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In India, never drink tap water without boiling it. In Delhi, when you ask nicely at any restaurant, they will usually get your bottles filled with hot water. There will be proper filtered water available in most hotels and hostels where you can fill up. The blue drums will be found in many places and you can ask where any of those are found.

Also, bottled water can be cheap compared to your country. You can get 1 liter for Rs. 20 (0.29$). Never hesitate to buy if you are running out.

Just a personal note. I have faced serious health issues, even being hospitalized while in Delhi, while drinking water without boiling or from tap. Be cautious about this.

I've lived in India for five years, and I believe it's better to err on the side of caution here.

Why?

It's important to remember that as a foreigner, your immune system is quite differently equipped compared to that of the locals. Indian ground water and tap water is often contaminated with various bacteria, other pathogens, as well as toxic chemicals from agriculture, industry, etc. The most common and most reliable way to truly remove all of this is a quality system of Reverse Osmosis (RO), preferably with ozonization or UV for redundancy. This system also needs regular maintenance and quality control, and the membrane has to be replaced regularly. Anything they give you in a restaurant or hotel that is referred to as "filtered water" or even "RO water" may or may not have been produced according to such standards. Or some employee may have washed a vessel with tap water before filling it with this water, etc, etc. In some cases it may be satisfactory for the immune system of most locals, but not for that of a foreigner.

Therefore

For drinking water, stay away from all fountains, taps, non-sealed and non-branded vessels , and the like. I would stick to this advice even in the airport and at hotels. Also use bottled water for brushing your teeth.

Buy bottled water of a known brand. (Bisleri, Aquafina, Kinley.) It's preferable to get them from a large supermarket or other trusted source, to avoid counterfeit bottles. Your hotel will probably sell them. Make sure the caps are sealed. You can typically get 5L or even 10L bottles. If you do this, though, make sure you don't get the type where the same bottles are reused, as the refilling process can often be unhygenic. Below is a picture of reusable bottles that you want to avoid. They feature quite sturdy plastic. These are 20L, but smaller varieties and shapes also exist.

If they ask for a deposit on the bottle, and the bottle is exchanged back for another bottle after use, then you know you're dealing with reusable bottles. Some of my friends have had water tests done of the reusable bottles (of a known brand) and found that they are more likely to contain various bacteria. The tests of the disposable bottles, on the other hand, turned out fine. If there is no deposit on the bottle, then you know that each bottle is new.

Stay away from reusable bottles, even from known brands. Any local refilling facility may or may not have their procedures in place. These bottles may in some cases be washed with tap water between use, or even not washed at all.

Here is an example of a disposable bottle (which is what you do want) from a trusted brand:

This bottled water is quite cheap here. A 1L bottle costs €0.26. A 5L bottle costs €0.87.

Personally, I have my own RO system, UV filter and ozonizer, but that's usually not an option for short-term visitors.

Since most of the bottled water here is RO processed, it's good to choose a type that has added minerals in it (Bisleri, Kinley). The RO process removes all bacteria and harmful chemicals from the water, but it also removes all the useful minerals.

Environmental aspect

For those concerned with the environmental impact of using plastic bottles, you can make sure the plastic is recycled (as opposed to ending up burned or dumped in a field somewhere). In India, there are people who collect plastic for a living in order to sell it for recycling. You could give it to those workers, perhaps along with a tip, as such people are typically poor.

Other than that, if you're worried about the plastic, then the only real and safe option that removes both pathogens and toxins is some sort of RO system, but for a short trip you'd have to consider if the environmental footprint of buying a whole system is really smaller than some plastic bottles.

Delhi has a few water ATMs which dispense potable water at a really cheap price(~ 0.07$/liter). Although you have mentioned that you don't want to purchase water bottles, I'd like to mention that water bottles are relatively cheaper (not more than ~ 0.30$/liter ) in India. If you are traveling really cheap, don't hesitate to knock on a roadside house door and ask for a free refill. People are more than happy to offer you water.

Almost everywhere I go, I carry a “Grayl.” This has allowed my to drink from irrigation ditches, small puddles, etc. with no ill effects.  Replacing a filter cartridge every three months or longer for $45 (US) definitely beats buying bottles of water every day.  Although I prefer the Grayl, it has many competitors, some of them quite good.

Please make sure to ABSOLUTELY NOT drink water on Railway stations from so called RO-filters. Those machines haven't actually had their filters replaced in years. You will get sick drinking that water.

Trust NO water in India except after carefully checking the spelling, lables 'Bisleri' and 'AquaFina'.

Source: I am an Indian.

