Why Rainwater Harvesting Makes Sense in Bangladesh
Bangladesh receives between 2,000 and 3,000 millimetres of annual rainfall, making it one of the most rain-abundant countries in Asia. Yet paradoxically, safe drinking water remains a daily challenge for millions of people β because the primary drinking water source in rural and semi-urban Bangladesh is groundwater, which in many areas carries arsenic, iron, microbial contamination and other hazards.
Rainwater, before it contacts any surface, is essentially distilled water β naturally soft (TDS of 10β30 ppm), free of dissolved minerals, and free of the arsenic and heavy metal contamination that affects Bangladesh's groundwater. These characteristics make it an exceptionally good source for drinking water β provided it is collected, stored and purified correctly.
In coastal Bangladesh, where saline intrusion makes groundwater completely unsuitable for drinking, rainwater harvesting has been the traditional primary drinking water source for centuries.
The Basic Rainwater Harvesting System
A household rainwater harvesting system has four components:
Component 1 β Catchment Surface
The roof of your building is the catchment surface. The type of roofing material significantly affects water quality:
- Corrugated iron/tin roof: The most common in Bangladesh. Water quality is generally good, but iron particles from rust contribute some iron content. Annual roof cleaning is recommended.
- Concrete/RCC roof: Excellent catchment surface β minimal contamination pickup. Mossy or algae-covered concrete roofs require cleaning before the monsoon.
- Clay tile roof: Good quality but tiles can harbour moss and biological material. Clean annually.
- Thatched roof: Not suitable for drinking water harvesting β introduces too much organic contamination.
Component 2 β First Flush Diverter
The most important component after the roof. The first 1β5 minutes of rain after a dry spell washes accumulated dust, bird droppings, pollution particles and other contaminants off the roof. A first flush diverter automatically diverts this initial, most contaminated flow away from your storage tank and only allows cleaner subsequent flow into the system.
A simple first flush diverter can be fabricated from PVC pipe at a cost of ৳500β৳1,500. Commercial versions with automatic reset are available for ৳1,500β৳4,000.
Rule of thumb: Divert the first 2 litres of rainfall per square metre of roof area. For a 50 square metre roof, divert the first 100 litres.
Component 3 β Storage Tank
The storage tank must be:
- Covered completely β mosquito breeding in open tanks is a serious public health issue; all inlets must be screened with fine mesh
- Food-grade material β food-grade polyethylene (PE) tanks are widely available and affordable; avoid galvanised metal tanks (zinc leaching)
- Sized for your dry season β calculate storage needed to bridge the longest dry period in your area
Tank sizing formula:
Storage required (litres) = Daily consumption Γ Longest dry period (days) Γ 1.5 (safety factor)
For a family consuming 20 litres per day bridging a 90-day dry period: 20 Γ 90 Γ 1.5 = 2,700 litres minimum tank capacity.
Available tank sizes in Bangladesh: 500L, 1,000L, 2,000L, 5,000L, 10,000L β all widely stocked.
Component 4 β Treatment and Purification
Harvested rainwater requires treatment before drinking despite being relatively clean at the time of collection. Storage contamination is the primary risk β tanks that are not perfectly sealed can be contaminated by insects, rodents, algae and airborne bacteria.
Minimum treatment for harvested rainwater:
- Sediment pre-filter β removes particles picked up during collection and storage
- UV purifier β kills bacteria and viruses from storage contamination; this is the single most important treatment stage for rainwater
- Activated carbon filter β removes any taste or odour compounds from the storage period
If TDS is above 50 ppm (indicates significant roof contamination or atmospheric pollutants): add an RO stage before UV.
Water Quality of Harvested Rainwater: What to Expect
Well-collected and stored rainwater in Bangladesh typically shows these characteristics:
| Parameter | Expected Range | Safe Level |
|---|---|---|
| TDS | 10β80 ppm | Any level below 500 ppm is acceptable |
| pH | 5.5β6.8 (slightly acidic) | Should be raised to 6.5β8.5 with a mineral filter |
| Iron | Low (0.05β0.2 mg/L) | Below 0.3 mg/L is acceptable |
| Bacteria | Variable β depends on storage | Must be treated; do not drink untreated |
| Arsenic | Essentially zero | This is a key advantage over groundwater |
| Lead | Low (atmospheric deposition) | Test if near industrial areas |
The slightly acidic pH of rainwater (due to dissolved COβ forming carbonic acid) is easily corrected with a small alkaline mineral cartridge after the UV stage.
Seasonal Considerations for Bangladesh
Monsoon (JuneβOctober): Peak collection season. Focus on maximising storage during this period to bridge the dry season. Heavy initial rains after the dry season are most contaminated β the first flush diverter is especially important for the first rains of the monsoon.
Post-monsoon (NovemberβJanuary): Good collection, lower intensity. Maintain tank levels.
Dry season (FebruaryβMay): Minimal rainfall. This is why adequate storage capacity is critical. The dry season in coastal districts can be 4β5 months with very little rain.
Cyclone events: After cyclones, atmospheric contamination is elevated. Increase first-flush volume and consider not collecting for the first few hours after a severe storm.
Cost of a Basic Household Rainwater Harvesting System
| Component | Cost Estimate |
|---|---|
| PVC guttering and downpipe | ৳2,000β৳5,000 |
| First flush diverter | ৳1,500β৳4,000 |
| 2,000L food-grade PE tank | ৳8,000β৳12,000 |
| Sediment pre-filter | ৳500β৳1,000 |
| UV purifier (point of use) | ৳6,000β৳12,000 |
| Carbon post-filter | ৳500β৳1,000 |
| Alkaline mineral cartridge | ৳600β৳1,000 |
| Installation and fittings | ৳3,000β৳6,000 |
| Total | ৳22,000β৳42,000 |
For arsenic-affected areas where this system completely replaces purchased or contaminated groundwater, the investment typically pays back within 12β18 months compared to bottled water costs.
Who Benefits Most From Rainwater Harvesting in Bangladesh
- Coastal district households (Satkhira, Khulna, Barguna, Patuakhali) β escaping saline groundwater
- Arsenic-affected district households β eliminating arsenic exposure at the source
- Rural households without reliable piped water supply
- Flood-prone areas β where groundwater quality deteriorates after flooding events
- Households seeking water independence from municipal supply disruptions