Thanks for that. The need for tablets is on a case by case basis and is a tricky personal decision.
Personally I'm getting concerned by my perception of the spread of contaminants in
watercourses across the country, especially viral ones. I fear many of these are human food chain related and bear as much on farming practices as water companies. e.g. meat slurry field injection as a fertiliser.
I agree, the more sophisticated filter systems can remove bacteria and many chemical (organic and/or inorganic depending on filtration media) contaminants,
but none of them will remove viruses.
To remove viruses, you must either boil, treat with disinfectant or use a particular wavelength of UV light (or x-rays/gamma rays of course

).
The other thing that may get through a filter is the sporulated ("cyst") state of bacteria. The ceramic filters used by Berkefeld systems specifically tell you that viruses and cysts will pass them.
Quite a few bacteria- including some relevant nasties such as Bacillus cereus (rice food poisoning), Bacillus anthracis (anthrax), Clostridium botulinum (botulism), Clostridium tetani (tetanus), Clostridium difficile (infection- colitis), Clostridium perfringens (gangrene/food poisoning)- have a spore form where the bacteria can curl up on themselves in a "hardened" dormant state to survive hostile conditions.
The spores are really tough- they can be resistant to dry heat, dessication, UV radiation from the sun, extreme freezing and household chemical disinfectants such as alcohols, quaternaty ammonium compounds and detergents. They can also survive in a viable form for hundreds, even thousands, of years.
Standard household bleach (10% sodium hypochlorite) in contact with spores for a minimum of several minutes can be effective- only a small proportion of spores can survive longer than 10 minutes. Higher concentrations won't help but a longer stand time might. Note this is in contact with the spores, so filter first them treat to have maximum effect. A good rolling boil is an alternative for treating water. Some shorter wavelength UV (UVC- the lamps producing this are different to standard "blacklights") or X-rays/gamma rays also disrupt spores.
The reason that the WHO recipe for hand sanitiser includes hydrogen peroxide is to denature the bacterial spores- which the standard alcohol sanitiser would not touch.
The immune system will deal with an amount of contamination- level depending on the individual- but if you drink from a watercourse that could have been polluted, please
please boil/treat with hypochlorite after filtering (the purifying UVC lamps used in a water treatment plant are typically a bit too big to lug around if camping).
GC