Selecting Filter Media for Wastewater Treatment Performance

Wastewater treatment specialists can choose from a wide range of filter media products, and new additions are entering the market every year. Faced with so many products and differing claims of efficiency and performance, it can be difficult to know which are the best products to choose for applications such as traditional trickling filters, SAF, MBBR and IFAS.

The performance of biological wastewater treatment systems depends heavily on the selection of suitable filter media. In applications such as trickling filters, submerged aerated filters (SAF), moving bed biofilm reactors (MBBR) and integrated fixed film activated sludge (IFAS) systems, filter media design directly impacts biofilm development, oxygen transfer efficiency and long-term operational reliability.

Which product features will help the biofilm to form most rapidly and uniformly?

Which types of filter media provide the most efficient operation in aerated and anoxic treatment plants?

And which of the numerous options will provide the best removal performance in the specific application you are planning?

In this article, I’m reviewing the results of recent industry/academic research collaborations which have led to scientific discoveries of importance for filter media performance. Focusing on the science behind filter media for wastewater treatment, my aim is to provide practical guidance to help specifiers and purchasing teams to make informed decisions and create best-in-class treatment plants.

Importance of biofilm formation in wastewater treatment

A key consideration when using any fixed film process is the creation and maintenance of an effective biofilm. But biofilms are extraordinarily difficult to see and analyse in any amount of detail with the naked eye.

An important contribution to this issue in recent years was the capture of a photograph of a biofilm. This magnified image demonstrates what it is that we are all trying to achieve when we create a new treatment plant involving filter media. See Figure 1.

The initial creation of the biofilm is crucial, so one of our early collaborative research projects with Cranfield University aimed to shed light on the physical properties of filter media products and their impact on start-up times.[1]

The results, published in an academic journal article in 2018, stated that spherical media achieved stable biofilm formation faster than cylindrical media (15-17 days compared to 23-24 days) with more uniform coverage.

Uniformity is important to the integrity of the biofilm, and reduces the risk of detachment and biofilm abrasion once the plant is fully operational. Oxygen mass transfer was also higher for spherical media due to their higher voidage.

This is because they have internal fins and large openings, offering a surface area of between 135m²/m³ and 220m²/m³ with high voidage (between 92% and 95%). The spherical designs enable the biomedia to form more rapidly and to retain their integrity. The structure and surface textures of the biomedia provide a protected environment to host the beneficial bacteria needed for biological treatment processes.

High voidage eliminates the problems relating to clogging and puddling which are typically associated with traditional gravel, slag and clinker filter beds. The fins can also interlock in the filter bed to protect resilience.

“A key challenge faced by water utilities is the need to meet ever more stringent discharge consent levels whilst avoiding spiralling operational costs.”

Selecting filter media for SAF

In aerated applications such as submerged aerated filters (SAF) or submerged aerated fixed film reactors, a number of other factors come into play. These applications combine high biomass concentration of fixed film processes with high oxygen transfer efficiency through forced aeration. With the soaring costs of energy in recent years, the efficiency of oxygen transfer has become a major consideration.

Efficiency of aeration formed the basis of a separate research project carried out by Warden Biomedia in collaboration with Cranfield University. To create a baseline for comparison, the research looked at oxygen transfer rates in clean water, with and without filter media.

It then carried out the same tests on wastewater, with different types of random filter media. We were able to properly quantify the improvements in aeration performance and hydraulic efficiency of using filter media of different shapes and dimensions.

The clean water baseline tests proved that media enhanced the overall oxygen transfer efficiency by 23%–45% and hydraulic efficiency by 41%-53%. These figures were compared with separate wastewater tests carried out in operational conditions where biofilms were present.

The three best-performing filter media designs of the five tested (which proved to be those with spherical designs) increased the hydraulic efficiency to 89%, 93% and 100% respectively.

Best choice for SAF: For SAF applications the best choice is a high surface area which gives optimum air to wet surface contact time to help eliminate odour issues. High voidage is also important, as it means a low pressure drop on the gas side, minimising fan power helping to contribute to environmental and sustainability targets.

Choosing media for MBBRs and IFASs

The research project mentioned above also provided valuable insights into the types of filter media that would perform best in moving bed biofilm reactors (MBBRs) and integrated fixed film activated sludge (IFAS) processes. In these applications the use of suspended buoyant media brings a number of additional factors into play.

A key challenge is that these advanced wastewater treatment processes make more complex demands on the filter media, and selecting products with the optimum design features can deliver valuable commercial and operational benefits. MBBRs and IFASs require, for example:

• High protected surface area to increase solids retention time (SRT) and biomass concentration for improved BOD₅ removal and nitrification (>80%)
• Large open areas to prevent media blockage by fast-growing aerobic heterotrophic biofilms under high load conditions
• Durable and rugged filter media with density close to water
• Controlled biofilm shear to minimise biomass thickness and detachment
• Low sensitivity to shock loads and temperature changes
• Flexibility for different configurations to achieve high quality effluent

Oxygen transfer efficiency and hydraulic performance remain paramount of course. Once in operation, the design of the filter media needs to be instrumental in the control of the biomass thickness, thereby optimising performance and, in aerobic applications, to reduce the amount of oxygen/aeration needed, and the costs associated with this.

