PFAS Filtration: Designing For Smaller OPEX And Footprint
By Conrad Hopp
Keys to bring down the cost of PFAS treatment for operations with limited resources — or any operation using media filtration.
In April of 2024, the U.S. EPA announced two regulatory actions targeting PFAS (per- and polyfluoroalkyl substances), also known as “forever chemicals” due to their persistence in the environment and associated health risks. The first of the two, published on April 10, outlines drinking water standards for six individual PFAS and combinations therein. Nine days later, the EPA announced its rule designating two PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic (PFOS), as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). These rulings will require the installation and operation of PFAS treatment technologies at municipal drinking water utilities as well as federal and industrial sites that utilized or manufactured PFAS.
The EPA has designated three technologies as best in class for PFAS treatment: granular activated carbon (GAC), ion exchange (IX), and reverse osmosis. Of the three technologies, GAC and IX, often collectively referred to as media filtration, are considered the gold standard for PFAS treatment due to cost advantages and performance reliability. When designing a media filtration system, there are two components that impact capital and operating costs:
- Media selection
- Mechanical design.
With thousands of systems due to come on-line in the years ahead, minimizing cost is essential to ensuring public money is used efficiently. While multiple factors must be considered in the design of filtration systems — including media selection — mechanical design is a centerpiece of cost and performance optimization.
Media Selection
Designing a media filtration system starts with selecting the most effective media given the unique water quality of each site. When comparing IX and GAC, the former is generally more cost-effective than the latter. While GAC is cheaper than IX resin per cubic foot, it requires a longer empty bed contact time to effectively treat PFAS. As a result, 5x more media are required to treat the same flow rate. This, in combination with a lower hydraulic loading rate, means that GAC systems require a larger footprint and higher capital costs.
In addition to capital costs, long-term operating costs such as media replacements must be considered. Organic compounds, including total organic carbon (TOC), compete with PFAS for adsorption sites on activated carbon. For the anion selective IX resin, anions like nitrate, sulfate, or chloride compete for exchange sites. The presence of co-contaminants can harm treatment performance and drive up the cost of ownership depending on media. Co-contaminants that require removal to a maximum contaminant level (MCL) may dictate media selection. For example, if removal of volatile organic compounds (VOCs), total trihalomethanes (TTHMs), or general TOC is critical, GAC may be preferred. Selecting the best media requires a deep understanding of the complex chemistry that underpins PFAS removal in the presence of a variety of co-contaminants.
Regardless of which media is right for the job, pressure vessels are required to facilitate the filtration process and have served water providers for decades to combat an extensive list of contaminants. However, system failures and expensive operational costs related to the mechanical design of pressure vessels can frustrate providers aiming to distribute clean, affordable water to their ratepayers.
Mechanical Optimization — 4 Tenets Of Pressure Vessel Design
Emphasis is often placed on media selection for longevity and operational costs; however, the mechanical design of pressure vessels is equally critical. Mechanical design not only impacts the performance and longevity of the media but also energy consumption, construction costs, and maintenance. Equipped with the four tenets of pressure vessel design, engineers can optimize vessel performance and maximize lifespan, providing customers the simplest and lowest cost of ownership. The four tenets are as follows:
- Hydraulic performance
- Media optimization
- Corrosion management
- Long-term operation & maintenance.
Hydraulic Performance
Designing for hydraulic performance is critical for minimizing energy consumption and maximizing media lifetime. When designing a pressure vessel for optimal hydraulic performance, engineers must pay attention to three regions:
- The overdrain — where water enters the system and is distributed onto the media
- The media bed — a resin or carbon-based media that removes contaminants
- The underdrain — nozzles or a slotted pipe that separates treated water from media.
The coordinated sizing and geometry of these three regions dictates the long-term performance of a pressure vessel and has considerable effects on corrosion rates, lifespan, and operational costs. Each region must work in harmony to create a flow pattern that is linear and uniform where water moves parallel to the media with minimum mixing to ensure even contact. Achieving “plug flow” within the media bed under ideal hydraulic conditions is critical for effective treatment.
