Polyethylene Tanks

Polyethylene tanks provide chemical storage and process containment in municipal water and wastewater treatment facilities, utilizing rotationally-molded or fabricated high-density polyethylene (HDPE) construction for corrosion resistance. These tanks store treatment chemicals like sodium hypochlorite, ferric chloride, and polymer solutions, with standard capacities ranging from 100 to 15,000 gallons for typical municipal applications. HDPE tanks offer 15-20 year service life with minimal maintenance compared to steel or concrete alternatives. However, they have temperature limitations (typically 140°F maximum) and require UV protection for outdoor installations to prevent degradation.

• Chemical Storage (Sodium Hypochlorite): PE tanks store 12.5% bleach solution for disinfection systems in 500-15,000 gallon capacities. Selected for chlorine resistance and UV stability. Connected upstream to bulk delivery trucks via cam-lock fittings, downstream to chemical feed pumps through CPVC piping with secondary containment.

• Polymer Feed Systems: 300-2,000 gallon tanks store diluted polymer solutions (0.25-0.5%) for dewatering operations. PE prevents polymer adhesion and allows easy cleaning. Fed from dry polymer makeup systems, discharged to belt presses or centrifuges via progressive cavity pumps.

• Lime Slurry Storage: 1,000-5,000 gallon tanks hold lime slurries (5-15% solids) for pH adjustment in water treatment. PE resists alkaline corrosion better than steel. Equipped with mixers to prevent settling, connected to lime feeders and process streams requiring pH control.

• Emergency Water Storage: 5,000-50,000 gallon tanks provide backup potable water during main breaks or contamination events. Selected for NSF-61 certification and long-term water quality preservation. Connected to distribution systems via bypass pumping arrangements.

Daily Operations: Operators check tank levels via SCADA systems and visual gauges, monitoring for proper chemical feed rates and storage volumes. Chemical deliveries require verification of proper connections and containment integrity. Level transmitters provide continuous monitoring with high/low alarms integrated into plant control systems.

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Maintenance: Monthly inspections include checking fittings for leaks, cleaning level sensors, and verifying vent operation. Annual tasks include containment area cleaning and fitting torque verification. Safety requires chemical-specific PPE and confined space procedures for internal inspections. Basic mechanical skills sufficient for routine maintenance, with vendor support for major repairs.

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Troubleshooting: Common failures include fitting leaks (5-10 year replacement cycle), level sensor fouling, and UV degradation of exposed surfaces. Warning signs include discolored tank walls, chemical odors, or erratic level readings. Properly maintained PE tanks typically serve 15-25 years, with chemical exposure and UV radiation being primary life-limiting factors.

• Tank Shell: Rotationally molded or fabricated polyethylene walls, typically 0.25-0.75 inches thick depending on capacity. HDPE or XLPE materials selected based on chemical compatibility and temperature requirements. Sizes range 100-50,000 gallons for municipal applications.

• Fittings and Connections: Molded-in or welded flanged outlets, typically 2-8 inch diameter. HDPE or polypropylene materials with EPDM gaskets. Positioned based on hydraulic requirements and maintenance access.

• Venting Systems: 2-4 inch atmospheric vents with flame arrestors for volatile chemicals. Emergency vents sized per tank capacity and stored chemical properties. Stainless steel or PVC construction depending on vapors.

• Level Instrumentation: Ultrasonic or pressure transmitters for continuous monitoring. Float switches for high/low alarms. Sight gauges for visual confirmation. Selection based on chemical compatibility and accuracy requirements.

• Secondary Containment: Concrete or HDPE-lined containment areas sized for 110% of tank volume per EPA requirements.

• Tank Capacity: 50-50,000 gallons typical for municipal applications. Small plants (0.5-5 MGD) commonly use 500-5,000 gallon units for chemical storage; larger facilities (10-50 MGD) require 10,000-50,000 gallon tanks for bulk chemical handling.

