Disclaimer: I am not an expert on chemicals or water filters. What is mentioned in this particular post about chemicals is a perception based on information sourced from the internet.
If you use an RO (Reverse Osmosis) filter and are familiar with the process of osmosis, then all you need to know is that RO is osmosis in the reverse direction which happens when water pressure is high. The heavy metals and salt in the water is retained within the confines of the membrane and pure water (including chlorine) gets across the membrane.
But gravity based filters aren't like that. They depend on just gravity to go through a few filters and viola! somehow, the water is purified. On asking the company how bacteria and viruses are killed, you'll get at best an ambiguous answer. In-fact, an investigation into Hindustan Unilever's 1 crore viruses kill claim was shown to be exaggerated.
So the first thing you learn about modern water filter companies is:
Don't trust their marketing. Ask extremely detailed questions. If you don't get proper answers, then don't buy the product.
My old water filter stopped functioning, and I had to get a new temporary water filter quickly. Looking at the reviews on Amazon, Hindustan Unilever's Pureit Advanced gravity based water filter seemed ok at that time, because I didn't need to filter borewell water (which would've had metallic impurities). I just needed it to filter the chlorinated corporation water. Ordered it via Pureit's helpline and it was promptly delivered.
After a few days of drinking it, I got an uneasy feeling in my stomach and lost my hunger. It was like the stomach needed time to repair damage it sustained. Oddly, even the construction workers we gave the water to, refused to drink it after a few days.
On reporting it, a Pureit technician came to inspect it, said it might be a problem with the carbon polisher and surprisingly, he himself refused to drink the water saying he's had throat problems after drinking the water at various customers houses. He seemed familiar with the problem.
After replacing the carbon polisher, the problem continued. This time, a different technician arrived, and he refused to drink the water too. Said he was familiar with the problem, replaced the 'germ-kill' kit and asked me to try using the filter again. Problem continued and I had to ask them to take back the product. They took it back for half the price.
What a gravity-based purifier may rely on:
1. A Germkill processor: This is essentially nothing but bleaching powder. The same white powder used to sanitize swimming pools, is used to disinfect your drinking water. This powder basically brings chlorine into the water which kills microorganisms. Problem is, if your water is already chlorinated by your municipal corporation, then the water purifier is just going to add more chlorine and that can be bad for your stomach.
In the case of Pureit, the germkill processor had this chemical in a little container which would slowly dissolve in the water and when it got over, the red piece of plastic at the top would have moved to the bottom and would be visible to the user as a sign that the germkill kit needed replacement.
2. An advanced microfiber mesh: It's nothing but a porous material which allows water through and does not allow dirt through. Nothing advanced about it. Gets dirty quickly, and the technician advised cleaning it every week. So honestly, it does the job, but well...clean it every week is a reality.
3. Carbon polisher: This is a carbon-based fabric supposed to remove pesticides, parasites (really? So the germkill kit doesn't work?) and chlorine, but that also means you need to change your carbon cartridge often because it can't adsorb (not absorb) chemicals forever.
4. Microcharged membrane: It's just another fancy name which claims to remove harmful parasites (really? So the germkill kit and the carbon polisher didn't work yet?) It's just a hollow cylinder with a clump of white wires at the bottom. The technician told me it was to give the water a sweet taste.
So that's the process. Chlorinate the water, remove dirt, remove chlorine, add a sweet taste. This is claimed to be "better than boiling", but it'd take a very gullible person to believe that.
The most visible and alarming indicator was that their own technicians were refusing to drink the "purified" water. I wrote to the company requesting them to do further checks and if necessary, initiate a product recall in the interest of public safety.
As for you, if you get municipal corporation water, boiling it would be a much more reliable way to get rid of bacteria. Although boiling doesn't get rid of all pathogens, it does reduce it to a safe level. An acquaintance who worked for a water purifier company informed me that it is the old water filters that used to be much better and more reliable than the modern filters. The modern ones bank on getting money out of you by having annual maintenance contracts and having components like the germkill kit and carbon polisher which need frequent replacement.
We need a better solution than relying on unethical corporations for a basic need such as clean drinking water.