Wastewater Operator Wastewater Laboratory Procedures Questions and Answers — Questions and Answers
Question 1: An operator is preparing to measure the pH of a raw influent sample. According to standard laboratory practice, which of the following describes the correct procedure for calibrating a pH meter?
- Calibrate using a single buffer solution closest to the expected sample pH.
- Rinse the probe with tap water, calibrate with pH 7.0 buffer, then measure the sample.
- Rinse the probe with deionized water, calibrate with a pH 7.0 buffer, rinse again, then calibrate with a second buffer (e.g., pH 4.0 or 10.0). (Correct answer)
- Immerse the probe in the sample first to acclimate it, then perform a two-point calibration.
Correct answer: Rinse the probe with deionized water, calibrate with a pH 7.0 buffer, rinse again, then calibrate with a second buffer (e.g., pH 4.0 or 10.0).
Proper pH meter calibration requires a multi-point check to ensure both accuracy at a neutral point (offset) and a correct response across a range (slope). A two-point calibration, typically starting with the pH 7.0 buffer and then a second buffer (pH 4.0 for acidic or pH 10.0 for basic ranges), is standard practice. Rinsing the probe with deionized water between each step is crucial to prevent cross-contamination of the buffers and sample.
Question 2: When performing a 5-day Biochemical Oxygen Demand (BOD₅) test, what is the primary reason for incubating the sample bottles in complete darkness at 20°C?
- To prevent the growth of anaerobic bacteria which would consume organic matter too quickly.
- To inhibit algae growth, which would produce oxygen through photosynthesis and interfere with the results. (Correct answer)
- To accelerate the chemical oxidation of inorganic compounds in the sample.
- To maintain a constant pressure inside the sealed BOD bottle.
Correct answer: To inhibit algae growth, which would produce oxygen through photosynthesis and interfere with the results.
The BOD test is designed to measure the amount of dissolved oxygen consumed by microorganisms as they decompose organic matter. Algae, if exposed to light, will photosynthesize and produce oxygen. This oxygen production would offset the oxygen consumption by bacteria, leading to an inaccurately low BOD reading. Incubation in the dark at a constant 20°C ensures that only the biochemical oxygen demand is measured.
Question 3: An operator is performing a Total Suspended Solids (TSS) analysis. After filtering a known volume of sample, the glass-fiber filter is placed in a drying oven. What is the standard temperature for drying the TSS filter?
- 550°C ± 50°C
- 103°C to 105°C (Correct answer)
- 20°C ± 1°C
- 180°C ± 2°C
Correct answer: 103°C to 105°C
According to standard methods for wastewater analysis, the filter used for Total Suspended Solids (TSS) must be dried at a temperature of 103°C to 105°C. This temperature range is high enough to evaporate all water without burning off the organic solids. The higher temperature of 550°C is used for the Volatile Suspended Solids (VSS) test, which is performed after the TSS measurement.
Question 4: During a fecal coliform test using the membrane filtration method, the operator places the membrane filter onto the culture medium in the petri dish. Which of the following is a critical step in this process?
- Placing the filter grid-side down to ensure direct contact with the medium.
- Rolling the filter onto the medium to avoid trapping air bubbles between the filter and the agar. (Correct answer)
- Exposing the plate to UV light for 30 seconds to sterilize the surface before incubation.
- Adding a few drops of the sample water on top of the filter after placement to ensure it is moist.
Correct answer: Rolling the filter onto the medium to avoid trapping air bubbles between the filter and the agar.
When placing the membrane filter on the agar or absorbent pad, it is crucial to do so with a rolling motion. This technique helps to ensure that no air bubbles are trapped between the filter and the nutrient-rich medium. Trapped air can prevent the bacteria on that portion of the filter from accessing the necessary nutrients, leading to inhibited growth and an inaccurate, lower-than-actual colony count.
Question 5: A laboratory is implementing a Quality Assurance/Quality Control (QA/QC) program. Which of the following is an example of a Quality Control (QC) activity?
- Writing a Standard Operating Procedure (SOP) for a new analytical method.
- Analyzing a proficiency testing (PT) sample from an external provider.
- Calibrating all laboratory thermometers annually against a NIST-certified thermometer.
- Analyzing a laboratory-fortified blank (LFB) with each batch of samples. (Correct answer)
Correct answer: Analyzing a laboratory-fortified blank (LFB) with each batch of samples.
Quality Control (QC) refers to the specific, routine technical activities intended to control the quality of the data. Analyzing a laboratory-fortified blank (LFB), also known as a spike, with each sample batch is a QC activity that checks for the accuracy and recovery of the analytical method in a clean matrix. Writing SOPs and annual calibrations are part of the overall Quality Assurance (QA) program, which establishes the system, while PT samples assess the lab's overall performance.
Question 6: When using the Winkler titration method to determine dissolved oxygen, a series of reagents are added to the sample in the field to 'fix' the oxygen. What is the purpose of this 'fixing' step?
- To calibrate the titration equipment before analysis.
- To precipitate the dissolved oxygen into a visible floc, which is later dissolved and titrated. (Correct answer)
- To adjust the sample pH to exactly 7.0 before transporting it to the lab.
- To remove any chlorine residual that would interfere with the titration.
Correct answer: To precipitate the dissolved oxygen into a visible floc, which is later dissolved and titrated.
In the Winkler method, the 'fixing' step involves adding reagents (manganous sulfate and an alkali-iodide-azide solution) that react with the dissolved oxygen to form an orange-brown precipitate (manganic hydroxide). This chemically binds the oxygen, preventing it from being lost or gained due to atmospheric contact during transport or storage before the final titration is performed in the lab. The amount of precipitate formed is proportional to the initial dissolved oxygen concentration.
Wastewater Operator Wastewater Laboratory Procedures Questions and Answers
An operator is preparing to measure the pH of a raw influent sample.
According to standard laboratory practice, which of the following describes the correct procedure for calibrating a pH meter?