Rectangular Clarifiers

Rectangular clarifiers remove suspended solids from water and wastewater through gravity sedimentation in long, rectangular basins. Raw water enters at one end and flows horizontally while particles settle to the bottom, creating a clarified effluent that exits via weirs at the opposite end. Typical surface overflow rates range from 500-1,200 gallons per day per square foot (gpd/sf) for water treatment and 300-800 gpd/sf for wastewater applications. The primary trade-off is footprint efficiency - rectangular clarifiers require significantly more land area than circular designs but offer superior hydraulic performance and easier integration with linear treatment processes.

• Primary Clarification (0.5-20 MGD): Rectangular primary clarifiers receive screened raw wastewater, removing 50-60% TSS and 25-35% BOD through gravity settling. Selected for smaller plants due to simpler construction and lower capital costs versus circular units. Typical loading rates: 800-1,200 gpd/sf surface area, 3-6 feet SWD. Feeds directly to secondary treatment.
• Secondary Clarification (1-15 MGD): Following activated sludge or trickling filters, rectangular clarifiers separate mixed liquor, achieving 85-95% TSS removal. Chain-and-flight mechanisms handle higher solids loading (15-25 lb/day/sf) better than circular scrapers. Common in package plants and retrofits where site constraints favor rectangular footprints.
• Tertiary/Polishing (2-50 MGD): Used after secondary treatment with chemical addition for phosphorus removal or advanced filtration pretreatment. Lower loading rates (400-800 gpd/sf) optimize floc settling. Rectangular design allows multiple parallel units for operational flexibility during maintenance.

Daily Operations: Operators monitor effluent turbidity, sludge blanket depth (target 2-3 feet), and surface scum accumulation. Flow distribution checks across inlet baffles ensure even loading. Weir adjustments maintain proper water levels. Typical monitoring includes TSS grab samples and sludge judge readings every 4-8 hours during peak flows.

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Maintenance: Chain-and-flight systems require weekly lubrication of drive mechanisms and monthly tension adjustments. Quarterly underwater inspections check for broken flights or chain wear. Annual drive motor and gearbox service typical. Confined space entry procedures required for basin access. Operators need basic mechanical skills for minor adjustments; major repairs require millwright support.

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Troubleshooting: Short-circuiting from uneven flow distribution causes poor settling - indicated by high effluent TSS on one side. Chain breaks create immediate sludge removal failure with rising blanket levels. Excessive scum indicates upstream process issues or inadequate surface removal. Drive mechanism failures show gradual speed reduction. Equipment service life: chains 8-12 years, drives

• Chain-and-Flight Collection System: Continuous chain drive with flights spanning basin width, moving settled sludge to hoppers. Carbon steel chains with polymer flights typical; stainless steel for corrosive conditions. Sizing: 6-12 inch flight height, 10-15 fpm speed. Selection based on solids loading and basin length (typically 50-200 feet).
• Inlet/Outlet Structures: Perforated inlet baffles distribute flow evenly across width; adjustable weirs control effluent level. Fiberglass or stainless steel construction. V-notch weirs provide ±2-inch level control. Sizing: 10-15 gpm/linear foot weir loading.
• Scum Removal: Surface skimmers or rotating drums collect floating materials. Typically covers 20-40% of surface area. Polymer or stainless steel construction. Beach or hopper collection with 1-2% slope requirements.
• Sludge Hoppers: Reinforced concrete collection points, typically 8-12 feet deep with 60-degree slopes. Located at inlet end for primary, distributed for secondary applications. Sized for 2-4 hour storage capacity.