The Overdrain
Overdrain design establishes the pressure differential and distribution patterns within the system to provide optimal plug flow. Designs can vary from simple inlet diffusers to multipoint distributors, depending on the characteristics of the media used. IX resin beads are spherical and uniform in size, whereas activated carbon is a mixture of coarse materials. When resin is used in large-diameter vessels, such as a typical 12-foot-diameter vessel, a multi-point distributor is installed to ensure better flow distribution over the resin bed to prevent channeling at the higher hydraulic loading rates or movement of the bed itself. With GAC applications, lower hydraulic loading rates mean a basic diffuser or splash plate can provide sufficient distribution.
The Media Bed
Plug flow rate of a water column through the media bed is designed to maximize treatment kinetics and allow enough time for the migration of contaminant compounds to the surface of treatment media. Unlike ion exchange resin beads, carbon granules are made up of a mixture of coarse material. When carbon is loaded into a pressure vessel, it must be backwashed to stratify the carbon bed and minimize pressure drop. The figure below shows pressure drop for backwashed and non-backwashed carbon beds with respect to changing superficial velocity. It is important to work with a vendor who can provide support from concept to commission.
The Underdrain
Four well-established underdrain designs play an equal and opposite role to the overdrain, maintaining appropriate outflow rates, plug flow, and pressure differentials. Each design can meet water quality and treatment goals. Underdrain design has improved incrementally to reflect the latest advancements in engineering, leading to the external ring header commonly used today when analyzed through the four tenets. An evolution of underdrain design is shown in the figure above. Older designs are still used today, even with advancements, due to vendors’ knowledge and manufacturing capabilities. The three main designs commonly employed today are:
- Header-lateral or hub-lateral — This design employs a horizontal drainage pipe with laterals to drain treated water. Some designs include drop-offs of the laterals down to the bottom of the vessel.
- Internal cone — This is similar to a colander and is welded inside the unit.
- External ring header — The external ring header uses nozzles and screens and fits flush with the vessel.
Using the external ring header — the latest evolution in underdrain design — reduces head loss throughout the system. Pressure drop for a 12-foot-diameter GAC system with 700 cubic feet of GAC operating at 1,000 gpm is 6 PSI for the external ring header vs. approximately 12 PSI for older designs. For end users that require multiple systems, this can translate to millions of dollars in energy savings over the operating lifetime.
Media Optimization
After media is selected for a specific job, optimizing its performance through mechanical design is critical to minimizing operational costs. There are two important factors to consider: the volume of fully utilized media and the establishment of an effective mass transfer zone (MTZ). This section focuses on minimizing underutilized media through underdrain design. Establishing an effective MTZ is accomplished by optimizing hydraulic performance and plugged flow, as discussed in the previous section.
Minimizing Underutilized Media
Making the most of the media from an initial fill goes a long way in delivering operational savings. Underdrain design plays an important role in minimizing underutilized media in a system. When plugged flow is achieved as the MTZ migrates linearly down through the bed, the first detection of contaminated water during the bed life occurs when the saturated media reaches the top of any nozzle where the screens allow water to filter through. This means any media sitting under the nozzles will not be 100% saturated before a change out is required. The image on the following page illustrates the media volume in the underdrain area for typical 12-foot-diameter vessels and how some designs underutilize a larger volume of media than others.
For a 12-foot-diameter system operating for 25 years, minimizing the underutilized media volume can save millions. The table below outlines lifecycle costs associated with the media volume contained below the underdrain, exclusive of future media price increases.
Corrosion Management
When using carbon steel, pressure vessel corrosion is certain. The design phase is an opportunity to anticipate and deter premature vessel corrosion. Engineers can ensure their solutions last with manageable operations costs by considering vessel materials, strong coating specifications, and underdrain design.
