⟵ 11 The Pillars of Water Treatment (Narrative) · Contents · 11 The Pillars of Water Treatment (Problem Set) ⟶

The Pillars of Water Treatment: Engineering Notes

“If you are reading this straight through, you can skip this section and lose nothing essential to the story. These notes are for the operators, engineers, and technicians who need to do the math.”

Why This Matters in the Field

Water treatment failures can be prevented by asking four diagnostic questions about a system:

1. What enters?
Water, heat, dissolved species, suspended matter, gases, treatment chemistry, biological load.

2. What leaves?
Water at changed conditions, heat, blowdown, steam, leaks, corrosion products, biomass, gases, solids.

3. What accumulates?
Deposits, sludge, biofilm, stagnant volume, corrosion products, concentrated dissolved species, retained heat.

4. What changes form?

Examples:

• calcium bicarbonate to calcium carbonate scale
• sulfite to sulfate
• dissolved oxygen to corrosion current
• planktonic cells to surface biofilm
• bicarbonate to carbon dioxide
• soluble iron to insoluble iron oxide

If you cannot answer all four questions, you do not yet understand the system.

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Bulk Water versus Surface Conditions

This is one of the most important technical distinctions in the chapter.

Most water tests describe the bulk water.
Most failures happen at the surface.

Examples:

• Bulk pH may look acceptable while tube skin temperature drives local CaCO₃ precipitation.
• Bulk inhibitor residual may test on target while deposits isolate the metal beneath them.
• Bulk biocide residual may be present while the inside of a biofilm remains chemically protected.
• Bulk turbidity may seem manageable while low-flow zones accumulate settled solids.

A system can look chemically acceptable on paper and still fail badly at the surface. So every free body diagram should include a separate question: What is likely happening at the surface that the bulk water does not reveal?

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Pillar Interaction Matrix

Interaction	Mechanism	Direction	Diagnostic Clue
Scale → Corrosion	Deposits create O₂ differentials; thermal stress cracks films	Scale promotes corrosion	Pitting beneath hard deposits
Corrosion → Scale	Iron oxide provides heterogeneous nucleation sites	Corrosion promotes scale	Scale with brown/red discoloration
Biology → Corrosion	Biofilm creates anaerobic zones; SRB produce H₂S	Biology promotes MIC	Pitting with black deposits, sulfide odor
Corrosion → Biology	Tubercles provide shelter and nutrients	Corrosion promotes biology	Biofilm on corroded surfaces preferentially
Biology → Scale	Biofilm insulates surface, raising skin temp; EPS traps ions	Biology promotes scale	Mixed organic/mineral deposits
Scale → Biology	Scale provides rigid, protective structure	Scale shelters biology	Biofilm persisting beneath hard scale
Solids → All	Deposits create O₂ differentials, shelter, nucleation sites	Amplification	Fouling in low-flow zones despite good bulk chemistry
pH ↑	Favors CaCO₃, weakens HOCl, may precipitate Ca₃(PO₄)₂	Scale ↑, Biocide ↓	Scaling + bio breakthrough after pH drift
pH ↓	Dissolves protective films, increases general corrosion	Corrosion ↑, Scale ↓	Rising iron; coupons worsen
Cycles ↑	All dissolved species concentrate; supersaturation increases	Scale ↑, Corr. variable	LSI increases; blowdown decreases
Biocide gap	Bloom → biofilm → deposit + corrosion	Biology → All	Rapid turbidity increase; slime on coupons

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Common Diagnostic Cascades

When multiple symptoms appear simultaneously, the mass balance and free body diagram helps identify whether they share a root cause. The most common cascades:

1. Lost Blowdown Control

Controller fails → cycles climb → dissolved species concentrate → LSI rises → scale forms on condenser surfaces → condenser approach increases → chiller efficiency drops. Simultaneous effect: higher TDS may increase corrosion risk, while concentrated organics can increase biocide demand.

2. Biocide Pump Failure

Biocide drops to zero → planktonic bacteria multiply → biofilm begins establishing on surfaces → biofilm insulates heat transfer → biofilm shelters corrosion → oxidant demand rises even after biocide is restored

3. Sidestream Filter Bypass

Solids accumulate in basin → particles distribute through system → deposits form on low-flow surfaces → under-deposit corrosion initiates → corrosion products generate more solids → biology colonizes deposits → biocide demand rises → chemistry appears to fail

4. pH Drift High (Cooling Tower)

CO₂ stripping or acid feed failure → pH rises → CaCO₃ supersaturation increases → HOCl shifts to OCl⁻ → calcium phosphate may precipitate if phosphate is present → scale + bio + solids problems intensify

5. Lost Condensate Return (Boiler)

Condensate pumps fail → cold makeup fraction rises → feedwater temperature drops → dissolved oxygen load rises → sulfite demand rises → conductivity rises → the boiler system is stressed

6. Lost Condensate Quality, Not Just Quantity (Boiler)

Condensate contamination (process leak, tube failure) → hardness enters boiler → deposits on heat-transfer surfaces → scale promotes under-deposit corrosion → boiler efficiency drops → if contamination is severe, boiler blowdown spikes → alkalinity imbalances follow

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Mass Balance Calculations

1. Weighted Average: Mixed Input Streams

When two streams combine, the resulting temperature, conductivity, or dissolved oxygen is the flow-weighted average of the two inputs. This is the calculation behind the boiler feedwater example in the chapter.

