Rotary Drum Cloth Filters

Rotary Drum Cloth Filters provide tertiary filtration in municipal wastewater treatment plants, removing suspended solids and phosphorus to meet stringent discharge limits. The system consists of a rotating cylindrical drum covered with fine-mesh cloth media that filters water from inside-out as it slowly rotates through a tank. Typical installations achieve effluent TSS concentrations below 5 mg/L with hydraulic loading rates of 2-8 gpm/sf. While highly effective for polishing secondary effluent, these systems require frequent cloth replacement and generate significant backwash volumes that must be returned to the headworks for retreatment.

• Secondary Clarifier Upgrade/Replacement (5-50 MGD): RDCFs replace aging secondary clarifiers where footprint expansion is impossible. The drum filters connect directly to biological treatment effluent, eliminating clarifier bypassing during peak flows. Downstream connects to disinfection or tertiary treatment. Selected for 90%+ TSS removal in 15-20% of original clarifier footprint.
• Tertiary Filtration (2-25 MGD): Installed post-secondary treatment for advanced nutrient removal compliance. Upstream from UV disinfection or membrane bioreactors where <5 mg/L TSS is required. Provides consistent 2-5 mg/L TSS effluent regardless of upstream biological process variations.
• Primary Treatment Enhancement (10-50 MGD): Used after primary clarifiers during plant capacity expansions. Captures additional suspended solids before biological treatment, reducing downstream loading by 20-30%. Particularly effective for plants with high industrial loadings or combined sewer systems.
• Sidestream Treatment (0.5-15 MGD): Treats centrate from dewatering operations or return activated sludge streams. Reduces internal plant loading and improves overall treatment efficiency.

Daily Operations: Operators monitor effluent turbidity (target <5 NTU), headloss across cloth media, and backwash frequency. Key adjustments include drum rotation speed (typically 0.5-1.5 rpm) and backwash pressure (80-120 psi). Visual inspection of cloth condition and spray nozzle alignment occurs during routine rounds. Flow rate monitoring ensures design loading rates (2-10 gpm/sq ft) aren't exceeded.

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Maintenance: Cloth replacement required every 2-4 years depending on loading conditions. Monthly bearing lubrication and quarterly spray nozzle cleaning prevent performance degradation. Requires confined space entry procedures for drum access and lockout/tagout for rotating equipment. Operators need basic mechanical skills for nozzle adjustment and cloth tensioning. PPE includes eye protection, gloves, and non-slip footwear around wet surfaces.

• Filter Cloth Media: 10-20 micron polyester or polypropylene fabric wrapped around rotating drum. Cloth selection based on required particle removal and chemical compatibility. Municipal applications typically use 6-12 foot diameter drums with 100-500 sq ft filter area per unit.
• Backwash System: High-pressure spray nozzles (80-120 psi) with automated sequencing. Uses 2-5% of treated effluent for continuous cloth cleaning. Includes filtrate pumps rated 50-200 gpm and programmable wash cycles every 30-180 seconds based on headloss.
• Drive Mechanism: Variable speed gear motor (0.1-2 rpm) with stainless steel shaft and bearings. Includes torque monitoring for cloth tension control and emergency stops. Motors typically 1-5 HP for municipal installations.
• Level Control System: Ultrasonic or float-based sensors maintaining 6-18 inch operating water levels. Controls influent flow rates and drum rotation speed. Includes overflow weirs and emergency bypass provisions.
• Support Structure: Stainless steel or concrete basin with influent/effluent channels. Designed for 15-25 year service life with removable drum assemblies for maintenance access.

• Flow Rate: 0.1-25 MGD per unit, with multiple units for larger plants. Standard sizing at 4-8 gpm/ft² cloth area.
• Hydraulic Loading: 4-12 gpm/ft² of cloth media surface area. Conservative municipal design typically uses 6-8 gpm/ft² for secondary effluent applications.
• Solids Loading: 10-100 mg/L TSS influent typical for tertiary filtration. Peak loading capacity up to 200 mg/L for short durations.
• Backwash Requirements: 15-25% of forward flow for cloth cleaning. Backwash pressure 15-30 psi, duration 10-30 seconds per cycle.
• Head Loss: Clean cloth starts at 6-12 inches, dirty cloth reaches 24-36 inches before backwash initiation. Terminal head loss typically set at 30 inches.
• Drum Specifications: Diameter 8-14 feet, length 6-20 feet depending on capacity. Rotational speed 0.1-1.0 rpm.
• Filtrate Turbidity: Consistently achieves <2 NTU, often <1 NTU with proper operation.
• Power Requirements: 0.5-2.0 hp per MGD for drum rotation and backwash pumps.
• Cloth Media: 10-20 micron polyester or nylon mesh, 2-5 year replacement cycle in municipal service.

• What peak hourly flow must the system handle? Drums sized for average flow cannot handle 2.5-3.0x peak flows common in municipal plants. Undersizing results in breakthrough and permit violations. Need 24-hour flow data and peaking factors.
• Will the system treat secondary or advanced secondary effluent? TSS concentrations >50 mg/L require more frequent backwashing and larger cloth surface area. Biological nutrient removal plants with polymer addition may need 25% additional capacity due to higher solids loading.
• What effluent turbidity limit must be met? <2 NTU requires standard operation, but <1 NTU for reuse applications may need redundant units or cloth media upgrades. Failure means costly permit violations and potential plant shutdown.
• How will solids handling integrate with existing systems? Backwash flow (15-25% of influent) returns to headworks, potentially overloading upstream processes. Must verify capacity of return pumping and primary treatment. Poor integration creates plant-wide hydraulic problems and increased operating costs.

• Division 40 - Process Integration
• Section 40 31 33 - Cloth Media Filtration Systems
• Primary specification covers complete rotary drum filter systems including drives, backwash systems, and controls. Secondary reference to 40 20 00 for process piping integration.

• Material/Equipment Verification:
• Verify cloth media specifications and NSF certification
• Confirm stainless steel grades for wetted components
• Review motor and drive specifications for continuous duty
• Installation Requirements:
• Level concrete pad within 1/4" over filter footprint
• Adequate overhead clearance for cloth media replacement
• Proper drainage for backwash collection
• Field Challenges:
• Media tensioning requires factory-trained technicians
• Electrical coordination for variable frequency drives
• Coordination Issues:
• 12-16 week lead times typical for custom configurations

• Veolia Water Technologies - ACTIFLO Turbo series for enhanced clarification
• Parkson Corporation - DynaSand continuous backwash filters with cloth media options
• WesTech Engineering - Trident cloth media filters for tertiary applications
• Huber Technology - RoDisc rotary disc filters with cloth media configurations
• All maintain strong municipal references across 1-50 MGD applications.

• Traveling Bridge Sand Filters - Lower O&M costs, proven reliability; 20-30% higher capital cost
• Membrane Bioreactors (MBR) - Superior effluent quality but 3-4x higher operating costs
• Conventional Dual Media Filters - Lowest capital cost option; requires more operator attention and backwash water

Establish direct relationships with manufacturer service teams early - cloth media replacement requires specific tensioning procedures that vary by model. Budget 15-20% contingency for site-specific modifications during installation. Consider standardizing on single manufacturer across multiple units to simplify spare parts inventory and operator training. Negotiate bulk cloth media pricing for multi-year supply agreements.