“The clean water baseline tests proved that media enhanced the overall oxygen transfer efficiency by 23%–45% and hydraulic efficiency by 41%-53%.”

Lastly, it is important that the filter media can be retrofitted cost-effectively to upgrade effluent quality and increase treatment capacity of established facilities, as many organisations are investing heavily to improve their existing WwTPs.

One of our most recent research projects in collaboration with Cranfield University enabled us to quantify the importance of these multiple design factors. A pilot plant was tested in different operational and hydraulic scenarios with media carriers of different protected surface areas and voidage ranges.

Through mathematical models developed, Warden Biomedia was able to become the first to introduce a new parameter -“media dimensionality” – which has proven to have a direct positive impact on biological start-up time and the process operation during steady state conditions. This provided scientific validation for spherical media products such as that in Figure 4.

Best choice for MBBRs and IFASs

These findings also helped us to set criteria for a new filter media product, designed specifically for MBBRs and IFASs. We returned to the drawing  board and, using the recently-calculated media dimensionality parameters, we invested R & D resources to create a new type of filter media product which incorporated the findings that had come from scientific testing under operational conditions.

With this new media product at prototype stage, Warden Biomedia then supported a 3-year PhD project as a Knowledge Transfer Partnership with Cranfield University to compare the performance of this product against other industry options. The result is shown in Figure 5.

This is a spherical media product, which improves biological commissioning times and process security. It also has higher biomass retention, which increases wastewater treatment capacity and enables the tank volume to be reduced.

A key challenge faced by water utilities is the need to meet ever more stringent discharge consent levels whilst avoiding spiralling operational costs. Effective oxygen transfer is important to facilitate degradation of organic matter, endogenous respiration and nitrification, as well as mixing and scouring of the biofilm.

In moving fixed film systems, dissolved oxygen (DO) set points are usually high (3–5 mg/L), compared to conventional ASP (1–2 mg/L). So aeration efficiency needs to be optimised to minimise energy consumption and operational costs, while still maintaining process performance.[2]

In addressing these commercial challenges, the choice of filter media can be crucial, and our research collaborations aimed to solve these very real issues faced by organisations involved in wastewater treatment.

“Uniformity is important to the integrity of the biofilm, and reduces the risk of detachment and biofilm abrasion once the plant is fully operational.”

The industry/academic research studies we have been involved in since 2013 have taken the guesswork out of achieving optimum performance by enabling us to quantify the advantages of different shapes and designs of filter media in specific industry applications.

Irrespective of the shape and dimensions of individual products, it is important to remember that these types of filter media are designed for sustainable, long-term use. The Warden Biomedia products, for example, are extremely resistant to wear. Almost all are manufactured in recycled material and every product is, in itself, recyclable.

A take-back scheme is in place to enable used filter media to be cleaned and made available for use in other applications. Responsible use is, of course, necessary to avoid filter media being released due to accidental spillage caused by mechanical/plant failture, but the inherent integrity of the products prevents materials relating to the filtering process from exiting the treatment plant in effluent and entering watercourses.

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[1] Dias, Bellingham, Hassan, Barrett, Soares, Stephenson, Bioresource Technology (2018)

[2] Dias, Bellingham, Hassan, Barrett, Stephenson, Soares, Chemical Engineering (2018)

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FAQs

What is filter media in wastewater treatment?

Filter media is the material used to support the growth of beneficial microorganisms in biological wastewater treatment systems. These microorganisms form a biofilm that helps break down organic pollutants and improve water quality.

Why is biofilm formation important in wastewater treatment?

Biofilm formation is essential because the microorganisms within the biofilm carry out the biological treatment process. A stable and uniform biofilm improves treatment efficiency, process reliability and pollutant removal performance.

What is the best filter media for MBBR systems?

The best filter media for MBBR systems typically combines high protected surface area, high voidage and durable construction. Spherical media designs are often preferred because they improve biomass retention and oxygen transfer efficiency.

How does filter media improve oxygen transfer efficiency?

Filter media improves oxygen transfer by increasing air-to-surface contact time and improving hydraulic mixing within the treatment system. Efficient oxygen transfer reduces aeration energy demand and supports better biological treatment performance.

What is the difference between SAF MBBR and IFAS systems?

SAF systems use fixed filter media with forced aeration while MBBR systems use suspended moving media within the reactor. IFAS systems combine suspended activated sludge with fixed film media to increase treatment capacity and biological performance.

Why is spherical filter media beneficial?

Spherical filter media promotes faster and more uniform biofilm growth while improving hydraulic efficiency and reducing clogging risk. Its design also helps maintain biofilm integrity during long-term operation.

Can wastewater filter media be recycled?

Yes, many modern wastewater treatment filter media products are manufactured from recycled materials and can also be recycled at the end of their operational life through specialist recovery and reuse programmes.