Water Conditioning & Purification Magazine

Testing for Commercial Water Treatment

More and more water treatment dealers who have traditionally served the residential market are getting more involved in treating commercial and industrial waters. The WQA has recently developed educational resources and a certification program dedicated to this market, as it has seen a need to help dealers appropriately treat these water problems. A small mistake in a commercial application can be a costly mistake, especially to a smaller company. Appropriate testing is even more critical to ensure proper size and design of commercial systems. In addition, there are regulatory requirements and agencies that must be dealt with, so a well thought-out plan with water quality data will be helpful in establishing yourself as a knowledgeable person to the local regulatory agency.

Chemical Feed

A variety of chemicals are used to treat water for many reason including: pH adjustment, disinfection, oxidation for precipitation and coagulation to name a few. When designing a chemical feed system for a public water supply you must use chemicals that are NSF 60 approved to ensure safety for potable water supplies. These chemicals are typically introduced into the water supply via a chemical feed pump. In order to properly size a feed pump a little testing must be done to determine the correct amount of chemical to fully treat the water. Jar testing is commonly used to simulate a full-scale water treatment process, and is an excellent tool for sizing a chemical feed pump. It is important to keep in mind that water quality will vary depending on the source and how it is influenced by outside factors. For example water that is coming from a surface source can vary in quality based upon the amount of rain and potential sources of contamination within the water shed. With the potential of changes in water quality, it is common to oversize a chemical feed in order to make adjustments in treatment when needed.

pH Adjustment

Many water treatment processes are dependent on the pH level, so adjustment is very common. It is often adjusted to help in preventing corrosion in water where the pH is lower and can be aggressive in attacking plumbing fixtures and pipes causing lead or other metals to leach into the water. In fact, whenever a public water supplies has a failure for lead or copper a corrosion control study must be done. This study not only includes pH but also takes into account alkalinity, which is directly related to the pH level. The pH is adjusted using acids to lower the pH or bases in order to raise it. Some of the bases used include the following: sodium bicarbonate, sodium carbonate, potassium carbonate and sodium hydroxide. Acids that can be used include: acetic acid, citric acid, hydrochloric acid, nitric acid, phosphoric acid and sulfuric acid.

You can easily raise the pH using sodium carbonate or more commonly known as soda ash and lower the pH using acetic acid commonly known as white vinegar. Solution strength will be very important in determining the amount of chemical needed, the stronger the solution the less you will need use to change the pH. To perform jar testing you can easily use the above-mentioned common household chemicals, as these are easily obtainable, inexpensive and relatively safe. If you are planning on using a stronger chemical, there are conversion factors that can be used so you can extrapolate the results. While you may be working with household chemicals it is still important to protect yourself by wearing protective eyewear and gloves. Below is a list of some common equipment you should have to perform the testing needed.

Equipment Needed:

  • (2) 1000 ml Beaker
  • (2) 100 ml Beaker
  • (1) Teaspoon sampler spoon
  • (2) Transfer pipette
  • Protective eyewear
  • Latex gloves
  • pH meter (probe style is easiest to work with)
  • Magnetic Stirrer (not necessary but helpful)

Since pH can change quickly upon exposure to the atmosphere it is important to use a fresh sample to get the most accurate results in your jar tests. It is easy to work with a sample size of 1 Liter or 1000 mL to make the calculations easier. So fill your 1000 ml beaker with your water sample and you should perform this test a couple of times to take into account any natural variation in the water quality. You need to take a baseline reading of the pH and then keep the pH probe placed in the beaker so you can see the pH as it changes. To bring the pH of the water up, add your base using a transfer pipette. You may want to use a magnetic stirrer to make sure you are completely mixing the chemical; otherwise you will need to manually stir the sample while adding your chemical. Continue to add your chemical until you reach your desired pH level. Once you have determined the amount of chemical need to change the pH to the desired level, some simple calculations can be used to help you in determining the size of your chemical feed pump.

