Filter Surface Wash Agitators

Filter surface wash agitators mechanically disrupt the filter bed surface during backwash operations to enhance solids removal and extend filter run times. These rotating arm assemblies create localized turbulence at the media interface, breaking up accumulated biofilms and compacted particles that standard air scour and water backwash alone cannot effectively remove. Typical installations achieve 15-25% longer filter runs and reduce headloss buildup by 30-40% compared to conventional backwash systems. The primary trade-off is increased mechanical complexity and maintenance requirements, with rotating seals and drive mechanisms requiring regular inspection and replacement in the aggressive backwash environment.

• Rapid Sand Filtration Plants (5-50 MGD): Surface wash agitators are installed above sand filter beds during backwash cycles to break up biofilm and debris accumulation on the top 2-3 inches of media. They connect to plant wash water systems (typically 15-25 psi) and discharge to backwash troughs. Essential for plants treating surface water with high organics where biological growth creates filter binding.

• Direct Filtration Systems (0.5-15 MGD): Used in plants without sedimentation where filters receive higher solids loading. Agitators prevent rapid headloss development by removing accumulated particles during surface wash sequences before air scour or backwash initiation.

• Dual Media Filter Retrofits: Installed when converting sand filters to anthracite/sand systems. Surface agitation prevents anthracite media migration and maintains proper stratification during backwash sequences.

• High-Rate Filtration (>5 gpm/sf): Critical for filters operating above 4 gpm/sf where conventional backwash alone cannot adequately clean media surfaces, particularly with algae-laden source waters.

Daily Operations: Operators verify surface wash activation during scheduled backwash sequences through SCADA monitoring. Check rotation speed and spray pattern coverage visually during daylight operations. Monitor wash water pressure (target 20-25 psi) and flow rates. Document cycle times and any irregular operation. Most systems operate automatically but require visual confirmation of proper nozzle spray patterns.

​

Maintenance: Monthly inspection of nozzle condition and spray patterns, quarterly lubrication of drive bearings and gear reducers. Semi-annual removal and cleaning of clogged nozzles using wire brushes and dilute acid solutions. Annual inspection of support structure bolting and seals. Requires confined space entry procedures and lockout/tagout. Maintenance staff need basic mechanical skills and lifting equipment for nozzle servicing.

​

Troubleshooting: Common failures include clogged nozzles (evidenced by uneven spray patterns), drive motor overload from debris accumulation, and seal leakage at rotating joints. Warning signs include increased backwash turbidity, shortened filter runs, and visible algae growth on media surface. Typical service life of 15-20 years for mechanical components, 5-8 years for nozzles.

• Rotating Arm Assembly: Typically 316 stainless steel construction with 8-12 foot radius for standard rectangular filters. Features adjustable nozzle spacing (6-12 inches) and rotation speed (1-3 RPM). Sized based on filter dimensions and required coverage area.

• Drive Mechanism: Gear-reduced electric motor (0.5-2 HP) with variable speed capability. Includes torque protection and position feedback for automated operation. Motor sizing depends on arm length and water resistance.

• Nozzle Configuration: Fixed or adjustable spray nozzles (typically 1/4" to 1/2" orifice) designed for 15-30 degree spray patterns. Stainless steel construction with removable tips for maintenance. Spacing determined by hydraulic coverage requirements.

• Support Structure: Heavy-duty pedestal or bridge mounting system capable of supporting rotating assembly plus water loading. Includes sealing mechanisms for submerged operation and access platforms for maintenance.

• Control System: Integration with plant SCADA for automated sequencing with backwash cycles, including timer controls, position sensing, and interlock protection.

• Flow Rate Requirements: 2-6 gpm per square foot of filter surface area, with 3-4 gpm/sf being standard for most municipal applications. For a typical 20' x 40' filter (800 sf), expect 2,400-3,200 gpm wash water demand.

