Modular Containment:
An Aboveground Storage Option
by Reed Margulis

(This article was originally presented in Pollution Engineering - April 01, 1995)

Modular tanks can be used for short-term storage of liquids, hazardous wastes and sludges.

In a growing number of applications, aboveground modular storage systems are used as alternatives to conventional round storage tanks. Modular systems reportedly offer lower per-gallon storage costs for permanent or emergency storage of materials.

By design, modular containment systems can be configured for irregular sites, quickly bolted together for use, and later dismantled for relocation or storage. Systems are equipped with chemical-resistant flexible membrane liners for handling and treating liquids, hazardous wastes and sludges.

Redefining the tank

Modularity is a common feature of many of today's typical round tanks, which are assembled from prefabricated curved panels sequentially bolted or welded together.

But modularity becomes a total design concept with the advent of low-profile rectilinear systems which can be customized into virtually any right-angle shape and accommodate millions of gallons.

Packaged, prefabricated modular systems have a designated number of steel panels and framing members of specified sizes and types. They are accompanied by an appropriate flexible membrane liner of proper fit, and standardized galvanized components.

Modular containment ranging from 300,000 to 500,000 gallons is typically less costly than fabricating and installing liners in ponds or berms of equivalent capacity. Modular rectangular tanks may be economical, short-term answers for liquid and solid transfer, and storage during pond repairs or closures. Similarly, aboveground modular systems could become practical alternatives to excavating ponds in rocky or frozen soils, or in areas with high water tables.

Rectangular vs. round

The round tank's practical storage limit is affected by hoop stress. The larger its diameter and height, the greater the rupture strength required of the round tank's structure to safely contain its contents.

Modular rectangular tanks are modest in height, ranging up to about six feet. Since wall pressure relates to the height of the liquid contained, the structural members of low-profile tanks can be fabricated of lighter-gauge, less-costly steel. Fewer bolts are required, and special, massive foundations are not needed.

However, modular systems have their limitations. Lower in height, rectangular tanks obviously have larger footprints than do higher round tanks of comparable volumes. If space is at a premium, a round tank may be the only viable choice.

Furthermore, depending on the storage application, a round tank might be more suitable for extended long-term permanent containment. Accordingly, the cost-efficiency of the modular rectangular tank could recommend it for shorter term high-volume service involving millions of gallons of liquids or sludges.

Flexible configurations

Modular design could help answer a common site planning question: How can tankage most efficiently utilize an irregularly shaped area? Depending on space restrictions, a conventional solution might be to nest a number of small, round tanks within the area, possibly interconnecting them with pumps and piping. Complexity often poses costly engineering, operational and maintenance challenges.

On the other hand, a single rectangular modular system could be configured for bolt-together assembly conforming to the contours of an odd-sized site. The tank's right-angled sections or "legs" could be extended in any direction congruent with the area's boundaries or intruding structures. For example, patterned as the letter L, a tank could be cornered around obstructions. In the same manner, the standard building blocks which compose the modular tank could be organized into T, X or U shapes, or into asymmetrical configurations corresponding with available space. A single membrane liner would serve the entire facility.

Liners

Chemical-resistant flexible membrane liners for these tanks are available in many application- specific materials and in various thicknesses. Industrial-quality liners are designated by their physical and chemical resistance properties. The appropriate liner is usually prefabricated at the shop to the tank's dimensions.

A critical variable is the height of the tank - an unreinforced liner supplied as a flat sheet would normally serve a low-height rectangular system of simple configuration. However, reinforced liners are recommended for modular round tanks above 15 feet.

Modular tanks accept a variety of leak detection systems. Floating covers and baffles, as well as piping and plumbing arrangements, also can be used.

Installation and relocation

Tank installation may be completed on leveled, compacted ground, or on concrete or asphalt surfaces. Engineered with a modest degree of flexibility, they will adjust to minor site variations and imperfections. Little supervision is usually needed over assembly crews employing ordinary hand tools.

Modular systems knock down for standby emergency containment or relocation from site to site. In addition, they can be reconfigured in the field by adding or deleting modular panels and installing a suitably sized replacement liner.

Reed Margulis is president of ModuTank Inc., Long Island City, N.Y., 718-392-1112.