The Blueprint of Water

The Solvent: Engineering Notes

8 min read

⟵ 03 The Solvent (Narrative) · Contents · 03 The Solvent (Problem Set) ⟶

The Solvent: 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 is rarely limited by water itself. It is often limited by what’s dissolved in it - and by whether you can measure it fast enough to stay ahead of scaling, corrosion, fouling, and process upset. This section builds four field skills:

  1. Distinguishing suspended vs. dissolved solids

  2. Converting and communicating concentration

  3. Using conductivity as a proxy for ionic load

  4. Recognizing which ions actually drive risk

Core Tools & Constants

Constant / ConversionValue
1 mg/L (dilute water)≈ 1 ppm
1 gal water≈ 8.34 lb
1 L water≈ 1,000,000 mg
TDS proxy (rule of thumb)TDS = 0.7 × Conductivity (µS/cm)
1 mS/cm= 1,000 µS/cm
1%= 10,000 ppm
Ultrapure water resistivity18.2 MΩ·cm (≈0.055 µS/cm)1

Field note: The TDS–conductivity factor (0.7) is an approximation. It varies with ion type, concentration, and temperature - but it is accurate enough for real-time operational control in most freshwater systems.

Measuring the Invisible

Dissolved ions are easy to ignore because they are invisible. But in industrial systems, their concentration determines whether water remains stable or begins to scale, corrode, or foul. To control them, we need ways to measure how crowded the solution has become.

The most fundamental measure is Total Dissolved Solids (TDS). In the classical test, a filtered sample is evaporated and the remaining residue is weighed. The result is reported in mg/L: the mass of dissolved material left behind after the water is removed.

This works well in the lab. In the field, it is too slow for real-time control. That is why conductivity becomes so valuable.

Ions in Motion - Conductivity

Conductivity measures how easily electricity moves through water. Pure water conducts very poorly, but dissolved ions act as charge carriers. The more ions present, the easier it is for current to flow and the higher the conductivity.

That makes conductivity a fast, continuous proxy for ionic load. In most industrial systems it is reported in µS/cm or mS/cm. Since 1 mS/cm = 1,000 µS/cm, unit mistakes can create order-of-magnitude errors.

A common rule of thumb is:

TDS (mg/L) ≈ 0.7 × Conductivity (µS/cm)

This is only an estimate. The exact relationship depends on ion type, concentration, and temperature.

Conductivity measures ionic load, not chemistry. Two waters with identical conductivity can behave entirely differently. The difference is not how many ions are present – but which ones.

Concentrations: ppm & mg/L

Parts per million (ppm) is simply a ratio:

One Part of SomethingOne Million Equivalent Parts of Something Else

In water treatment, that ratio should almost always be treated as mass over mass:

One Part of Solute (by Mass)One Million Parts of Water (by Mass)

Because 1 liter of water weighs very close to 1,000,000 mg under normal conditions:

1 ppm ≈ 1 mg1,000,000 mg ≈ 1 mgkg ≈ 1 mgL

This equivalence is very good for dilute aqueous solutions, but becomes less exact as concentrations rise and density departs from that of pure water.

Added Substance Calculation

For certain dosing calculations, it is often helpful to rearrange the ppm mass ratio.

By multiplying both the top and bottom of the equation by one million, the following formula can be used to calculate ppm from added substances:

Mass of Substance AddedMass of Resulting Solution × (1,000,000) = ppm Substance in Solution

Example: Adding 17 lb of sodium chloride to 300 gallons of water

Step 1: Convert Everything to Mass

Sodium Chloride = 17 lb

Water = 300 gal × 8.34 lb/gal = 2,502 lb

Step 2: Calculate Resulting Mass of Solution

17 + 2,502 = 2,519 lb

Step 3: Divide Mass of Substance by Mass of Solution & Multiply by 1,000,000:

17 lb Sodium Chloride2,519 lb of Solution × (1,000,000) = 6,750 ppm

This equation is derived from the standard dosage formula:

Dosage (lb) = Volume gal × 8.34 lbgal × ppm1,000,000

Percent to ppm

Another useful shortcut is converting mass percent to ppm. This allows us to quickly convert % active in a product and provide a reference for fully saturated sodium chloride in a softener brine tank (~26% by weight at room temperature).

A 1% solution is equal to 10,000 ppm

A percent (%) literally means “parts per hundred,” so a 1% solution contains 1 Part of Solute for every 100 Parts of Total Solution.

