Desalinator Watermaker

The watermaker or desalinator is a portable device that allows the removal of contaminants from normal saltwater, to clean drinking water.

1/ Introduction
Our original questions were why was it so expensive? We scoured the internet for units which all ranged between AUD$5000 – AUD$15000 and with many of these packages, there was no generator or battery source as part of that package, or the unit was so small in output, it was almost no worth having and going to that expense. Building a unit ourselves was the only viable alternative.

Once tested, we will make available 'Pure Majek Desalination', where the data listed below will be available in much greater detail.

By nature, we human beings will always try and find an alternative or cheaper way to build or repair an item and this ‘calculated risk’ needs to be just that...very carefully calculated. Many marine environmental challenges can exacerbate component failure, forcing costs go up and the manufacturer becomes the easiest scapegoat for ones shortcutting.

Based on Professor James Reason’s ‘safety model’ sometimes referred to as the ‘Swiss Cheese Theory’, components are specifically chosen for their particular qualities plus safety reserve and modification of these components (as an example, switching to a cheaper component, or replacing a stainless steel nut with a plastic nut or upgrading output without increasing the pump components) only further adds to system failure.

Those who test these boundaries appear be the minority and are the ones we consistently meet conducting repairs and complaining of how labour intensive the desalination task is or how bad a particular unit is and when you dig a bit further you find the dollars they saved initially during set-up cutting corners, come home to roost.

Understanding some of this waffle will go a long way in helping grasp why certain items appear to be over-the-top with our choices and as stated previously, there are many ways to skin-a-cat when making a desalinator.

 

2/ Calculating the freshwater needs
The summary of the desalination argument was that we needed to generate sufficient fresh water to cover our use [the bold type being the key words] not that we have anything against those who enjoy the spiritual virtues of salt-water bathing.

For the seasoned sailor, the next few paragraphs are not for you. For the amateur builder hoping to win the heart of his water-loving wife/partner and kids, continue reading.

We jotted down the water requirements we thought would be our comfortable minimum per person and added the required buffer.

The other consideration was the running of a generator to drive the desalinator. It was not to exceed one hour a day…period.

The Water Matrix provides a minimum freshwater requirement per day and becomes a key for calculations further on. To calculate freshwater tank size, we then multiplied the daily figure by two.

That would give us a comfortable two days of freshwater or a good six days if rationed supplies with a non-functioning desalinator were that to ever eventuate.

 

3/ Selecting the High Pressure Pump
The time that one wishes to have a motor purring (rumbling, vibrating or pumping fumes) needs to be carefully assessed as this will reflect the pump size that needs to be purchased. Our choice limited this to one hour per day.

This meant that in 40 minutes, we had to generate all the water we needed to top up the tanks. The other 20 minutes of the 1 hour run time was to be used for any charging or 240VAC requirements, such as a washing machine (another wish-list item).

For those about to start the numbers journey themselves, you will soon notice the challenges about to be faced in selection of a high pressure pump, pump motor and whether AC or DC motors should be used.

The pump size is directed by the freshwater needs plus an additional 85% to provide the total saltwater uplift. The membranes require this volume amongst other reasons, to keep the system cleared of the brine and concentrated salts. Only 12-15% of saltwater taken through the high pressure membrane in an amateur build, is returned as fresh water.

This type of pump is known as a ‘high pressure pump’ (HPP) as it increases saltwater inlet pressure from low pressure - around 0.55 bar (8 psi) to high pressure - around 65 bar (950psi).

There are some good and bad high pressure pump manufacturers out there too, our final choice being a Cat Pump given that many of the off-the-shelf desalinator companies use Cat equipment for their reliability, pump RPM and component build.

The cost of the various HPP types does vary from vendor to vendor and bronze is cheaper by 20%, so it pays to shop around. A close search was made for a second hand HPP unit on Ebay and we did find a few.

BUT, with all of them – Do the internal components need replacing? Has the pump been dropped? Has the pump seized? (one respondent here stated ‘no, I can still turn it with a big wrench’) etc. We could not get a component guarantee for this critical high pressure part, which is why we chose to buy new (remembering the Swiss Cheese theory).

 

4/ Motor to drive the HPP
The choices of motor that drives this HPP unit are varied and can be very confusing. We found that the amount of power required to run a largish electric motor (3hp in our case), was enormous.

It becomes very restrictive beyond 1-1.5hp electric motor sizes, which is why many tend toward the belt-drive units. The challenge with AC motors is that they need three times their running power requirement just to break the inertia during start-up. As a rough guide, this meant that to start 3hp motor, one would need a 5-6hp generator to get it running (this is not strictly correct, but a good guide for an amateur build).

