Precast Concrete Tanks

Precast concrete tanks serve as primary containment structures for water storage, treatment processes, and solids handling in municipal facilities from 0.5 to 50 MGD. These factory-manufactured concrete vessels are cast off-site under controlled conditions, then transported and assembled at the treatment plant using crane installation. Typical residential detention times range from 2-24 hours depending on application, with standard modules handling 50,000 to 2 million gallons per individual tank section. The key trade-off involves transportation limitations that restrict individual section dimensions to approximately 12 feet wide by 40 feet long, requiring multi-section assembly for larger capacities and creating potential leak points at joints.

• Equalization Basins (2-20 MG capacity): Precast tanks provide flow equalization ahead of treatment processes, smoothing diurnal flow variations. Connected upstream from headworks screening and downstream to primary treatment. Selected for rapid installation, watertight construction, and ability to handle variable hydraulic loading without structural concerns.
• Contact/Disinfection Tanks (0.1-2 MG capacity): Final chlorine contact chambers before distribution, providing required CT values per EPA Surface Water Treatment Rule. Positioned after filtration with inlet baffling and outlet to clearwell. Chosen for reliable contact time calculations, minimal leakage risk, and compliance with disinfection requirements.
• Chemical Storage (500-5,000 gallons): Contained storage for liquid treatment chemicals like sodium hypochlorite, ferric chloride, or polymer solutions. Connected to chemical feed systems with secondary containment. Selected for chemical compatibility, structural integrity, and regulatory compliance with spill containment requirements.
• Sludge Storage/Thickening (0.5-3 MG capacity): Temporary storage of waste activated sludge or primary sludge before dewatering. Connected between clarifiers and dewatering equipment. Preferred for odor control capabilities, structural durability under varying liquid levels, and ability to integrate mixing equipment.

Daily Operations: Operators monitor liquid levels using SCADA or local gauges, checking for proper inlet/outlet flow distribution and any signs of structural movement or joint leakage. Routine inspections include verifying access covers are secure and observing surface conditions for unusual odors or foam formation. Chemical feed systems require daily verification of proper dosing rates and storage levels.

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Maintenance: Monthly joint inspections for signs of infiltration or exfiltration, particularly during seasonal temperature changes. Annual confined space entry for interior coating inspection and cleaning requires proper ventilation, gas monitoring, and rescue equipment. Maintenance staff need confined space training, fall protection certification, and chemical safety awareness. Joint resealing typically required every 10-15 years depending on ground movement.

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Troubleshooting: Common failure modes include joint seal deterioration (visible as wet spots or vegetation growth), structural cracking from settlement.

• Wall Panels (6-12 inch thickness): Reinforced concrete sections forming tank perimeter, typically 8-20 feet wide by 12-30 feet tall. Steel reinforcement designed for hydrostatic pressure and soil loads. Selection based on tank diameter, liquid depth, and seismic requirements per ACI 350.
• Base Slab (8-18 inch thickness): Reinforced concrete foundation providing structural support and leak prevention. Includes keyways for wall panel connection and penetrations for piping. Sizing determined by soil bearing capacity and uplift resistance requirements.
• Joint Sealant Systems: Waterstop materials (PVC, rubber, or hydrophilic) preventing infiltration/exfiltration at panel joints. Critical for maintaining structural integrity and preventing groundwater contamination. Selection based on expected joint movement and chemical compatibility.
• Access Hatches/Covers: Aluminum or fiberglass covers providing personnel access and equipment installation points. Standard 24-36 inch diameter openings with locking mechanisms. Required for confined space entry compliance and routine maintenance access.
• Inlet/Outlet Connections: Penetration sleeves and pipe connections integrated during manufacturing. Sized for design flow rates with proper hydraulic transitions. Include flexible connections to accommodate thermal expansion and settling.

