Treatment Processes: Coagulation, Flocculation, Sedimentation, Filtration
The treatment train as one system
Coagulation destabilizes charge; flocculation builds settleable floc; sedimentation removes it; filtration polishes. Alum (aluminum sulfate) consumes alkalinity at roughly 0.5 mg/L as CaCO3 per mg/L of alum and depresses pH, so low-alkalinity water may need lime or caustic. Iron salts work over a wider pH band. Optimum dose and pH come from jar tests; streaming current or zeta potential can guide real-time tweaks. Flocculation uses tapered velocity gradient (G), faster first stages then gentler ones, so floc grows without shearing.
Sedimentation basins are rated by surface overflow rate and weir loading. Short-circuiting, density currents, and wind all degrade performance. Sludge must be wasted on schedule or the blanket rises and carries over.
Filters are rated in gpm/ft². Dual-media (anthracite over sand) allows higher rates; backwash must expand the bed enough to scour it - often about 20% - with air scour or surface wash for tough floc. Watch for negative head, mudballs, media loss, and turbidity breakthrough. Filter-to-waste protects the finished water after backwash.
Traps
- Confusing G (mixing intensity) with detention time.
- Assuming a higher coagulant dose always improves clarity - overdosing restabilizes particles.
- Ignoring the finished-water turbidity requirement: 0.3 NTU in at least 95% of monthly readings and never above 1 NTU for conventional filtration.
Worked example
A filter is 20 ft by 30 ft (600 ft²) and receives 2,400 gpm. Loading rate = 2,400 ÷ 600 = 4 gpm/ft². If flow climbs to 3,600 gpm, the rate is 6 gpm/ft² - check the design rate before accepting it.