WWTP Step-by-Step Design Calculator | Activated Sludge Process

Civil & Environmental Engineering Manual

Wastewater Treatment Plant (WWTP) Process Calculations

Step-by-Step Engineering Design Calculator — Activated Sludge Process (ASP)

1. Introduction to Activated Sludge Process (ASP)

The Activated Sludge Process (ASP) remains standard for biological municipal and industrial wastewater treatment. Engineered as a continuous-flow suspended-growth system, ASP relies on a dense culture of microorganisms (primarily heterotrophic bacteria) kept in suspension to stabilize organic matter under aerobic conditions. Organic pollutants measured as Biochemical Oxygen Demand ($\text{BOD}_5$) and Chemical Oxygen Demand ($\text{COD}$) are bio-converted into cellular biomass, carbon dioxide ($\text{CO}_2$), and water.

2. Process Train & System Components

A complete ASP treatment facility operates on two distinct lines: the liquid stream (water clarification) and the solids stream (sludge management).

• Preliminary Treatment: Mechanically cleaned bar screens and grit channels protect downstream pumps and prevent physical damage or blockage.
• Equalization Tank: Dampens diurnal peak factors and organic load shocks, ensuring constant feed to the biological reactors.
• Primary Clarifier (PST): Settles heavy organic solids, reducing overall BOD loading prior to biological treatment.
• Aeration Basin: The core reactor where mixed liquor suspended solids (MLSS) interact with diffused oxygen to degrade dissolved organics.
• Secondary Clarifier (SST): Separates biological flocs from clarified effluent, yielding clear supernatant and recycling return activated sludge (RAS).
• Sludge Handling (Thickener & Filter Press): Concentrates waste activated sludge (WAS) and primary sludge to produce dewatered sludge cakes suitable for disposal.

3. Interactive Process Design Calculator

Input project-specific baseline parameters (flow, BOD, target effluent) and advanced operational criteria below to calculate volumetric requirements, oxygen demand, clarification areas, and sludge mass balances.

0 Design Basis — Input Parameters

Average Flow Rate (Q) m³/day

Peak Factor

Influent BOD₅ mg/L

Influent COD mg/L

Influent TSS mg/L

Required BOD Outlet mg/L

Advanced Assumptions (editable)

Equalization HRT hr

Bar Screen Velocity m/s

Primary Clarifier SOR m³/m²·day

F/M Ratio kg BOD/kg MLSS·day

MLSS mg/L

Secondary Clarifier SOR m³/m²·day

BOD Removal Efficiency %

O₂ Factor (kg O₂ / kg BOD)

Air / O₂ Ratio (kg air / kg O₂)

Air Density kg/m³

Primary Sludge Factor × BOD load

Biological Sludge Yield × BOD removed

Thickener Solids Loading kg/m²·day

Filter Cake Solids %

1 Equalization Tank Design

2 Bar Screen Design

3 Primary Clarifier Design

4 Aeration Tank Design (Activated Sludge)

5 Air Requirement

6 Secondary Clarifier Design

7 Sludge Generation

8 Sludge Thickener Design

9 Filter Press Sizing

★ Summary Table & Process Flow

Yogendra Gopal Borse

Civil Engineer | Assistant Engineer Grade-I, Maharashtra PWD

B.Tech (Civil) from VJTI Mumbai. Experienced in bridge design, road works, estimation, project monitoring and digital engineering tools. Creator of YogiPWD – practical technical resources for civil engineers.

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