Wastewater Operator Preliminary and Primary Treatment Questions and Answers — Questions and Answers
Question 1: What is the primary purpose of a grit chamber in the preliminary treatment phase of a wastewater treatment plant?
- To remove large solid objects like rags, plastics, and sticks.
- To allow for the settling of organic solids and reduce BOD.
- To remove heavy, abrasive inorganic particles like sand and gravel. (Correct answer)
- To shred and grind large solids into smaller, more uniform pieces.
Correct answer: To remove heavy, abrasive inorganic particles like sand and gravel.
The main function of a grit chamber is to remove dense inorganic solids such as sand, gravel, and cinders from the wastewater. This is crucial for protecting downstream mechanical equipment from abrasion and excessive wear, and to prevent the accumulation of these materials in pipelines and digesters.
Question 2: A properly functioning primary clarifier can be expected to remove approximately what percentage of Biochemical Oxygen Demand (BOD)?
- 5-15%
- 25-40% (Correct answer)
- 50-70%
- 80-95%
Correct answer: 25-40%
A conventional primary clarifier, through the process of sedimentation, typically removes 25% to 40% of the influent BOD. It also removes a higher percentage of total suspended solids (TSS), usually between 40% and 60%.
Question 3: An operator at a treatment plant notices that the primary clarifier's effluent has a high concentration of suspended solids and the sludge blanket is rising. Which of the following is the MOST likely cause?
- The influent flow rate is too low, causing solids to decompose.
- The sludge withdrawal rate is too low. (Correct answer)
- The comminutor located upstream is malfunctioning.
- The grit chamber is removing too much organic material.
Correct answer: The sludge withdrawal rate is too low.
When the sludge withdrawal rate is too low, solids accumulate at the bottom of the clarifier. This leads to an excessively thick sludge blanket which can be stirred up by hydraulic currents or begin to decompose, causing gas bubbles to lift solids into the effluent. High influent flow can also cause issues, but insufficient sludge removal is a very common operational problem leading to these symptoms.
Question 4: Which of the following pieces of equipment is designed to cut and shred large solids in the wastewater stream without removing them from the flow?
- Bar Screen
- Grit Classifier
- Primary Clarifier Skimmer
- Comminutor (Correct answer)
Correct answer: Comminutor
A comminutor is a device that grinds or shreds coarse solids present in wastewater into smaller, more manageable particles that can pass through the plant without causing blockages in pumps or pipes. Unlike a bar screen, which physically removes solids from the flow, a comminutor processes them within the channel.
Question 5: The main objective of preliminary wastewater treatment processes is to:
- Disinfect the wastewater to kill pathogens.
- Remove dissolved organic matter through biological action.
- Protect downstream equipment and subsequent treatment processes. (Correct answer)
- Stabilize sludge before final disposal.
Correct answer: Protect downstream equipment and subsequent treatment processes.
Preliminary treatment, which includes screening, comminution, and grit removal, is designed to remove large solids and abrasive materials that could damage or clog pumps, pipes, and other equipment later in the treatment plant. This protection is its primary function, ensuring the reliability and efficiency of the overall treatment system.
Question 6: In a primary clarifier, the physical process responsible for the removal of settleable solids is:
- Filtration
- Chemical precipitation
- Gravity (Correct answer)
- Biological uptake
Correct answer: Gravity
Primary clarifiers, also known as sedimentation tanks, work by slowing the velocity of the wastewater to a near-quiescent state. This allows solids that are denser than water to settle to the bottom of the tank due to the force of gravity, while lighter materials like grease float to the surface.
Wastewater Operator Preliminary and Primary Treatment Questions and Answers
What is the primary purpose of a grit chamber in the preliminary treatment phase of a wastewater treatment plant?