Material Selection
The anodic (most active) and cathodic (least active) metals used to build pressure vessels will inevitably interact. Accounting for this, a critical aspect of corrosion control is choosing materials that are close within the galvanic series, to decelerate the exchange of electrons between the two metals.
Linings and Coatings
Coatings are another important consideration of corrosion control. Some vessel specifications exclusively call for the coating of the anodic member, as this is the electron donor that erodes. However, NACE (now AMPP) recommends coating both cathodic and anodic metals to reduce the interaction between them.
Surface Preparation
Approximately 70% of coating failures are due to inadequate surface preparation. By utilizing industry best practices generated from the Society of Protective Coatings, SSPC SP-5, or NACE Standard RP0178-2007, as well as coating manufacturer recommendations, engineers can ensure that vessel materials are free of contaminants that affect mechanical adhesion of the coating or lining system. Welding specifications also lay the groundwork for long-term life and reduced corrosion rates, like NACE RP0178, which requires welds and sharp edges be ground down to create a smooth surface to build proper dry film thickness and mitigate voids.
Underdrain Design
A properly designed underdrain can prevent electrolyte buildup that corrodes vessel outlets. Per the welding specifications, avoiding unintentional crevices within the vessel will prevent water and media stagnation, which wears coatings and vessel materials.
- Header-lateral: The internal structure of this design challenges the lining and can cause corrosion with dissimilar metals, crevasses, and welds.
- Internal cone: Because of its shape, welding seams, and sharp edge hydrospheres, this design has lining challenges and can be prone to corrosion.
- External ring header: This features one homogenous lining to avoid corrosive crevices and is fully rated to the ASME VIII design criteria.
Long-Term Operation & Maintenance
The final tenet of pressure vessel design considers how the vessel will be operated and maintained. The design choices made earlier in the process dictate the standard operating procedures required, particularly for the underdrains.
Ease of Maintenance
When vessel professionals discuss simpler designs, they’re typically referring to ease of inspection during service events. Especially during media exchange, accessibility and lack of confined space protocol simplifies upkeep for operators.
- Header-lateral: Media must be removed from the vessel for any underdrain maintenance and requires confined space protocol.
- Internal cone: To gauge lining integrity, confined space entry is required both above and below the internal cone. The false bottom is not fully rated to the pressure vessel rating in upflow and downflow.
- External ring header: This doesn’t require carbon removal for maintenance, and it uses a simple forklift to remove the ring header for any potential maintenance.
For all designs, a 2:1 elliptical head should be designed to create a circular bottom to allow the easy flow of media to the bottom center for removal without any flat areas where media can accumulate.
System Footprint
Pressure vessel systems often operate inside a building to protect them from the elements and to prevent them from becoming eyesores in the community. Considering height during design can affect the facility’s broader operational costs. The annual electric costs related to HVAC and pumping water to the overdrain are directly affected by vessel height. Using the external ring header saves nearly three feet of height when compared to the internal cone design and just over three feet when compared to the headerlateral, as shown in the image above for a 12-foot-diameter, fivefoot side shell external ring header system.
As discussed, there are several important factors that must be considered in the design of media filtration systems, many of which are associated with underdrain design. The table above highlights the key mechanical impacts of underdrain design.
With the EPA’s announcement of regulatory actions regarding PFAS, considering the tenets of pressure vessel design is crucial to providing quality and cost-effective treatments. Thousands of systems are due to come on-line in the coming years, and working with a vendor that can provide support from concept to commission is critical for ensuring the long-term success of these systems.
About The Author
As Manager of Strategic Initiatives, Conrad Hopp supports the AqueoUS Vets CEO in four main capacities: planning and alignment, strategic partnerships and corporate development, strategic projects, and direct support. Conrad brings a deep understanding of the emerging contaminants market to the position and has a proven record of success leading the Advisory Services team at BlueTech Research, a global provider of water technology market intelligence.