• Overflow Rate (Surface Loading Rate): Primary clarifiers: 600-1,200 gpd/sf (typical: 800 gpd/sf), Secondary clarifiers: 400-800 gpd/sf (typical: 600 gpd/sf)
• Solids Loading Rate (Secondary only): Normal operation: 15-25 lbs/day/sf, Peak operation: 30-40 lbs/day/sf maximum
• Detention Time: Primary clarifiers: 1.5-2.5 hours (typical: 2.0 hours), Secondary clarifiers: 2.0-4.0 hours (typical: 3.0 hours)
• Weir Loading Rate: Effluent weirs: 10,000-20,000 gpd/linear foot, Maximum recommended: 15,000 gpd/lf for consistent performance
• Dimensional Parameters: Length-to-width ratio: 3:1 to 5:1 (optimal: 4:1), Side water depth: 10-16 feet (typical: 12-14 feet), Bottom slope: 1% toward sludge collection mechanism
• Sludge Collection: Chain and flight speed: 2-4 feet per minute, Traveling bridge speed: 4-8 feet per minute, Return sludge pumping rates: 25-100% of influent flow

• What peak hourly flow must the clarifier handle while maintaining acceptable overflow rates? Threshold: Secondary clarifiers exceeding 800 gpd/sf show significant solids carryover. Need 24-hour flow data and peaking factors (typically 1.8-2.5 for small plants, 1.4-1.8 for large plants). Wrong sizing leads to permit violations and downstream process upset.
• Will solids loading exceed 25 lbs/day/sf during normal operation? Critical for secondary clarifiers with activated sludge. Requires MLSS concentration data, SVI testing, and return sludge rates. Exceeding threshold causes sludge blanket rise and poor settling. Need biological process design parameters and sludge age calculations.
• Should you specify chain-and-flight or traveling bridge sludge collection? Chain-and-flight suitable for <20 MGD, lower maintenance but limited to rectangular geometry. Traveling bridge for larger plants, higher capital cost but better flexibility. Decision depends on redundancy requirements, maintenance capabilities, and basin dimensions.
• What effluent weir configuration optimizes hydraulic performance within available footprint? Peripheral weirs provide better distribution but require more space. V-notch weirs improve low-flow performance but increase cost. Need hydraulic modeling for complex geometries and consideration of future expansion requirements.

• Division 40 - Process Integration: Section 40 32 00 - Clarification Equipment (Primary), Section 40 44 26 - Secondary Clarifiers
• Secondary section 33 44 00 (Water Treatment Equipment) may apply for potable water clarifiers, but municipal wa

• Material/Equipment Verification: Concrete mix designs meeting ACI 350 requirements, Stainless steel grade verification (316SS minimum for submerged components), Drive unit torque calculations and safety factors
• Installation Requirements: Crane access for collector mechanism installation, Concrete curing time (28 days minimum before equipment setting), Electrical coordination for VFD integration
• Field Challenges: Levelness critical for proper sludge removal (±1/8" tolerance), Underground utilities conflicts during excavation
• Coordination Issues: 16-20 week lead times for custom collector mechanisms, Sequencing with concrete contractor for anchor bolt placement

• WesTech Engineering - FlexFloat™ and FlexSweep™ systems for rectangular clarifiers up to 300 ft length
• Evoqua Water Technologies - Envirex® rectangular clarifiers with chain-and-flight collection systems
• Lakeside Equipment - Raptor® rectangular clarifiers with traveling bridge collectors
• Ovivo - Density current baffles and longitudinal collectors for municipal applications ranging 1-50 MGD

• Circular Clarifiers - 15-20% lower capital cost, better for <5 MGD plants with space constraints
• Lamella Clarifiers - 50% smaller footprint, 30% higher cost, ideal for plant expansions
• Dissolved Air Flotation (DAF) - Superior for low-density solids, 40% higher O&M costs but faster startup

Specify redundant drive systems on collectors over 200 ft - single motor failures create major operational issues. Negotiate maintenance contracts during procurement; manufacturer service rates increase 15-20% post-warranty. Consider oversizing electrical components by 25% for future process modifications. Request factory acceptance testing for complex drive systems to avoid costly field troubleshooting and startup delays.