Chem-Tainer TC5971DC 500 gal 59" x 71" Double Wall Dual Containment Tank - This molded double wall dual containment tank is designed for stringent environmental concerns and codes. Outer containment of the double wall tank complies with federal regulation 40CFR-264.193 capacity requirements. Tanks come standard with a vented manway. Non-vented and hinged manways are also available. Non-vent manways are recommended for applications where chemical fumes should be vented away from the tank area.