Despite the seemingly endless varieties of H2O, there are three categories in Hong Kong: tap, distilled and mineral water, the latter an umbrella that includes everything from purified tap water with added minerals to top-of-the-range natural spring waters from France.

According to the Water Supplies Department, Hong Kong has one of the safest tap flows in the world, complying chemically and bacteriologically with World Health Organisation (WHO) standards. It is soft water, has low to moderate mineral content and is low in organic matter.

About 80 per cent of Hong Kong's water comes from Dongjiang's East River; the rest is collected from local rainfall. It is stored in reservoirs before being pumped to local water-treatment works for purification, filtration and treatment with chlorine and lime to remove bacteria. No minerals are specially added, apart from fluoride to minimise dental decay.

But although tests to certify its quality are rigorously and continuously conducted, critics say the river is adversely affected by urbanisation and industrialisation. What's more, the water meets WHO standards only at, and immediately after, leaving the treatment works; ancient storage tanks and unlined galvanised pipes (which were once widely used in Hong Kong but have been prohibited since 1995), can contaminate water on its way to taps and alter its taste.

'The present problem of water quality is not from the treatment system,' says water specialist Dr K C Ho, the programme leader in environmental studies at the Open University of Hong Kong. 'It is from the rusty distribution system, which is not properly maintained. Broken water pipes and dirty water tanks are the problem.'

However, Mimi Sham, a registered dietician in private practice, says Hong Kong's tap water is safe to drink and cheap too. The added fluoride is important for dental development and the only problem can be too much of it.

'Chinese people boil up huge pans of water with which to make soup for daily consumption,' she says. 'This leads to an over-concentration of fluoride in this water, which isn't good for children whose teeth are still forming because it affects the enamel, causing discoloration to the outer layers. If you're worried about that happening, pay close attention to your child's teeth and cut down on the amount of soup you give him or her to drink.'

Distilled water is simply the condensed steam of boiled tap water, and although it is free from harmful bacteria, it is devoid of all good minerals too. Freshwater fish perish if they are kept in a tank of distilled water because they need minerals to survive. In the past, doctors, nutrition experts and scientists have commented that if distilled water is ingested by humans over time it can leach out valuable body minerals such as potassium, magnesium and calcium, which are respectively required by the body for regulating the acid base and water balance in blood and body tissues, maintaining healthy teeth and bones and for the body's metabolic function.

Sham disagrees. 'Theoretically, we don't depend on water for our daily doses of minerals and trace elements - although fish do,' she says. 'The best thing about distilled water is that it's clean, although critics would pan it for its lack of fluoride, for its lack of anything. Most people cook using tap water - even if all they drink is distilled bottled water. But even drinking and cooking exclusively with distilled water, you would still get the minerals you need from food.'

According to Ho, saying mineral water is better than distilled - or vice versa - is controversial. 'Every kind of bottled water has its advantages and disadvantages,' he says. 'Some minerals are good, but sometimes an overdose of them may cause problems. Distilled water isn't favoured by most nutritionists because it doesn't contain the necessary amount of minerals for the human body. Moreover, some scientists claim that distilled water is too 'pure', that it causes a lack of osmotic balance in blood and cells.'

Watson's Water, which has been selling distilled H2O in Hong Kong since 1903, recently introduced a version with added minerals such as potassium, calcium, magnesium and sulphate.

Chris Atkins, the general manager of the company, echoes Sham's sentiment. 'There is no nutritional benefit to the minerals in any water, whatever the source,' he says. 'From a health standpoint, the inclusion of minerals is therefore not relevant and it then becomes a question of personal preference. We decided to introduce water with added minerals to increase consumer choice with the purest water available. There are no legal guidelines for any style of mineral waters in Hong Kong, unlike other countries such as the United States, which has minimum mineral-level specifications of 250 milligrams a litre. Our water exceeds this standard and has the benefit of starting with a base of pure water to which minerals are added - as opposed to the filtered waters used by other brands.'

While Atkins won't divulge Watson's mineral formula - this is, after all, Asia where copying is rife - he stresses the company has looked to international specifications to ensure its mineral content is healthy.

