Cloth Media Filtration: How It Works and When to Use It

Introduction

Cloth media filtration — most commonly implemented as a rotating disc filter or rotating drum filter — has become one of the most popular tertiary filtration technologies at municipal wastewater treatment plants over the past 20 years. It delivers excellent effluent quality (< 2 NTU turbidity, < 5 mg/L TSS) in a compact, low-energy footprint that is well-suited for tertiary filtration, reuse, and phosphorus polishing applications.

If you've been specifying conventional sand filtration for tertiary treatment, cloth media deserves a serious look as an alternative.

How Cloth Media Filtration Works

The basic operating principle: wastewater flows through a fine woven fabric (cloth) that captures suspended solids while allowing clear filtrate to pass. The continuous backwash mechanism keeps the cloth clean without interrupting flow.

Disc Filter Configuration (Most Common)

A series of rotating disc-shaped panels, each covered with fine woven polyester fabric, are partially submerged in a tank. Wastewater flows from inside the discs outward through the cloth, with filtered water collected in a launder around the outside.

Operating cycle:

1. Wastewater enters the central hub and flows outward through the cloth panels
2. Solids accumulate on the inside face of the cloth
3. Solids buildup causes a rise in water level inside the disc (head buildup)
4. When head differential exceeds a setpoint, a backwash pump activates
5. Low-volume high-pressure spray (from the outside) removes the cake from the cloth face
6. Backwash slurry falls into a collection trough for return to the plant headworks
7. Discs rotate continuously (typically 1-4 RPM) to pass all panel sections through the backwash zone

Key advantage: Backwash volume is very low — typically 1-3% of feed flow, vs. 10-15% for sand filtration. This is a significant water savings for reuse applications.

Drum Filter Configuration

Rotating drum with cloth on the outside surface. Feed enters inside the drum, and filtrate passes outward through the cloth. Less common than disc for municipal applications but used for industrial and stormwater.

Performance Characteristics

Cloth Media for Phosphorus Removal

Cloth media filters are widely used for tertiary phosphorus polishing when combined with upstream chemical addition (alum or ferric chloride). The filter captures the fine chemical floc that carries the precipitated phosphorus.

Typical TP performance:

• With upstream coagulant: 0.05-0.2 mg/L TP in filtrate
• Without coagulant (TSS removal only): 0.2-0.5 mg/L TP depending on influent

For ultra-low phosphorus limits (< 0.05 mg/L), membrane filtration (UF) may be required instead.

Key advantage over sand for P removal: The low backwash volume means less return of recovered floc to the plant headworks, maintaining better overall phosphorus mass balance.

Cloth Media for Water Reuse

Water reuse regulations (California Title 22, NSF 350, state rules) typically require:

• Turbidity < 2 NTU (unrestricted reuse)
• TSS < 5 mg/L

Cloth media disc filters reliably meet these requirements with secondary effluent (after biological treatment). Combined with UV disinfection downstream, cloth filtration is a complete tertiary train for unrestricted reuse.

Advantages for reuse:

• Low backwash volume (conserves recovered water)
• Compact — fits in existing treatment plant footprint
• Simple operation — automated, minimal operator intervention
• Well-documented regulatory acceptance in most states

Cloth Media vs. Other Tertiary Filtration Options

vs. Continuous Backwash Sand Filters

Cloth media wins on:

• Footprint (50-70% less)
• Backwash volume
• Energy use
• Headloss

Sand wins on:

• First cost (at small scale)
• Industry familiarity
• Very high SS removal (sand can achieve < 2 mg/L)

vs. Granular Media (Dual Media) Filters

Cloth media wins on:

• Footprint
• Backwash water volume
• Headloss
• No filter-to-waste during backwash

Dual media wins on:

• Deeper solids removal (dual media captures solids through bed depth)
• Cost at small scale
• Better for high solids loads

vs. Membrane Filtration (UF/MF)

Cloth media wins on:

• Cost (much lower capital)
• Simplicity
• Energy use

Membrane wins on:

• Effluent quality (< 0.1 NTU vs. 0.5-2 NTU for cloth)
• Pathogen removal log credits
• Consistent performance regardless of feed variability
• Applications requiring 4-log virus or protozoa removal

Sizing Cloth Media Filters

Design inputs:

• Peak flow (GPM or MGD) — cloth media is sized for peak week or peak day
• Influent TSS and turbidity — affects backwash frequency
• Target effluent TSS and turbidity

Typical sizing:

• Hydraulic loading: 5-15 gpm/ft² of cloth area (varies by manufacturer and application)
• Number of units: N+1 minimum redundancy (one unit out of service during backwash doesn't affect overall performance)

Leading Tertiary Filtration Manufacturers

• Municipal Filtration Co. (MFC) — fixed-plate (stationary) cloth-media disk filters, no internal moving parts (10 GPM–6 MGD)
• Entex Technologies — FlowTex disc filter
• Aqua-Aerobic Systems (AquaDisk) — the original cloth media disc filter for wastewater; widely installed in US municipal market
• Parkson (DynaSand) — continuous-backwash sand filter (not cloth media); competes in the same space
• Hydrotech (Veolia) — drum filter for industrial and municipal
• Kruger (Veolia) — DISCFILTER systems
• Alfa Laval (Ashbrook) — disc filters for industrial

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