2.4 Water Chemistry Fundamentals for Operators

CHEMISTRY appears as a scored objective on Drinking Water B, C and D, all four Wastewater exams, and Distribution System 2 and 3; GENERAL SCIENCE adds items on all four Wastewater exams. Together they are worth around 21 items across the family. This section builds the chemistry vocabulary the rest of the guide assumes.

Concentration Units

• mg/L (milligrams per liter) is the working unit for nearly everything an operator measures. In dilute aqueous solution, 1 mg/L = 1 ppm (part per million) because one liter of water weighs almost exactly one kilogram (one million milligrams).
• µg/L (micrograms per liter) = ppb (parts per billion); 1,000 µg/L = 1 mg/L. Lead, arsenic and disinfection byproduct limits are often quoted in µg/L.
• Percent solution: 1% weight/volume equals 10,000 mg/L. Commercial hypochlorite is commonly labeled by weight percent, so converting a 12.5% product to mg/L also requires its specific gravity; do not assume that 12.5% by weight equals 125,000 mg/L.
• Grains per gallon (gpg) survives in hardness work: 1 gpg = 17.1 mg/L as CaCO₃.

The bridge from concentration to a physical quantity of chemical is the pounds formula:

$\text{lb/day} = \text{Flow (MGD)} \times \text{Dose (mg/L)} \times 8.34$

The 8.34 is the weight of one gallon of water in pounds. Worked example: dosing 2.4 mg/L of chlorine into 0.65 MGD requires

$0.65 \times 2.4 \times 8.34 = 13.0 \text{ lb/day}$

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pH and the Logarithmic Scale

pH is the negative logarithm of hydrogen ion activity:

$\text{pH} = -\log[\text{H}^+]$

The scale runs 0 to 14, with 7.0 neutral at 25 °C. Because it is logarithmic, each whole unit is a factor of ten. Water at pH 6.0 has ten times the hydrogen ion concentration of water at pH 7.0, and one hundred times that of pH 8.0.

Why operators care:

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Alkalinity: The Buffer That Holds Everything Together

Alkalinity is the water's capacity to neutralise acid, reported as mg/L as CaCO₃. In most natural water it is carbonate chemistry: bicarbonate (HCO₃⁻) dominant near neutral pH, carbonate (CO₃²⁻) appearing above pH 8.3, hydroxide (OH⁻) only at high pH.

Alkalinity is not the same as pH. pH tells you the current hydrogen ion activity; alkalinity tells you how hard it is to change. Water at pH 7.5 with 15 mg/L alkalinity and water at pH 7.5 with 220 mg/L alkalinity behave completely differently when acid is added.

Processes that consume alkalinity:

• Alum coagulation: each 1 mg/L of alum consumes about 0.45 mg/L of alkalinity as CaCO₃.
• Chlorine gas: each 1 mg/L of Cl₂ consumes about 1.4 mg/L of alkalinity, because chlorine gas hydrolyses to hypochlorous and hydrochloric acid.
• Nitrification: each 1 mg/L of ammonia nitrogen oxidised consumes about 7.1 mg/L of alkalinity — the single largest alkalinity demand in wastewater treatment, and the reason nitrifying plants on soft water must add caustic or lime.

Denitrification returns roughly 3.6 mg/L of alkalinity per mg/L of nitrate nitrogen reduced, recovering about half of what nitrification consumed. This is a major operational argument for placing an anoxic zone ahead of aeration.

Worked example. A plant nitrifies 22 mg/L of ammonia nitrogen. Alkalinity demand is $22 \times 7.1 = 156$ mg/L. If the influent carries only 130 mg/L of alkalinity, nitrification will drive the pH down and stall, and the operator must either add alkalinity or recover it through denitrification.

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Oxidation, Reduction and Valence

Oxidation is the loss of electrons; reduction is the gain of electrons. The mnemonic is OIL RIG. An oxidant takes electrons from something else and is itself reduced.

Water treatment oxidants, in rough order of oxidising power: ozone > chlorine dioxide > free chlorine > potassium permanganate > chloramine.

Common operator applications:

• Iron and manganese: soluble ferrous iron (Fe²⁺) and manganous manganese (Mn²⁺) are oxidised to insoluble ferric (Fe³⁺) and manganic (Mn⁴⁺) forms that can be filtered out.
• Hydrogen sulfide is oxidised to elemental sulfur or sulfate, removing the rotten-egg odour.
• Dechlorination with sulfur dioxide or sodium bisulfite reverses the process: the sulfite reduces free and combined chlorine back to chloride.

Valence is the combining capacity of an element. Calcium is Ca²⁺, sodium Na⁺, chloride Cl⁻, sulfate SO₄²⁻. Valence explains why one calcium ion replaces two sodium ions on an ion exchange resin, and why hardness and alkalinity are both reported "as CaCO₃" — expressing everything in equivalents of a single reference compound lets operators add and subtract them directly.

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Gas Laws and Solubility

• Gas solubility falls as temperature rises. This is why summer lagoons and warm rivers hold less dissolved oxygen, and why DO saturation tables are indexed by temperature.
• Chlorine cylinder pressure rises with temperature, which is why a chlorine cylinder must never be heated with an open flame and why the room, not the cylinder, is warmed.
• Henry's law governs how much of a dissolved gas will strip out in an aerator, which is the basis of packed tower aeration for radon, carbon dioxide and volatile organics.