Classroom activity Grade 10 Engineering & Design
Vellamore Reverse Osmosis Troubleshooting Rounds
A grade 10 station rotation where teams read operating data from four fictional membrane trains, calculate flow and salt passage changes, and diagnose each fault.
Objectives
- Students will calculate percent changes in permeate flow and differential pressure from baseline and current readings.
- Students will calculate salt passage from feed and permeate concentrations and compare it against a baseline.
- Students will diagnose a membrane fault from operating data and select the corrective action that matches it.
Vocabulary
Key words: permeate, differential pressure, salt passage, fouling, scaling, recovery, normalized data
Teacher overview
An operator watching a reverse osmosis plant does not see the membranes, so every decision comes from three numbers tracked against a startup baseline: how much water the train makes, how much pressure it loses across the elements, and how much salt gets through. In this rotation, teams of three visit four stations, each holding one train's data card from a fictional brackish groundwater plant at Vellamore, and each fault leaves a different fingerprint in those three numbers. This matters because the same symptom, a drop in permeate flow, has three different causes here, and the fourth train's fault barely changes flow at all, so only the pattern across all three readings separates them. A misconception worth naming before the rotation is that any decline in production calls for a chemical cleaning, yet one of these four trains needs no cleaning at all and another cannot be fixed by cleaning at any dose.
Activity Directions
- Before class, print the four train data cards from the Student Handout, one per station, and post them at four separated spots in the room. Print the diagnostic reference sheet from the Student Handout, one per team; it describes four fault patterns for the fictional Vellamore plant, which are fouling, scaling, a temperature effect, and a mechanical leak, each with the readings that point to it. Set the timer for nine-minute intervals.
- Setup and framing, 6 minutes. Form teams of three and give each student a handout and each team a reference sheet. Define the three tracked numbers aloud, then state the two formulas the rotation uses: percent change equals the difference divided by the baseline, times one hundred, and salt passage equals permeate concentration divided by feed concentration, times one hundred. Note that every train in this plant runs on the same feed water.
- Station rotation, 36 minutes. Teams start at different stations and rotate on the timer, spending nine minutes at each of the four trains. At every station a team answers the four numbered questions for that train on the handout, in order, so that the two calculations are complete before the diagnosis is written.
- Circulate and probe. Ask one team at each station to name the reading that rules out a competing diagnosis. At Train B, listen for students who notice the feed temperature before they reach for a cleaning recommendation.
- Formative check, 3 minutes. After the last rotation, ask for a show of hands on Train B: which teams recommended no cleaning? If fewer than three quarters of the class did, work questions 5 through 8 aloud before the debrief.
- Debrief, 5 minutes. Ask two closing questions. First, which train would get worse if the crew cleaned it instead of correcting the real cause? Second, why does an operator record a baseline on the day a train is started rather than comparing this week with last week?
Student Handout
Vellamore is a fictional brackish groundwater plant created for this rotation. All four trains treat the same feed water, which measures 2,400 mg/L of total dissolved solids. Baseline readings were taken on the day each train was started. Use these two formulas. Percent change equals the difference divided by the baseline, times 100. Salt passage equals permeate concentration divided by feed concentration, times 100.
Diagnostic reference sheet. Four fault patterns account for everything happening at this plant. Compare a train's three tracked numbers against these descriptions before writing a diagnosis.
Fouling of the lead elements. Particles, organic matter, or biological growth block the feed channels at the front of the train. Production falls off while the pressure lost across the elements climbs, and the membrane surface is still rejecting salt normally, so salt passage barely moves. Cartridge filters upstream that need changing more often are a supporting sign.
Scaling of the tail elements. Sparingly soluble salts come out of solution where the brine is most concentrated, at the back of the train. Salt passage rises well above baseline while flow and differential pressure move only modestly. Raised recovery or an interrupted antiscalant feed is what sets it up.
Temperature effect. Cold feed water is more viscous and crosses the membrane more slowly, so production drops with no fault present at all. Differential pressure stays near baseline and salt passage improves, because colder water also carries less salt through the membrane.
Mechanical leak in a vessel. A failed O-ring, a leaking interconnector, or a damaged element lets feed water bypass the membrane. Flow and differential pressure stay near baseline while salt passage rises, and probing the vessels one at a time finds a single vessel far above the rest.
Train A data card. Permeate flow was 190 gpm at baseline and is 152 gpm today. Differential pressure across the first stage was 18 psi at baseline and is 34 psi today. Permeate total dissolved solids was 48 mg/L at baseline and is 50 mg/L today. Feed temperature is unchanged at 21 degrees Celsius. The cartridge filters ahead of the train now need changing twice as often as they did at startup.
- Find the percent decline in permeate flow for Train A, using the baseline of 190 gpm and today's 152 gpm.
- Find the percent increase in differential pressure for Train A, using the baseline of 18 psi and today's 34 psi.
- Which fault pattern do the Train A readings point to?
- What corrective action should the crew take on Train A?
Train B data card. Permeate flow was 200 gpm at baseline and is 158 gpm today. Differential pressure was 17 psi at baseline and is 18 psi today. Permeate total dissolved solids was 52 mg/L at baseline and is 44 mg/L today. Feed temperature was 22 degrees Celsius at baseline and is 14 degrees Celsius today. This plant uses a rule of thumb that permeate flow changes by about 3 percent for each degree Celsius of temperature change.
- Find the percent decline in permeate flow for Train B, using the baseline of 200 gpm and today's 158 gpm.
- Using the 3 percent per degree rule and the 8-degree temperature drop, what percent decline in permeate flow would temperature alone explain?
- Which fault pattern do the Train B readings point to?
- What corrective action should the crew take on Train B?
