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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water Chemistry for Operators › Lesson 10
AGR-WATE71-02 · Lesson 10 · 14.5 clock hours · practice

Chemical Dosing Calculations and Feed Systems

Objective. Students will solve multi-step chemical dosing problems involving unit conversions, percent solutions, and feed-pump settings for the chemicals commonly used in water and wastewater treatment.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Dosing Calculations Matter

Chemical dosing is one of the most safety-critical and legally significant tasks a water or wastewater operator performs. Too little chemical leaves pathogens or contaminants in the water; too much wastes money, may harm public health, and can violate permit limits. Every state licensing exam tests your ability to move fluently between concentration units, apply the dosing formula, and translate a dose into a practical feed-pump setting. Accuracy is not optional—it is the job.

The Core Dosing Formula

Almost every chemical feed problem in water and wastewater treatment reduces to one relationship:

Dose (mg/L) × Flow (MGD) × 8.34 lb/gal = Feed Rate (lb/day)

The factor 8.34 is the weight of one gallon of water in pounds. This equation connects the desired treatment dose (expressed as milligrams of chemical per liter of water) to the actual pounds of chemical you must add each day to a known flow.

Remember the units:

If flow is given in gallons per minute (gpm), convert first: MGD = gpm × 1,440 min/day ÷ 1,000,000.

Common Treatment Chemicals

Coagulants (alum, ferric chloride, ferric sulfate): Used to destabilize suspended particles so they clump and settle. Doses vary by raw-water quality and are determined by jar testing. Always check the Safety Data Sheet (SDS) before handling—ferric salts are corrosive.

Lime and soda ash: Used for pH adjustment and softening. Lime (calcium hydroxite or calcium oxide) is a severe irritant to eyes, skin, and lungs; respiratory protection (at minimum an N95, often a half-face respirator with P100 cartridges) and chemical splash goggles are mandatory when handling bulk lime. Soda ash (sodium carbonate) is less hazardous but still requires eye and skin protection.

Fluoride (sodium fluoride, sodium silicofluoride, hydrofluosilicic acid): Added at very low doses for dental health. Hydrofluosilicic acid is a corrosive liquid; full face shield, chemical-resistant gloves, and an apron are required. Overdose is toxic—fluoride systems must have automatic shutoffs and continuous monitoring.

Chlorine compounds: Covered in depth elsewhere; always treat gaseous chlorine as immediately dangerous to life and health (IDLH) and maintain proper lockout/tagout (LOTO) on feed equipment during maintenance.

Percent Solutions

Many chemicals arrive as solutions, not dry powders. If a coagulant is supplied as a 48% solution by weight, you need more gallons of solution to deliver a given number of pounds of active chemical:

Gallons of solution per day = (lb active chemical/day) ÷ (% concentration as decimal × 8.34 lb/gal)

Always use the concentration of the as-delivered product, which appears on the product label and SDS.

Feed-Pump Settings and Calibration

Chemical metering pumps—most commonly diaphragm or peristaltic types—are set by stroke rate, stroke length, or speed control to deliver a target flow in milliliters per minute (mL/min) or gallons per hour (gph).

Converting lb/day to mL/min (for a solution feed pump):

1. Convert lb/day to gallons/day using the solution density.

2. Convert gallons/day to mL/min: 1 gal = 3,785 mL; 1 day = 1,440 min.

Pump calibration (catch method): Place a graduated cylinder under the pump discharge. Run the pump for a measured time (commonly 1 minute). The volume collected is the actual output. Compare to the desired output; adjust the pump setting up or down and recheck. Target: within ±5% of desired rate.

Calibration formula:

Actual output (mL/min) = Volume collected (mL) ÷ Time (min)

Percentage error = |Actual − Target| ÷ Target × 100

Safety During Chemical Feed Operations

Common Errors to Avoid

1. Forgetting to convert flow to MGD before using the 8.34 formula.

2. Using the percent concentration as a whole number (e.g., 48) instead of a decimal (0.48).

3. Inverting the concentration in the denominator when solving for solution volume.

4. Arithmetic errors when chaining unit conversions—write every unit and cancel step by step.

5. Confusing dose of active chemical with dose of product as supplied.

Key terms8

Dose (mg/L)The concentration of chemical to be added to water, expressed as milligrams of chemical per liter of water; numerically equal to parts per million (ppm) for dilute aqueous solutions.
Feed rate (lb/day)The mass of chemical that must be added to a treatment system per day to achieve a desired dose at a given flow; calculated using the core dosing formula.
8.34 lb/galThe weight of one gallon of pure water at standard conditions; used as the conversion factor in the standard dosing formula to relate mg/L and MGD to lb/day.
Percent solution (% by weight)The mass of active chemical present in 100 mass units of solution; used to determine how many gallons of a liquid product must be fed to deliver a required number of pounds of active chemical.
Pump calibration (catch method)A field procedure in which the actual output of a chemical metering pump is measured by collecting its discharge into a graduated cylinder for a timed interval and comparing the result to the desired feed rate.
CoagulantA chemical such as alum or ferric chloride added to water to destabilize colloidal particles, allowing them to aggregate into larger floc that can be removed by settling or filtration.
Lockout/tagout (LOTO)An OSHA-required safety procedure that uses physical locks and warning tags to ensure that hazardous energy sources—electrical, pneumatic, chemical—are fully isolated before maintenance work begins on equipment.
Safety Data Sheet (SDS)A standardized document, required by OSHA's Hazard Communication Standard, that provides information on a chemical's hazards, safe handling, PPE requirements, and emergency response procedures.

