When a bilge pump operates it pushes into the marine environment all the water that has built up in the bilge of any vessel – and that can include oil, microplastics and detritus from on board.

We all recognise that the ocean is critical for life on Earth. It regulates our climate and is home to an incredible array of marine life and plant species. But our seas are facing several threats, among them are oil spills and microplastics.

Boaters are increasingly aware of personal impact. Just one drop of oil or fuel in the water will create a visible ‘blue-shine’ which can spread for up to 6m. If you can see oil in the water – in marinas or out at sea when dirty bilge water is automatically pumped overboard – this equates to around 25 parts per million (ppm) of oil to water.

The legal limit set by MARPOL (the international convention for the prevention of pollution from ships) is 15ppm, reducing to just 5ppm in specially protected areas.

Microfibres and microplastics from clothes, ropes and onboard bedding are also ending up in the bilges.

Microplastics are said to be the most pervasive pollutant of our time, having been found in the Mariana Trench, Antarctic ice, soil, human and animal bodies, and the air around us.

Retrofit solution

To avoid pumping pollutants overboard, boaters can fit a compact bilge filter.

Starting at £100, bilge filters can take less than an hour to fit; some filters have been designed to achieve the highest discharge levels of 5ppm to meet requirements when going from coastal to inland waterways.

Former Royal Marine Justin Morton, who sailed with marine biologist and TV presenter Monty Halls on his Generation Sea Change environmental project last summer, helped to fit a Wavestream 1 on Monty’s Colvic Victor 34.

Justin, the project’s sailing advisor and fixer, said: “Generation Sea Change involved two months of sailing around the south-west coast filming Monty and his family interviewing and highlighting individuals or groups making changes in ways that were positive, inspiring and most important of all, repeatable by most of us.

“For parts of it, we took two boats; Monty and family on his ketch Sobek, and me and the camera team on mine, a Nelson 42 powerboat.

“We met people doing amazing things from making kayaks out of plastic they found around Salcombe to a surfer who has invented a retrofit filter that takes the microfibres out of waste washing machine water.

“On the way we learned about the excellent work people in Dartmouth are doing to highlight water quality issues and initiatives in Falmouth Harbour to regenerate seagrass beds and keep their water clean,” he said.

“With such a positive environmental backdrop, it made sense to make sure we did what we could to reduce our own footprint as much as we could in our boats.”

Water ingress

During the voyage, Sobek had a number of engine issues.

“It transpired that water was getting into the fuel tank through the fuel filler cap in the cockpit floor,” Justin said. “So much water, in fact, that from a clear water separator, we would have almost pure water at the fuel injection pump after just four or five hours of motoring.

“As you can imagine, that is a lot of watery fuel and mess to deal with and despite our best efforts some of it inevitably found its way into the bilge.

“This got us thinking. As habitual bilge pumpers, more for sucking out stale air (and any gas) than water, it was worrying that every now and again we would leave an, albeit light, but oily trail.

“We got in touch with Wave International, which makes various filtration systems for fresh, grey and bilge waters, and it provided us with suitably sized, Wavestream 1, filters.”

Justin said the Navy had drummed into him the need to keep bilges clean so nothing would block the bilge pump. So he was initially sceptical about the filter.

“It didn’t make sense to put something designed to block things into a pipe we wanted kept clear,” he said.

Concerns alleviated

However, having a bilge filter proved not to be an issue for two reasons. Firstly, the filter in the bilge system removes oily contaminants but lets water pass almost unhindered.

“Secondly, and perhaps surprisingly to some, the vast majority of us do not have bilge pumps (automatic or manual) big enough or fast enough to stop our boats sinking from a catastrophic failure anyway – unlike the Navy.

“They are generally only capable of pumping the water out once a big leak has been plugged. Also, a strum box or filter on the pick-up end of the bilge pipe will most likely block long before the filter system would.

“Even though the filter will also stop solid objects including micro-fibres, they are unlikely to block the filter as it is designed with a large flow area and it would take an awful lot of debris to cause an issue.”

It is recommended that you change the filter every time you do an oil change, engine service or annually, but the filter’s lifespan is directly related to how much oily contaminant it has to soak up. Replacement cartridges cost just under £30.

Easy fit

The filter ideally should be installed upright – so the housing can be unscrewed without the contents spilling everywhere – and with enough room underneath to unscrew the filter.

Justin added: “Fitting couldn’t have been more straightforward because we had a spot it would fit in, in an easily accessible place. For Sobek, it took less than an hour to fit after about an hour of working things out.

“We basically just followed the instructions. The two hose connectors and jubilee clips we needed to buy were available from a local chandlery.

“As Sobek’s filter has a see-through housing it’s easy to monitor what is going on. It is pretty much fit and forget. So far we have a few flecks of paint, some plastic, grit, and a few soft looking things we don’t recognise sat at the bottom of the housing.

“And it is working properly. Despite a thorough clean we still have an oily residue in the bilge, but we are glad to report it’s not automatically going over the side as we pump the bilge; something we’d previously been becoming increasingly reluctant to do.”

Fitting a Wavestream bilge filter step-by-step guide

Step 1) The Wavestream needs to be fitted vertically, inline between the bilge pump and the bilge outlet. Decide where you will fit it and note your pump output. The smaller Wavestream 1, measuring 315 x 130mm, copes with a flow rate of up to 40lt/min and costs £144.

