The most common question I get from Bali villa owners setting up water treatment is whether they need RO, UV, or both. The answer depends on what's in your specific source water — which is why testing matters. But there's a general framework that applies to most Bali sources, and I want to explain it clearly here.
What Each System Removes
Reverse osmosis (RO) uses a semi-permeable membrane to remove dissolved solids. It removes dissolved minerals (reduces TDS from 500 ppm to 20–50 ppm), heavy metals including arsenic and lead, nitrates from agricultural contamination, and most bacteria and viruses through size exclusion. It does not reliably remove all viruses, and very small bacteria can sometimes pass damaged membranes. RO is primarily a chemical and mineral treatment with significant biological benefit.
UV sterilisation uses ultraviolet light to disable biological organisms by disrupting their DNA. It eliminates bacteria, viruses and protozoa — including organisms that can pass through RO membranes. It removes nothing from the water chemically — TDS, minerals and dissolved compounds are unchanged. UV is a biological treatment only.
When RO Alone Is Sufficient
For most Bali well water with moderate TDS (200–400 ppm) and no confirmed biological contamination, a quality 5-stage RO system provides adequate drinking water treatment. The membrane removes the majority of biological organisms through physical size exclusion. This covers most Canggu, Seminyak and Kuta well water situations.
When UV Is Essential
For Ubud spring-fed water with lower TDS but confirmed biological contamination risk, UV is the primary treatment required. The spring water may already be relatively low in dissolved minerals — RO may be over-treatment for mineral removal — but biological safety requires UV.
When to Use Both
For coastal properties with high TDS from saltwater intrusion (Jimbaran, coastal Kuta, parts of Nusa Dua in dry season) and biological contamination risk: RO handles the TDS and most biology; UV at the post-RO stage provides the final biological safety step. Combined systems are the right answer for high-risk source water.
Which for Your Property?
A TDS test (IDR 0 — we do it when visiting) tells you whether your source water needs RO or not. If TDS is above 150 ppm, RO is beneficial. If below 150 ppm with good organoleptic quality, UV alone may be sufficient for biological treatment. WhatsApp us your location and current source — we can advise the most likely answer before testing.