Train C data card. Permeate flow was 185 gpm at baseline and is 170 gpm today. Differential pressure was 16 psi at baseline and is 21 psi today. Permeate total dissolved solids was 48 mg/L at baseline and is 96 mg/L today. Feed temperature is unchanged. Recovery was raised from 70 percent to 80 percent last month, and the antiscalant feed pump has been running off-stroke.
- Find the baseline salt passage for Train C, using the baseline permeate of 48 mg/L and the feed of 2,400 mg/L.
- Find today's salt passage for Train C, using today's permeate of 96 mg/L and the feed of 2,400 mg/L.
- Which fault pattern do the Train C readings point to?
- What corrective action should the crew take on Train C?
Train D data card. Permeate flow was 188 gpm at baseline and is 186 gpm today. Differential pressure is unchanged at 17 psi. Combined permeate total dissolved solids was 50 mg/L at baseline and is 96 mg/L today. Probing the individual vessels shows vessel 7 producing permeate at 420 mg/L while every other vessel reads about 50 mg/L.
- Find the salt passage from vessel 7 on Train D, using its permeate of 420 mg/L and the feed of 2,400 mg/L.
- Find the salt passage from a normal vessel on Train D, using a permeate of 50 mg/L and the feed of 2,400 mg/L.
- Which fault pattern do the Train D readings point to?
- What corrective action should the crew take on Train D?
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: 20.0 percent.
The decline is 190 minus 152, which is 38 gpm, and 38 divided by 190 is 0.20, or a 20.0 percent decline.Answer: about 88.9 percent.
The increase is 34 minus 18, which is 16 psi, and 16 divided by 18 is 0.889, or about an 88.9 percent increase.Answer: Fouling of the lead elements.
Flow is down sharply, differential pressure is up sharply, and salt passage has barely moved. That combination means material is blocking the feed channels at the front of the train rather than damaging the membrane surface. The rising cartridge filter change-out rate confirms that particles are reaching the elements.Answer: Clean in place with a high-pH cleaner and correct the pretreatment.
A high-pH clean removes organic and biological fouling from the lead elements. Cleaning alone would not hold, so the crew must also find why the cartridge filters are loading faster, which usually points to the upstream filtration or a change in the source water.Answer: 21.0 percent.
The decline is 200 minus 158, which is 42 gpm, and 42 divided by 200 is 0.21, or a 21.0 percent decline.Answer: 24 percent.
The temperature fell 8 degrees Celsius, and at about 3 percent of permeate flow per degree, that accounts for 8 times 3, which is a 24 percent decline.Answer: A temperature effect, not a membrane fault.
Temperature alone would explain about 24 percent, which is more than the 21 percent actually observed. Differential pressure is nearly unchanged and salt passage has improved, both of which are normal in colder water. The membranes are performing as expected.Answer: Normalize the data for temperature and do not clean.
Cleaning would cost chemicals and downtime and would shorten membrane life without improving anything. The crew should compare temperature-corrected values from now on so that a seasonal change is not mistaken for a fault.Answer: 2.0 percent.
Salt passage is 48 divided by 2,400, which is 0.02, or 2.0 percent at baseline.Answer: 4.0 percent.
Salt passage is 96 divided by 2,400, which is 0.04, or 4.0 percent today, double the baseline value.Answer: Scaling on the tail elements.
Salt passage has doubled while flow is down only modestly and differential pressure is up modestly. Raising recovery from 70 percent to 80 percent concentrates the brine at the back of the train, and an off-stroke antiscalant pump removes the protection that keeps those salts in solution.Answer: Lower recovery, repair the antiscalant pump, then clean at low pH.
Returning recovery toward 70 percent and restoring the antiscalant feed stops the scale from forming. A low-pH cleaning dissolves the mineral scale already deposited. Doing the cleaning without fixing the feed pump would let the scale return within weeks.Answer: 17.5 percent.
Salt passage from vessel 7 is 420 divided by 2,400, which is 0.175, or 17.5 percent.Answer: about 2.08 percent.
Salt passage from a normal vessel is 50 divided by 2,400, which is 0.0208, or about 2.08 percent, close to the train's baseline.Answer: A mechanical leak in vessel 7.
Flow and differential pressure are both normal, so nothing is blocking or coating the membranes. One vessel passing more than eight times the salt of its neighbors points to a physical bypass, most often a failed interconnector O-ring or a damaged element.Answer: Probe and repair vessel 7, replacing the O-ring or the damaged element.
The crew should profile the vessel to find which element or connector is leaking, then replace that part. Cleaning cannot repair a mechanical bypass, and running the train as it is sends high-salt water into the finished product.
Differentiation
Support
Give these teams a partly completed handout in which the two calculations at Train A are already worked, so their first task at the fictional Vellamore plant is to read the pattern rather than to compute it. Provide the diagnostic reference sheet as a three-column table with the words up, down, and unchanged filled in for each fault, so the diagnosis becomes a matching task.
Extension
Ask these teams to write the data card for a fifth train showing a fault they invent, with baseline and current readings that point to exactly one cause and rule out the other three. Have them trade cards with another team and check whether the intended diagnosis is the only one the readings support.
ELL supports
Post a labeled diagram of a pressure vessel showing feed entering, permeate leaving through the membrane, and concentrate leaving the far end, with arrows for each stream. Provide a term strip pairing permeate with the clean water produced, differential pressure with pressure lost along the train, and salt passage with the share of salt that gets through, and allow the diagnosis to be given orally.
Extensions
Have teams research how a plant reports normalized permeate flow and why the industry normalizes for temperature, pressure, and feed concentration before comparing any two days. Teams can also request the operating log summaries a nearby membrane plant publishes in its annual report, if any are available, and compare which of the four fault patterns that plant reports most often.