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Safe drinking water for yachts, and stop using plastic bottles

Many bluewater yachts travel to far flung places where the water supply may be contaminated by parasites and bacteria.  Indeed, it is a certainty that you will be exposed to parasites such Cryptosporidium, Giardia, and nematodes.  You will also be exposed to bacteria such as Vibrio cholera and viral infections, so how do you prevent disease and illness.

There are two aspects to water quality, water chemistry and water pathology. 

Water chemistry

Some filters will remove, chlorine, chlorine disinfection by-products, fluoride and dissolved organics such as herbicides and pesticides if they contain active carbon, but they will not remove dissolved salts. It is therefore useful to have a TDS meter to measure the salt content of the water.  The TDS meter can be used to make sure the RO unit is working properly; it can also be used to check if your fresh water has not been contaminated by salt water or other salts in ground water or from the sea.  The TDS should be less than 500mg/l, do not consume if it is above 1000mg/l.

The rest of this document will centre of the biology of drinking water and how to ensure that your drinking water is safe to consume.  It also means that you will not need to purchase plastic bottles of drinking water.  It is a false assumption that bottled water is safe to drink. I have travelled to many locations where the bottled water was filled from a contaminated source water, so don’t trust bottled water in low-income countries.

Step 1.

Always use your own hose to connect to the water supply, and let it run for at least 5 minutes, or until you feel the temperature of the water drop.  Water can be sitting in the supply pipe for many hours, it gets warm, and this can incubate bacteria, so let it run to discharge this plug of potentially contaminated water.

The water can be disinfected by use of chlorine or chlorine dioxide. Both these chemicals sound the same, but they are very different. Chlorine is usually supplied as a solution of hypochlorite (bleach), but there may be other chemicals in the solution, so only use Calcium Hypochlorite (CalHypo) which will be supplied as a tablet, usually 20g tablet which you can obtain in swimming pool stores.


If you using chlorine as CalHypo, the available amount of chlorine should be in the region of 30%, so a 30g tablet has 6g of chlorine. You want to try and achieve a concentration of about 2mg/l in your water tank. If there are 6g of chlorine in the tablet then you need 1/3rd of the of the tablet ( 6.6g) to achieve 2mg/l in 1000m3, or 1/6th of the tablet for 500 litre tank.

2mg/l is actually quite a high concentration of chlorine, however you don’t know the quality of the water, so it is always best to be on the high side.  If you have added sufficient chlorine, you should be able to smell the chlorine when you turn on the shower.  The chlorine smell isn’t chlorine, it is a reaction product between the chlorine and ammonium and organic matter that forms other chemicals such as chloramines and chloroform.   You can test the water for residual chlorine, however 2mg/l should kill all bacteria and viri in the water when given a contact time of around 1 hour.

The chlorine will not kill parasites, or water insects and bacteria and viri can hide inside these organisms, so they need to be removed.

Chlorine Dioxide

Chlorine dioxide can also be purchased as a 20 g tablet, but they are not so common and may be more difficult to obtain.  A 20 g tablet will contain 2 g of chlorine dioxide, but you only need to achieve 1 mg/l with chlorine dioxide for the chemical to be effective. So ½ a tablet or 10g will treat 1000 litres of water, or ¼ of a tablet 5g will treat 500 litres of water.

Again, give the water at least 1 hour to allow the chlorine dioxide to work, or better still fill that tank at night and leave it to sit overnight to work.

Chlorine dioxide is better than hypochlorite because it does not form chlorine disinfection by-products and is much more effective against parasites. However, to be sure, the water is safe to drink, you need a second stage to the treatment process.


Stage 2.

The water has now been treated with either calcium hypochlorite, or chlorine dioxide, but it is not yet safe to drink.  It is best to remove the residual chlorine and chlorine dioxide as well as any disinfection by-products.  The chemicals can be removed by passing the water slowly through a carbon filter.  This must be a slow process, no more than about 500ml in 1 or 2 minutes. The flowrate will depend upon the size of the filter, but it will be in this time frame. Filters fitted to the water supply in-line when you are filling your tank will never remove the chlorine or by products, indeed the filter will do almost nothing to improve the quality of the water.  Filtration must be a slow process, or it will not work.

The carbon will remove the chlorine and by-products, but it will not remove water insects and parasites. To be sure that most parasites are removed, the filtration level should be down to at least 1 micron. This is the size of most bacteria, so if you achieve this level of filtration, combined with an active carbon filter, you can be pretty sure that the water is safe to drink.

There are many filters available that can achieve this task, and some are very expensive.  Onboard s/v Copepod we used a ceramic carbon filter from a UK company called Doulton The filter that we use is the UltraCarb which is a combination of active carbon and ceramic mechanical filter combined in the one filter element.

On s/v Copepod we also use a version of UlrtaCarb filter element that is OBE (open at both ends), the reason for using an older design is that the filter element is compatible with industrial standard 10” filter housings, so if the filter breaks (which shouldn’t happen) it should be possible for us to get a replacement from a swimming pool store.

UltraCarb OBE filter element

10” water filter housing

Tap example

In addition, you will need the hose and fittings etc, to connect to your pressurised water supply.  However, the total cost should come to under 100 Euro. One filter element will provide you with in the region of 2000 litres of safe water.  This means you save having the purchase expensive bottled water and avoid the single use plastic, and you can be sure the water is safe.

Depending on how good or poor the quality of your source water, you will need to remove the ceramic filter every few weeks or months to give is a clean to remove some of the scale and solids on the surface of the element to restore the water flow.

Be safe, drink plenty of water, and make sure that it is safe water.

By Howard & Diane Dryden, s/v Copepod



Life on earth depends upon healthy Oceans, we have 10 years to stop toxic chemical pollution, or life on earth may become impossible

Dr. Howard Dryden, CSO

Goes Foundation

Roslin Innovation Centre
The University of Edinburgh
Easter Bush Campus
Midlothian EH25 9RG