Secondary Treatment - Clarification

Physical separation of biological solids from treated effluent using gravity settling and sludge thickening.

Overview

Clarification is the final step in secondary treatment, separating biomass from the treated mixed liquor to produce low-TSS effluent while thickening return sludge for biological process control.

Unit Operations

Secondary-Clarifier

Circular settling tanks with Takács double-exponential settling model for standard activated sludge applications.

Rectangular-Clarifier

Rectangular geometry for large plants, phased construction, and constrained linear sites.

Lamella-Clarifier

High-rate inclined plate settlers achieving 3-5× conventional overflow rates in compact footprint.

RAS-Pumping

Return and waste activated sludge pumping for MLSS control and SRT management.

Key Design Parameters

Surface Overflow Rate (SOR):

• Conventional: 15-30 m³/m²·d (average)
• Lamella: 60-120 m³/m²·d (projected area)

Solids Loading Rate (SLR):

• Target: 80-120 kg/m²·d (average)
• Peak: <200 kg/m²·d

Sludge Volume Index (SVI):

• Good: 80-120 mL/g
• Acceptable: 120-150 mL/g
• Bulking: >150 mL/g

Process Selection

Criterion	Circular	Rectangular	Lamella
Plant size	Any	>20 MLD preferred	Any
Site constraints	Open area	Linear/constrained	Severely constrained
Footprint efficiency	Medium	Medium	High (1/3 conventional)
Capital cost	Low	Medium	High (per m²)
SVI tolerance	Good (<150)	Good (<150)	Requires <120
Maintenance	Low	Medium	Medium-High

References

See individual unit pages for detailed references.