Making a Salt Water Distiller for Remote Area Sea Kayakers [77/78]

by David Winkworth

Coleridge’s The Rime of the Ancient Mariner was in my head. It was 8 August 2008. My companion Ron and I were on the remote north western coast of the Gulf of Carpentaria, midway through our paddle from Karumba to Darwin. We had just completed a rough crossing from Gooninnah Island to the mainland at Point Arrowsmith — a paddle of only 10 kilometres but one that took us two and a half hours!

Sitting on the beach in the shade of mangroves we realized we were in deep trouble! We’d taken on our last load of water at Groote Eylandt several days ago which was to take us to Nhulunbuy, but bad weather and severed thumb tendons — an injury I sustained while rockhopping — had seriously slowed our progress. It was very likely we were going to run out of fresh water.

My map indicated a building about 40 km inland. There was no indication of water tanks or occupancy there, so it was a risky trek and one I declined to tackle. Our other option was to dig for water and that’s what we did.

In a dry creek bed, we used a tent peg to dig holes about a metre deep. Four hours later we carefully scooped brackish seepage from the bottom of the holes. The taste was awful but it may have saved our lives.

Later in the trip, my plan to paddle to the northern tip of the Wessels Islands was aborted because of strong winds and daily water requirements. I sat on a beach on Marchinbar Island, surveying the waters of the Arafura Sea… all that water and none that I could drink.

On my return home I listed fresh water security as a priority project before my next expedition. The answer would be: carrying more water in the kayak, my reverse osmosis pump, or a saltwater distiller… but which one?

Fresh water is heavy stuff — I routinely carry up to 40 litres in my Nadgee for remote area paddling. This 40 kg load is in addition to perhaps four weeks food, gear and of course me! It makes for a heavy boat. I always carry some extra water as a safety margin but it would be nice to leave that ‘safety margin’ water behind and make it up another way only if needed. I ruled out carrying any more water.

Reverse osmosis watermakers are heavy too. I have one. With all its tubes and bits it weighs around five kilograms. It’s about the size of a large grease gun and it works — when pumped — by forcing saltwater through very fine membranes at a very high pressure. The makers claim it can produce five litres of fresh water per hour but my experience is well less than that. It’s hard work, some concentration is required, and if the pumping stops, it stops producing. But the biggest drawback for me is that the watermaker has to be used where the water is… which means sitting at the water’s edge in the tropical sun or stationary out in my kayak with the crocs. I ruled out the desalinator. (Incidentally Andrew McAuley used a desalinator on his Tasman crossing when his water supplies ran out. He had no other option of course but it can’t have been easy pumping water for hours on cold windy seas.)

This left the saltwater distiller — my D-SAL project.

When you Google ‘saltwater still’ there are many hits on the principles of distillation but nothing on small portable practical examples. This was going to be a ‘from scratch’ project.

My plan was to boil seawater in a lightweight sealed container, collect all the steam and hopefully condense it to fresh water. I figured I’d be able to do this in the shade of coastal trees, tending the fire and replacing the cooling water occasionally. It just might work.

My design points for the D-SAL were these: it had to be light, simple, robust and fit in the kayak through a 20 cm round hatch. It also had to produce a practical amount of fresh water. No sense boiling saltwater all day for one litre of fresh water.

For the Mark 1 D-SAL I used a 1.5 litre aluminium Sigg bottle — the largest size available. This tall and narrow bottle had a plastic screw lid which I replaced with an O ring sealed aluminium one to which was welded a 700 mm length of aluminium tube.

To the aluminium tube outlet I attached four metres of 10 mm ID ‘food grade’ plastic tubing which was coiled in a collapsible 10 litre water container (Sea to Summit ‘Kitchen Sink’.) This would hold the cooling water. The coiled plastic tubing connected to a condensate catch bottle. Another ‘kitchen sink’ was used to fetch more cooling water. So far it was ticking all the design point boxes. Testing was next.

I filled the Sigg bottle with a litre of salt water and lit a small fire under it. The bottle was in danger of tipping over, even with the water ballast in it so a stake in the ground near the fire and a cord tie to the plastic tubing steadied the whole affair. Boiling commenced quickly and steam moved along the tube and out at the catch bottle. However the temperature of the steam exiting the catch bottle told me that the plastic tubing in the water bath was not a satisfactory conductor of heat out of the system.

Worse was to come. The boiling activity in the Sigg bottle became so vigorous due to the tall shape of the bottle (low surface area to volume ratio) that some of the salt water was carried up into the aluminium tubing where steam pressure forced it over the top… and then to the catch bottle. The condensate was contaminated. Bugger!

The two problems with the Mark 1 D-SAL were: wrong shape boiling vessel and poor thermal conductivity of the plastic tubing. The consequence of those problems was that I would be drinking salty water… and not much of it!

I hoped the Mark 2 D-SAL would address these problems.

I believed the condensate recovery percentage and rate of recovery to be a function of the thermal conductivity of the condenser tubing and the boiler surface area respectively. I began the search for a wider boiler. After skulking around home with a tape measure in hand, I nicked an 18 cm aluminium saucepan from the kitchen. It was now destined for better things than boiling cabbage! My local engineer welded the lid on and also added two large diameter threaded hole bosses to the lid. One hole was for the tube cap from the Mark 1 version and the other was a filler hole with cap for replenishing the salt water without removing the unit from the fire.

The condenser tube now became a l.6 m length of coiled 10 mm copper pipe with short lengths of plastic tube connecting each end to the boiler tube and the catch bottle. The copper was a little heavier than I wanted but the total weight of the Mark 2 was a respectable 850 grams.

In set-up the wider saucepan was very stable and no stake was required. Half full in the saucepan was 1.5 litres of seawater. On the fire and steam began to flow very soon. But then something different to the Mark 1 unit’s performance happened: the steam was fully condensed in the copper tube and pure water was flowing full in the tube to the catch tank — a 100% recovery rate! The boil-dry time for the 1.5 litres was about 50 minutes. The D-SAL thus has the potential to deliver 12+ litres of fresh water a day.

The taste test showed very very slight contamination of the condensate but the water was in no way unpalatable or objectionable. I think that if super-pure water is needed, the condensate could be run through the system a second time. In future testing I may try an expansion chamber on the aluminium tube or a baffle at the base of the tube. I’ll also test the salinity with a hydrometer.

Future testing may include larger diameter tubing but as the tubing diameter increases, the ratio of circumference (tubing wall) to cross sectional area (steam) changes so I think I may already be using the optimal size tubing. I will test thin wall copper and aluminium tubing for the condenser which will reduce the weight of the unit to under 700 g, significantly less than a litre of water. The thermal conductivity of copper is much greater than aluminium so the weight saving may be balanced by reduced efficiency — has to be tried though!

Using this D-SAL unit I believe a remote area paddler could partly or fully replenish fresh water supplies on a daily basis. Seawater and firewood are all that is required, and I’ve only camped on a few sand cays where both of these ingredients weren’t available. If I could leave 30 or so kilograms of water behind, I could replace it with at least another 30 days worth of dried food! This for example, would make a crossing of the Great Australian Bight feasible without any re-supply whatsoever — no more reliance on water and food drops.

If there is a downside to this distiller, it is that the operator has to keep the condensing water fairly cool, necessitating regular trips to the ocean… well… in between cups of tea that is! There is also the chore of keeping a small fire burning under the pot.

So, there you have it so far. It’s not perfect, more refinement is required and it may only have appeal for a few paddlers, but it does open up possibilities for lengthy island stays in the tropics and elsewhere.

I’d be pleased to hear of any suggestions you may have to improve the operation of the D-SAL.