Water chemistry fundamentals

You run the chemistry, the exam asks why

You know the chemicals your plant feeds and the numbers you hold on the board. The exam asks about the chemistry underneath: why pH changes disinfection, what alkalinity actually is, why alum needs alkalinity, which oxidant hits hardest. None of it is trick material, but the daily job does not make you put the why into words, and the exam does.

pH, alkalinity, and hardness are three different things

This is the most common mix-up on the test, so pin it down first. pH is a snapshot of how acidic or basic the water is right now, on a base-10 log scale, so one whole unit is a tenfold change in hydrogen ions: pH 5 is ten times more acidic than pH 6 and a hundred times more acidic than pH 7. Alkalinity is something else entirely, the water's buffering capacity, its ability to soak up acid without the pH crashing, and it comes mostly from bicarbonate and carbonate. A water can sit at a comfortable pH and still have almost no buffer. Hardness is a third thing again, the calcium and magnesium content, usually reported as mg/L as calcium carbonate.

Chlorine chemistry

Free chlorine is really two species: hypochlorous acid (HOCl), the strong disinfectant, and hypochlorite (OCl), the weaker one. Combined chlorine is chlorine bound to ammonia as chloramines, and total is free plus combined. The part the exam leans on is that pH controls the free chlorine split. Low pH favors HOCl, high pH favors the weaker hypochlorite, and the two are about even near pH 7.5. That is why disinfection gets harder as pH climbs even when the residual on the meter has not moved.

Breakpoint chlorination

When you add chlorine to water that carries ammonia, the residual does something odd. It rises, then falls as the added chlorine destroys the chloramines, then rises again as free chlorine appears once the ammonia is used up. The low point is the breakpoint. If your plant holds a combined residual on purpose, you stay short of breakpoint by keeping the chlorine to ammonia-nitrogen ratio below about 5 to 1 and the pH near neutral, which favors monochloramine.

Coagulation

The fine clay particles that make water turbid carry a negative surface charge that keeps them apart, so they will not settle on their own. A coagulant like alum or ferric chloride neutralizes that charge so the particles can collide and stick into floc. The catch the exam wants is that alum consumes alkalinity and pulls the pH down, so a low-alkalinity water can fall below the effective coagulation range, roughly pH 5.5 to 7.5, and the operator adds lime or soda ash to hold it there.

Oxidant strength and the units

Among the common treatment oxidants, ozone is the strongest and chloramine is the weakest, with free chlorine and permanganate in between. Keep the concentration units straight too, because the exam mixes them: in dilute water 1 mg/L equals 1 ppm, one grain per gallon equals 17.1 mg/L as calcium carbonate, and a 1 percent solution is 10,000 mg/L. One more direction worth knowing is the Langelier index. A negative value means the water is undersaturated and tends to corrode, a positive value means it tends to lay down scale, and near zero is balanced. It points a direction, not a corrosion rate.

Where operators lose points

• confusing pH, alkalinity, and hardness, which measure three different things
• reading the pH scale as if it were linear, when one unit is a factor of ten
• getting the chlorine and pH link backward; HOCl is stronger and is favored at low pH
• thinking alum raises pH, when it lowers pH and eats alkalinity
• mixing up mg/L, ppm, percent, and grains per gallon

Point your study time

Chemistry sits right next to the microbiology and the process control on the exam, so study them as a set. The full layout of the test is in what's on the operator exam, and the dosing math that uses these ideas is in the free practice tool. The program details where you test sit on your state's page.