Consumer choice may ostensibly be Watson's main motive for launching this new product, but figures from market researchers AC Nielsen (comparing the period from August 2000 to July 2001 with the previous 12 months) show that distilled water seems to be losing ground to its mineral-rich rivals. The former grew a mere 0.6 per cent compared with a 24 per cent increase in mineral-water sales (excluding the consumption of carboys, the large water bottles used with dispensers in homes and offices).

Atkins points out that the growth in the mineral-water segment is from a relatively small base whereas distilled water makes up about 80 per cent of the market.

Mineral water is collected and bottled at the point of emergence from a protected underground source. According to European laws, it must contain a consistent level of natural mineral salts and trace elements to qualify for the name, although there is no minimum content requirement. In the US, the Food and Drug Administration (FDA) regulatory body defines mineral water as 'untreated water from a natural source containing no less than 250mg a litre of minerals'. In Hong Kong, labelling laws don't regulate this definition or enforce any distinction between various types. You might assume that a bottle labelled 'mineral water' would be naturally occurring, mineral-rich H2O such as Volvic, but it could simply be purified tap water with minerals added afterwards, such as the Bonaqua brand.

'There is no natural spring in Hong Kong, so technically we're the only local 'mineral water' per se,' says Neil Waters, the sales and marketing director of Swire Coca-Cola, which bottles and distributes Bonaqua from its plant in Sha Tin. 'We did, and still could, market Bonaqua as 'mineral water' [older 18-litre bottles still bear this label], but we wanted to use the correct term 'mineralised water' because we want our customers to know that they are receiving the same standard and taste of water, year after year.'

Natural mineral water gets its taste and content depending on the type of rocks the rain falls on and how long its drops take to filter through. According to Waters, its content can apparently change - albeit slightly. Bonaqua draws water from the government water supply, imitates the filtration process for natural mineral water, passes the water over two sand filters and an active carbon filter, adds a concoction of minerals specified by Coca-Cola's strict guidelines and then purifies it.

'We try to replicate the natural process, so our water is as near to the natural product as possible,' says Waters. 'We also obtained the water industry's version of the ISO 9002 [the international certificate awarded to companies that conform to certain standards of quality and service] from the American National Sanitary Federation, so we have to satisfy its people routinely - it's not something you earn once and for good - as well as going through Coca-Cola's hoops to deliver the quality it requires.'

This year, Bonaqua stepped up its advertising and marketing campaigns to create a strong brand awareness, and in Waters' opinion 'to educate Hong Kong people about the difference between distilled and mineral water'. One of its more recent television commercials showed a man giving distilled water to his robotic dog - reinforcing the idea that you use it in appliances such as steam irons and car batteries - while choosing to imbibe Bonaqua himself. Another, featuring a thirsty office worker, implied that by picking distilled water she was drinking nothing.

'Distilled water satisfies your thirst - and that's all,' says Waters. 'There's nothing in it. You might get only a fraction of your daily requirement of essential minerals from drinking water but at least Bonaqua and other such mineral waters are assisting in the intake, however small.

'When you lose mineral salts playing sport or hiking, you need to replenish them as quickly as possible. At least by drinking mineralised water you start to replace what you have sweated out. It's better than drinking nothing.'

That said, there are some reservations about the benefits of mineral water.

'It's fine for a healthy individual to drink any kind of mineral water,' says Sham. 'But if a person has kidney problems or hypertension, he or she must avoid drinking water with a high sodium content. [Badoit fans, beware: anything more than 110mg is considered high; Badoit contains 150mg a litre.] Similarly, if you suffer from calcium stones you don't want to be consuming more calcium than your body can take.'

Carbonated mineral water can also exacerbate bloating, and if distilled water is your regular tipple, buy only bottles from well-known companies.

'I have come across a lot of bottled waters that claim to be distilled but are really tap water, especially on the mainland,' says Ho. 'So it is very important to buy a reputable brand.'

Everyone agrees the regular consumption of water is a must - eight glasses a day, more in hot weather or if you're exercising. So whether you prefer the cachet of having Italy's finest on your desk or are happy with H2O from the Dongjiang, the best advice is to drink up.