by Torin Fastnedge
Microfibres from our clothes make up around 35% of the primary microplastics found in our oceans – plastics that originate from household and industrial appliances – and form between 15% to 31% of all plastics in the ocean [J. Boucher & D. Friot 2017 Primary microplastics in the oceans: a global evaluation of sources]. It is also estimated that each person in the UK produces on average 243g of microplastic fibres per year when washing their clothes in a standard washing machine. Due to this astounding quantity, many countries have introduced laws requiring new washing machines to filter out microfibres when draining wastewater. White goods companies, such as Beko and Grundig with their Fibercatcher® filter (www.grundig.co.uk/fiber-catcher), are working on integrating microfibre filters into their washing machines. However, the current conventional dead-end mesh filters used clog too quickly to be practical. Oxford mathematicians Torin Fastnedge, Chris Breward and Ian Griffiths have been collaborating with Graham Anderson, Konstantinos Pantelidis, Simge Tarkuç and Melih Toklu from Arçelik and Beko R&D to integrate a biomimetic process to increase the lifespan of such dead-end filters, coined, ricochet separation.
Ricochet separation, proposed by Divi et al. [R.V. Divi et al. 2018 Manta rays feed using ricochet separation, a novel nonclogging filtration mechanism, Sci. Adv.], involves a particle-rich flow over a series of angled gill-like branched channels, indicated in figure 1. Unlike cross-flow filters, where particles are captured along and on top of the wall pore structure tangentially to the flow, ricochet separation acts to allow water through the branched channels at an angle to the flow, but particles may ricochet back into the free-stream flow, and then subsequently out of the end. Within a washing machine, this method removes some ‘clean’ microfibre-free water from the microfibre-rich water flowing into the dead-end Fibercatcher® filter. This lowers the pressure drop over some already fouled microfibre dead-end filter, increasing the time before the filter needs to be cleaned, which occurs when a critical pressure is reached.
By using mathematical and homogenisation techniques, we have found analytical expressions for the high-Reynolds-number flow in this device, so that we need not rely on lengthy numerical simulations. We perform particle tracing in the flow field to understand how fibrous particles behave in such a device. This will reduce computation time in the optimisation of the trade-off between minimal particles and maximal fluid flux through each branched channel, whilst ensuring a reduced pressure drop over the dead-end filter.
This work is a crucial step in modelling branched channel filters and their effective use in washing machines. We will be extending this work to optimise Beko’s filtration devices and to prevent further microplastic pollution from entering our already heavily polluted oceans.