Tube Settlers

Tube settlers are inclined tubular media installed in sedimentation basins to increase settling capacity by reducing the vertical distance particles must travel to be removed from suspension. These hexagonal or square tubes, typically installed at 55-60° angles, create multiple parallel settling zones that can increase basin capacity by 200-400% compared to conventional clarifiers. Surface loading rates can be increased from typical 800-1,000 gpd/sf to 1,500-2,500 gpd/sf while maintaining equivalent removal efficiency. The primary limitation is increased maintenance requirements due to solids accumulation and potential for uneven flow distribution if not properly designed.

• Primary Clarifiers (2-25 MGD plants): Tube settlers retrofit existing circular clarifiers to double surface overflow rates from 800 to 1,600 gpd/sf while maintaining 85-90% TSS removal. Installed above existing sludge collection mechanisms, receiving raw wastewater and discharging to aeration basins. Selected when plant expansion isn't feasible but capacity increases are needed.

• Secondary Clarifiers (0.5-50 MGD): Most common application, achieving 1,200-2,000 gpd/sf surface loading with MLSS concentrations up to 4,500 mg/L. Positioned between aeration and disinfection processes, tube modules occupy 60-80% of clarifier surface area. Selected for their ability to handle peak hydraulic loads during wet weather while maintaining effluent TSS below 30 mg/L.

• Tertiary Clarification: Used after biological nutrient removal processes, handling flows up to 1,500 gpd/sf with chemical precipitation. Critical for phosphorus removal plants requiring effluent TP below 1.0 mg/L.

Daily Operations: Operators monitor clarifier effluent turbidity (target <10 NTU), sludge blanket levels using sludge judges, and flow distribution across tube modules. Surface loading rates are calculated and logged. Effluent weir adjustments maintain proper hydraulic balance between parallel units.

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Maintenance: Monthly inspection of tube modules for algae growth or solids accumulation, requiring confined space entry procedures and fall protection. Annual high-pressure washing removes biofilms. Support structure inspection every six months for corrosion. Maintenance requires two-person teams with gas monitoring equipment for confined space work.

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Troubleshooting: Uneven effluent quality indicates flow maldistribution - check inlet baffling and weir settings. Rising effluent TSS suggests tube plugging from filamentous bacteria or debris. Excessive sludge carryover indicates improper sludge wasting rates. Properly maintained systems achieve 15-20 year service life with minimal tube replacement.

• Tube Modules: PVC or polypropylene honeycomb structures, typically 24" or 36" deep with 2" internal diameter tubes at 60° inclination. Standard 4'x8' modules cover 32 sf each. Selection based on clarifier geometry and required surface loading rates.

• Support Structure: Hot-dip galvanized steel framework spanning clarifier walls, designed for 15-20 psf live load plus tube weight. Includes access walkways and lifting points for maintenance.

• Inlet/Outlet Baffling: Perforated steel or fiberglass baffles ensuring uniform flow distribution across tube field. Positioned 6-12" upstream and downstream of modules.

• Sludge Collection: Existing chain-and-flight or rotating bridge mechanisms operate beneath tubes. Clearance typically 18-24" minimum between tube bottom and sludge blanket level.

• Flow Control: Adjustable weirs and gates for hydraulic balancing between multiple clarifier trains, critical for optimizing settling performance.

• Surface Loading Rate: 0.5-1.5 gpm/ft² for primary clarifiers; 0.3-1.0 gpm/ft² for secondary clarifiers. Higher rates (1.2-1.5 gpm/ft²) acceptable for plants with consistent influent characteristics.

• Tube Angle: 60° from horizontal is standard for municipal applications. Steeper angles (55°) increase settling efficiency but reduce hydraulic capacity.

• Tube Length: 36-48 inches typical for municipal clarifiers. Minimum 24 inches for effective particle capture; maximum 60 inches due to structural limitations.

• Tube Diameter: 2-inch nominal diameter most common. Smaller tubes (1.5-inch) provide better efficiency but higher headloss; larger tubes (2.5-inch) reduce maintenance but decrease performance.

• Headloss: 0.1-0.3 feet through clean media at design flow. Plan for 0.5 feet maximum with fouling.

• Depth Below Surface: Install 12-18 inches below normal water level to prevent scour and ensure even flow distribution.

• Support Spacing: Maximum 8-foot spans for fiberglass modules; 6-foot spans for PVC assemblies.

• Sidewall Clearance: Minimum 6 inches from basin walls for maintenance access and flow distribution.

• What surface loading rate should I specify based on my influent characteristics? Plants with high seasonal variation or industrial discharge should use conservative rates (0.3-0.8 gpm/ft²). Consistent domestic wastewater allows higher rates (0.8-1.2 gpm/ft²). Wrong decision leads to poor effluent quality during peak flows or oversized, expensive installations.

• Should I retrofit existing clarifiers or build new ones with integrated tube settlers? Existing circular clarifiers >60 feet diameter retrofit well; rectangular clarifiers with length-to-width ratios >3:1 are ideal. Clarifiers with center-feed mechanisms or insufficient freeboard (<4 feet) are poor retrofit candidates. Requires detailed hydraulic modeling and structural analysis.

• What material should I specify for my operating environment? PVC suitable for standard municipal applications with pH 6-9. Fiberglass required for aggressive environments, high temperatures (>100°F), or chlorine contact applications. Polypropylene for extreme chemical resistance. Material choice affects 20-year lifecycle costs significantly.

• How do I size the sludge removal system for increased solids loading? Tube settlers increase solids capture by 40-60%, requiring proportional increases in sludge pumping capacity and thickening processes. Underestimating leads to septicity and performance degradation.

• Primary: Division 46 23 13 - Packaged Water Treatment Equipment

• Secondary: Division 40 32 00 - Clarifiers (for retrofit applications)

• Note: Tube settlers are typically specified as components within clarifier systems rather than standalone equipment sections.

• Material/Equipment Verification: Verify PVC/PP tube material meets NSF-61 certification; Confirm structural loading calculations for existing clarifier retrofits; Check tube spacing matches hydraulic design (typically 2" spacing)

• Installation Requirements: Crane access for module lifting (typical modules weigh 500-2000 lbs); Temporary dewatering of clarifier during installation; Support frame anchoring to existing concrete structure

• Field Challenges: Maintaining proper slope during installation (typically 60° angle); Alignment of multiple modules for uniform flow distribution; Access for future cleaning and maintenance

• Coordination Issues: 8-12 week lead times for custom-sized modules; Coordinate with plant shutdown schedules for retrofit installations

• Parkson Corporation - LAMELLA-PAK systems, widely used in 1-50 MGD plants with 60° inclined plates

• WesTech Engineering - SUPRACLARIFIER tube modules, popular for retrofit applications

• Evoqua Water Technologies - ENVIREX tube settler packages for circular clarifiers

• Walker Process Equipment - CLARI-PAC lamella clarifiers, common in smaller municipal facilities (0.5-10 MGD)

• Conventional clarifiers with longer detention times - preferred for new construction when land is available, typically 30-40% lower capital cost

• Dissolved Air Flotation (DAF) - better for low-density floc, 20-30% higher cost but superior performance with algae-laden water

• Ballasted flocculation (ACTIFLO) - handles higher loading rates, 50-70% higher cost but smaller footprint for space-constrained sites

Work closely with manufacturer field service during startup - improper flow distribution causes immediate performance issues. Specify removable modules for easier maintenance access. Consider phased installation for large clarifiers to maintain treatment capacity. Negotiate spare tube modules (10% extra) in initial purchase - individual tube replacement is costly and time-consuming compared to full module swapping.