Rainwater Harvesting for Drinking Water in Bangladesh: Collection, Storage and Purification Guide

Why Rainwater Harvesting Makes Sense in Bangladesh

Bangladesh receives 1,500–3,000mm of rainfall annually — far above the global average of approximately 750mm. The monsoon season (June–October) delivers the majority of this rainfall in concentrated bursts. Yet paradoxically, large parts of Bangladesh face chronic water quality problems: arsenic in groundwater, saline intrusion in coastal areas, and microbially contaminated surface water and shallow wells.

Rainwater is naturally free of dissolved minerals (TDS close to zero), arsenic-free, and initially free of microbial contamination when it falls. Collected correctly and purified appropriately, it can provide genuinely clean drinking water — particularly valuable in:

• Coastal districts with saline intrusion where groundwater is unsuitable for drinking
• Arsenic-affected areas where safe well water is unavailable
• Flood-prone areas where all other water sources are temporarily contaminated
• Rural households without access to municipal supply

Understanding Rainwater Quality: What Is Actually in It?

Fresh rainwater from clean clouds is very pure — essentially distilled water with tiny amounts of dissolved atmospheric gases. By the time it reaches your collection surface, it has picked up:

From the atmosphere:

• Dust, pollen and particulates
• Atmospheric pollutants — sulphur dioxide and nitrogen oxides (acid rain components) near industrial areas
• Microorganisms from dust

From the collection surface (roof):

• Bird and animal faeces — the primary source of E. coli and other bacterial contamination
• Dust, leaves and organic debris
• Chemicals from roofing materials — particularly from painted or treated corrugated iron sheets
• Lead from old paint on roofs

From gutters and storage containers:

• Algae growth in sunlit storage
• Bacterial growth during warm storage periods
• Insects and larvae if containers are not covered

The first rain of the season and the first flush of each rain event (the initial rainfall that washes the roof surface) are the most contaminated. Discarding the first 10–20 litres of runoff from each rain event dramatically improves collected water quality.

The Three Components of a Safe Rainwater System

Component 1 — Collection Surface

The roof is the collection surface. Different roofing materials have different contamination risks:

Roof maintenance: Sweep the collection surface before the monsoon season. Remove leaves, debris and bird droppings. Trim overhanging tree branches that deposit organic material.

Collection area calculation: Every 1mm of rainfall on 1 square metre of roof = 1 litre of potential runoff (minus approximately 20% for evaporation and first-flush discard). A 40 square metre roof receiving 200mm of rainfall in a month can theoretically collect up to 6,400 litres.

Component 2 — First Flush Diverter

This is the most important single component for improving rainwater quality. A first flush diverter automatically discards the first 10–20 litres of runoff from each rain event — the water that washes the roof clean of accumulated contamination — before diverting subsequent cleaner rainfall into the storage tank.

A basic first flush diverter can be made from a vertical PVC standpipe (4-inch diameter, 1.5 metres tall) connected to the downpipe. The standpipe fills with the first dirty flush. Once full, subsequent water flows over and into the storage tank via a bypass. A small drain hole at the base of the standpipe slowly drains between rain events, resetting it.

Capacity rule: Allow 1 litre of first flush diverter capacity per 25 square metres of roof surface.

Component 3 — Storage Tank

Material options:

• HDPE (High-Density Polyethylene) tanks — the standard recommendation. UV-stabilised, food-grade, available in 500L–10,000L sizes. Most widely used for rainwater harvesting in Bangladesh.
• Ferrocement tanks — excellent choice where locally available; durable, cool interior temperature slows bacterial growth, relatively inexpensive to construct
• Traditional earthen/clay pots — suitable only for daily use; too small for meaningful storage
• Repurposed drums — only if confirmed food-grade and never used for chemicals

Tank placement: Keep the tank covered and shaded. Direct sunlight warms the water and promotes algal and bacterial growth. Elevated tanks provide gravity pressure for distribution; ground-level tanks are easier to fill and maintain.

Tank sizing: For a family of four using 8 litres per day for drinking and cooking, a minimum 1,000-litre tank provides approximately 125 days of drinking water from a full tank. For year-round reliance on rainwater, a 5,000–10,000 litre tank is needed to bridge dry season gaps.

Purification of Collected Rainwater

Collected and first-flush-diverted rainwater still requires purification before drinking. The contamination profile (low TDS, microbial risk, possible particulates) indicates this treatment sequence:

Step 1 — Sediment filtration (mandatory):

Pass water through a 5-micron sediment filter before any other purification. Removes particulates, debris and some biological load. Change every 3–6 months depending on turbidity.

Step 2 — Choose your primary purification based on your concern:

Important: Do NOT use a full RO system on very low TDS rainwater (below 50 ppm) without adding a mineraliser post-filter. Ultra-pure water can leach minerals from body tissue and tastes flat. Add a mineral cartridge post-filter if using RO.

Step 3 — Safe storage of purified water:

Store purified rainwater in a clean, covered container — glass, food-grade stainless steel, or HDPE. Never store in open containers or clear plastic left in sunlight.

Seasonal Planning for Bangladesh

A well-designed rainwater harvesting system with a 5,000–10,000 litre tank in a high-rainfall area of Bangladesh can supply a family's complete drinking water needs year-round — eliminating dependence on groundwater (and its arsenic risk), bottled water (and its cost and plastic waste), or an unreliable municipal supply.