• Hydraulic Loading: Flow rates: 0.5-50 MGD (347-34,700 gpm); Hydraulic retention time: 2-8 hours for contact basins, 0.5-2 hours for pump stations; Surface overflow rates: 300-800 gpd/sf for clarifiers; Weir loading rates: 10,000-20,000 gpd/linear foot
• Structural Parameters: Tank depths: 8-20 feet typical, 25 feet maximum for transportation limits; Wall thickness: 6-12 inches depending on depth and soil conditions; Live loads: 40-100 psf for walkways and equipment platforms; Dead loads: Include 150 pcf concrete plus equipment weights
• Dimensional Constraints: Maximum shipping dimensions: 12' wide × 14' high × 40' long; Joint spacing: 20-30 feet maximum between expansion joints; Freeboard: Minimum 12 inches, 18 inches preferred
• Performance Standards: Concrete strength: f'c = 4,000-5,000 psi minimum; Water tightness: Maximum 0.05 gallons/day/100 sf at design head; Joint deflection: ±0.5 inches maximum; Service life: 50+ years with proper design and installation

• Single large tank vs. multiple smaller precast sections? Threshold: Tanks >40 feet in any dimension typically require sectional construction. Single-piece tanks limited to ~15,000 gallon capacity. Wrong decision leads to transportation issues, excessive crane requirements, or costly cast-in-place alternatives. Need: site access analysis, crane capacity assessment, and construction schedule constraints.
• Buried vs. above-ground installation? Threshold: Burial depth >8 feet requires enhanced structural design and waterproofing systems. Above-ground tanks need freeze protection in climates with temperatures <20°F. Wrong decision causes structural failure or operational issues. Need: geotechnical report, groundwater levels, frost line depth, and aesthetic requirements.
• Standard vs. custom internal configurations? Threshold: Custom internals add 15-30% to costs and 4-6 weeks to delivery. Standard baffle arrangements suit 80% of municipal applications. Wrong decision leads to budget overruns or suboptimal hydraulic performance. Need: process requirements, mixing intensity needs, and maintenance access requirements.
• Joint sealing system selection? Threshold: Chemical-resistant seals required for chlorine contact >2 mg/L or pH <6.5. Standard seals adequate for most applications. Wrong decision causes premature seal failure and infiltration/exfiltration issues. Need: water quality parameters and chemical feed locations.

• Primary: 03 41 00 - Precast Structural Concrete - Covers precast concrete tank manufacturing, transportation, and installation requirements.
• Secondary: 33 11 00 - Water Utility Distribution Piping - For associated piping connections and appurtenances integrated with precast tank systems.

• Material/Equipment Verification: ACI 350 compliance documentation; Concrete mix design approval (4,000-5,000 psi typical); Joint sealant and waterstop specifications
• Installation Requirements: Crane access for 20-40 ton panel placement; Level foundation pad ±1/4" tolerance; Temporary shoring during assembly
• Field Challenges: Weather delays during joint sealing; Dimensional coordination with mechanical equipment; Access limitations in tight sites
• Coordination Issues: 8-12 week lead times for custom panels; Early structural drawings needed for process equipment anchoring

• Oldcastle Infrastructure - Modular rectangular clarifiers and contact basins, widely used in 1-20 MGD plants across the Southeast and Midwest.
• Norwalk Concrete Industries - Standard circular clarifiers and rectangular basins, strong presence in California and Pacific Northwest municipal projects.
• Precast Concrete Structures - Custom rectangular tanks and pump stations, notable installations in Texas and Southwest regions.
• Shea Concrete - Modular treatment basins and storage tanks, established supplier for Northeast municipal markets.

• Cast-in-place concrete - Preferred for custom geometries or restricted crane access; 15-20% lower material cost but 30-40% longer construction schedule.
• Steel tanks with liner systems - Faster installation for large storage applications; similar installed cost but higher long-term maintenance.
• HDPE or fiberglass tanks - Limited to smaller applications (<1 MG); 40-50% lower cost but requires more frequent replacement and limited structural loading capacity.

Establish manufacturer relationships early - Oldcastle and Norwalk offer value engineering that can reduce project costs 10-15% through standardized panel sizes. Request factory pre-assembly mock-ups for complex geometries to avoid field fit-up issues. Consider modular designs over monolithic pours for sites with crane access - installation time drops from weeks to days, offsetting higher material costs through reduced construction management expenses.