What is a Rainwater Harvesting Tank? Types, System Components & Benefits
Created on 2025.04.16
What is a Rainwater Harvesting Tank? Types, System Components & Benefits
A rainwater harvesting tank (also known as a rainwater storage tank or cistern) is a specialized water containment vessel engineered to collect, filter, and store runoff from rooftop catchments or land surfaces during rainfall events.
Part of a broader rainwater harvesting system (RWHS), these storage units capture water that would otherwise become urban stormwater runoff, preserving it for later use in irrigation, toilet flushing, cooling towers, washing, and—when paired with proper filtration—potable drinking water supplies.
How a Rainwater Harvesting System Works
A rainwater harvesting tank does not operate in isolation. It relies on several interconnected components to collect clean water and maintain long-term water quality:
1. Catchment Surface: The roof or paved surface where rainfall falls. Metal, tile, and concrete roofs deliver the cleanest runoff.
2. Transportation Infrastructure: Gutters and downspouts channel water from the catchment toward the storage facility.
3. Primary Filtration & First-Flush Diverter: A pre-tank mesh filter removes leaves and twigs. A first-flush diverter isolates and discards the first few millimeters of rainfall, which carries ambient dust, bird droppings, and atmospheric pollutants.
4. Storage Tank: The main sealed vessel that holds the collected water safely away from sunlight, preventing algae growth and vector breeding.
5. Distribution & Post-Treatment: Submersible or external pumps deliver water to end-use fixtures. Optional secondary filters (sediment filters, carbon blocks, or UV sterilizers) prepare the water for high-purity applications.
Types of Rainwater Harvesting Tanks
Storage tanks vary significantly based on installation location, material construction, and overall storage capacity requirements:
1. By Installation Location
● Above-Ground Tanks: Installed on flat concrete pads adjacent to buildings. Easy to inspect, maintain, and connect via gravity flow or simple pumps.
● Underground Tanks: Buried beneath driveways, lawns, or parking lots. Ideal for urban sites with limited footprint space or extreme winter climates where freezing is a concern.
2. By Material Construction
● Polyethylene (HDPE) Tanks: Popular for residential and light commercial projects up to 10,000 gallons (38,000 liters). They are lightweight, UV-inhibited, and easy to transport.
● Glass-Fused-to-Steel (GFS) / Bolted Steel Tanks: The gold standard for municipal, commercial, and industrial stormwater harvesting (50 m3 to over 10,000 m3). They combine the structural strength of steel with the chemical inertness of enamel glass, delivering a design life exceeding 30–50 years with zero internal wall corrosion.
● Concrete Cisterns: Cast-in-place or precast concrete tanks used primarily for underground installations. Concrete naturally neutralizes acidic rainwater, though it can crack under ground movement over time.
Comparative Matrix: Rainwater Storage Tank Materials
Material Type | Capacity Range | Ideal Installation | Lifespan | Maintenance Needs | Relative Cost |
Polyethylene (Plastic) | 100 L - 50,000 L | Above-Ground / Small Burial | 15–25 Years | Low (Clean sediment) | Low |
Glass-Fused-to-Steel (GFS) | 20 m3 - 60,000+ m3 | Above-Ground Industrial/Municipal | 30–50+ Years | Very Low (Glass coating) | Moderate to High |
Reinforced Concrete | 5,000 L - 500,000+ L | Underground Burial | 30–40 Years | Moderate (Check for cracks) | Moderate |
Galvanized Bolted Steel | 10,000 L - 2,000,000 L | Above-Ground Commercial | 20–30 Years | Low (Replace inner liner) | Moderate |
Core Benefits of Utilizing Rainwater Harvesting Tanks
● Water Bill Savings: Replaces municipal grid water with harvested rainwater for toilet flushing, landscaping, laundry, and industrial cooling.
● Stormwater Runoff & Flood Mitigation: Capturing peak rainfall volume reduces local storm sewer overflow, lowers urban erosion, and prevents local flooding.
● Drought Resilience: Provides an independent, on-site water buffer during dry seasons or municipal water restrictions.
● Environmental Sustainability: Reduces demand on overworked groundwater aquifers and municipal treatment plants, lowering energy consumption for city water distribution.
Frequently Asked Questions (FAQ)
Q1: Is water stored in a rainwater harvesting tank safe to drink?
Answer: Raw harvested rainwater stored in a tank is suitable for non-potable uses like garden irrigation, toilet flushing, and vehicle washing. It is not safe for drinking without secondary treatment (such as multi-stage filtration, activated carbon filtering, and UV sterilization or chlorination) to remove pathogens and airborne particulates.
Q2: How do you size a rainwater harvesting tank?
Answer: Tank size depends on three factors: average annual rainfall in your region, total catchment roof area (m2), and daily water consumption needs. A general calculation formula is:
Q3: How do you prevent mosquitoes from breeding in a rainwater tank?
Answer: Mosquitoes require access to standing water to lay eggs. Rainwater tanks prevent mosquito breeding by using fine mesh screens (less than 1 mm aperture) over all inlet pipes, overflow outlets, and ventilation ports, alongside keeping the tank hatch completely sealed.
Q4: What maintenance does a rainwater storage tank require?
Answer: Routine maintenance includes cleaning gutter screens and leaf filters monthly, inspecting and emptying the first-flush diverter after heavy rains, checking inlet/outlet mesh annually, and removing accumulated bottom sediment (sludge) every 2 to 3 years.
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