Chapter 11 · The Pillars of Water Treatment · Ten playable problems

Ch 11 · P1 — Before you touch a pump

PROBLEM 101 · THE PILLARS OF WATER TREATMENT — THE FOUR QUESTIONS

Conductivity up, pH up, oxidant residual weak. Before adjusting any feed pump, match each finding to the diagnostic question it answers.

Live model

enters · leaves · accumulates · transformsHigher conductivity, higher pH, weaker oxidant. Match each finding to the diagnostic question it answers.

Ch 11 · P2 — Drawing the boundary

PROBLEM 102 · THE PILLARS OF WATER TREATMENT — SYSTEM BOUNDARIES

Basin, condenser tube, feedwater tank, dead leg — each needs the same treatment. What makes a good free-body framing of a component?

Live model

a control volume, not a parts listWhat makes a good free-body framing of a condenser tube?

Ch 11 · P3 — Name the primary pillar

PROBLEM 103 · THE PILLARS OF WATER TREATMENT — CASE TRIAGE

Three field findings. Click a case, then the pillar bin it primarily belongs to — the amplifiers come with the verdict.

Live model

every case has a primary — and an amplifierClick a case, then the pillar bin it primarily belongs to.

Ch 11 · P4 — Four gauges, one failure

PROBLEM 104 · THE PILLARS OF WATER TREATMENT — THE INPUT BALANCE SHIFTS

Condensate return drops from 80% to 35% — cold 65°F makeup floods in to replace 180°F condensate. Assign a direction to each gauge.

Live model

a different input balance, not a failing programCondensate return collapses from 80% to 35%. Assign a direction to each gauge.

Ch 11 · P5 — Which pillar first

PROBLEM 105 · THE PILLARS OF WATER TREATMENT — THE CONDENSER TUBE FBD

A condenser tube: 4 cycles, pH 8.3, 220 mg/L makeup calcium, visible suspended solids, intermittent oxidant. All four pillars are present — which do you address first?

Live model

clean surfaces first — chemistry second4 cycles, pH 8.3, visible solids, intermittent oxidant. Which pillar first?

Ch 11 · P6 — Two years, no answers

PROBLEM 106 · THE PILLARS OF WATER TREATMENT — THE STAGNANT BRANCH

A valved-off branch, stagnant two years in a nitrite-treated loop. Nothing enters, nothing leaves, the chemistry inside is long spent. Dominant failure mode?

Live model

chemically isolated · biologically occupiedA branch valved off for two years in a nitrite loop. Dominant failure mode?

Ch 11 · P7 — Four symptoms, one cause

PROBLEM 107 · THE PILLARS OF WATER TREATMENT — RISING APPROACH, COUPONS, BIOCIDE DEMAND, SILT

Four complaints on one work order. Click the cascade from the single root cause in order.

Live model

solids → deposits → corrosion → biology → approachRising approach, rising coupon rates, rising biocide demand, silt in the basin. Click the cascade in order.

Ch 11 · P8 — Besides more acid

PROBLEM 108 · THE PILLARS OF WATER TREATMENT — SCALE PRESSURE RISING

High calcium, phosphate inhibitor, scale pressure rising — and more acid would destabilize the rest of the program. Pick two legitimate alternatives.

Live model

0 / 2 PICKED

every response ripples through all four pillarsScale pressure rising; more acid is off the table. Pick two alternatives. 0 of 2 picked.

Ch 11 · P9 — 6.5 MPY with perfect numbers

PROBLEM 109 · THE PILLARS OF WATER TREATMENT — BULK VS SURFACE

Residuals on target, pH and conductivity in range, biocide on schedule, no visible scale — and the coupons read 6.5 MPY. Pick the chapter's two explanations.

Live model

0 / 2 PICKED

bulk data never guarantees surface protectionEvery bulk number in range, coupons at 6.5 MPY. Pick the two chapter explanations. 0 of 2 picked.