Worked examples4

1. A plant treats 2.5 MGD. The desired alum dose is 12 mg/L. How many pounds of dry alum must be fed per day?
  1. Write the dosing formula: lb/day = Dose (mg/L) × Flow (MGD) × 8.34
  2. Substitute values: lb/day = 12 × 2.5 × 8.34
  3. Multiply 12 × 2.5 = 30
  4. Multiply 30 × 8.34 = 250.2 lb/day
Answer: 250.2 lb/day of dry alum
2. A treatment plant needs to feed 185 lb/day of ferric sulfate. The product arrives as a 42% solution (by weight) with a density of 9.2 lb/gal. How many gallons per day of solution must the pump deliver?
  1. Identify what is known: active chemical needed = 185 lb/day; solution is 42% by weight (0.42 as a decimal); solution density = 9.2 lb/gal.
  2. Calculate pounds of solution needed per day: lb solution/day = lb active/day ÷ fraction active = 185 ÷ 0.42 = 440.5 lb solution/day
  3. Convert pounds of solution to gallons: gal/day = lb solution/day ÷ solution density = 440.5 ÷ 9.2 = 47.9 gal/day
Answer: Approximately 47.9 gallons per day of 42% ferric sulfate solution
3. A pump is set to deliver fluoride solution at 55 mL/min. During calibration, the operator collects 162 mL in 3 minutes. Is the pump within ±5% of the target? If not, should it be adjusted up or down?
  1. Calculate actual pump output: actual = volume ÷ time = 162 mL ÷ 3 min = 54 mL/min
  2. Calculate percentage error: % error = |54 − 55| ÷ 55 × 100 = 1 ÷ 55 × 100 = 1.8%
  3. Compare to the ±5% tolerance: 1.8% is less than 5%, so the pump is within tolerance.
  4. Direction check: actual (54) is slightly below target (55), so if adjustment were needed, it would be upward; however, since the error is within tolerance, no adjustment is required.
Answer: The pump output is 54 mL/min, which is 1.8% below target—within the ±5% tolerance. No adjustment is required.
4. A plant flow is 800 gpm. The desired chloramine dose requires 3.5 mg/L of ammonia-nitrogen. How many lb/day of ammonia-nitrogen must be fed?
  1. Convert flow from gpm to MGD: MGD = 800 gpm × 1,440 min/day ÷ 1,000,000 = 1,152,000 ÷ 1,000,000 = 1.152 MGD
  2. Apply the dosing formula: lb/day = 3.5 mg/L × 1.152 MGD × 8.34
  3. Multiply 3.5 × 1.152 = 4.032
  4. Multiply 4.032 × 8.34 = 33.6 lb/day
Answer: Approximately 33.6 lb/day of ammonia-nitrogen

The math in this lesson2

Learners compute alum feed lb/day = 12 mg/L × 2.5 MGD × 8.34 = 250.2 lb/day.
A-SSE.1N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor-led worked examples: converting between mg/L, lb/day, and gallons per day; calculating dose for coagulants (alum, ferric), lime, soda ash, and fluoride; students follow along with calculators and complete a parallel problem set. · Feed-pump calibration simulation: using a peristaltic pump, graduated cylinder, and stopwatch, students measure actual pump output at two speed settings, compare to the desired feed rate, and adjust the setting to hit a target within 5%. · Error-analysis exercise: students receive five worked dosing calculations that contain deliberate errors (wrong unit conversion, inverted formula, arithmetic mistake) and must identify and correct each error, explaining in writing what went wrong.
Materials: Scientific calculators, dosing calculation worksheet, peristaltic pump, graduated cylinder, stopwatch, chemical feed tubing, water (plain, for simulation), unit-conversion reference card.
Check: Calculation worksheet: at least 8 of 10 problems solved correctly within 5% of correct answer (80% pass mark); pump calibration output within ±5% of target; all five errors in the error-analysis exercise correctly identified and explained.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Treatment — Chemical Dosing
1. A plant flow is 3.0 MGD and the desired dose of soda ash is 15 mg/L. What is the required feed rate in lb/day?
Water Treatment — Chemical Feed Systems
2. An operator collects 210 mL from a metering pump in 4 minutes during calibration. The target feed rate is 55 mL/min. What is the percentage error?
Water Treatment — Chemical Dosing
3. A chemical feed solution is labeled 48% by weight with a density of 10.0 lb/gal. The plant needs 240 lb/day of active chemical. How many gallons per day of solution must be pumped?
Safety and Health — Chemical Handling
4. Which personal protective equipment is specifically required when handling bulk dry lime?
Water Treatment — Chemical Dosing
5. A student's dosing calculation shows: lb/day = 8 mg/L × 1.5 MGD × 8.34 = 100.1 lb/day. What error, if any, is present?
Safety and Health — Lockout/Tagout
6. Before changing a feed tube on a chemical metering pump that is wired to run automatically, what must the operator do first?
Water Treatment — Chemical Dosing
7. A plant flow is 1,200 gpm. The lime dose is 22 mg/L. What is the feed rate in lb/day? (Use: 1 MGD = 1,000,000 gal/day; 1 day = 1,440 min)
Water Treatment — Chemical Dosing
8. Which of the following statements about percent-solution calculations is correct?
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