Coagulation, Flocculation, and Sedimentation
This domain focuses on the chemical and physical processes that remove suspended and colloidal particles before filtration. The exam tests your understanding of coagulant chemistry, jar testing, flocculation energy, and sedimentation theory.
Coagulation Chemistry
- Coagulants such as aluminum sulfate (alum), ferric chloride, and polyaluminum chloride neutralize the negative surface charge of colloids, allowing them to aggregate. The optimum pH range for alum is typically 5.5–6.5; for ferric salts, 4.0–6.0.
- Charge neutralization and sweep floc are the two primary mechanisms. Sweep floc occurs at higher coagulant doses where metal hydroxide precipitates enmesh particles.
- Alkalinity is consumed by coagulant addition: 1 mg/L of alum consumes about 0.45 mg/L of alkalinity as CaCO3. If alkalinity is insufficient, pH drops and coagulation fails.
Flocculation
- Flocculation gently mixes the water to promote collisions and grow floc. Velocity gradient (G) typically ranges from 10–75 s⁻¹, with a tapered profile (higher G at the start, lower later). Detention time is usually 15–45 minutes.
- Overmixing can shear floc, reducing settleability. Undermixing leaves small, slow-settling floc.
Sedimentation
- Type I settling is for discrete particles (e.g., sand), Type II for flocculent particles (size increases as they settle), Type III for hindered settling (sludge blanket), and Type IV for compression.
- Surface overflow rate (SOR) is the key design parameter: SOR = flow rate / surface area. Typical values for conventional sedimentation are 0.5–1.0 gpm/ft² (1.2–2.4 m/h).
- Weir loading rate affects effluent quality; high rates can resuspend solids.
Worked Example
A sedimentation basin has a surface area of 2,000 ft² and receives a flow of 1,200 gpm. Calculate the surface overflow rate in gpm/ft² and compare to a typical design range of 0.5–1.0 gpm/ft².
- SOR = 1,200 gpm / 2,000 ft² = 0.6 gpm/ft².
- This is within the typical range, so the basin is adequately sized for that flow.
- If flow increased to 2,200 gpm, SOR = 1.1 gpm/ft², exceeding the range and risking poor settling.
Common Traps
- Forgetting that coagulant addition consumes alkalinity and can depress pH below optimal.
- Using the wrong surface area (e.g., including weir areas) in SOR calculation.
- Assuming higher G is always better; excessive flocculation shear breaks floc.
- Confusing Type I and Type II settling; flocculent particles do not settle at constant velocity.
- Neglecting temperature effects: cold water increases viscosity and slows settling.