­Engineers and developers are faced with many challenges during the process of finding an acceptable wastewater treatment solution for their development projects.

Besides the capital investment and other financial aspects of the project, they must also consider if the treatment solution will meet the present and future needs of the community and development. Will it provide a positive impact on the environment? Will it offer its stakeholders the most efficient use of their invested dollars?

More and more developers are finding steel circular treatment systems, commonly referred to as field-erected treatment plants (FETPs), as a viable solution. These treatment systems are in a circular layout and encompass most of the treatment equipment in concentric steel-walled tanks on a concrete slab. FETPs appeal to developers, as they are quick to install, easy to operate, flexible in treatment options, easily expandable, and more economical, yet just as effective in treatment as other advanced treatment options.

FETP Benefits
Residential developers realize tremendous value using FETP systems principally because of the quick timeframe in which the systems can be designed, permitted and installed. Compared to conventional construction, the short concept-to-completion schedules of FETP systems allow developers to start selling homes sooner, thus quickly establishing positive cash flow.

Clearing and excavating the site and pouring the concrete slab can be done while the supplier manufactures steel walls and internal components. The components are then shipped to the site and erected in a matter of weeks instead of months. In addition to helping meet the developer’s schedule, these plants’ factory-built nature removes much of the scheduling uncertainty and delays caused by inclement weather during construction.

Because most of the treatment equipment is contained in the circular tank, the footprint or area that the wastewater system needs to occupy is smaller than most other types of treatment. This allows the development to devote more land to profitable uses.

With the inherent flexibility of FETP steel-tank construction, expanding the wastewater treatment system is relatively easy as the needs of the community or development grow. For example, an additional circular steel plant can be added or modifications of the process within the existing tank walls can be cost-effectively made to increase treatment capacity.

The FETP system can be designed to incorporate virtually any wastewater treatment processes, depending on the effluent discharge requirements. From the basic treatment processes to the most advanced biological nutrient removal designs, FETP systems utilize conventional, proven technologies that are quickly and easily accepted by regulatory and permitting agencies.

Because the FETP design can incorporate the most sophisticated and advanced wastewater treatment process available, developers can make a significantly positive impact on the environment. Through a combination of advanced biological treatment and advanced filtration, the wastewater treatment system can produce valuable reusable water. Developers can use this water to irrigate lawns, golf courses or neighboring farmlands, which is especially beneficial in drought-stricken areas.

Wastewater that is not going to be reused should still meet advanced treatment requirements such as low levels of nitrogen and phosphorus, to maintain natural ecosystems when released back to rivers and streams.

FETP Features
How do FETPs differ from other types of construction? They are designed for optimum application flexibility and footprint utilization. Regardless of the process employed, they all share the same basic “tank in tank” configuration. Although the tanks are typically epoxy-coated carbon steel, other materials such as concrete or stainless steel are available. The hydrostatically reinforced steel inner wall contains the secondary clarifier.

Using a series of bulkhead walls, the space between the inner and outer walls is divided into different compartments to create the various process zones necessary for full and effective treatment. There are virtually limitless possibilities for how this volume can be utilized.

The technology employed in a FETP is time-tested, with the first units installed in the mid-1960s. Since then, more than 1,000 of these systems have been commissioned throughout the world, in such diverse locations as the Florida Keys, Upstate New York, Mexico, and Saudi Arabia. The municipal and industrial environments in which these plants are operating are just as varied as their locales, from being only a few yards away from the ocean to fully buried in the ground for winter thermal protection.

Unlike conventional, site-built construction where individual unit processes are usually isolated in separate tanks and interconnected with a network of piping, wiring and walkways, FETP systems contain the entire treatment process within a single structure. This design not only reduces the footprint required but also reduces the construction cost by simplifying the yard piping and conduit runs needed.

A FETP uses conventional process designs that incorporate conservative safety factors readily accepted by regulatory agencies. This results in a system that operates reliably and consistently over a wide range of conditions. Unlike other types of wastewater processes, these systems typically do not depend on sophisticated control strategies or a myriad of control valves, sensors and other sensitive equipment to operate effectively. In fact, most operators claim that once they get a few key process control parameters dialed in, field-erected plants virtually run themselves.

Using a FETP allows for a single point of responsibility during the entire project, where the manufacturer is responsible for supplying (and in most cases installing) all the components required for a fully functional treatment system, from raw influent to treated effluent, in a single package. This eliminates the finger-pointing and blame-shifting that is unfortunately all too common during the construction and commissioning phases of wastewater jobs.

Flexibility of use is another outstanding feature of the steel tanks. Not only are they built to last for decades, their functional utility can be easily and economically modified to meet future needs as the community grows. A FETP designed to treat a small community’s flow can be incorporated into that community’s capacity expansion plans by retrofitting the internals so the existing tankage can be used as a sludge digester or for other functional purposes.

For example, a city in the Florida panhandle installed a pair of field-erected plants in the 1980s. About 10 years later, the community needed to expand its capacity.

During the construction of two new FETPs, the existing plants were converted to aerobic digesters by removing the bulkhead walls in the outer ring, removing the clarifier mechanisms in the center tank and adding coarse-bubble diffusers. The ability to cost-effectively retrofit this existing tankage instead of building new saved the city hundreds of thousands of dollars.

If additional process capacity is not required at a particular location, existing plants can still extend their value and prolong their useful life. In fact, FETPs can be easily cut down, moved and re-erected at other sites where additional treatment is needed, resulting in a plant with the same warrantee and life expectancy of a new plant, but at a fraction of the capital cost.

In contrast to concrete tanks where demolition is essentially the only option when it comes to expansions or upgrades, the steel tanks used in FETP systems maintain a residual value and useful life long after they have fulfilled their initially intended purpose.

Conclusion
In summary, field-erected treatment systems offer robust and consistent ­treatment to meet virtually any effluent limit. They utilize conventional, conservative biological process designs that are reliable and simple to operate. Superior quality, factory-assembled components allow for quick and economical installations that provide decades of useful ­service. SLDT