Tanks: Versatile Water Storage Solutions

Access to clean, reliable water remains one of the most pressing challenges for households and farms across the world, especially in regions where rainfall is seasonal or groundwater levels fluctuate. Tanks – whether installed above ground or buried underground – offer one of the most practical and time-tested solutions to this problem. By capturing rainwater from rooftops or land runoff and storing it in a covered, enclosed structure, tanks help secure a usable water supply while minimising contamination and reducing losses from evaporation. Understanding the different types of tanks, the materials they are made from, and how they function is essential for making the right choice for any water harvesting setup.

What are water storage tanks? 🔗

In the context of rainwater harvesting, a tank is a dedicated storage structure used to collect and hold water until it is needed. Rainwater tanks are installed to make use of rainwater for later use, reduce dependence on mains or groundwater supplies, and support self-sufficiency in water management. They can be positioned above ground or buried underground, and they come in a wide range of sizes – from small household rain barrels of a few hundred litres to large farm or community tanks holding hundreds of thousands of litres.

The water stored in these tanks is typically sourced from two places: rooftop catchment, where rain falls on a roof surface and is channelled through gutters and downpipes into the tank; and land surface runoff, where rainwater flowing over fields, paved areas, or other impervious surfaces is directed into a collection point. A well-designed rooftop system can yield around 1,000 litres of water from just 1 cm of rainfall on a 100 m² roof, which makes even modest rainfall events worth capturing.

Above-ground vs underground tanks 🔗

The choice between an above-ground and an underground tank is one of the first decisions to make, and it has practical implications for water quality, cost, ease of maintenance, and available space.

Above-ground tanks 🔗

Above-ground rainwater tanks have a wider selection of tank types, construction materials, and storage volumes compared to their underground counterparts. They are generally easier to install, inspect, and maintain. However, exposure to sunlight is a significant drawback – direct sunlight warms the water, encourages algae growth, and accelerates evaporation. For this reason, above-ground tanks should be opaque and, where possible, placed in shaded locations. Cylindrical above-ground polyethylene tanks typically range in capacity from 100 gallons up to 10,000 gallons, making them suitable for a wide range of household and agricultural needs.

Underground tanks 🔗

Underground tanks solve several of the challenges posed by above-ground storage. Since these tanks are stored underground, there is no exposure to wind, and the temperature remains constant, which prevents both evaporation and bacterial or algal growth. Underground installation also means they do not occupy valuable surface space – an important consideration in densely populated areas or on small farms. The trade-off is cost: in-ground storage tanks tend to be more expensive than above-ground tanks because of excavation costs and the need for more heavily reinforced construction. They also require a pump to bring the water up for use.

Tank materials: options and considerations 🔗

The material a tank is made from affects its durability, water quality, cost, and suitability for different climates. Rainwater tanks may be constructed from materials such as plastic (polyethylene), concrete, galvanised steel, fiberglass, and stainless steel. Each material has specific strengths and limitations worth understanding.

Plastic (polyethylene and polypropylene) 🔗

Plastic tanks are the most widely used material for residential and small-scale agricultural rainwater storage. They are lightweight, come in many sizes and colours, and are affordable. Polyethylene is a flexible plastic, while polypropylene is more rigid – both are commonly available in opaque finishes, which is important for blocking sunlight and limiting algae growth. Polyethylene storage tanks can be installed above or below ground and are available for both potable and non-potable applications, in a wide range of capacities, shapes, and dimensions. One limitation is UV degradation over time when exposed to direct sunlight, which can shorten the usable life of above-ground plastic tanks to around 20 years.

Concrete tanks 🔗

Concrete tanks are durable, strong, and heavy, and can be installed above ground or below ground. There are two main types: ferro-concrete, where a concrete mixture is sprayed onto a metal frame (a technique common in developing countries for its affordability), and monolithic-pour concrete, where the structure is either cast in place or prefabricated. One notable characteristic of concrete tanks is that lime from the concrete can leach into the stored water, slightly raising its pH – which actually helps neutralise the natural acidity of rainwater. However, concrete tanks are expensive to install underground due to their weight and the excavation required.