• Operating Pressure: 15-25 psi at the agitator nozzles, requiring 35-50 psi at the wash water pump discharge to overcome piping losses. Higher pressures (up to 30 psi) may be needed for filters with deeper media beds or high turbidity loading.

• Agitator Spacing: 4-6 feet on center in both directions, with closer spacing (3-4 feet) for anthracite or multimedia filters. Typical municipal installations use 5-foot spacing as standard.

• Traverse Speed: 1-3 feet per minute for effective surface cleaning, with 2 fpm being optimal for most applications. Slower speeds increase wash duration but improve cleaning effectiveness.

• Wash Duration: 3-8 minutes total cycle time, including 1-2 minutes for agitator positioning and 2-6 minutes active washing. Heavily loaded filters may require up to 10 minutes.

• Power Requirements: 2-5 HP motors for agitator drive mechanisms, plus 50-150 HP for wash water pumps depending on system capacity and head requirements.

• What wash water flow rate should be specified for the specific filter media configuration? Standard sand filters typically require 3-4 gpm/sf, while anthracite or GAC caps may need 4-6 gpm/sf. Undersizing by 1 gpm/sf can result in 20-30% reduction in cleaning effectiveness and shortened filter runs. Engineers need media specifications, expected loading rates, and historical turbidity data.

• Should the system use fixed or rotating agitators? Fixed rotary arms are suitable for filters up to 30' wide and cost $15,000-25,000 less per filter. Traveling bridge systems are required for filters over 30' wide but cost $40,000-60,000 more. Decision depends on filter dimensions, building clearances, and maintenance access requirements.

• What level of automation is required for the wash sequence? Basic timer-controlled systems cost $8,000-12,000 less but require operator intervention. Fully automated systems with turbidity monitoring and variable wash duration add $15,000-20,000 but reduce labor costs by 2-3 hours per week. Consider staffing levels and operational philosophy.

• How should the system integrate with existing filter controls? Standalone systems are simpler but require separate HMI panels. Integration with plant SCADA adds $5,000-8,000 but provides centralized control and data logging capabilities essential for regulatory reporting and optimization.

• Primary: Division 40-25-00 (Water Treatment Equipment)

• Secondary: Division 40-25-13 (Filtration Equipment) for integrated specifications with filter systems

• Material/Equipment Verification: Verify 316SS construction for all wetted components, Confirm AWWA C651 compliance for rotating assemblies, Check motor IP ratings and explosion-proof classifications

• Installation Requirements: Coordinate filter drain timing with surface wash installation, Verify adequate crane access for rotating arm assemblies, Plan temporary bypass during retrofit installations

• Field Challenges: Filter box dimensional variations affect arm clearances, Existing piping conflicts common in retrofits, Lead times: 16-20 weeks for standard units, 24+ weeks for custom configurations

• Infilco Degremont - ULTRAFOR surface wash systems, dominant in North American municipal market

• WesTech Engineering - WWASH rotating arm systems, strong municipal presence

• Evoqua Water Technologies - Leopold surface wash equipment, established municipal installations

• Roberts Filter Group - ROTOWASH systems, growing municipal market share

​

All maintain extensive municipal reference lists for 1-50 MGD facilities.

• Fixed spray nozzles - 30-40% lower cost, suitable for smaller filters (<500 sf), limited cleaning effectiveness

• High-rate backwash only - Eliminates surface wash equipment entirely, requires 20-30% higher backwash rates, common in package plants

• Air scour systems - 50-75% higher capital cost, superior cleaning performance, preferred for high-turbidity applications or when minimizing backwash water usage is critical

Establish direct relationships with manufacturer field service teams early - they're invaluable for troubleshooting hydraulic issues and optimizing wash sequences. Budget 15-20% contingency for retrofit projects due to unforeseen interferences. Consider standardizing on single manufacturer across multiple filters to simplify spare parts inventory and operator training. Many plants achieve 10-15% longer filter runs after proper surface wash optimization.