Formula:

Result = (Q₁ × C₁ + Q₂ × C₂) ÷ (Q₁ + Q₂)

Where Q = flow rate and C = the property being calculated (temperature, conductivity, DO, etc.)

Example – Normal Operation (75% condensate return):

System	Condensate	Makeup	Feedwater (calculated)
Flow Rate	75 gpm	25 gpm	(75 + 25) = 100 gpm
Temperature	170°F	70°F	(75×170 + 25×70) ÷ 100 = 145°F
Conductivity	50 µS	500 µS	(75×50 + 25×500) ÷ 100 = 163 µS
Dissolved O₂	0.2 mg/L	8.5 mg/L	(75×0.2 + 25×8.5) ÷ 100 = ~2.3 mg/L

2. Cycles of Concentration: What Leaves Determines What Stays

In a cooling tower, pure water leaves as evaporation. Dissolved solids do not. They stay behind and concentrate. Cycles of concentration (COC) describes how much more concentrated the tower water is compared to the makeup.

Formula:

COC = Conductivity of tower waterConductivity of makeup water

Or equivalently:

COC = Makeup rateBlowdown rate

Example: Makeup conductivity = 400 µS, Tower conductivity = 1,600 µS

COC = 1,600400 = 4 cycles

At 4 cycles, dissolved species in the makeup – calcium, alkalinity, chloride, sulfate – are present at four times their makeup concentration in the cooling tower water.

Scaling consequence:

If makeup calcium = 80 mg/L as CaCO₃, tower calcium at 4 COC = 320 mg/L as CaCO₃. If COC drifts to 6 due to blowdown failure: tower calcium = 480 mg/L as CaCO₃.

That 50% increase in calcium concentration – from one control failure – is the difference between a manageable LSI and an aggressive scaling condition. The chemistry didn’t change. The balance changed.

3. Chemical Demand: What Changes Form Inside the System

When dissolved oxygen load increases, sulfite demand increases proportionally. When COC rises, inhibitor demand rises proportionally. These are not mysterious consumption events – they are form-change consequences of a shifted mass balance.

Sulfite demand from dissolved oxygen:

Stoichiometric ratio: ~7.9 mg sulfite consumed per 1 mg dissolved O₂

Sulfite demand (mg/L) = DO (mg/L) × 7.9

Using the example from the chapter:

Condition	DO (mg/L)	Sulfite Demand (mg/L)
Normal (75% Condensate Return)	2.3	2.3 × 7.9 = ~18 mg/L
Upset (25% Condensate Return)	6.4	6.4 × 7.9 = ~51 mg/L

Sulfite demand from Dissolved Oxygen increase:

Nearly three times the sulfite demand – from the same root cause that raised conductivity and dropped temperature. The technician who turns up the sulfite pump without running this calculation may get a residual back temporarily, but hasn’t solved anything.

Inhibitor demand from COC increase:

If a program targets 20 mg/L inhibitor residual at 4 COC, and COC climbs to 6, the tower volume now contains 50% more dissolved species competing for treatment surface area. Inhibitor demand rises accordingly – even if no other variable changed.

Free Body Diagram: Field Guide

A field free body diagram does not need to be pretty. It only needs to be complete. For each component, define the following:

1. Boundary
What exactly are you analyzing?
Examples:

• Tower basin
• Condenser tube bundle
• Feedwater tank
• Deaerator
• Boiler drum
• Branch dead leg

2. Inputs
List all entering streams and conditions:

• Flow
• Temperature
• Pressure
• pH
• Conductivity
• Inhibitor residual
• Oxidant residual
• Hardness, alkalinity, silica, chloride, sulfate
• Suspended solids
• Dissolved oxygen
• Microbiological load

3. Outputs
List all exiting streams and conditions:

• Flow out
• Blowdown
• Evaporation
• Steam
• Return flow
• Leaks
• Vented gases
• Solids removed by filtration
• Heat rejected or absorbed

Use the weighted mass balance to determine what, if anything, is accumulating in the system.

4. Internal accumulation
Ask what is being stored inside the boundary:

• Scale
• Sludge
• Biofilm
• Corrosion products
• Stagnant water
• Concentrated dissolved solids

If accumulation cannot account for the mass balance, identify possible transformations.

5. Transformations
Ask what reactions or phase changes are occurring:

• Oxidation-reduction
• Acid-base shifts
• Precipitation
• Gas stripping
• Evaporation
• Biological growth
• Inhibitor consumption

If transformation is occurring, identify which pillars might be contributing.

6. Pillar Analysis
Ask which pillar is primary, and which may be secondary or symptomatic:

• Corrosion
• Scale
• Biological growth
• Suspended solids

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⟵ 11 The Pillars of Water Treatment (Narrative) · Contents · 11 The Pillars of Water Treatment (Problem Set) ⟶