Water Quality Laboratory
Objective - To measure water quality parameters most relevant to portable water filters.
Water Parameters most relevant to portable water filters:
Turbidity - The tendency of water to scatter light at 90 degrees. Turbidity is a measure of water clarity. It is caused by suspended solids (thus, turbidity is an indirect measure of suspended solids). Measured in NTUs, using a Turbidimeter. For most people, water with less than 5 NTUs looks clear. The US primary drinking water standards require that water to be disinfected should have no more than 0.1 NTU.
Color - True color is caused by dissolved compounds in water. It can be natural or anthropocentric. Dissolved and suspended solids (together) cause apparent color. For example, brown colored water could be the result of dissolved byproducts of plant biodegradation (true color) or suspended clay particles (apparent color) or both (also apparent color). Color is measured in Platinum-Cobalt units (Pt-Co). The US secondary drinking water standards recommend drinking water have no more than 15 Pt-Co. A Platinum-Cobalt solution mimics natural water color. Originally, color was determined by comparing a water sample with standard water solutions made with Platinum Cobalt. We will use a Spectrophotometer (Spec), a device that creates a light beam of a specific wavelength and measures how that light is absorbed by a water sample. Depending on the method used to prepare the water and the wavelength selected, spectrophotometers can be used to measure hundreds of parameters, from color to iron concentration. Color can be measured using light with a wavelength of 455 nm. This wavelength of light is adsorbed by the color typically found in natural waters.
Pathogens - Pathogens are disease-causing microorganisms. US primary drinking water standards set a goal of zero pathogens in our drinking water. We'll measure coliform bacteria. While usually not pathogenic, the presence of coliform bacteria indicates that a sample has been contaminated with fecal matter and, consequently, is probably contaminated with pathogens. We'll use both the presence-absence test and the membrane filtration test. The presence-absence test merely indicates the presence or absence of coliform bacteria in a water sample. The membrane filtration test can be used to estimate the concentration of coliform bacteria in a water sample.
Water Samples - We will test some or all of the following samples: natural water (from the stream or pond near Rowan Hall), tap water, and treated water. You will create the last sample by treating approximately a liter of natural water with your portable water filter.
Methods
Turbidity - If you need to, you can also look at the instructions on the quick reference sheet found with the turbidimeter.
1. Fill a clean turbidity sample bottle to near the neck. Avoid air bubbles. Cap the sample bottle. Caution: TOUCH THE TURBIDITY SAMPLE TUBES AT THE TOP (THREADS) OR CAP ONLY. Smearing body oils on the bottle will interfere with the measurement.
2. Holding the bottle by the cap, put a thin line of oil down the side of the bottle. Wipe the oil around the bottle using the supplied cloth. The oil has the same reflectivity as the glass bottle, thus it makes any cracks "disappear".
3. Put the bottle in the Turbidimeter (Hach 2100). Watch the reading. If it goes up, the cause is probably air bubbles. If it drops, solids in the sample are settling out. Avoid both.
4. Take the sample bottle out and empty the contents in a sink. Rinse the bottle out with tap water.
5. If you need to reuse a turbidity bottle, rinse it carefully with DI water and shake as much of the water out as possible. Wipe the outside with ChemWipes only.
Color - If you need to, you can also look at the instructions in the Spectrophotometer Handbook.
Apparent Color
1. Fill a Hach DR/4000 spectrophotometer sample bottle with filtered DI water. This is the "blank".
2. Fill another DR/4000 sample bottle with the sample. This is the "sample".
3. Make sure that the Spectrophotometer is running program 1670. If it isn't, press the soft key under HACH PROGRAM, then press "1670" and ENTER. The display should show HACH PROGRAM: 1670 Color, 455 NM
4. Put the blank in the cell holder and close the light shield. Press the soft key under ZERO. Once the display shows "0 units Pt-Co, go to the next step.
5. Put the sample in the cell holder and close the light shield. Record the Pt-CO units.
True Color
1. Set up a filtration apparatus using a clean filtration flask. If a clean flask is unavailable, rinse a flask with DI water and shake as much of the water out as possible. Connect the flask to a vacuum valve using the black tubing. Place a filter support base into the top of the filtration flask. This can be a fritted-glass base or one that holds a stainless steel screen. [For stainless steel screen - Put a Teflon gasket in the screen holder. Place the stainless steel screen on the gasket.] Place a 0.45 mm filter paper on the filter support base. Place the funnel on the screen holder and clamp it in place.
2. Turn on the vacuum and filter about 50mL of sample into the clean filter flask. NOTE: turn the vacuum only high enough to filter the sample.
3. Follow the steps for Apparent Color, but use the filtered sample water.
Pathogens
Presence-absence
1. Collect 100 mL of sample in sterile container. Shake sample to mix.
2. Add sample to the presence-absence bottle, to the fill-to line. Note, the bottle comes with a broth that only coliform bacteria can easily consume. The broth also contains a compound that changes color with pH.
3. Incubate at 35 +- 0.5 C for 24-48 hours. If coliform bacteria are present, they consume the broth and the ensuing biochemical reactions change the pH of the sample, resulting in a color change.
4. A color change to yellow or yellow-brown indicates the presence of coliform bacteria.
Membrane Filtration
1. Before each run, clean the filter apparatus thoroughly with tap water, then rinse with RO water (in containers by windows in Room 302). Set up filter apparatus. Use sterile tweezers to handle filter membrane. Run tap water samples first, then samples of natural water passed through your water filter, then untreated natural water. Clean the filter apparatus thoroughly before each use.
2. Collect 100 ml of each sample in a sterile container. Shake each sample to mix.
3. Filter 100 ml of sample through the gridded membrane filter. Use buffered water to rinse any sample on the side of the apparatus through the membrane filter. Any bacteria in the sample should now be retained on the membrane filter. Waters with high concentrations of coliform bacteria may need to be diluted to reduce the number of coliforms retained on the membrane filter. Diluted or not, one always passes 100 ml of water through the membrane filter.
4. Using sterile tweezers, and with a slight rolling motion, place the membrane filter grid side up in a petri dish with the appropriate pad and agar (the agar is food for the coliforms; it allows colonies to grow around each coliform deposited on the membrane filter).
5. Cap petri dish and place upside down in incubator at 35 +- 0.5 C for 24 hours.
6. Count colonies. Red and blue colonies indicate total coliforms and blue specify E. coli (a specific type of coliform that has historically been used as an indicator of pathogen contamination). Report coliform as colonies per 100 ml. If the colonies have grown together, making them difficult to count, you need to repeat the experiment with a more dilute sample. If you use a diluted sample, you'll have to estimate the number of colonies expected in 100 ml of undiluted sample. For example if you dilute 1 ml of sample to 100 ml of water. You'll need to multiply the number of colonies counted on the filter membrane by one hundred.
Activities
Each team will measure Turbidity, True color, Apparent color, and Coliform bacteria (presence-absence and concentration) for the following.
The class will record the natural and tap water measurements on the white board. Each team member is responsible for recording ALL of the natural (untreated) and tap water data on the white board in her/his lab notebook. Teams will use the entire classes' data on natural (untreated) and tap water.
If your data are unacceptable, you will be expected to repeat experiments outside of normal class time.
Reporting
Results from this lab will be included in the mid semester report and presentation and the final report and presentation.
For the midsemester report and presentation, just report average values. For the final report and presentation we'll use our knowledge of statistics and present more than just averages.
Resources
Turbidity - Quick Reference, Science
Coliform - Presence Absence Test, Membrane Filtration Procedure, MF Troubleshooting