The adage “less is more” is highly applicable to the municipal water industry. Despite the large volumes of water that are treated in municipal drinking water facilities, small tools and equipment can provide great benefits to cost control, efficient upgrading, ease of operations and community support. One valuable outcome of advancements in materials engineering, design approaches and other technology is the ability to achieve equal (or better) performance using less space and fewer materials. An example of this can be found in pressure vessel design.
Pressure Vessel Size Problems
Used for a variety of treatment applications, pressure vessels take up a lot of space. For drinking water treatment, they are typically used for water remediation and purification. In climates that reach sustained, below-freezing temperatures, they must be enclosed to prevent damage from the cold. Buildings erected around them are built only as large as they need to be to protect the pressure vessel. As a result, when the vessel needs to be upgraded, the size of the replacement vessel will be limited by the existing building that houses it—or a new structure will have to be built. This often severely limits options for plant managers and operators or threatens to strain municipal budgets.
Another scenario in which vessel size is highly relevant is where pressure vessels are not housed in buildings but are part of a decentralized treatment system, such as those that use wells in suburban areas. These are out in the open, often in and around neighborhoods, and can be highly visible to the community. In these instances, tall pressure vessels expose utilities to public criticism from residents who view such structures as eyesores.
Going Low
With a shorter vessel height, water and wastewater treatment plants can avoid an expensive capital investment and/or project delay due to the need to rebuild the housing structure. When a structure is necessary and already built, utilities can choose a shorter pressure vessel that will fit inside the existing building and use existing infrastructure such as connections and site piping, thereby speeding up the project and avoiding significant capital expenses. For new construction, buildings can be built shorter, which is another route to reducing capital costs.
For systems located close to the community and without need of a building, a smaller pressure vessel can adhere to local height restrictions. This will help move project approval along more quickly for both new builds and upgrades. In some cases, proactively choosing a shorter vessel when upgrading technology may offer the opportunity for a utility to build community trust and support by demonstrating its willingness to reduce vessel size and noticeability while improving performance.
How the Innovation Works
The squat nature of this lower profile vessel is attributed to its use of mechanical design changes like short radius bends and adjusted media outlet placement to reduce height without impacting performance. Media capacity and empty bed contact time remain unaffected by this shorter design. With less steel inside the tank, there is less risk of corrosion over time, making long-term maintenance more affordable.
An additional benefit of the lower profile is lower hydraulic head loss, which provides additional energy efficiency—up to 25% less energy is required to operate than in taller systems, with equivalent performance. Unique to this design is an external ring header underdrain, a feature that allows the height to be lowered while also maintaining easy access. Nearly all repairs and maintenance can be done externally, which sharply increases the ease of maintenance and reduces the need for a closed space installation.
Where Shorter Vessels Shine
In jurisdictions where height restrictions are mandated by law or demanded by public will, a short vessel can be a ready-made solution. In operations where replacing a building is not an option, a shorter vessel may be the preferable path to a treatment upgrade. In cases where a new building is absolutely necessary, a shorter pressure vessel helps control construction costs by reducing building height.
Case study #1: Massachusetts town upgrades existing infrastructure to treat for PFAS
To comply with state maximum contaminant levels (MCLs) for per- and polyfluoroalkyl substances (PFAS), the town of Norwell, Massachusetts, began upgrading its treatment systems in 2020. The town needed vessels that could accommodate the PFAS treatment media, fit into an existing treatment plant and be delivered within 90 days.
To accomplish this, newly fabricated American Society of Mechanical Engineers (ASME) pressure vessels were outfitted with dual distributers to use either granular activated carbon (GAC) or ion exchange (IX) resin, depending on the needs of the town. The system included two 500 gallon per minute (gpm) vessels that could be installed in a parallel or lead-lag configuration. With a height of 14 feet and 6 inches, the pressure vessels easily fit within the town’s existing drinking water plant.
Case study #2: California well water treatment system designed for state’s largest water supplier
In 2017, a well site in Rancho Cordova, California, was developed in tandem with a local contractor and national engineer to construct a 1.4 million gallon per day (mgd) GAC well water treatment system to remove perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), which were detected in the local supply at 180-200 parts per trillion (ppt) combined. As the site was located in a suburban area, a solution was sought that would achieve public expectations for aesthetics and minimal visibility.
The resulting design included two 475 gpm two-vessel systems that could be used in either parallel or lead-lag configuration to achieve target levels of PFAS. The overall system height to the top of the pipe was 15 feet and 10 inches, and, as an added touch, a shade of forest green paint was utilized that would allow the structure to blend in with the natural surroundings.
The Savings of Going Small Add Up
With increasing regulations for drinking water contaminants and rising treatment costs, municipal operations are seeking cost savings in every corner. A shorter pressure vessel can offer savings—including possibilities for utilities to use existing infrastructure and buildings, provide energy savings from less hydraulic head loss, reduce maintenance costs due to easier access and minimize public impact when installing treatment operations located in communities.
