Requirements for biological treatment end equipment, Part 2: Activated sludge process and its modifications
1982 Edition

Overview of Scope and Materials (Clauses 2.0 & 3.1)

• Defines the components and construction methodologies for aeration chambers and biological treatment apparatus.
• Specifies construction materials detailed in Table 1 including reinforced concrete, masonry, metals, and plastics with relevant IS references.

Material Reference Table:

• Utilizes SI units such as metre (m), newton (N), and pascal (Pa) for measurement standardization.

Clarification of Terms and Units (Clause 2.0 onwards)

• Establishes consistent terminology throughout the standard.
• Defines key equipment, e.g., the Vortair Entrainment Aerator, which employs turbine-induced hydraulic action just beneath the liquid surface to enhance oxygen transfer.

SI Units and Symbols:

• Specifies material and construction practices conforming to durability and compatibility requirements.

Material Selection and Construction Guidelines (Clause 3.1 & 4.1.11)

• Details construction materials with IS references covering concrete, masonry, metals, and plastics.
• Civil works include reinforced/plain concrete, brick, and stone masonry.
• Mechanical components like motors, gears, and bearings are specified with their respective IS standards.
• Requires valves, gates, stop planks, and weirs for flow regulation at inlets and outlets.
• Hydraulic design must accommodate peak instantaneous loads.

Material and IS Code Summary:

Design Principles and Construction Specifications

• Civil works utilize reinforced or plain concrete (IS 456-1978, IS 3370), brick masonry (IS 2212-1962), and stone masonry (IS 1597).
• Mechanical components such as motors, starters, gears, and turntables adhere to their respective IS codes.
• Design validation requires provision of oxygen transfer data including characteristic curves and operational performance metrics.
• Employs SI units for all engineering parameters.

Summary Table of Key Materials:

Guidelines for Aeration Tank Configuration (Clause 4.1)

• Tank volume is calculated based on organic loading rates and volatile suspended solids (VSS), with detailed data referenced in Appendix A.
• Two primary tank types defined:
  • Diffused aeration tanks where oxygen is introduced through bottom diffusers.
  • Mechanical or surface aeration tanks employing surface aerators such as rotors or paddles.
• Oxygen transfer capacity must meet biochemical oxygen demand (BOD) removal and maintain VSS levels.
• Designers must submit oxygen transfer characteristic curves or long-term operational data.

Typical Aeration Tank Volume Calculation:

[ V = \frac{Q \times L}{k \times X} ]

Where:

• (V) = tank volume (m³)
• (Q) = flow rate (m³/day)
• (L) = organic loading (kg BOD/m³)
• (k) = reaction rate constant (day⁻¹)
• (X) = active biomass concentration (kg VSS/m³)

Oxygen Transfer Rates for Aerators:

Requirements for Aeration Devices (Clauses 4.2.2 & 4.2.3)

• Equipment capacity must be aligned with BOD removal needs and VSS maintenance.
• Oxygen transfer performance must be demonstrated via characteristic curves or long-term operating data.
• Aerator types include diffused air systems (ridge & furrows, spiral flows) and mechanical systems (impeller draft tubes, brush, paddle-wheel with diffused air).
• Design considerations emphasize oxygen transfer rate (OTR), energy efficiency, and uniform oxygen distribution.

Oxygen Transfer Rate Formula:

[ OTR = K_L a \times (C^* - C) ]

Where:

• (K_L a) is the overall oxygen transfer coefficient (1/min)

• (C^*) is saturation oxygen concentration (mg/L)

• (C) is actual dissolved oxygen concentration (mg/L)

• Performance data should include oxygen transfer efficiency, power input per volume, and airflow rate.

Design Criteria for Return Sludge Recycling (Clauses 4.4.1 & 4.4.2)

• Return sludge flow is calculated to maintain the Mixed Liquor Suspended Solids (MLSS) concentration in the aeration tank:

[ Q_r = \frac{MLSS_{aeration} \times V}{MLSS_{return}} ]

Where:

• (Q_r) = return sludge flow (m³/day)

• (MLSS_{aeration}) = target MLSS in aeration tank (mg/L)

• (V) = aeration tank volume (m³)

• (MLSS_{return}) = suspended solids concentration in return sludge (mg/L)

• Pumping capacity must be sized for 100% of calculated flow with an additional standby pump.

Recommended Loading and Operational Parameters (Appendix A):

Sludge Management Practices (Clauses 4.5 & Appendix A)

• Recommended parameters for various activated sludge systems include loading rates, MLSS concentrations, sludge retention times (SRT), and air requirements.

• Waste activated sludge can be directed to primary tanks, concentration tanks, aerobic or anaerobic digesters, or sludge drying beds, particularly in extended aeration systems.

• Important formula relating process loading (U), yield coefficient (Y), and decay rate (k_d):

[ U = \frac{kg ; BOD ; removed}{kg ; VSS \cdot day}; \quad Y = yield ; coefficient; \quad k_d = decay ; rate ]

• For temperatures below 15°C, temporary increases in loading rates are advised.

Operational Guidelines for Activated Sludge Systems (Clause 3.3 & Appendix A)

• Typical operational parameters by system type include loading rates, MLSS concentrations, sludge retention times, and air requirements.

• Sludge Retention Time (SRT) is calculated as:

[ SRT = \frac{Y \times U}{k_d} ]

Where:

• U = process loading (kg BOD removed/kg VSS/day)

• Y = yield coefficient (mg VSS/mg BOD removed)

• k_d = decay rate of microorganisms (day⁻¹)

• Oxygen transfer equipment capacity must be supported by oxygen transfer data or long-term plant performance information.

• Construction materials must comply with relevant IS standards.

Loading Rate Recommendations for Different Activated Sludge Systems

• In cold conditions (below 15°C), loading rates should be temporarily increased.
• Aeration tank volume calculations rely on these loading rates and VSS concentrations.
• SRT is calculated using the formula:

[ SRT = \frac{Y \times U}{k_d} ]

Mechanical Aerator Variants and Their Characteristics

• Impingement Aerator: Employs water jets impacting surfaces to entrain air.

• Surface Aerator: A rotating flat plate with radial blades submerged just below the water surface, creating vortex and hydraulic jumps to promote air entrainment.

• Surface Impeller with Draft Tube: Draws liquid through the draft tube and disperses it over the surface.

• Vortair Entrainment Aerator: Utilizes vortex formation to draw air into the water body.

• Capacity and performance must correspond to BOD removal requirements and VSS maintenance.

Typical Surface Aerator Specifications:

• Refer to IS 8413 Part 2 figures and tables for detailed design and power specifications.