Free WW Laboratory Analysis Procedures and Quality Control Questions and Answers — Questions and Answers
Question 1: What is the standard incubation temperature and duration for the BOD5 test?
- 35°C for 24 hours
- 25°C for 3 days
- 20°C for 5 days (Correct answer)
- 4°C for 7 days
Correct answer: 20°C for 5 days
The Biochemical Oxygen Demand (BOD5) test measures the amount of oxygen consumed by microorganisms over a five-day period. To ensure consistent and comparable results, the standard method specifies an incubation temperature of 20°C for 5 days in the absence of light.
Question 2: When is a 'grab sample' more appropriate than a 'composite sample'?
- When measuring average daily BOD loading
- When measuring pH and chlorine residual (Correct answer)
- When determining total weekly phosphorus discharge
- When complying with a 24-hour average TSS limit
Correct answer: When measuring pH and chlorine residual
A grab sample is a single sample collected at a specific time and place, representing conditions only at that moment. It is used for parameters that can change rapidly, such as pH, dissolved oxygen, or chlorine residual. A composite sample, collected over time, would average these values and mask any fluctuations.
Question 3: In laboratory quality control, what is the purpose of analyzing a 'blank'?
- To calibrate the instrument
- To check for contamination of reagents or equipment (Correct answer)
- To determine the precision of the analyst
- To verify the concentration of a known standard
Correct answer: To check for contamination of reagents or equipment
A blank is a sample of distilled or deionized water that is treated exactly like a wastewater sample. Its purpose is to check for contamination in the reagents, glassware, or analytical instruments. An ideal blank should yield a result of zero or below the detection limit.
Question 4: The procedure for Total Suspended Solids (TSS) requires filtering a sample through what?
- A pre-weighed glass-fiber filter (Correct answer)
- A standard coffee filter
- A piece of cheesecloth
- A calibrated sedimentation cone (Imhoff cone)
Correct answer: A pre-weighed glass-fiber filter
The TSS test measures the total amount of solid material that is not dissolved in a sample. The standard method involves filtering a known volume of water through a pre-weighed glass-fiber filter paper with a specific pore size, typically 1.5 micrometers. The filter is then dried and weighed again to determine the mass of the captured solids.
Question 5: How often should a pH meter be calibrated?
- Once per month
- Once per week
- Daily, before use (Correct answer)
- Only when it provides a reading that seems incorrect
Correct answer: Daily, before use
A pH meter's electrode can drift over time, leading to inaccurate readings. To ensure accuracy, it must be calibrated regularly using standard buffer solutions. Best practice, and a requirement for many regulated labs, is to calibrate the meter at the beginning of each day or before each set of measurements.
Question 6: What is the holding time and preservation method for a sample that will be tested for fecal coliform?
- Cool to <10°C, analyze within 8 hours (Correct answer)
- Add sulfuric acid to pH <2, analyze within 28 days
- Freeze at -20°C, analyze within 7 days
- No preservation needed, analyze within 48 hours
Correct answer: Cool to <10°C, analyze within 8 hours
Bacterial populations in a sample can change rapidly after collection. To get an accurate count, fecal coliform samples must be analyzed as soon as possible. They should be cooled to <10°C and analyzed within 8 hours of collection.
Question 7: A 'split sample' is sent to an external lab as part of a quality assurance program. What is the purpose of this?
- To reduce the workload of the in-house lab
- To get the results faster
- To verify the accuracy of the in-house laboratory's results (Correct answer)
- To have a backup sample in case of a spill
Correct answer: To verify the accuracy of the in-house laboratory's results
A split sample is a single sample that is divided into two or more parts, with one part analyzed in-house and the other sent to an external, certified laboratory. Comparing the results helps to validate the accuracy of the in-house lab's procedures, equipment, and analyst technique. It is a common component of a Quality Assurance/Quality Control (QA/QC) program.
