15.1 Chemical Dosing Processes: Coagulation, Flocculation, pH Adjustment & Dosage Calculations
Key Takeaways
- Chemical feed rate in pounds per day equals flow in MGD multiplied by dose in mg/L multiplied by 8.34 pounds per gallon, and this single relationship covers nearly every dosing calculation on the exam.
- When a chemical is supplied as a solution or at less than 100 percent purity, the calculated pounds of active chemical must be divided by the decimal purity or solution strength to find the pounds of product actually required.
- Coagulation destabilizes charged colloidal particles in seconds with rapid mixing, while flocculation grows the destabilized particles into settleable floc over many minutes with gentle tapered mixing.
- Alum and ferric salts consume alkalinity and depress pH, so a plant that both nitrifies and doses metal salts must manage a single shared alkalinity budget.
- Jar testing is the standard method for selecting a coagulant dose because the optimum dose depends on the actual water and cannot be read from a table.
15.1 Chemical Dosing Processes: Coagulation, Flocculation, pH Adjustment & Dosage Calculations
Exam Focus: "Chemical dosing — Coagulation/flocculation, Nutrient removal/enhancement, pH adjustment" is a Treatment Process line item, and Treatment Process carries nine of the exam's ten calculation items. Dosage math is very likely to appear.
1. The Dosage Calculation
Every chemical feed calculation rests on one relationship:
Pounds per day = Flow (MGD) x Dose (mg/L) x 8.34 lb/gal
The 8.34 is the weight of one gallon of water in pounds, and it is what converts a concentration and a volume into a mass.
Worked Example 1 — Straightforward Dose
A plant treating 3.2 MGD doses ferric chloride at 18 mg/L. How many pounds per day of ferric chloride are required?
lb/day = 3.2 MGD x 18 mg/L x 8.34 = 480.4 lb/day
Worked Example 2 — Correcting for Solution Strength
The same plant buys ferric chloride as a 38 percent solution. The 480.4 lb/day above is the active chemical required; the plant must feed more product than that because the product is only 38 percent active.
lb/day of product = 480.4 / 0.38 = 1,264 lb/day of 38% solution
The rule: divide by the decimal purity or strength. Dividing by a number less than one always increases the answer, which is the sanity check — you always need more product than active chemical. Candidates who multiply by 0.38 instead get a number smaller than the active requirement, which is physically impossible.
Worked Example 3 — Working Backward to a Dose
A plant feeds 210 lb/day of a chemical at 100 percent strength while treating 1.5 MGD. What is the dose?
Dose (mg/L) = lb/day / (MGD x 8.34) = 210 / (1.5 x 8.34) = 210 / 12.51 = 16.8 mg/L
2. Coagulation and Flocculation
Fine colloidal particles do not settle because they carry like electrical charges and repel one another, keeping the suspension stable. Removing them requires two distinct steps.
Coagulation adds a coagulant — typically alum (aluminum sulfate) or a ferric salt (ferric chloride, ferric sulfate) — whose multivalent metal ions neutralize the particle charge. Once charge is neutralized the particles no longer repel and can stick together on contact. Coagulation is a chemical step that happens in seconds and requires intense rapid mixing to disperse the coagulant before it hydrolyzes.
Flocculation is the physical step that follows: gentle mixing over 20 to 30 minutes brings destabilized particles into contact so they aggregate into large, dense, settleable floc. As covered in Section 10.2, too much mixing energy in the flocculator shears the floc apart, and sheared floc does not re-form.
A flocculant aid polymer may be added after coagulation to bridge particles together and strengthen the floc.
Where Coagulation Is Used in a Wastewater Plant
- Chemical phosphorus precipitation — the most common application at a municipal plant.
- Chemically enhanced primary treatment to boost primary clarifier removal during wet weather.
- Improving solids capture ahead of tertiary filters.
- Sludge conditioning ahead of dewatering (usually a polymer rather than a metal salt).
Jar Testing
The correct coagulant dose depends on the actual characteristics of the water on that day, so it is determined empirically. A jar test runs several beakers of the same water side by side at different doses through a simulated rapid mix, flocculation, and settling sequence, and the operator compares floc formation, settling rate, and supernatant clarity. Jar testing is the standard method for selecting and adjusting coagulant dose, and it must be repeated when water characteristics change.
3. pH and Alkalinity Adjustment
Alkalinity is the water's capacity to neutralize acid — its buffer against pH change. pH is the current hydrogen ion condition. A water can sit at a comfortable pH and still have almost no alkalinity, and that water will crash the moment any acid-producing process starts.
| To Raise pH / Add Alkalinity | Notes |
|---|---|
| Sodium hydroxide (caustic soda) | Fast-acting liquid, easy to feed, no slurry handling; expensive and a severe burn hazard; freezes at moderate temperature at high concentration |
| Lime (calcium hydroxide) | Inexpensive per pound of alkalinity; requires slurry make-up, continuous agitation, and abrasion-resistant equipment; scales piping |
| Soda ash (sodium carbonate) | Easy to handle as a dry chemical; costlier than lime |
| Magnesium hydroxide | Safer to handle, self-limiting on pH, slower acting |
| To Lower pH | Notes |
|---|---|
| Carbon dioxide | Self-limiting and safer; forms carbonic acid |
| Sulfuric acid | Fast and inexpensive; severe hazard, requires dedicated containment and PPE |
The Alkalinity Budget
Several processes draw on the same alkalinity reserve simultaneously:
- Nitrification consumes about 7.14 lb as CaCO3 per lb of ammonia nitrogen oxidized (Section 14.2).
- Metal salt coagulants — alum and ferric — also consume alkalinity and depress pH.
- Denitrification returns about 3.57 lb as CaCO3 per lb of nitrate nitrogen reduced.
A plant that nitrifies and doses ferric for phosphorus removal is drawing on that reserve twice. Monitoring effluent alkalinity as a routine process control test — and holding a residual, commonly targeted at 50 to 100 mg/L as CaCO3 — is what keeps both processes stable.
4. Practical Dosing Discipline
- Change one variable at a time, and give the process time to respond before judging the result.
- Verify actual feed rate against a calibration cylinder or a tank draw-down, not the pump dial (Section 9.2).
- Track chemical usage per million gallons treated — a sudden change in usage at a stable dose means a feed system problem.
- Watch for over-dosing. Excess coagulant can re-stabilize particles by reversing their charge, producing worse clarity at a higher dose. This is one of the main things a jar test reveals.
- Respect the safety data sheet for every chemical, and never mix incompatible chemicals or store them where a spill could combine them (Section 10.2).
A plant treating 2.5 MGD must feed alum at a dose of 22 mg/L. The alum is delivered as a 48 percent solution. How many pounds per day of the 48 percent solution are required?
An operator increases coagulant dose beyond the optimum found by jar testing and observes that settled water clarity gets worse rather than better. What explains this result?
A plant that nitrifies to meet an ammonia limit also doses ferric chloride for phosphorus removal, and it is experiencing intermittent nitrification failure. Which interaction most likely explains this?