What filtered water tap to buy?

[geem]

If the RO is actually working then there won't be anything bigger than a molecule left so "residual bacteria" is the wrong way of thinking about it. Any bugs are coming from the water storage and plumbing system. which should be regularly cleaned through since the water doesn't have chlorine.
Not saying UV is the wrong answer, just clarifying that RO removes all bacteria and viruses if it's working. What it cannot remove is heavy metals, so a carbon filter is definitely required, alongside sensible water sourcing (for instance, never use one in the Fal as it's full of arsenic!)

Bacteria removal efficiency of seawater RO is 99.9-99.999% it's not perfect. Bacteria concentrations in permeate are less than 10 cells/mL to zero.
For the cost of a UV system, it's belt and braces. We never put anything else in our water tank once it has been chlorinated. This was done in July. Everything in the tank since has been RO water with UV. The tank was fully refurbished in 2021. New WRAS approved polyurethane two pack paint to the whole tank interior. The only place bacteria can now get in is through the vermin screen on the tank breather.

 

[geem]

Bacteria removal efficiency of seawater RO is 99.9-99.999% it's not perfect. Bacteria concentrations in permeate are less than 10 cells/mL to zero.
For the cost of a UV system, it's belt and braces. We never put anything else in our water tank once it has been chlorinated. This was done in July. Everything in the tank since has been RO water with UV. The tank was fully refurbished in 2021. New WRAS approved polyurethane two pack paint to the whole tank interior. The only place bacteria can now get in is through the vermin screen on the tank breather.

This is what GPT says about RO combined with UV filtration:

Combining UV disinfection with Reverse Osmosis (RO) is one of the most effective and reliable ways to ensure safe, microbe-free water. The two technologies complement each other perfectly — each covers the other’s weaknesses.


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How RO + UV Work Together

ProcessWhat It DoesMicrobes TargetedStrengthsWeaknesses

Reverse Osmosis (RO)Physical filtration through semi-permeable membrane (~0.0001 µm pores)Removes most bacteria, protozoa, and many virusesExcellent for removing dissolved salts, heavy metals, and microorganisms by size exclusionDoesn’t kill microbes; small viruses or leaks may allow passage
UV DisinfectionExposes water to ultraviolet light (typically 254 nm)Inactivates bacteria, viruses, protozoa DNA/RNAKills or renders any microbes harmless — even those that slipped past RODoesn’t remove chemicals, turbidity, or minerals



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Why the Combination Is So Effective

1. Redundancy & Complementarity

RO removes pathogens (size exclusion).

UV kills/inactivates anything that might still be present.



2. Coverage for All Microbial Types

RO provides near-complete removal of bacteria and protozoa.

UV adds complete viral inactivation — including small viruses that could penetrate RO.



3. Post-treatment Protection

RO storage tanks and pipes can experience bacterial regrowth over time.

A UV sterilizer after the tank ensures that any regrown microbes are neutralized before use.



4. Chemical-Free Disinfection

UV doesn’t add taste, odor, or byproducts (unlike chlorination).

Combined Removal Efficiency

MicroorganismRO AloneUV AloneRO + UV Combined

Bacteria99.9 – 99.999 %99.9 – 99.999 %≈100 % (5–7 log reduction)
Viruses90 – 99.999 %99 – 99.999 %≈100 % (5–7 log reduction)
Protozoa (Giardia, Crypto)≈100 %99–100 %≈100 %


> In practice: RO + UV systems can achieve sterile-grade water (<1 viable cell per mL), suitable for lab, medical, or high-purity applications.


Note that chat GPT suggests adding UV after the storage tank. I looked at doing that but it was difficult to switch on the UV light so that it ran whilst the tap was running. Instead, I added before the tank. Not ideal, but it still ensures everything going into the tank is sterile water.
We used to have a UV filter on our spring water supply at home. So I have plenty of experience of using them