Standard Model Water
Updated: Sep 25, 2020
I've been teaching civil engineering students how to perform jar tests since 2014. In my experience, I've noticed that even with a detailed step-by-step procedure many people (including PhD students) find their first jar test experience intimidating and miserable. Most first-timers spend up-to 4 hours on a single jar test. Unfortunately, the only way to learn jar testing is by "getting your hands dirty". My research adviser is a strong proponent of the "learning from your mistakes" approach. Making mistakes can be frustrating in the short-term, especially after spending 4 hours jar testing only to learn that your data is garbage; but this process forces you to reflect on what went wrong and motivates you to find ways to improve and ultimately allows you to develop the skills necessary for critical thinking. It takes most people about a month to be comfortable with the process and at least 6 months to perfect their jar testing skills.
The figure above shows results from an experiment I asked some students to perform. I gave them the same water and asked them to perform a jar test where they replicated three jars at pH 6.5 and three jars at pH 6.75 (the coagulant dose was constant). I wanted to measure each student's accuracy (how close they were to the target pH) and precision (how far apart were the data points). Also, I wanted to see how the students compared to each other. The figure shows that most students were fairly accurate at pH 6.5 and were able to consistently reproduce their removal results. On the other hand, the results at pH of 6.75 were highly variable. The turbidity removal range was -14 to 93%.
So how do we know which results to keep and which to disregard? how confident are we that the student that reported a removal of -14% made a mistake? how do we even know what and/or where they made a mistake? A mistake can be made in practically every step of the jar test process. Mistakes can be anything from calculation errors to adding the wrong or no chemicals to the jars.
There is a need for a "standard water" that can be used universally for jar test training and procedure validation purposes. Having a standard water allows users to measure their accuracy and precision by comparing their results to a set of verified values. The section below describes the standard water preparation procedure and provides jar test results using the "Next-Generation Jar Test Procedure". The experiment should take approximately 1.5 - 2 hours to complete.
The standard water is prepared using four main ingredients:
1) Distilled or de-ionized (DI) water
2) Particles: kaolin
4) Organics: instant coffee
Most drinking water treatment plants should have a supply of DI water, if not, most grocery stores sell distilled water for about $1/gallon. Kaolin is the only ingredient that I recommend to be exclusively purchased from the link provided since the size distribution of the particles will vary between suppliers.
Water Quality Characteristics
The table below shows the water quality characteristics of the standard model water. The "measured" column shows the average values from 12 jars. In general, the measured values should be within 5% of the target values.
The ingredients are added from stock solutions. A spreadsheet with instructions for preparing the stock solutions can be found here. Simply enter the volume of stock solution you would like and follow the instructions provided. The kaolin and sodium bicarbonate solutions can be stored in the fridge for up-to 3 months, while the instant coffee solution must be prepared on the day of the experiment.
Once the stock solutions have been prepared, follow the instructions outlined in the "Standard Water Preparation Instructions" table.
Note: It's best to prepare the water overnight to allow the water to come to equilibrium before the experiment.
1. Enter the number of jars in your jar tester (cell B9)
2. Enter the volume of each jar (cell B10)
3. Partially fill (~25%) a appropriate size container with DI water
4. Add the volumes of ingredients from the stock solution as shown in spreadsheet
5. Continue filling the container with DI water until the recommended batch volume is reached
Note: To minimize variability between the jars, make sure you the batch is completely mixed. I recommend mixing the batch each time you fill a new jar.
pH Adjusting Chemicals
The jar test procedure is based on a single-variable optimization approach where the pH is held constant as the coagulant dose is varied. Titrations are performed to determine the appropriate volumes of acid or base required to hold pH constant. I recommend preparing 0.1N hydrochloric acid and sodium hydroxide solutions for adjusting the pH. The instructions below use 38% hydrochloric acid (HCl) solution and sodium hydroxide pellets (NaOH).
Note: Please follow all laboratory safety procedures for your facility, especially when working with concentrated chemicals. If you are unsure, then please seek advice from a safety professional prior to performing any laboratory work.
For a 1000-mL 0.1N HCl solution: Slowly add 8.2 mL of the 38% hydrochloric acid solution to 250 mL DI water. Adjust the final volume of solution to 1000 mL with deionized water.
For a 1000-mL 0.1N NaOH solution: Add 4g of NaOH pellets to 1000 mL DI water.
Jar Test Results
The following results were obtained using the standard model water and by following the "Next-Generation Jar Test Procedure". The experiment should take approximately 1.5 hours to perform. The table below shows the experimental conditions used:
The results of the experiment are shown in the table and figure below.