11.2 Water & Wastewater Chemistry Fundamentals

The Chemistry an Operator Is Actually Tested On

Pennsylvania does not test operators as chemists. It tests whether an operator can predict what a chemical will do to a process and recognize when a laboratory result is impossible. Six concepts carry nearly all of the chemistry questions.

1. pH, Acids and Bases

pH measures hydrogen ion activity on a logarithmic scale from 0 to 14, with 7.0 neutral at standard temperature. Because the scale is logarithmic, each whole unit is a tenfold change:

• pH 6.0 water is ten times more acidic than pH 7.0 water.
• pH 5.0 water is one hundred times more acidic than pH 7.0 water.

This is why a small pH excursion matters so much. Coagulation with aluminum sulfate works best between about pH 5.5 and 7.5; hypochlorous acid, the effective form of free chlorine, dominates below about pH 7.5 and gives way to the far weaker hypochlorite ion above it; and nitrifying bacteria in an aeration basin stall below about pH 6.5.

2. Alkalinity: The Buffer That Gets Consumed

Alkalinity is the water's capacity to neutralize acid. It is measured by titration and reported as mg/L as calcium carbonate. Three forms exist, and which one is present depends on pH:

Coagulants are acids in solution. Feeding alum destroys roughly 0.5 mg/L of alkalinity for every 1 mg/L of alum applied, and ferric chloride destroys somewhat more. A low-alkalinity raw water will therefore lose pH control as coagulant is increased, which is the standard reason a plant supplements with lime, soda ash or caustic soda.

3. Hardness and the Calcium Carbonate Basis

Hardness is the sum of divalent cations, dominated by calcium and magnesium. Both hardness and alkalinity are converted to mg/L as CaCO3 so that they share a common basis and can be subtracted:

• Carbonate hardness (temporary hardness) is the portion of hardness matched by alkalinity; it precipitates when heated or when lime is added.
• Noncarbonate hardness (permanent hardness) is hardness in excess of alkalinity and is associated with sulfate and chloride; removing it requires soda ash in lime softening or an ion exchange resin.

Water below about 75 mg/L as CaCO3 is soft, 75 to 150 is moderately hard, and above 300 is very hard.

4. Oxidation and Reduction

An oxidizing agent gains electrons and is itself reduced. Chlorine, chlorine dioxide, permanganate and ozone are the oxidants operators feed. They convert soluble reduced species into insoluble oxidized species that a filter can capture:

• Ferrous iron (Fe2+, soluble and colorless) oxidizes to ferric iron (Fe3+, red-brown, filterable).
• Manganous manganese (Mn2+, soluble) oxidizes to manganese dioxide (black, filterable).
• Sulfide oxidizes to sulfate, removing rotten-egg odor.

Reducing agents run the reaction backwards. Sodium bisulfite, sodium metabisulfite and sulfur dioxide are fed to dechlorinate a wastewater effluent, and ascorbic acid is used to neutralize chlorine in a sample bottle.

5. Solution Strength, Specific Gravity and Equivalent Weight

Chemicals are purchased as solutions and solids of stated strength, and the pump must move product, not pure chemical:

Specific gravity is the ratio of a liquid's density to water. Multiply 8.34 pounds per gallon by the specific gravity to get the weight of a gallon of product. DEP's modules note the field tool for this: a hydrometer tests liquid chemical strength by measuring specific gravity, which is how an operator verifies that a delivery matches the purchase specification before it is pumped into a day tank.

Two conversions round out the arithmetic:

• Pounds per day = dose in mg/L x flow in MGD x 8.34. A 2.5 MGD plant dosing 32 mg/L of alum feeds 32 x 2.5 x 8.34 = 667 pounds per day of dry alum equivalent. If the alum is a 48.5 percent liquid weighing 11.1 pounds per gallon, that is 667 / (11.1 x 0.485) = about 124 gallons per day of product.
• Grains per gallon. Hardness is still reported to customers in grains per gallon in Pennsylvania. One grain per gallon equals 17.1 mg/L as calcium carbonate, so 171 mg/L hardness is 10 grains per gallon.

Finally, know which chemicals move pH in which direction. The chemicals fed to raise pH are sodium hydroxide, calcium hydroxide (hydrated lime), sodium carbonate (soda ash) and potassium hydroxide; the chemicals that lower pH are carbon dioxide, sulfuric acid and the acidic coagulants themselves.

6. Temperature and Gas Solubility

Gas solubility falls as temperature rises. Dissolved oxygen saturation in clean fresh water is roughly 14.6 mg/L at 32 degrees F, about 9.1 mg/L at 68 degrees F, and about 7.6 mg/L at 86 degrees F. Three operating consequences follow: an aeration basin needs more blower output in summer to hold 2.0 mg/L, cold water increases viscosity and therefore backwash drag, and cold water slows every biological and chemical reaction, including chlorine disinfection, which is why CT tables demand more contact time at low temperature.