Bolted Steel Tanks

Bolted steel tanks provide modular, field-assembled storage for potable water, process water, and biosolids in municipal treatment facilities. These tanks use factory-manufactured steel panels connected with high-strength bolts and gaskets, allowing rapid on-site construction without welding. Typical capacities range from 100,000 to 10 million gallons with assembly times 40-60% faster than welded alternatives. The primary trade-off is higher initial cost per gallon compared to concrete, though this is offset by reduced construction time and easier future expansion or relocation.

• Finished Water Storage: Bolted steel tanks serve as clearwells and distribution storage, typically 0.5-5.0 MG capacity for 2-25 MGD plants. Connected downstream from filtration/disinfection, upstream to high-service pumps

• Chemical Storage: Sodium hypochlorite, ferric chloride, and polymer solutions stored in 5,000-50,000 gallon epoxy-lined tanks. Connected to chemical feed systems via transfer pumps

• Biosolids Thickening: Gravity thickener tanks, 30-100 feet diameter for 5-50 MGD plants. Receives waste activated sludge, discharges thickened solids to digesters

• Equalization Basins: Flow equalization for 1-10 MGD plants using 0.25-2.0 MG tanks. Receives peak flows upstream of treatment processes

Daily Operations: Operators monitor tank levels via SCADA or local gauges, checking for proper fill/drain cycles and overflow conditions. Visual inspections include checking for panel alignment, gasket integrity, and foundation settlement. Water quality sampling from multiple tank levels ensures proper mixing and detention time compliance.

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Maintenance: Annual bolt torque verification and gasket inspection required, typically 8-16 hour task requiring confined space entry permits and gas monitoring. Epoxy lining touch-ups every 3-5 years depending on stored chemicals. External coating maintenance every 10-15 years. Tasks require millwright skills and fall protection equipment for roof access.

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Troubleshooting: Panel joint leakage indicates gasket failure or improper bolt tension, typically appearing within first two years. Foundation settlement causes panel misalignment and potential structural failure. Coating deterioration leads to corrosion, particularly at waterline interfaces. Expected service life 25-40 years with proper maintenance, compared to 50+ years for concrete alternatives.

• Shell Panels: Galvanized or epoxy-coated steel sheets, typically 4'×8' or 5'×10', bolted with EPDM gaskets. Panel thickness ranges 1/4" to 1/2" based on liquid height and specific gravity

• Foundation Ring: Concrete ring beam or steel angle foundation, sized for soil bearing capacity and tank diameter. Municipal installations typically require 3,000-4,000 PSI concrete with proper drainage

• Roof Structure: Self-supporting aluminum domes or steel truss systems for tanks over 30 feet diameter. Includes access hatches, vents, and equipment penetrations

• Appurtenances: Manholes, level instrumentation, overflow piping, and drain valves. Stainless steel or coated carbon steel based on stored liquid. Sizing follows AWWA D103 standards for municipal applications

• Storage Volume: 50,000 to 5,000,000 gallons (typical municipal range 100,000-2,000,000 gallons)

• Tank Diameter: 20 to 150 feet (most common 40-100 feet for municipal applications)

• Sidewall Height: 8 to 48 feet (typical 16-32 feet for water storage, up to 40 feet for biosolids)

• Design Pressure: Atmospheric to 15 psig (most municipal applications atmospheric with 2-5 psig venting)

• Foundation Loading: 62.4 psf per foot of water depth plus dead load (typically 80-120 psf total)

• Seismic Design: Per ASCE 7, typically Zone 2-4 requirements for municipal facilities

• Wind Loading: 90-150 mph basic wind speed per local code requirements

• Liquid Specific Gravity: 1.0 for potable water, up to 1.2 for lime slurry, 1.05 for activated sludge

• Temperature Range: -40°F to 140°F (ambient to heated digesters)

• Coating Systems: NSF-61 approved epoxy for potable water, glass-fused-to-steel for aggressive applications

• Access Requirements: 24-inch minimum manways, OSHA-compliant platforms and ladders per height

• What liquid specific gravity and maximum operating temperature will the tank handle? Standard glass-lined steel handles SG up to 1.2 and 200°F, while epoxy-coated limits to SG 1.05 and 140°F. Wrong choice leads to coating failure and costly recoating within 5-10 years

• Does the application require NSF-61 certification for potable water contact? Potable water storage mandates NSF-61 approved coatings and gaskets. Non-compliant tanks require complete recoating, costing $2-4 per square foot of surface area

• What seismic zone and wind load govern the foundation design? Seismic Zone 3+ or wind speeds >120 mph require engineered foundations and anchor bolt systems. Undersized foundations cause tank settlement and shell distortion, requiring costly releveling

• Will the tank operate under positive pressure or vacuum conditions? Pressures >5 psig or vacuum >2 inches WC require structural upgrades to roof and shell. Standard atmospheric tanks fail catastrophically under these conditions, requiring complete replacement

• Division 46 - Water and Wastewater Equipment

• Section 46 73 00 - Packaged Water and Wastewater Treatment Equipment

• Alternative: Division 33 - Utilities, Section 33 56 00 - Potable Water Storage Tanks (for finished water storage applications only)

• Material/Equipment Verification: Steel certifications (ASTM A36/A572), Coating specifications and thickness verification, Bolt torque requirements and documentation

• Installation Requirements: Level concrete pad within 1/4" tolerance, Crane access for assembly (60-80 ft radius typical), 12-16 week lead times for custom municipal sizes

• Field Challenges: Weather delays during coating application, Soil bearing capacity verification, Utility conflicts with large diameter foundations

• Coordination Issues: Early structural/civil coordination for foundation design, Electrical rough-in timing for instrumentation

• CST Industries - Aquastore glass-fused-to-steel tanks, dominant in municipal potable storage

• Caldwell Tanks - Pedesphere and field-erected models, strong wastewater presence

• ModuTank - Modular bolted systems, popular for temporary/emergency applications

• Tank Connection - LIQ and DRY series, growing municipal market share with competitive pricing

• Cast-in-Place Concrete: 20-30% higher initial cost but 50+ year life, preferred for large diameter (>100 ft) applications

• Prestressed Concrete: Premium option for seismic zones, 40% cost premium over bolted steel

• Welded Steel: Field-erected for >2 MG capacity, similar cost to bolted but longer construction time

Establish manufacturer relationships early - CST and Caldwell offer technical support throughout design phase. Request factory training for operations staff during construction. Specify redundant level instrumentation from day one rather than retrofitting. Consider shared foundations when installing multiple tanks to reduce costs 15-20%. Always verify local crane availability and costs before finalizing tank diameter selections.