Ch 11 · P10 — The first-visit walkthrough

PROBLEM 110 · THE PILLARS OF WATER TREATMENT — SIX STOPS

Design the first-visit walkthrough: move physically and logically through the system. Click the six stops in order.

Live model

walk it the way the water movesDesign the first-visit walkthrough: click the six stops in order.

Problem set

The playable drawings need JavaScript. Every prompt and authored answer remains available below.

Problem 1
A cooling tower suddenly shows higher conductivity, higher pH, and weaker oxidant residual. Before adjusting any feed pump, list: What may have changed in what enters? What may have changed in what leaves? What may now be accumulating? What transformations may be occurring faster than before?
Authored answer
Likely possibilities include: What enters: Makeup fraction may have increased, incoming solids may have increased, contamination may have entered, or inlet temperature may have changed. What leaves: Blowdown may have decreased, a valve may have failed, filtration may have lost effectiveness, or another removal pathway may have changed. What accumulates: Dissolved solids, deposits, biomass, and sludge may now be accumulating. What changes form: Bicarbonate may be shifting toward carbonate at higher pH, oxidant demand may be increasing through reaction with organics or biofilm, and dissolved species may be precipitating as solids.
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Problem 2
For each of the following components, write one sentence defining the system boundary and two likely inputs and outputs: Cooling tower basin Condenser tube Boiler feedwater tank Dead leg in a chilled water system
Authored answer
A strong answer clearly defines each component as a bounded control volume and identifies plausible inflows and outflows appropriate to that component. The key is not one exact answer, but correct diagnostic framing: a clear system boundary realistic inputs realistic outputs treats the component as part of a larger system
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Problem 3
For each case, identify the primary pillar and the most likely amplifier: Deep pitting beneath brown tubercles on mild steel White mineral deposit on a warm tube with soft slime beneath it Rapid oxidant demand increase after weeks of poor filtration Stable bulk chemistry but increasing condenser approach temperature
Authored answer
These cases allow for some interpretation, but strong answers will generally follow this logic: Primary: corrosion. Amplifier: solids/deposit formation. The deposit creates the oxygen differential and helps localize attack. Primary: scale. Amplifier: biology. The mineral deposit is the dominant feature, but biofilm likely contributed by insulating the surface and trapping ions. Primary: biology. Amplifier: solids. Poor solids control creates shelter and surface area for biological growth, increasing oxidant demand. Primary: scale or solids. Amplifier: biology, if biofilm is part of the insulating layer. The key insight is that the problem is likely at the surface rather than in the bulk water.
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Problem 4
A feedwater tank normally receives 80% condensate return at 180°F and 20% makeup at 65°F. Suddenly the condensate return drops to 35%, while total flow remains unchanged. Explain qualitatively what happens to: Feedwater temperature Conductivity Dissolved oxygen load Sulfite demand Then explain why increasing sulfite feed alone may not solve the problem.
Authored answer
When condensate return collapses and makeup rises: Feedwater temperature decreases Conductivity increases Dissolved oxygen load increases Sulfite demand increases Increasing sulfite feed addresses one symptom, but the deeper problem is the change in the system’s mass and heat balance. The correct diagnosis is not simply “the sulfite program is failing,” but “the feedwater system has shifted to a different input balance.”
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Problem 5
A condenser tube receives tower water at 4 cycles, pH 8.3, calcium hardness 220 mg/L as CaCO₃ in the makeup, visible suspended solids, and intermittent oxidant feed. Write a free body diagram that includes the following, along with which pillar you would address first. System boundary Inputs Outputs Accumulations Transformations Likely surface conditions not captured by the bulk water tests
Authored answer
A strong answer includes: System boundary: condenser tube or condenser tube bundle Inputs: warm recirculating water, dissolved calcium and alkalinity, suspended solids, intermittent oxidant, heat-transfer load Outputs: water leaving at changed temperature, consumed chemistry, any released corrosion products or biological fragments Accumulations: scale, solids, biofilm, corrosion products Transformations: precipitation, oxidant consumption, biological attachment and growth, corrosion reactions Surface conditions not captured by bulk tests: elevated skin temperature, sheltered deposits, localized concentration effects, reduced chemistry penetration beneath deposits The most defensible first priority is often solids control if visible suspended matter is high, because solids amplify deposition, shelter biology, and promote under-deposit corrosion. A stronger oxidant strategy may also be necessary, but cleaning and solids control often come first.
