Pressure Sand Filter & Activated Carbon Filter Vessel & Media Calculation for Water & Wastewater Treatment Plant

1. Working Principles

Pressure Sand Filter

• Uses a bed of graded sand/gravel media in a pressurized vessel
• Water flows downward through media layers under pressure
• Removes suspended solids, turbidity, and particulate matter
• Typical media arrangement (top to bottom):
  • Fine sand (0.5-1.0 mm)
  • Coarse sand (1-2 mm)
  • Fine gravel (2-4 mm)
  • Coarse gravel (4-8 mm)

Activated Carbon Filter

• Uses activated carbon media in a pressurized vessel
• Removes organic compounds, chlorine, odor, and taste
• Works through adsorption process
• Types of carbon:
  • Granular Activated Carbon (GAC)
  • Powdered Activated Carbon (PAC)

2. Key Design Parameters & Calculations

A. Vessel Size Calculations

1. Filter Area (A) calculation:

A = Q / DFV

Where:

A = Filter area (m²)

Q = Flow rate (m³/hr)

DFV = Design Filtration Velocity (m/hr)

2. Vessel Diameter (D) calculation:

D = √(4A/π)

Where:

D = Diameter (m)

A = Filter area (m²)

3. Media Bed Height:

• PSF: Typically 0.9-1.2m
• ACF: Typically 1.0-1.5m

B. Backwash System Design

1. Backwash Flow Rate:

Qbw = A × Vbw

Where:

Qbw = Backwash flow rate (m³/hr)

A = Filter area (m²)

Vbw = Backwash velocity (m/hr)

2. Air Scour Rate:

Qair = A × Vair

Where:

Qair = Air flow rate (m³/hr)

A = Filter area (m²)

Vair = Air velocity (m/hr)

C. Operating Parameters

1. Service Flow Rate:

SFR = Q / A

Where:

SFR = Service Flow Rate (m³/m²/hr)

Q = Flow rate (m³/hr)

A = Filter area (m²)

2. Daily Treatment Capacity:

DTC = Q × Operating Hours

Where:

DTC = Daily Treatment Capacity (m³/day)

Q = Flow rate (m³/hr)

3. Design Considerations

Typical Design Values

• Design Filtration Velocity (DFV): 10-15 m/hr for PSF, 8-12 m/hr for ACF
• Backwash Velocity: 30-45 m/hr for PSF, 25-35 m/hr for ACF
• Air Scour Velocity: 50-60 m/hr
• Operating Hours: Usually 16-24 hrs/day
• Service Flow Rate: Should not exceed maximum recommended by media manufacturer

Safety Factors

1. Vessel Height:

• Add 40-50% to media height for expansion during backwash
• Additional 20% freeboard for water distribution

2. Pressure Rating:

• Design pressure: typically 3-6 bar
• Test pressure: 1.5 × design pressure

Important Notes

1. Media Specifications:

• Use proper gradation for even flow distribution
• Ensure media meets AWWA standards
• Consider local water quality for media selection

2. Maintenance Requirements:

• Regular backwashing schedule based on pressure drop
• Media replacement every 3-5 years for PSF
• GAC replacement based on breakthrough (typically 1-3 years)