While living/travelling in several locations, me and my wife, usually, we get by drinking potable water using:

• the free water bottles the hotel provide as courtesy;


• distilled/filtered/boiled water in restaurants given as courtesy;


• buying them from local supermarkets/stores - 2L/5L, and leaving it in the room for (re)filling up 20cl bottles a few times;


• we also carry often a metal water container when travelling by car that is refilled at least daily;


• if going out for the day, buying at least a couple of 1/1.5L bottles;


• if residing a couple of weeks in a single location e.g. family home (my wife is an expat Filipina), we buy one or more of those 20L blue container/drums ourselves or whatever we find available in that location (the more hassle free to get are usually 5L-7L bottles pretty much everywhere around the globe).

As a rule, just avoid buying bottles at the hotel or restaurants for tourists, where the prices are way inflated. Otherwise, they are fairly cheap compared to our prices back home (in Europe).

PS. We stayed in a 5 start hotel in my wife anniversary for a couple of days two weeks ago in Manila, and we bought a 2-liter bottle of water in a supermarket that we left in the room. The hotel left a lot of complementary bottles seeing that bottle, which was a nice gesture of them.

PPS Concerning the quality of bottled water, we usually aim for bottles that state "mineral" water. Beware that Coca Cola/Pepsi/Nestlé owned water brands (that are found in pretty much any continent/country) are usually distilled/boiled/"purified"/"mineralized" tap water (e.g. Nestlé, Dasani and that Aquafina brand in the comments). We only buy those latter brands when not getting any bottle of mineral water, the price difference is not that significative.

see Pepsi Admits Aquafina Bottled Water Is Plain Tap Water

Pepsi released a statement admitting that Aquafina -- its brand of
bottled water -- is not purified or sourced from some majestic
mountain stream, it's just plain old tap water.

Don't reuse disposable plastic water bottles. This is a practice can cause significant health risks, even in a First World country with a reliable supply of tap water.

These bottles were designed by their manufacturers to be used once, then thrown away, and all of their safety testing is designed around that assumption. As a result, the plastic begins to rapidly degrade when the bottles are reused, and this can cause two main health risks: first, the tiny crevices and abrasions that form in the plastic as a result of this degradation can serve as breeding grounds for bacteria - and washing the bottle in water hot enough to kill them just accelerates the process of this degradation. Secondly, the degradation of the plastic can release potentially-toxic chemicals into the water inside it that can be harmful for your health.

Going against the grain here: Water is not the only place where bacteria live, so if you haven't been to a country before, that usually means you will stumble across some new ones. You cannot avoid touching door handles, and you wouldn't lick them anywhere else in the world, so you will have moderate exposure anyway.

Make sure your vaccinations are adequate and leave some time before the trip for the immune system to get back to normal. Your doctor should also be able to give you additional information I might have forgotten.

Take it easy during the first week, leave some room to acclimatize yourself in a low-stress environment and expect to get slightly sick whatever you do, so prepare for that:

• paracetamol against fever/headaches


• loperamide against diarrhea


• dissolvable electrolytes (to replenish after diarrhea)


• something to wash down the taste of the electrolytes except if you believe bad taste is an important part of medicine

Delhi is in a malaria zone, so NO aspirin/ibuprofen/diclofenac!

Regarding water, I just went with bottled water for the first week, and then started refilling from public taps at train stations (but buying a new small bottle every morning), had no issues there.

OK, maybe we did things a little differently, but we travelled for over 5 months in India and maybe bought about 5 bottles of water the entire time. We had a Steripen Ultra, which we charged once every 5-7 days, then sterilised water from taps in a 1l nalgene bottle. Some restaurants provide hot water (very common in Kerala, even train stations had boiling water dispensers), some provide "filtered", but you can't be sure filters are effective.

We got sick 3 times, one virus, no obvious source, one was food poisoning and one was after holi festival (crazy streets, lots of strangers spreading colour on our face).
AFAIK it was never the water. Definitely treat the water, but India has a huge plastic problem, try and reduce your footprint as much as possible.

Also, give the water a smell test, if it smells off (some Mumbai water smelt foul), then go out and buy some.

I don't know what country you're from, so I don't know the availability of Sawyer filters where you're at. In the US you can purchase them in places like Walmart, Amazon or more upscale places like REI. For about $20 USD, you can filter thousands of gallons of water (for their 0.1 micron filter, they advertise 100,000 gallons of water with appropriate maintenance (basically back flushing with clean water)). We keep one in our camping equipment.

If you're worried about heavy metals, pesticides, chemicals and other similar contaminants, look at their S1 & S3 filter bottles. They're more expensive, and don't last as long, but they're very good at filtering out the pollutants as well as bacteriological contaminants.