When it comes to liquid assets that cost the Earth, have you got the bottle to drink tap water?

What's up with water? Some consumers are willing to pay up to US$100 for a litre of the stuff in a bottle, in the belief it is healthier than the water that flows freely from the tap. The pricey liquid in question, 420 Volcanic, is marketed as natural spring water filtered through 200 metres of volcanic rock.

But while the World Health Organisation acknowledges that in some instances (such as in third-world countries) bottled is preferable to piped supplies, it says certain packaging materials and micro-organisms can have a greater negative impact on bottled water than the free-flowing alternative. Hong Kong's Water Supplies Department discourages domestic water filters as these can provide breeding grounds for harmful bacteria. Corrosion-prone unlined galvanised pipes for domestic water supplies have been banned since 1995. Those living in older buildings are advised to run a tap for several minutes in the morning to flush out impurities that may have built up overnight.

Hong Kong teacher Kathryn Kelly drinks about three litres of both bottled and tap water a day. Her choice is driven by convenience, availability, taste and sometimes aesthetics. 'I have bought this new spring water from Fiji in the nice bottles with artwork that is clearly trying to make its contents look as 'natural' as possible, but I don't think I actually believe the water has been captured in volcanic springs,' says Kelly.

'I tend to buy one or two bottles of water each week and then top them up in my gym or workplace, either from the tap or the water fountain.'

At home, however, Kelly buys bottles of the cheapest natural spring water from the supermarket. Her flat is in an old apartment block and she's not sure how clean the pipes are, she says.

While the body needs water to survive, it does not look to water as a source of minerals, preferring instead food sources such as meat, vegetables, fruit and grains. Therefore drinking bottled water to boost mineral intake is not necessarily the best course of action. The main minerals the body requires are calcium, magnesium, sodium, potassium, phosphorus, chlorine and sulphur, yet water is not the best source for any of these. Even among the required trace elements (a requirement of less than 100mg/day) only fluorine occurs naturally in water. Many bottled waters contain little or no fluoride, while some water authorities, including Hong Kong, add it to the water supply in areas where naturally occurring fluoride is low.