Out To Dry

How much of the synthetic microfibres from clothes washing get removed in the water treatment plants varies depending on the design of the treatment plant. Studies have shown that up to 99% of microplastics can be removed in some designs, but on average the figure is more likely to be between 95%-99%. Given that these treatment plants can process billions of litres of waste water a day, even very small percentages that get through can add up to huge amounts.

When the sewage from our homes and businesses reaches the treatment plant it is first allowed to settle, so that heavier particles sink to the bottom of the tank forming a sludge. This includes a substantial amount of microplastics. Since dumping this sludge at sea was banned in 1998, it is now mostly used as fertilizer for farmland. According to Assured Biosolids "Currently around 87% of sludge is recycled to agricultural land as biosolids, 4% is incinerated, 3% goes for industrial use (e.g. as a fuel for cement production) and 6% is used for land reclamation or restoration."

After the solids have been allowed to settle, the liquid from the top is passed into a secondary treatment tank where air is bubbled through it to encourage bacteria to digest the remaining solids. After these bacteria and solids are allowed to settle out again the liquid is finally passed through a filtration bed of sand and rock (or sometimes a field) to catch any floating or suspended particles that are left, before being pumped into the river or sea. As some of the microplastic particles are so small and can remain suspended or floating they can make their way through the system and out the other side. Capturing these remaining particles can require expensive additional equipment and so there is little incentive to try and capture the last 1-5% of particles that are getting through.

One study (Pedrotti et al, 2020) that analysed water coming out of a typical treatment plant found that about 18,500 synthetic fibres are being emitted per cubic metre of water (1000 litres). Using this figure we can estimate how much synthetic fibre is being emitted by a water company or treatment plant. Thames Water state on their website that they treat 4.6 million cubic metres a day. We can calculate, based on these numbers, that around 85 billion synthetic microfibres are being emitted from their system each day and if we take a typical length of a microfibre to be 1/2 a millimetre, then this is 43,000 Km of microfibre per day, enough to wrap around the earth with some left over. Over the course of the year this amounts to a length of fibre that could be wrapped about 400 times around our planet.

Thames Water also overflowed 24 million cubic metres of raw sewage into the environment in 2020. Pedrotti et al measured 13,000,000 microfibres per cubic metres in the water entering treatment plants, if we allow for rainwater dilution of the raw sewage (say 5 to 1 rainwater to sewage), then we get enough microfibre for approximately another 600 times around the Earth, giving us a total of 1,000 times around per year, or roughly 40 million Km. This is for one water company for one year.

A recent report by researchers at the University of California estimated that "5.6 million tons of synthetic microfibers were emitted from apparel washing between 1950 and 2016. Half of this amount was emitted during the last decade, with a compound annual growth rate of 12.9%. Waterbodies received 2.9 million tons, while combined emissions to terrestrial environments (1.9 million tons) and landfill (0.6 million tons) were almost as large and are growing."

If we assume that a common polyester thread thickness is 50 Wt (50 weight, or 50 Km for one Kg), then we can use the number above to roughly calculate the length of all the synthetic fibre produced between 1950 and 2016. This is anbout 2.5 Trillion Km, enough to reach Pluto 50 times.