CWS Filtration Media & Technologies 3 — Questions and Answers
Question 1: What is the recommended maximum service flow rate for a greensand filter treating iron-bearing water?
- 1–2 gpm/ft²
- 3–5 gpm/ft² (Correct answer)
- 6–8 gpm/ft²
- 10–12 gpm/ft²
Correct answer: 3–5 gpm/ft²
Greensand filters are typically rated for 3–5 gpm per square foot of bed area to allow adequate contact time for iron oxidation and filtration.
Question 2: In a pressure vessel filter, the term 'differential pressure' refers to:
- The pressure drop across the filter media from inlet to outlet (Correct answer)
- The difference between system pressure and atmospheric pressure
- The variance in flow rate between backwash and service cycles
- The pressure at which the vessel is rated for safe operation
Correct answer: The pressure drop across the filter media from inlet to outlet
Differential pressure (ΔP) is the pressure drop across the filter bed from influent to effluent, and rising ΔP indicates media loading and signals backwash need.
Question 3: Manganese greensand regeneration in the continuous regeneration (CR) mode uses which oxidant?
- Chlorine
- Potassium permanganate (Correct answer)
- Hydrogen peroxide
- Ozone
Correct answer: Potassium permanganate
In continuous regeneration mode, potassium permanganate (KMnO₄) is added to the influent to continuously re-oxidize the greensand coating as it removes manganese.
Question 4: Which media type would be MOST appropriate for a filter designed to target arsenic removal?
- Granular activated carbon
- Iron-based adsorptive media (GFH or similar) (Correct answer)
- Calcite
- Anthracite
Correct answer: Iron-based adsorptive media (GFH or similar)
Granular ferric hydroxide (GFH) and similar iron-based adsorptive media are highly effective for arsenic adsorption, particularly As(V), due to strong ligand exchange with iron hydroxide sites.
Question 5: What is the purpose of a support gravel bed beneath filter media in a pressure filter?
- To add alkalinity to the effluent
- To provide even distribution and prevent media from entering the underdrain (Correct answer)
- To further remove turbidity from filtered water
- To increase the filter's contact time with oxidants
Correct answer: To provide even distribution and prevent media from entering the underdrain
Graded gravel underbedding supports the filter media, prevents fines from escaping into the underdrain, and promotes even backwash flow distribution across the entire bed.
Question 6: A slow sand filter differs from a rapid sand filter primarily in that it:
- Operates at lower flow rates and relies on a biological schmutzdecke layer (Correct answer)
- Uses coarser media and higher backwash pressures
- Requires chemical coagulation ahead of the filter
- Produces water with higher hardness
Correct answer: Operates at lower flow rates and relies on a biological schmutzdecke layer
Slow sand filters operate at very low flow rates (0.04–0.1 gpm/ft²) and depend on the schmutzdecke, a biological layer on the sand surface, for pathogen and particle removal.
Question 7: During backwash of a sand filter, what percentage of bed expansion is generally targeted?
- 5–10%
- 15–25%
- 30–50% (Correct answer)
- 60–75%
Correct answer: 30–50%
A bed expansion of 30–50% during backwash is standard, providing enough turbulence to dislodge trapped particles without washing media out of the vessel.
CWS Filtration Media & Technologies 3
What is the recommended maximum service flow rate for a greensand filter treating iron-bearing water?