Biofouling on Filter Cloth: Causes and Cleaning Strategies

Mechanisms of Biofouling and Blinding

Biofouling refers to the accumulation of microorganisms (biofilm) on the surface and within the pile of filter cloth. Over time, bacteria and other microbes attach to the fibres, forming a slimy matrix that can reduce permeability. In parallel, grease, oils, and fibrous materials (such as hair or textile fibres) can lodge within the cloth pile, creating blinding that further impedes water flow.

Blinding and biofouling typically develop due to:

• High organic or grease loads in the influent
• Elevated suspended solids, especially fine fibres
• Insufficient or ineffective backwashing
• Extended filtration cycles without cleaning

The result is a gradual increase in resistance to flow, leading to higher differential levels and shorter filtration cycles.

Warning Signs of Biofouling and Blinding

Operators should regularly monitor the following indicators of filter cloth fouling:

• Rising differential water level across the filter unit (often from 20-40 cm up to 60 cm or more)
• Reduced filtration cycle times (e.g., from 60-90 min to 20-30 min)
• Increased backwash frequency without corresponding improvement in performance
• Visible slime, discoloration, or matting on the cloth surface

A gradual decline in filtrate quality may also signal the onset of blinding or biofilm buildup.

Routine Backwash Logic and Effectiveness

Routine backwash is the primary cleaning mechanism for pile cloth media filtration. Most systems use an air or water backwash (sometimes both), triggered by a preset differential level or elapsed filtration time. Typical backwash velocities range from roughly 4-10 m³/(m²·h), with durations of 1-3 minutes per segment.

Backwash removes most loose solids and some loosely bound biofilm. However, it is less effective against established biofilms or grease/fibre mats. Operators should verify that:

1. Backwash intervals are optimized for influent load and filter type
2. Backwash flow and pressure are within design ranges
3. Nozzle alignment and distribution are uniform across the filter area

If backwash alone cannot restore performance, enhanced cleaning may be required.

Periodic Chemical and Enhanced Cleaning Approaches

Periodic chemical or enhanced cleaning is necessary when biofouling or blinding persist despite routine backwash. Common approaches include:

• Oxidizing agents (e.g., sodium hypochlorite) to break down biofilm
• Surfactants or mild detergents for grease and oil removal
• Hot water or steam cleaning for thermal disruption of biofilm
• Manual brushing or low-pressure washing for physical removal

Cleaning intervals vary from monthly to annually, depending on influent characteristics and operational history.

Care must be taken to ensure chemical compatibility with the filter cloth material. Most modern pile cloths are made from polyester or polyamide, which tolerate mild oxidants and surfactants but may degrade with strong acids or prolonged high-temperature exposure.

Preventive Operating Practices

Preventing filter cloth biofouling and blinding is more effective than frequent remedial cleaning. Key preventive measures include:

• Optimizing primary and secondary treatment to reduce solids and grease load
• Maintaining regular and effective backwash cycles
• Monitoring influent for sudden changes in load or composition
• Periodically inspecting filter cloth condition and surface appearance
• Implementing scum removal or grease traps upstream if needed

Routine operator training and record-keeping help identify trends and address issues before major fouling occurs.

Materials Compatibility Considerations

Before applying any chemical or thermal cleaning method, confirm compatibility with the specific filter cloth material. Most pile cloths used in tertiary filtration are made from:

• Polyester (PET): good chemical resistance, moderate heat tolerance
• Polyamide (PA): higher temperature resistance, slightly less chemical resistance

Avoid strong acids, strong alkalis, and oxidants at concentrations or temperatures beyond manufacturer recommendations. Prolonged exposure to high temperatures (>90°C) or harsh chemicals can degrade pile structure and shorten filter life.