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Problem 6
A valved-off 4-inch branch line has been stagnant for 2 years in a nitrite-treated chilled water loop. Using the four questions, predict: What no longer enters What no longer leaves What may have accumulated What chemical and biological transformations likely occurred Then identify the dominant failure mode.
Authored answer
Strong answers may include: What no longer enters: Fresh inhibitor, fresh water, oxidant, or meaningful flow What no longer leaves: Water, corrosion byproducts, or biological material What may have accumulated: Corrosion products, stagnant water, depleted chemistry, biomass What likely changed form: Dissolved oxygen is consumed, inhibitor residual is depleted, localized microbial communities may establish, and corrosion proceeds in chemically isolated conditions Dominant failure mode: localized corrosion, often with significant MIC risk.
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Problem 7
A facility reports: Rising condenser approach Higher coupon corrosion rates Higher biocide demand Visible silt in the tower basin Use the free body diagram approach to identify the most likely root cause and explain how one problem can create all four symptoms.
Authored answer
Most likely root cause: failure of solids management, often due to filtration bypass, poor basin cleanliness, or excessive suspended loading. A strong cascade explanation would be: Solids accumulate in the basin and low-flow zones Deposits form on heat-transfer and metal surfaces Deposits create differential aeration and under-deposit corrosion Deposits shelter biology, which raises biocide demand Deposit and biofilm insulation increase condenser approach This is exactly the kind of multi-symptom problem that the free body diagram is meant to untangle.
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Problem 8
A cooling tower has high calcium hardness and a phosphate-based corrosion inhibitor. Scale pressure is rising. Lowering pH would help with CaCO₃ control, but may destabilize other parts of the treatment strategy. Give two alternative responses besides simply feeding more acid. For each, explain the consequences across all four pillars.
Authored answer
Strong answers may include two or more of the following: Increase blowdown to reduce concentration of calcium and alkalinity Improve solids removal and deposit control Optimize dispersant or polymer feed Shift to a different corrosion inhibitor strategy Improve control stability and monitoring rather than making a large acid adjustment The best answers trace the consequences through all four pillars: reduced scale pressure possible effects on corrosion protection indirect effects on biology through cleaner surfaces changes in solids accumulation and transport
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Problem 9
A condenser system shows: Inhibitor residual on target pH in range Conductivity in range Biocide schedule maintained No visible heavy scale But corrosion coupons show 6.5 MPY. Explain at least two ways this can happen using the distinction between bulk-water chemistry and surface conditions.
Authored answer
Two strong explanations are: Under-deposit corrosion: Bulk chemistry may be acceptable, but deposits isolate the metal beneath them from inhibitor protection and create local oxygen differentials. MIC or localized biofilm attack: The water column may test well while the surface microenvironment remains biologically active and corrosive. Other acceptable explanations include: flow-related erosion-corrosion galvanic effects poor coupon location coupon exposure that reflects a particularly aggressive part of the system rather than the average system condition The key idea is that bulk water data does not guarantee surface protection.
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Problem 10
Design a 6-step first-visit walkthrough for a cooling tower system using the logic of this chapter. For each stop, state: What component you are evaluating What you are looking for Which of the four questions you are answering Which pillar(s) you are primarily evaluating
Authored answer
A strong answer should move physically and logically through the system. A common sequence would include: Basin or sump Fill or distribution deck Sample point and control station Condenser-side performance review Blowdown and conductivity control equipment Filtration or side-stream solids removal equipment The best answers explain, at each stop: what the component is what evidence is being gathered which diagnostic question is being answered which pillar or pillars are being evaluated
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