Once a property manager asked me to install a UV system in a Seminyak villa whose source water TDS was 480 ppm with elevated iron and manganese. UV would have killed any bacteria — and then delivered water with 480 ppm dissolved solids and visible tannin colouring through the filter. The guests would have complained. UV is not the right tool for high-TDS high-mineral water. This guide is to help you understand which treatment addresses which problem.
What UV Does
UV sterilisation exposes water to germicidal UV-C light that destroys the DNA of bacteria, viruses and parasites, preventing them from reproducing. It is effective against E. coli, coliform bacteria, Giardia and most other biological contaminants. It does not remove dissolved minerals, chlorine, heavy metals, nitrates or any chemical contaminants. The water chemistry is completely unchanged — only the biological load is treated. UV is also not effective for turbid water — particles in the water block UV penetration, protecting bacteria behind the particle. A sediment pre-filter is always required.
What RO Does
Reverse osmosis forces water through a semi-permeable membrane at pressure, removing particles above approximately 0.0001 microns — essentially everything except water molecules. This includes dissolved minerals (reducing TDS by 90–99%), heavy metals, nitrates, chlorine, most pharmaceuticals, and biological contaminants including bacteria and viruses. RO produces genuinely clean water from almost any source. Trade-offs: it wastes 3–4 litres per litre of product water, removes beneficial minerals alongside harmful ones, and requires pre-filtration to protect the membrane from sediment and chlorine.
When to Use Each
Use UV when: source water TDS is below 200 ppm, biological contamination is the primary concern, you want to preserve the natural mineral content of the water, or you're treating high-volume flow where RO waste water is impractical.
Use RO when: source water TDS is above 200 ppm, there are taste or odour problems from dissolved compounds, the water is from a deep well with unknown chemistry, or you want the broadest treatment spectrum for drinking water.
Use both when: high TDS plus confirmed biological contamination — RO for chemical and biological removal, UV as a final safety stage on the product water.
For Bali specifically: most south Bali well water benefits from RO for drinking water. Ubud spring water often needs UV more than RO. Coastal properties with salt intrusion need RO. PDAM connected properties need at minimum a carbon filter; RO optional for premium drinking water quality.