A new water treatment filter inspired by the manta ray

Research shows that filter feeders strike a natural balance between permeability and selectivity that could inform water treatment systems.

New research from the Massachusetts Institute of Technology (MIT) shows that filter feeders strike a natural balance between permeability and selectivity that could inform the design of water treatment systems.

Filter feeders are everywhere in the animal world, from tiny crustaceans and certain types of coral and krill to various molluscs, barnacles, and even massive basking sharks and baleen whales. Now, MIT engineers have found that one filter feeder has evolved to sift food in ways that could improve the design of industrial water filters.

In a paper appearing this week in the Proceedings of the National Academy of Sciences, the team characterizes the mobula ray’s filter-feeding mechanism. Mobula rays are a family of aquatic rays that includes two manta species and seven devil rays. They feed by swimming open-mouthed through plankton-rich regions of the ocean and filtering plankton particles into their gullet as water streams into their mouths and out through their gills.

The floor of the mobula ray’s mouth is lined on either side with parallel, comb-like structures, called plates, that siphon water into the ray’s gills. The MIT team has shown that the dimensions of these plates may allow incoming plankton to bounce across the plates and further into the ray’s cavity rather than out through the gills. Moreover, the ray’s gills absorb oxygen from the outflowing water, helping the ray simultaneously breathe while feeding.

“We show that the mobula ray has evolved the geometry of these plates to be the perfect size to balance feeding and breathing,” says study author Anette “Peko” Hosoi, the Pappalardo Professor of Mechanical Engineering at MIT.

The engineers fabricated a simple water filter modelled after the mobula ray’s plankton-filtering features. They studied how water flowed through the filter when fitted with 3D-printed plate-like structures. The team took the results of these experiments and drew up a blueprint, which they say designers can use to optimize industrial cross-flow filters, which are broadly similar in configuration to that of the mobula ray.

“We want to expand the design space of traditional cross-flow filtration with new knowledge from the manta ray,” says lead author and MIT postdoc Xinyu Mao, PhD ’24. “People can choose a parameter regime of the mobula ray to potentially improve overall filter performance.”

Hosoi and Mao co-authored the new study with Irmgard Bischofberger, MIT associate professor of mechanical engineering.

A better tradeoff

The new study grew out of the group’s focus on filtration during the height of the Covid pandemic, when the researchers were designing face masks to filter out the virus. Since then, Mao has shifted focus to study filtration in animals and how certain filter-feeding mechanisms might improve filters used in industry, such as in water treatment plants.

Mao observed that any industrial filter must balance permeability (how easily fluid can flow through a filter) and selectivity (how successful a filter is at keeping out particles of a target size). For instance, a membrane studded with large holes might be highly permeable, meaning a lot of water can be pumped using very little energy. However, the membrane’s large holes would let many particles through, making it very low selectivity. Likewise, a membrane with much smaller pores would be more selective yet require more energy to pump the water through the smaller openings.

“We asked ourselves, how do we do better with this tradeoff between permeability and selectivity?” Hosoi says.

As Mao looked into filter-feeding animals, he found that the mobula ray has struck an ideal balance between permeability and selectivity: The ray is highly permeable, letting water into its mouth and out through its gills quickly enough to capture oxygen to breathe. At the same time, it is highly selective, filtering and feeding on plankton rather than letting the particles stream through the gills.

The researchers realized that the ray’s filtering features are broadly similar to industrial cross-flow filters. These filters are designed so that fluid flows across a permeable membrane that lets through most of the fluid. At the same time, any polluting particles continue flowing across the membrane and eventually out into a reservoir of waste.

The team wondered whether the mobula ray might inspire design improvements to industrial cross-flow filters, so they investigated the dynamics of mobula ray filtration further.

A vortex key

As part of their new study, the team fabricated a simple filter inspired by the mobula ray. The filter’s design is what engineers refer to as a “leaky channel” — effectively, a pipe with holes along its sides. In this case, the team’s “channel” consists of two flat, transparent acrylic plates glued together at the edges, with a slight opening between the plates through which fluid can be pumped. At one end of the channel, the researchers inserted 3D-printed structures resembling the grooved plates that run along the floor of the mobula ray’s mouth.

The team then pumped water through the channel at various rates, along with coloured dye, to visualize the flow. They took images across the channel and observed an interesting transition: At slow pumping rates, the flow was “very peaceful,” and fluid easily slipped through the grooves in the printed plates and out into a reservoir. When the researchers increased the pumping rate, the faster-flowing fluid did not slip through but appeared to swirl at the mouth of each groove, creating a vortex similar to a small knot of hair between the tips of a comb’s teeth.

“This vortex is not blocking water, but it is blocking particles,” Hosoi explains. “Whereas in a slower flow, particles go through the filter with the water, at higher flow rates, particles try to get through the filter but are blocked by this vortex and are shot down the channel instead. The vortex is helpful because it prevents particles from flowing out.”

The team surmised that vortices are the key to mobula rays’ filter-feeding ability. The ray can swim at just the right speed, and water, streaming into its mouth, can form vortices between the grooved plates. These vortices effectively block any plankton particles — even those smaller than the space between plates. The particles then bounce across the plates and head further into the ray’s cavity, while the rest of the water can still flow between the plates and out through the gills.

The researchers used the results of their experiments, along with dimensions of the filtering features of mobula rays, to develop a blueprint for cross-flow filtration.

“We have provided practical guidance on how to filter as the mobula ray does,” Mao offers.

“You want to design a filter such that you’re in the regime where you generate vortices,” Hosoi says. “Our guidelines tell you: If you want your plant to pump at a certain rate, then your filter has to have a particular pore diameter and spacing to generate vortices that will filter out particles of this size. The mobula ray is giving us a really nice rule of thumb for rational design.”

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