The Norwesco 3525-gallon multi-use underground tank is a single-piece polyethylene septic and liquid-storage vessel, built for buried service where a concrete tank would mean a crane, a pour schedule, and a much heavier install. At 211 inches long, 102 inches wide, and 51 inches tall, it holds 3,525 gallons and carries a 1.9 specific-gravity rating — structural headroom, not a chemical-compatibility claim, and the spec most buyers skip past on an underground tank this size.
Will it work for your application
This is an underground septic tank, molded in white polyethylene, and the 1.9 SG rating describes the load the shell is engineered to carry once it's buried and full. In real numbers: filled with water (SG 1.0), the contents weigh about 29,398 lb; at the full 1.9 rating, the tank is built to carry up to 55,857 lb of contents. That margin against hydrostatic and burial load is why this size shows up in septic and underground storage duty — and why a 3,525-gallon buried poly tank is also a legitimate candidate for rural and remote fire-water reservoir storage; our guide on fire-water reservoir tank design and NFPA 22 sizing walks through that use case. The footprint is 102.0 x 211.0 inches, so plan the excavation, bedding, and backfill envelope around that before you order. No outlet, riser, or bulkhead hardware is spec'd on this listing — because fitting, gasket, and seal selection is the compatibility-critical interface for anything other than plain water, confirm your fitting and riser plan against the technical drawing on this page rather than the listing photo.
Anti-flotation: the empty-tank failure mode
The single most important install fact on a buried 3,525-gallon poly tank is that an empty void wants to float in saturated ground. Treating the 1.9 SG rating as a license to skip burial engineering is the mistake to avoid: the rating describes liquid density the shell is built for, but it does not replace proper bedding, anti-flotation provisions for high water tables, and compacted backfill. A dewatered or improperly backfilled underground polyethylene tank behaves very differently than a buried concrete one. Size the system around real demand and real soil conditions — our Tank Sizing Calculator and Tank Sizing Guide help you match capacity to draw before you commit to an excavation this size. If this is a septic application crossing into Canada, read our guide on Norwesco IAPMO and CSA B66 septic tank approvals first for the code nuance.
Setting the tank in the ground
A buried tank this large rides on its bedding, and getting the base wrong is the most common field failure on underground polyethylene. For the engineering framework behind a stable base, our write-up on concrete pad sizing for polyethylene tanks covers the ACI references that inform support design, and the same load discipline applies below grade.
Norwesco molds this tank at three geographically spread plants — Fairfield, TX, Hanford, CA, and St. Bonifacius, MN — which means shorter LTL lanes to most of the contiguous US on a 211-inch tank where freight dominates total cost. It ships LTL freight, quoted live to your ZIP from the nearest of those plants, so run your delivery point through the Freight Cost Estimator for an accurate number. Questions on burial depth, riser configuration, or which plant serves your region fastest? Call us at 866-418-1777 and we'll work it out with you before you order.
