CWS Reverse Osmosis Systems 3 — Questions and Answers
Question 1: Which water quality parameter most significantly accelerates RO membrane fouling by biological growth?
- High total dissolved solids
- Elevated temperature
- Low chlorine residual in feedwater (Correct answer)
- High silica concentration
Correct answer: Low chlorine residual in feedwater
Insufficient chlorine residual in the feedwater allows bacteria to colonize and form biofilm on the membrane surface.
Question 2: What is 'concentration polarization' in an RO system?
- Buildup of rejected solutes at the membrane surface reducing flux (Correct answer)
- Electrical charge buildup on the membrane
- Uneven pressure distribution across the membrane element
- pH shift in the permeate stream
Correct answer: Buildup of rejected solutes at the membrane surface reducing flux
Concentration polarization occurs when rejected ions accumulate at the membrane surface, locally increasing concentration and reducing driving pressure.
Question 3: A CWS technician measures a permeate TDS of 45 mg/L with a feedwater TDS of 500 mg/L. What is the rejection rate?
- 91% (Correct answer)
- 9%
- 45%
- 55%
Correct answer: 91%
Rejection rate = (1 - permeate TDS / feed TDS) × 100 = (1 - 45/500) × 100 = 91%.
Question 4: Which pretreatment is required when feedwater contains more than 0.1 mg/L of free chlorine for TFC membranes?
- Softening
- Iron filtration
- Carbon adsorption (Correct answer)
- UV disinfection
Correct answer: Carbon adsorption
Activated carbon adsorption must remove free chlorine below 0.1 mg/L before the TFC membrane to prevent oxidative degradation.
Question 5: What is the recommended maximum SDI (Silt Density Index) value for feedwater entering most RO systems?
- SDI < 1
- SDI < 5 (Correct answer)
- SDI < 10
- SDI < 15
Correct answer: SDI < 5
An SDI of less than 5 is generally required to protect RO membranes from excessive particulate fouling.
Question 6: How does increasing feedwater temperature affect RO membrane permeate production?
- Decreases flux due to increased viscosity
- Increases flux due to decreased water viscosity (Correct answer)
- Has no significant effect on flux
- Increases rejection but decreases flux
Correct answer: Increases flux due to decreased water viscosity
Warmer water is less viscous and diffuses through the membrane more easily, increasing permeate flux approximately 3% per °C.
Question 7: What is the function of the auto-shutoff valve (ASO) in a residential RO system?
- Closes the drain when tank is full to stop membrane operation (Correct answer)
- Regulates permeate pressure to constant level
- Prevents backflow from storage tank to membrane
- Bypasses the carbon filter during high-demand periods
Correct answer: Closes the drain when tank is full to stop membrane operation
The ASO valve senses storage tank pressure and shuts off feed flow to the membrane when the tank reaches about 2/3 of line pressure.
CWS Reverse Osmosis Systems 3
Which water quality parameter most significantly accelerates RO membrane fouling by biological growth?