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Purifying Fresh Water Systems

Annual routine

Looking after your fresh water system is an annual routine.

  • Drain in Cold Climates. First, if leaving the boat in a cold climate, to avoid frost damage, drain the whole system, pumps, accumulators, calorifiers, everything.
  • Purify. Second, commissioning for the next season (or if at any time you think you've filled from a suspect water supply) disinfect the system. This note describes a simple chlorination process which is effective against bacteria, viruses and other micro-organisms.

Sodium Hypochlorite

Sodium Hypochlorite is routinely used to purify drinking water supplies - at 1/5000 dilution. Higher concentrations are used in swimming pools. It's available as:

  • ordinary household bleach (not 'thick bleach') in a 3% to 5% solution
  • as a brewery cleaning product at a higher concentrations
  • as "Milton", a branded <1% solution at many times the price.

Purification Routine

The cleaning routine is commercially well established. The most difficult part is ensuring every part of your water system is exposed to hypochlorite.for a long enough time.

  1. Partially fill your system with 100 parts water to one part household bleach (the 3% to 5% stuff).
  2. Run this solution through all your pumps, pipes, accumulators, calorifiers so they are full. Leave for about half an hour (half the concentration = 4 times the exposure).
  3. Empty the system through each of your outlets; all taps, hot and cold, showers.
  4. Flush twice with a large amount of fresh water.
  5. The system will still smell slightly of chlorine. That doesn't matter. To reduce the smell, flush all pipes through with a dilute acid - 1/10 solution of vinegar will do - or add 5cc hydorgen peroxide per 100 litres of water. Alternatively, use a charcoal filter for the drinking water tap.
  6. Final flush and fill. You've done it.

In Season.

Keep micro-organisms at bay. If needed in season, kill micro-organisms with a spoonful of bleach (10cc) per 50 litres. Sensitive noses should just be able to detect a faint whiff of chlorine at the tap. Silver or other noble metal compounds in pill form are also marketed - higher in price, less smelly.

Mould Growth

If any light gets into your water storage system (through transparent pipes, or translucent tanks) then mould growth is likely. That's a scrub out job before disinfecting the tanks. Portable tank storage should always be in black containers.

Water Filters

Charcoal filters remove smells and improve taste effectively. They don't filter bacteria. Other filters, depending on pore size, may be able to remove bacteria or protozoa.

Filters which can remove viruses are very small pore, and need pressure feed. Seagull filters are an example. Costs from around £300. A spare filter will be needed once a year for live-aboards

Water Makers

Sometimes you may end up cruising an area where potable water is not available. You'll either need big tanks (500 to 1,000 litres), or a water-maker, which is quite expensive.  These pump sea water at high pressures through membranes to create reverse osmosis - filtering out salt ions. They cost £1,000 plus, and use a moderate amount of power. Regular use and maintenance is needed to keep the filters operational. They also remove viruses.




Reviewed May 2016


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