitrates, Phosphates, Sulfides, to name a few. You will need to make sure that you weigh up the advantages, disadvantages and overall requirements before making a decision.
Titrimetric Methods
A sample is isolated and an indicator reagent(s) is introduced to produce a color. A color change occurs after a titrant, or reacting reagent, is added dropwise to the sample mixture. The endpoint is the point at which the color changes.
There are many titrant dispensing apparatuses: Drop count¬ – in which a calibrated dropper dispenses equal size drops. Once you reach the endpoint, the required number of drops needed to get to the endpoint is counted and multiplied by a conversion factor. E.g., one drop equals 5 ppm.
Laboratory
Automatic burettes are generally not portable. They are a dispensing apparatus with a calibrated scale on the barrel. The titrant is dispensed up until the endpoint. The volume used is then taken from the calibrated scale. The number of milliliters used often equates to the result in ppm. Direct Reading Microburettes are syringe size calibrated microburettes which dispense the titrant to the endpoint. Results are usually read directly off the calibrated scale in ppm. This procedure is totally portable like drop count methods.
Digital
When the titrant is dispensed from a cartridge inserted into a micro dispensing device, the quantity dispensed is read on a digital venier, normally in ppm. Overall, titration methods are quite inexpensive, and are the preferred method for many procedures. The most common tests include testing for Acidity, Alkalinity, Carbon Dioxide, Hardness, Dissolved Oxygen, and Chlorine. In this case, convenient packaging and simplicity are the key to their portability and accuracy. This method is preferred to determine the extent of corrosion in water supplies and offers the user an easy and inexpensive away to meeting lead/copper requirements.
Turbidimetric Methods
Some unique wastewater test processes determine results in a different way, i.e., not through color, by introducing turbidity or cloudiness to a sample. The greater the turbidity, the higher the concentration. In a similar way to colorimetric methods can be "read" using a visual comparator, or meter, turbidimetric methods can also be read. Results here are also expressed in ppms or mg/L. Typical tests using this method involve Potassium and Sulfates. Again, this method can be fully portable and easily packaged into a kit.
Electrometric Methods
One of the most common methods is an electrode inserted into a water sample. A small current or voltage is passed, amplified and read on a meter. Common tests include pH and conductivity, but a wide range of parameters using ion specific electrodes (ISE) can be measured including Calcium, Nitrates, Chlorine, etc. Almost all electrometric procedures require a meter calibration and/or sample pre-treatment. Examples of these are the 4, 7 & 10 pH buffers used to calibrate pH meters. In general, electrometric methods are more expensive initially and require a higher degree of care and maintenance because of the electrode systems. Nowadays, cheaper pocket pH, conductivity, and ORP meters are on the market. Even though they are designed to be disposed of after a period, care must be utilized in their use and maintenance. They also rival the costs of colorimetric or titrimetric methods. However, they are generally not a good choice for reporting purposes, but are ideal for quick system checks.
Nephlometric Methods
This method is for water turbidity. Suspended particles within the sample are measured through a specially designed meter that sends a concentrated light beam through the water sample. Suspended solids, dirt, and silt all scatter the light. The light scattering is measured using a photodiode at a 90~ angle incident to the light source. The results in this process are expressed as Nephlometric Turbidity units (NTU's) and are more qualitative in nature than quantitative. For field use, portable battery powered units are available. Private and municipal water and wastewater treatment systems that use surface water supplies such as lakes, streams, etc., are required to undertake routine turbidity measurement to monitor the various wastewater treatment systems, such as settling basins, and the performance of sand filters. Continuous monitoring turbidity meters and recorders are becoming the norm rather than the exception.
Gravimetric Test Methods
In essence, these are physical test procedures and include settleable solids, settle ability tests commonly used as operational guideposts in water and waste treatment facilities. A one liter sample is normally taken, mixed and allowed to settle. Imhoff cones, and settlometers are common containers. The samples are timed at different intervals to determine the solid ratio and volume of solids serried. Results are transferable to plant operations to determine the actual flocculant dose, expected sludge volumes, waste adjustment and the sludge in wastewater treatment facilities. These are relatively simple tests which requires no chemical or reagent preparation to undertake (apart from when determining the flocculant dosage) and provides valuable data to a water or wastewater operator.
Sampling Technique
Test methods above require a proper sample. Accurate sample volumes required for the tests are important. Some important points to remember.
- Choose the correct point in the water system for your sample. Let the spigot run a short period of time to obtain a representative sample. (Note if this is a first draw sample. For lead or copper disregard this step.) Pour the correct volume of sample into the test tube or jar. Accurate results require accurate sample volumes.
- Once the test is complete, dispose of waste reagent/sample properly, and clean all test tube thoroughly.
- Follow the test kit manufacturers' directions specifically. Do not alter the procedure to suit your needs or to take shortcuts risking skewed results.
- Do not mix different manufacturers' reagents, particularity colorimeteric ones, unless they are the exact same concentration.
Summary
We have taken a look at six different water test methods, suitable for use by a water and wastewater operator. So what's right for you?
- A careful review of which tests are needed.
- Choose the test method that suits your testing skill level.
- What accuracy do you need? Know the test's limitations.
- Review test kit (method) expense versus expected results. - Look to the marketplace for manufacturers of test equipment and kits. Review their products.
- Safety and reagent disposal requirements. - Are the results reportable, does the procedure follow standard methods or EPA manual? State approved?
Safety & Environmental Factors
Many test kits and instruments can contain dangerous reagents. Read all safety related manuals. You should also thoroughly review the test procedures before running a test. Use manufacturer supplied Material Safety Data Sheets (MSDS) to brush up on specific hazards and waste reagent disposal. Know the shelf-life of specific reagents and replace when it is needed.
Some test kit reagents have been banned for water testing in the home due to the presence for heavy metals. These include tests for Lead, Cadmium, Mercury, etc., as they can potentially contain highly dangerous materials such as Carbon Tetrachloride and Sodium Cyanide. These tests should be left for an outside certified laboratory to perform. Almost all of the test methods defined and described here are standard inorganic tests. Any tests for pesticides, aromatic hydrocarbons (gasolene's), PCB's, etc should also be left to a qualified certified laboratory to undertake as they possess the proper equipment to do so.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.