Metallic Specialty Tanks

Metallic specialty tanks serve critical storage and process functions in municipal water and wastewater treatment, including chemical feed tanks, contact basins, and specialized process vessels. These engineered steel or aluminum structures provide controlled environments for chemical mixing, detention time requirements, and intermediate storage between treatment processes. Typical detention times range from 15 minutes to 4 hours depending on application, with volumes from 1,000 to 500,000 gallons for municipal facilities. The primary trade-off involves balancing corrosion resistance through protective coatings and cathodic protection systems against initial capital costs, as metallic tanks require ongoing maintenance to prevent structural degradation from chemical exposure.

• Chemical Storage Systems: Metallic specialty tanks store concentrated chemicals like ferric chloride, alum, and sodium hypochlorite at 1-50 MGD plants. They connect upstream from chemical feed pumps and downstream to process points like rapid mix or disinfection contact chambers. Selected for corrosion resistance and structural integrity under varying chemical loads.
• Solids Handling: Thickened sludge storage tanks (10,000-100,000 gallons) hold concentrated biosolids at 5-50 MGD facilities between thickeners and dewatering equipment. These tanks maintain 4-8% solids content with slow-speed mixers, selected for structural strength and resistance to abrasive solids.
• Process Water Storage: Clear well storage tanks (50,000-2M gallons) provide treated water storage before distribution at 2-50 MGD plants. Connected downstream from filtration and upstream from high service pumps, selected for potable water compatibility and seismic resistance requirements.

Daily Operations: Operators monitor tank levels via SCADA or local gauges, checking for proper chemical feed rates and mixing operation. Daily visual inspections verify no leaks, proper ventilation, and mixer operation. Level alarms require immediate response to prevent overflow or pump cavitation.

​

Maintenance: Monthly mixer inspections, quarterly tank level calibration, and annual internal inspections during shutdowns. Confined space entry requires gas monitoring, ventilation, and three-person crews. Welding repairs need certified personnel and hot work permits. Coating touch-ups every 5-7 years extend service life.

​

Troubleshooting: Level instrument fouling causes false readings - clean monthly in chemical service. Mixer bearing failure shows increased vibration and noise, typically after 3-5 years continuous operation. Tank coating failure appears as rust staining or chemical odors, requiring immediate assessment to prevent structural damage.

• Tank Shell: Carbon or stainless steel construction, 1/4" to 1/2" wall thickness depending on capacity (1,000-500,000 gallons). Material selection based on stored media - 316SS for aggressive chemicals, epoxy-lined carbon steel for potable water.
• Foundation System: Concrete ring wall or full slab foundation sized for soil bearing capacity and seismic loads. Ring walls typical for tanks >50,000 gallons with 2-4 foot width.
• Access Systems: Manholes (24" minimum), level indicators, and sampling ports. OSHA-compliant ladder systems for tanks >8 feet height with cage protection above 20 feet.
• Mixing Equipment: Variable speed drives (0.5-10 HP) with propeller or turbine impellers for chemical storage tanks, sized for 1-2 turnovers per hour based on tank geometry.

• Hydraulic Parameters: Operating pressure: 5-150 psi (atmospheric to high-pressure applications), Design pressure: 1.5x operating pressure minimum per AWWA D100, Flow velocity: 2-8 ft/s in connecting piping to minimize erosion, Detention time: 15 minutes to 8 hours depending on application (contact tanks, surge vessels)
• Structural Design: Live loads: 40-100 psf for walkways, 300 psf for equipment platforms, Wind load: Per ASCE 7 local requirements (typically 90-120 mph design), Seismic design: Site-specific per ASCE 7, typically 0.1-0.4g acceleration, Foundation bearing: 2,000-6,000 psf typical for municipal sites
• Material Specifications: Steel thickness: 3/16" to 1" depending on diameter and height, Corrosion allowance: 1/8" minimum for potable water, 1/4" for wastewater, Tank diameter: 8-120 feet typical municipal range, Height-to-diameter ratio: 0.5-3.0 for structural stability
• Coating Systems: Interior: NSF-61 certified epoxy for potable water (8-12 mils DFT), Exterior: Polyurethane or acrylic systems (4-6 mils DFT), Expected life: 15-25 years with proper maintenance

• What operating pressure range and safety factors are required? Municipal applications typically operate at 5-50 psi, but chemical feed systems may require 100-150 psi. Under-designing pressure rating leads to structural failure; over-designing increases costs by 20-40%. Need: system pressure analysis, pump curves, and elevation profiles.
• What corrosion protection strategy matches the stored media and expected service life? Potable water tanks require NSF-61 coatings ($3-5/sq ft), while chemical storage needs specialized linings ($8-15/sq ft). Wrong coating selection leads to premature failure within 5-10 years versus 20+ year design life. Need: water quality analysis, chemical compatibility data, and maintenance budget.
• What seismic and wind design criteria apply to the site location? Seismic zones 3-4 require special anchoring and flexible connections, adding 15-30% to foundation costs. Inadequate design risks catastrophic failure and regulatory violations. Need: geotechnical report, local building codes, and site exposure assessment.
• How will the tank integrate with existing piping and electrical systems? Nozzle locations, elevation matching, and access requirements affect installation costs by $10,000-50,000. Poor coordination requires expensive field modifications. Need: existing utility surveys, P&ID development, and constructability review.

• Primary: Division 46 - Water and Wastewater Equipment
• Section 46 71 13 - Potable Water Storage Tanks
• Section 46 73 13 - Waste

• Material/Equipment Verification: Mill test certificates for steel plates, Coating system specifications and application records, AWWA compliance documentation
• Installation Requirements: Crane access for large diameter tanks, Foundation curing time (14-28 days typical), Weather restrictions for coating application
• Field Challenges: Wind load calculations during erection, Coating inspection and touch-up coordination
• Coordination Issues: 16-24 week lead times for custom tanks, Utility connections timing with tank completion

• Caldwell Tanks - Pedesphere elevated tanks, strong in Midwest municipal market with 1-5 MG capacity range
• CB&I Storage Solutions - Aquastore glass-fused steel tanks, popular for ground storage 0.5-10 MG
• CST Industries - Specialty steel tanks including Permastore for potable water
• McDermott/CB&I - Large welded steel tanks >2 MG capacity for major municipal systems

• Prestressed Concrete Tanks - 15-20% higher capital cost but 50+ year life vs. 30-year steel. Preferred for large ground storage >5 MG.
• Welded Steel Tanks - 20-30% less than specialty tanks, suitable for non-potable applications.
• Composite Tanks - Emerging option for smaller capacities <1 MG, 25% cost premium but corrosion-resistant.
• Glass-fused steel offers middle ground at 10% premium over carbon steel.

Establish manufacturer relationships early - Caldwell and CB&I offer value engineering during design phase that can reduce costs 10-15%. Specify standard sizes when possible (1.0, 1.5, 2.0 MG increments) rather than exact calculated volumes. Consider factory-applied coatings over field-applied to reduce weather delays and improve quality. Standardize on one manufacturer per utility system for parts commonality and service efficiency.