Why the Wrong Choice Is Expensive and Unsafe
Choosing a water purifier based on advertising or showroom recommendations without understanding your actual water problem leads to one of two outcomes: you buy an unnecessarily expensive system (common when RO is recommended for low-TDS water that needs only UV treatment), or you buy an inadequate system (common when a gravity filter is used for water with chemical contamination that requires RO).
Both outcomes are worse than doing nothing β the inadequate system gives false confidence while failing to address the actual contamination, and the unnecessarily expensive system wastes money and in the case of RO, wastes three litres of water for every litre purified.
Understanding the Three Approaches
Gravity Filters
Gravity filters use no electricity and no pressure. Water poured into the upper chamber passes through a ceramic, carbon or composite filter element by gravity into a lower storage chamber. Flow is slow β typically 1β4 litres per hour β but the filter operates completely independently of electricity and water pressure.
What they remove:
- Bacteria (ceramic and hollow-fibre elements with pores below 0.2 microns)
- Protozoa and cysts (Giardia, Cryptosporidium)
- Suspended particles and turbidity
- Some organic compounds (carbon-containing elements)
What they do NOT remove:
- Viruses (most gravity filters cannot achieve reliable viral removal)
- Dissolved chemicals β TDS, arsenic, nitrates, heavy metals
- Fluoride, chlorine (unless a specific carbon stage is included)
Total cost of ownership (5 years):
- Purchase: ৳3,000β৳10,000
- Replacement elements: ৳1,000β৳2,500/year
- Total 5 years: ৳8,000β৳22,500
Reverse Osmosis (RO) Purifiers
RO systems use an electric pump to force water through a semi-permeable membrane at high pressure. They are the only household technology that reliably removes dissolved chemical contaminants β TDS, arsenic, lead, fluoride, nitrates and hundreds of organic compounds.
What they remove:
- All of the above (gravity filter capability)
- Dissolved salts and TDS (90β97% reduction)
- Heavy metals including arsenic, lead, cadmium
- Fluoride, nitrates, pesticides
- Viruses (when UV stage is included)
What they do NOT do well:
- Function without electricity
- Avoid water wastage (2β4 litres wasted per litre purified)
- Retain beneficial minerals (removed along with harmful ones, unless mineraliser stage added)
- Function adequately without regular filter maintenance
Total cost of ownership (5 years):
- Purchase: ৳15,000β৳45,000
- Annual filters and service: ৳4,000β৳8,000/year
- Total 5 years: ৳35,000β৳85,000
UV Purifiers
UV purifiers pass water past a germicidal UV-C lamp. The UV radiation kills or inactivates all bacteria, viruses and protozoa in the water stream. UV is the gold standard for microbial purification β more effective than any membrane-based technology for viruses.
What they remove/kill:
- All bacteria (99.9999% log-6 reduction at correct UV dose)
- All viruses (99.99% log-4 reduction)
- All protozoa and cysts
What they do NOT remove:
- Any dissolved chemical contaminants
- TDS, heavy metals, arsenic, fluoride, nitrates
- Particles and turbidity (turbid water must be filtered before UV)
Total cost of ownership (5 years):
- Purchase: ৳6,000β৳18,000
- Annual lamp and filter replacement: ৳1,500β৳3,000/year
- Total 5 years: ৳13,500β৳33,000
The Decision Matrix
Use your water test results to navigate this decision:
| Your Water Situation | Best Choice | Reason |
|---|---|---|
| Low TDS (below 150 ppm) + microbial risk | UV + sediment pre-filter | Chemical treatment not needed; UV handles biology |
| Low TDS + power cuts frequent | Gravity UF filter | No electricity needed; handles bacteria and protozoa |
| Moderate TDS (150β500 ppm) + microbial risk | RO + UV | RO for TDS; UV for complete microbial safety |
| High TDS (above 500 ppm) | RO + UV | RO essential for TDS reduction |
| Arsenic-affected area (any TDS) | RO + UV | RO is the only reliable household arsenic removal |
| Turbid water + low TDS | Gravity ceramic + UV | Sediment and microbial removal; no RO needed |
| Budget severely limited | Gravity UF | Lowest cost; handles primary microbial risk |
| Off-grid, no electricity | Gravity UF | Only option without power |
The Combination That Covers Almost Every Situation
For most Bangladesh households β particularly urban and peri-urban β the combination of RO membrane + UV lamp + carbon pre-filter + sediment pre-filter in a single unit covers all contamination categories comprehensively:
- Sediment pre-filter: particles, turbidity, iron flakes
- Carbon pre-filter: chlorine, organic compounds
- RO membrane: TDS, heavy metals, arsenic, dissolved chemicals
- UV lamp: bacteria, viruses, protozoa (belt-and-suspenders for microbial safety)
- Post-carbon: final taste polish
This combination costs more than a gravity filter or UV-only system, but it is the only approach that addresses the full range of contamination risks found across Bangladesh's varied water sources β from arsenic-laden groundwater in southern districts to chlorinated but microbially risky municipal supply in Dhaka.
The One Question That Simplifies the Decision
What is your source water TDS?
- Below 150 ppm: UV or gravity UF. Skip RO β it wastes water and removes minerals your water doesn't have too much of anyway.
- 150β500 ppm: RO+UV recommended but not mandatory. A quality carbon+UF system may be adequate if you have no specific chemical contamination concerns and are not in an arsenic district.
- Above 500 ppm: RO is the only option. No other technology reduces TDS meaningfully.
- Arsenic district, any TDS: RO is the only option regardless of TDS.
This single number β your source water TDS β eliminates the confusion and points to the correct solution in the majority of cases.