Coagulation and Flocculation Calculations: Optimizing Water Treatment Processes (2023)

Introduction to Coagulation and Flocculation

In the realm of water treatment, coagulation and flocculation play pivotal roles in ensuring the delivery of safe and clean water to consumers. After undergoing screening and various pretreatment processes, the next step in conventional water treatment plants involves the addition of chemicals during the coagulation process.

Coagulation: A Chemical and Mechanical Symphony

Coagulation involves a series of chemical and mechanical operations where coagulants, such as aluminum sulfate, sodium aluminate, ferric sulfate, ferrous sulfate, ferric chloride, and polymers, are applied to water. The process consists of two phases: rapid mixing to disperse coagulant chemicals, and flocculation to agglomerate small particles into well-defined floc. The goal is to create a uniform, feather-like material that entraps suspended particles, enhancing the sedimentation process.

The Importance of Flocculation

Flocculation follows coagulation, employing a physical process to slowly mix coagulated water. This increases the probability of particle collision, forming a dense, strong floc that settles rapidly in the basin. Polymers are often introduced to expedite floc formation, enhancing its strength and weight.

Calculations for Optimal Operation

Efficient operation of coagulation and flocculation units demands precise calculations for chamber or basin volume, chemical feed calibration, chemical feeder settings, and detention time.

Chamber and Basin Volume Calculations

To determine the volume of a square or rectangular chamber or basin, utilize the following formulas:

  • Volume, ft³ = Length, ft × Width, ft × Depth, ft
  • Volume, gal = Length, ft × Width, ft × Depth, ft × (7.48 gal/ft³)

Example: For a flash mix chamber of 4 ft square with a water depth of 2.5 ft: Volume, gal = 4 ft × 4 ft × 2.5 ft × (7.48 gal/ft³) = 299.2 gal

Detention Time Calculation

Detention time, crucial for coagulation reactions, varies between flash mixers and flocculation basins. The formula for detention time is:

  • Detention Time (minutes) = Volume of Basin (gal) / Flow to Basin (gpm)

Example: For a flocculation basin 40 ft long, 15 ft wide, and 12 ft deep with a flow of 2800 gpm: Detention Time = 53,856 gal / 2800 gpm = 19.23 minutes

Chemical Feeder Settings

Accurate dosing of chemicals involves determining dry chemical feeder settings, lb/day, and chemical solution feeder settings, gpd.

Dry Chemical Feeder Setting

  • Chemical added, lb/day = Chemical, mg/L × Flow, MGD × 8.34 lb/gal

Example: For alum dose of 9 mg/L and a flow of 1,800,000 gpd: Chemical added, lb/day = 135.11 lb/day

Chemical Solution Feeder Setting

  • Chemical, lb/day = Chemical, mg/L × Flow, MGD × 8.34 lb/gal

Example: For alum dose of 8 mg/L, flow of 1.85 MGD, and alum solution containing 5.22 lb/gal: Chemical added, lb/day = 123.43 lb/day

Determining Chemical Solution Feeder Setting, mL/min

Some feeders dispense chemicals in mL/min. Convert gpd to mL/min using the formula:

  • mL/min = (Chemical, lb/day / Solution Strength, lb/gal) × (1 gal/3,785 mL) × (1 day/24 hours) × (1 hour/60 minutes)

Example: For alum dose of 8 mg/L, flow of 1.85 MGD, and alum solution containing 5.22 lb/gal: Ml/min = 212.76 mL/min

Determining Percent of Solutions

To find the percent strength of a solution:

  • Percent Strength = (Weight of Solute / Total Weight of Solution) × 100

Example: For 8 ounces of dry polymer in 12 gallons of water: Percent Strength = (8 oz / 100.08 lb) × 100 = 7.99%

Determining Chemical Usage

Calculate average chemical use for your plant:

  • Average Chemical Use (lb/day) = (Sum of Daily Use) / Number of Days

Example: For daily use over a week (72, 85, 77, 82, 85, 90, 89 lb/day): Average Chemical Use = 580 lbs / 7 days = 82.86 lb/day


Mastering coagulation and flocculation calculations is paramount for water treatment plants. Precise calculations ensure optimal chemical dosing, basin design, and operational efficiency, ultimately leading to superior water quality and prolonged filter runs. As operators, understanding these calculations empowers us to forecast chemical needs, maintain adequate inventory, and deliver consistently high-quality water to communities.

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