• Specific Gravity: 1.0-1.9 maximum for polyethylene construction. Critical for structural integrity and wall thickness calculations. Standard tanks handle water-based solutions; specialized resins required for SG >1.5.

• Operating Temperature: -40°F to 140°F continuous operation. Higher temperatures require UV-stabilized resins and may necessitate insulation or cooling systems.

• Chemical Compatibility: Must verify compatibility with stored chemicals. Common municipal chemicals (sodium hypochlorite, ferric chloride, polymer solutions) are compatible with XLPE or HDPE construction.

• Wall Thickness: 0.25-2.0 inches depending on capacity and specific gravity. Calculated per ASTM D1998 standards for rotational molding applications.

• Hydrostatic Test Pressure: 1.5x design pressure minimum. Typical municipal applications operate at atmospheric pressure with 2-5 PSI pump discharge pressure.

• UV Resistance: 2% carbon black minimum for outdoor installations. Indoor applications may use natural polyethylene with cost savings.

• What tank capacity and configuration meets detention time requirements? Chemical feed systems typically require 3-7 days storage capacity. Undersized tanks increase delivery frequency and operational costs; oversized units waste space and capital. Need daily usage rates, delivery schedules, and site constraints.

• Does the application require atmospheric or pressurized operation? Atmospheric tanks cost 30-40% less but require separate transfer pumps. Pressurized vessels (15-50 PSI rating) enable direct feed but require ASME compliance above 15 PSI. Consider pump energy costs versus vessel premium.

• What chemical compatibility and temperature rating is required? Standard HDPE handles most municipal chemicals below 120°F. Specialty applications (hot polymer solutions, concentrated acids) require XLPE or fluoropolymer linings, increasing costs 2-3x. Verify with chemical supplier data sheets.

• Should the tank be vertical or horizontal configuration? Vertical tanks minimize footprint but require higher foundations and may need mixers for chemical uniformity. Horizontal tanks ease maintenance access and mixing but consume more floor space. Consider building height restrictions and mixing requirements.

• Division 40 - Process Integration

• Section 40 05 00 - Common Work Results for Process Integration (primary)

• Section 46 07 13 - Liquid Chemical Storage and Feed Equipment (chemical feed applications)

• Note: Some specifications place under Division 33 - Utilities for water treatment plant applications.

• Material/Equipment Verification: Verify FDA/NSF certification for potable water applications; Confirm UV stabilization ratings for outdoor installations; Check wall thickness specifications against design requirements

• Installation Requirements: Level, compacted gravel pad with proper drainage; Adequate clearance for maintenance access (minimum 3 feet); Proper venting and overflow connections

• Field Challenges: Limited crane access for large tank placement; Foundation settlement in poor soil conditions; Temperature expansion joint requirements

• Coordination Issues: Electrical rough-in timing for level sensors; Piping connections must align with tank fittings; Lead times: 2-6 weeks for standard sizes, 8-12 weeks for custom fabrication

• Snyder Industries: GO Series tanks (500-20,000 gallons) - dominant in small municipal applications

• Norwesco: Vertical storage tanks (35-15,000 gallons) - popular for chemical storage

• Poly Processing: Custom fabricated tanks (1,000-40,000 gallons) - specialized municipal systems

• Tank Depot: Standard vertical tanks (100-12,000 gallons) - cost-effective municipal solutions

• Fiberglass tanks: Better chemical resistance, higher cost (20-30% premium), longer lifespan for aggressive chemicals

• Steel tanks with coatings: Higher capacity options (40,000+ gallons), requires more maintenance, 15-25% cost increase

• Concrete tanks: Permanent installation, significantly higher cost (3-5x), better for large volume storage (100,000+ gallons)

Always specify translucent tanks for chemical storage to enable visual level monitoring. Work directly with manufacturer technical support for custom fittings - they often provide engineering services at no charge for municipal projects. Consider bulk purchasing multiple tanks for standardization and cost savings. Establish preferred vendor relationships for faster delivery and better pricing on replacement parts.