Other avenues we did look at were soft-starters which proved well in theory, but as we found out would not be suitable for an amateur desalinator build.

Searching for a motor to drive the belt-drive, took some time. Our decision to minimise holes in the hull meant that saltwater cooling became a last resort. This left freshwater or fan cooled motors our preferred option.

Finally, our choice was between the industrial diesel fresh water cooled generator from a Yanmar (TF60 - 67kg) or the unleaded air cooled industrial Honda (GX200QXE - 26kg). Weight and cooling requirements dictated the direction here as they were the same price.

Another motor unit that came in at the same price was the Honda 20i petrol (roughly 2hp (20i) vs 5.5hp (GX200), 21kg vs 26kg). The Honda 20i could not physically power the desalination unit unless we inserted an electric motor and downsized on the HPP unit as discussed earlier. Also, in this situation would be a very inefficient way of making water.

 

5/ Seacock
So do we use stainless steel, bronze or plastic? As far as we are concerned with in our situation, stainless steel or bronze would be fine. The continual opening and closing, will take its toll on the seacock unless correctly installed on a very solid base.


6/ Raw Water Strainer
We found that options here are limited and expensive. Given that the pressure in the lines prior to the Low Pressure Pump (LPP) are very low around 0.55 bar (8 psi), a standard plastic strainer from a reputable manufacturer could be an alternative to the bronze types. The Filter insert should be of the stainless steel type and easily replaceable.

Strainer units with a bowl at the base make it very easy for cleaning and the odd mud-skipper or snail can be quickly removed before decomposition occurs

 

7/ Low Pressure Pump (LPP)
This would be the second most critical component in the whole desalinator system. The unit must be capable of providing a positive flow of corrosive fluid at a rate higher than that required by the manufacturer of the HPP. During the planning stages, the flow rate should be calculated plus a safety reserve that covers environmental variations, aging and ‘build characteristics.

Failure of the LPP at any stage could cost in repair as the feed source of the whole system now stops.

Two LPP options are available here, the first being a centrifugal type pump and the second, a self priming non centrifugal unit.

 

8/ Filtration
This is provided by way of three 10 inch canisters, two canisters for desalination filtration and one for mains water filling.

Firstly mains water, this contains chlorine which is very harmful to the system. To counter this, we have inserted a filter housing with a good quality carbon filter. Once filling is complete, the carbon filter is removed and then dried for at least a week before storage.

The other two canisters must be capable of corrosive fluid transfer and carry a safety valve. These too must be in a very easily accessible position for maintenance, some saltwater will also be lost below the canisters during the filter changes.

 

9/ Membrane Housing
Housings for high pressure membranes (that are used in desalinator's) are normally found in Stainless Steel or fibreglass. Rigid PVC is an option if the correct pressure rating can be found. We chose 2 x (2.5” x 40”) Fibreglass housings, commonly listed as FRP.

At the time of writing we found that an initial higher expense in the more popular 2.5” x 40” housings, linked in series as appropriate proved cheaper in the long run with replacement membranes and as a last resort, redundancy.

In the event one membrane failed with no replacement membrane – just removing the membrane and continuing with the desalination on the remaining membrane will still provide fresh water at a reduced rate.

We found that increasing the number of housings did not increase the amount of freshwater given our careful planning of the pump size

10/ Membrane choice
Here there is a choice between saltwater units, brack-water and standard house-water membranes.

All are specific, especially the saltwater units and these can sometimes be picked by the prefix ‘SW’ on the membrane item number. It is important to ensure that this is closely followed, as error here will cost dearly down the line with component failure.

Other digits on the membrane can indicate their unit size (as in our case, 2540 indicates a 2.5” diameter x 40” long), simple when you know how. Not all membranes are listed like this though.


11/ Valves – Solenoid / Motorised
There are two main types of valves used with desalination units, those that are Motorised and those operated by a Solenoid.

The main reason people choose one over the other is the speed of closing / opening of the valve. The motorised units are a lot slower, anywhere from 4 – 12 seconds from one position to the other extreme. When compared to the solenoid speed of ½ a second, there is a huge difference.


13/ Flow Meters
Flow Meters are used to provide a visual cue to the operation of the whole system. There are normally two placed in the system and the preferred position is ‘panel-mounted’ somewhere.

Panel mounting does incur additional plumbing requirements, but allows affective use of the flow meters. The meters show freshwater output per minute and brine water output per minute, the latter having much higher flow increments per min or hour than its freshwater counterpart.

These proved the hardest to track down. While there are a few quality manufacturers around many have agents throughout the world, some quick money spinners and others better priced.


13/ Gauge Monitoring
Both Vacuum and Pressure gauges are installed to ensure smooth operation of the system, but more importantly, the effectiveness of the components with fouling and wear.