We can use this to “scale up” into the ppm range:

1% of 1,000,000 = (0.01) × (1,000,000) = 10,000

So a 1% solution contains:

10,000 Parts of Solute1,000,000 Parts of Solution = 10,000 ppm

The ppm concentration can therefore be calculated by:

ppm = percent active × 10,000

  • 3.5% Product Active = 35,000 ppm Product Active
  • 12% Sodium Hypochlorite = 120,000 ppm Sodium Hypochlorite
  • 26% Sodium Chloride = 260,000 ppm Sodium Chloride

The Passengers and What They Do

Back in Chapter 1, we referred to the list of dissolved passengers. This is the manifest. They’re sorted not by how dangerous they are – that depends entirely on your system – but by how they cause trouble: the ones that build scale, the ones that drive corrosion, the ones that mostly ride along, and the control variable. Knowing which category an ion falls into tells you what to watch for when its number climbs on a lab report.

Scaling Drivers

Calcium (Ca²⁺)

  • Why we care: The concentration of calcium and its anionic partners is often the limiting factor in cooling tower cycles of concentration and devastating in boilers. Its compounds generally exhibit “inverse solubility,” meaning that they are more likely to precipitate at high temperatures (like boilers and heat exchangers).
  • Controls: Softening, RO/DI, inhibitors, pH/alkalinity control

Magnesium (Mg²⁺)

  • Why we care: May form insoluble deposits in both cooling tower and boiler systems. Combined with sufficient silica, it can form magnesium silicate in cooling systems at relatively low concentrations. In boilers, magnesium silicate and magnesium hydroxide will form but do not generally form scale deposits on surfaces. For boilers treated with phosphate, it is critical to maintain boiler OH alkalinity to ensure that magnesium precipitates as a fluid sludge rather than sticky scale.
  • Controls: Softening, RO/DI, inhibitors, alkalinity strategy in boiler programs

Silica (reactive silica as Si(OH)₄)

  • Why we care: Silica molecules can link together (polymerize) to form larger chains of polymeric, colloidal, and eventually particulate silica. The solubility of silica is typically estimated at 150 mg/L, but is heavily influenced by temperature, pH, total dissolved solids, and the concentration of divalent and trivalent cations. Silica scale is extremely hard, glass-like, and difficult to remove. Its presence can severely limit Reverse Osmosis recovery rates and cooling tower cycles.

  • Controls: RO/DI, lime softening, Mg precipitation routes, specialized approaches (electrocoagulation, etc.)

Alkalinity / Carbonate System (HCO₃⁻ / CO₃²⁻ / OH⁻)

  • Why we care: Produces CO2 in steam/ condensate lines. Can drive boiler carryover. While bicarbonate (HCO3-) is generally considered soluble in water, carbonate (CO32-) species will quickly deposit with calcium and iron.
  • Controls: Acid treatment, RO/DI, and dealkalization.

Corrosion Concerns

Chloride (Cl⁻)

  • Why we care: Chloride is considered a corrosive anion that disrupts the protective oxide layer of metal surfaces, increasing corrosion. Cooling tower manufacturers generally limit the acceptable levels of chloride to 250-300 mg/L in the circulating water. In boilers the chloride limit is typically 300 mg/L for low pressure boilers (<300 psi), and less as the pressure of the boilers increases.
  • Controls: RO/DI, cycle control, inhibitor strategy, metallurgy selection

Sulfate (SO₄²⁻)

  • Why we care: Sulfate forms insoluble precipitates with calcium, barium, and strontium, and must be monitored. Sulfate, like chloride, is also considered a corrosive anion.
  • Controls: RO/DI, softening, inhibitors.

Iron (Fe²⁺ / Fe³⁺)

  • Why we care: Ferrous (Fe²⁺) iron is generally soluble, but it can be quickly oxidized to the ferric (Fe³⁺) form, which precipitates rapidly as iron hydroxide. High iron levels foul equipment and encourage “Iron-Related Bacteria,” leading to Microbiologically Influenced Corrosion (MIC).
  • Controls: Aeration/filtration, coagulation, greensand (oxidation), lime softening, RO/DI, cation exchange, corrosion source control.

Copper (Cu⁺ / Cu²⁺)

  • Why we care: The presence of copper in water is most often attributed to corrosion of copper pipes and heat exchangers. It can impact inhibitors and lead to downstream deposits.
  • Controls: Azoles, pH/alkalinity, corrosion source control

Passive Ions and Tracers

Sodium (Na⁺)/ Potassium (K⁺)

  • Why we care: We generally don’t. Sodium and potassium rarely form scale and are mostly passive passengers in the water treatment world. Due to their high solubility, they are useful for accurately determining cycles of concentration.
  • Controls: Reverse osmosis, demineralization

Control Variable

pH

  • Why we care: The pH of water can be extremely detrimental to metal surfaces, as well as inhibitor and biocide efficacy. It also has a significant impact on several solubility equilibria.
  • Controls: Acids or bases.

⟵ 03 The Solvent (Narrative) · Contents · 03 The Solvent (Problem Set) ⟶

Footnotes

  1. ASTM D1193, “Standard Specification for Reagent Water,” ASTM International, which defines Type I reagent water (about 18.2 MΩ·cm, 0.055 µS/cm at 25 C). Confirm the current edition at time of publication. ↩

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