Alkalinity

Since alkalinity is so closely related to pH, it is often required whenever a corrosion control study is done, and if it were not required I would highly recommend running some samples to be safe rather than sorry. Alkalinity refers to the ability of the solution to neutralize acids, and it typically made up of carbonates, bicarbonate and hydroxides, but can also include phosphates and silicates. These compounds act as buffering agents and can help in maintaining a stable pH. If the alkalinity is too low anything added to the water will immediately affect the pH, which can cause scaling or corrosion. If the alkalinity is too high it will cause a gradual upward drift in pH, which can cause scale to form. It is important to maintain a moderate alkalinity level of about 30-70 mg/L to help in maintaining pH and minimizing corrosion. There are several titration test kits on the market that allow you to easily test for alkalinity. You can also determine the alkalinity, by performing a titration using a standard sulfuric acid solution to a specific pH endpoint. In order to measure total alkalinity, you will add the sulfuric solution while continually measuring and mixing the pH of the sample water, so it may be a good idea to use a magnetic stirrer to ensure a complete mixing of the sample. You will need to add the sulfuric solution until the pH reaches an endpoint of 4.9, 4.6, 4.5 or 4.3 depending on the type of sample. The amount of sulfuric acid added can be used to estimate the total alkalinity of the sample, depending on the strength of the sulfuric acid being used.

Quality Assurance and Control

In order to ensure you are getting the most accurate results you must take certain steps to maintain your testing equipment. First when using pipettes to transfer your chemical solution, you need to make sure they are free from any previous chemical residual that could throw off your results. A good way to ensure this doesn’t happen is to always use the same pipette for the same chemical every time or use a disposal pipette every time. It is important to choose the right cleaning agent to clean all your beakers and other glassware. You will need that is effective at cleaning yet does not leave behind a residual. There are several such cleaning agents on the market and can usually be found by asking your glassware supplier.

Maintenance & Storage

When using a meter to measure parameters such as pH, Conductivity/TDS or ORP, it is important to properly maintain and calibrate the instrument to ensure your results are highly accurate. Probes designed for pH measurement need to be stored in an appropriate storage solution. In a pinch you can use a pH 4 buffer, but this should not be used as a permanent solution as the color in this buffer can build up on the probe causing off reading. You can make a salt saturated solution by adding table salt to distilled, DI or RO water, but make sure not to use salt that contains Iodine. Keep in mind this is a temporary solution and you should use the proper storage solution as recommended by the manufacturer, whenever possible. Many meters that are currently on the market today contain a microprocessor, and it is important to not store these types of meters in any extreme heat or cold. They should be treated with the same care you give to say your laptop.

Calibration allows the user of the meter to ensure the meter is reading accurately. When it comes to calibrating a pH meter there are three standard buffers that are commonly used. These buffers are solutions with a known pH value, with the common buffers being 4, 7 and 10. The pH electrode is extra sensitive to changes so it should be calibrated on a more regular basis than most other electrodes. It is a good idea to calibrate a pH electrode every time before it is used to ensure the most accurate result. You should begin with the 7 buffer by placing the electrode in the 7 buffer and waiting a few moments for the meter to get a reading. The reading should be somewhat close to 7, but you may need to adjust up or down to the 7.0 the meter is supposed to be reading. You should then move to a 4 or 10 buffer and repeat the process making adjustments when necessary. It is a good idea to use all three buffers, although some manufacturer claim they can be calibrated using two points, using the three point calibration is going to yield higher accuracy levels. When your probe is going bad, the meter will drift further and further from the standard buffers. The pH electrode is replaced on a more frequent basis than other electrodes, depending on the application it should be replaced once very 6 months to a year. Of course if the electrode is not calibrated and cared for it can fail much quicker. A conductivity electrode does not need as frequent calibration as the pH electrode, but should be calibrated on a regular basis about once a month. You should use a solution that has conductivity close to what you are expecting to measure, so you would not want to use a 30 μS solution to calibrate when you are consistently measuring samples of high conductivity.

When testing to properly size a commercial treatment system it is important to take more than one sample, as water quality can greatly fluctuate depending on the source. By taking the time to properly test, you will save your reputation by providing your customer a system that is designed to take into account these natural changes.

 

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