Step 2) Measure the diameter of your bilge pipe. The Wavestream 1 has connections measuring ¾in BSP/19mm. Purchase the correct size and shaped connectors considering how the bilge pipe will run in and out the bilge filter, ideally with the least number of bends.

Step 3) The filter comes with a mounting bracket and screws. Other tools needed: jubilee clips, thread tape/thread sealing compound, screwdriver, drill, spanner, hacksaw or sharp knife, torch, pencil, cable ties, kitchen towel, tape measure, silicone grease/Vaseline.

Step 4) Screw the fitting bracket onto the top of the bilge filter to check and mark the best location for its vertical fit, allowing space below to unscrew the filter housing. Check what’s underneath, as water may spill during filter change.

Step 5) Remove the bilge filter from the bracket. Mark the position for the bracket screw holes, and drill the holes. Screw the bracket securely in place.

Step 6) Noting the direction arrows on top which indicate the in and out flow of water, screw in the connectors. Use sealing compound and gently turn with a spanner to be watertight. Screw the bilge filter to its fixing bracket.

Step 7) Mark the bilge pipe cuts, ensuring both ends can be easily connected to the bilge filter. Cut to length, add jubilee clips and push on to connectors.

Step 8) Now bleed out the air. Operate the bilge pump then press the green button on top. When all air has been expelled the system is ready to go.

Step 9) Lastly, look at the outlet from the boat to see if clean water is emitted. Check around the engine, replace drip trays, and make a note of engine hours.

Microscopic paint particles ‘caught’

The filter from Monty Halls’ boat was sent to a specialised microplastics laboratory for analysis and photographs were taken under 20x magnification.

The findings showed a multitude of paint particles and fibres. In particular, red, white, blue and black fibres which were hypothesised by the researchers to come from clothing and other fabrics on board such as seat covers and bedding.

Paint and resin particles are released from general wear and tear of using a boat, as well as when maintenance occurs. The analysis also identified dark coloured organic matter such as sludge and slime.

Monty Hall said that without having a filter in place, “a large amount of microscopic paint particles, microplastics as well as microfibres, would have been emitted overboard into the marine environment.”

A small unit with a great impact

Sailor and adventurer Ella Hibbert, who is attempting a solo, non-stop Arctic Circle circumnavigation in 2025 aboard her Bruce Roberts 38 yacht, has also installed a bilge filter.

Following five months usage and sea trials in Norway of her 38ft steel-hulled ketch, Yeva, Ella was “amazed to see all the gunk and oil collected”.

She said: “The Wavestream bilge water filter is an excellent bit of kit that could be instrumental in helping all boaters reduce their impact on the environment we love to enjoy so much – the sea.

“Installation is quite simple. The filter gets installed in the pipework that leads from your bilge pump to where the pipe discharges overboard.

“I had to cut across the bilge water hose to separate it into two sections, and then attached the connectors to the top of the Wavestream that would allow me to join the hose ends to the filter.

“I wrapped the ends of these connectors with PTFE tape for extra water-tightness and peace of mind, and used jubilee clips (two on each hose tail, facing opposite directions from each other) to secure the pipes snugly to the filter.

“The filter comes with a metal plate, and screws to connect the plate to the filter top, which allows you to use the plate to secure the filter top to your bilge.

“This allows you to be able to unscrew the base of the unit to swap out an old filter for a new one, and empty the filter of any debris such as microplastics and microfibres it has caught, without having to disconnect any of the pipework around the top of the unit.

“There are plenty of different sizes that these filters come in, to suit all cruising vessels, sail and motor. Such a small unit can have such a great impact.”

Ella’s 2025 mission is to raise awareness of how the Arctic is under immense stress and at the edge of irreversible damage, which will impact the rest of the globe – as ice melts, it leads to rising water levels, wreaking havoc on our coastlines.

The once unthinkable passage is now a possibility due to melting sea ice. Ella, 27, is scheduled to depart Haslar Marina, Portsmouth, at the end of May in Yeva, sailing up the east coast of England, to a start point on the 66.5°N latitude between Norway and Iceland.

The route, estimated to take five to six months, will travel clockwise around Iceland, Greenland, through the Northwest Passage, Alaska, Russia, and northern Norway – before recrossing the start point and heading back to Gosport.

With temperatures 1.5°C above pre-industrial levels, the Greenland ice sheet is said to be losing an average of 30 million tonnes of ice an hour.

“This isn’t a record attempt,” said Ella, “it’s a wake-up call.”

Ella, who achieved her RYA Yachtmaster Instructor qualification two years ago, and is also a PADI Divemaster, is sailing solo, with planned rough stops if required, depending on wind and ice cover.

During these stops, she will not disembark but could resupply with food and fuel. The ketch has been equipped with foam insulation, “spares of everything, including filters”, solar panels, hydro- generator, wind vane self-steering gear and watermaker.

The Arctic voyage will be filmed, with fixed cameras, action cameras and drones on board Yeva.

Ella has pledged to auction Yeva at the end of the expedition, and donate all proceeds to both Polar Bears International and Ocean Conservancy.