Metal tanks (galvanised steel and stainless steel) 🔗

Corrugated steel tanks are often used because of their availability, price, and aesthetic value, and can range in size from a few hundred gallons to tens of thousands of gallons. Most modern steel tanks come with a plastic inner lining to prevent rust, protect water quality, and extend the tank’s lifespan. Without this lining, galvanised steel tanks can leach zinc into stored water, making the water unsuitable for drinking. Stainless steel tanks are more resistant to corrosion but are significantly more expensive, which limits their use to situations where water quality and longevity are the primary concerns.

Fiberglass tanks 🔗

Fiberglass tanks are strong, relatively lightweight, and can be installed both above and below ground. They are well-suited to large-volume storage – for tanks larger than 10,000 gallons, fiberglass is one of the best options. They resist corrosion and do not leach harmful substances into the water. The main drawback is cost: fiberglass tanks are labour-intensive to manufacture and therefore more expensive than plastic alternatives. They must also undergo a specific coating process before they can be considered suitable for storing potable water.

Wooden tanks 🔗

Wooden tanks, particularly those made from hardwoods like redwood, were once a common choice in certain regions. They are now less practical – they have become expensive, are harder to source, and present a structural problem in dry climates: the wood dries out and shrinks, causing leaks. To prevent leaking, a wooden tank must be kept consistently full, or lined with an impermeable material. Their use today is largely limited to heritage installations or contexts where locally sourced timber is the only affordable option.

How tanks prevent contamination 🔗

A core function of any water storage tank is to keep the stored water clean. Rainwater, despite starting as relatively pure precipitation, picks up contaminants as it flows over rooftops and land surfaces – including dust, animal droppings, pollen, pesticide residues, and microbial pathogens. Rainwater harvested from roofs can contain bird and animal faeces, windblown dust, particulates from urban pollution, and dissolved gases. A well-designed tank system addresses this through several protective features.

The first line of defence is the first flush diverter – a device that automatically discards the initial flow of rainwater at the start of each rain event, which tends to carry the highest concentration of roof-surface contaminants. The first flush diverter collects and diverts some of the first, dirtiest rainwater that washes debris from the roof, removing materials like sediment, animal droppings, pollen, and potential chemicals before water reaches the tank. This is followed by inlet screens or mesh filters at the tank entry point to keep out insects, leaves, and debris.

The tank itself should be fully enclosed with a sealed lid. Tanks should be fitted with airtight lids or covers that not only prevent water evaporation but also keep out dust, insects, and other potential contaminants. Opaque tank materials or coatings prevent sunlight penetration, which would otherwise promote algae growth inside the tank. Where the stored water is intended for drinking or cooking, further treatment through ultraviolet (UV) light or chemical disinfection is recommended to remove microbiological pathogens that cannot be removed by physical filtration alone.

How tanks minimise evaporation losses 🔗

Open water surfaces lose a significant volume to evaporation, particularly in hot, dry, or windy conditions. Tanks address this directly through their enclosed design. Evaporation is the most common cause of water loss in above-ground tanks, and in warmer climates, losing several litres regularly due to heat is common – a problem that is entirely avoided with underground installation. For above-ground tanks, placing tanks in shaded or cooler areas, or using insulating materials or coatings that reflect sunlight and reduce heat absorption, can significantly minimise evaporation. Sealed fittings and robust tank walls that prevent seepage are equally important – any crack or gap is not just a pathway for contamination but also a source of ongoing water loss.

Suitable conditions for tank-based water storage 🔗

Tanks are most effective in regions that receive moderate and reasonably reliable rainfall, where there is enough water to justify the investment in storage infrastructure. They are also well-suited to areas where groundwater levels are relatively stable, meaning the stored water supplements rather than replaces a broader water management strategy. Areas with less frequent precipitation may require larger tanks to provide more storage capacity between water recharge events, and the size of the catchment area – typically the roof – directly determines how much water can be collected even from modest rainfall.

For households and small farms, the primary uses of tank-stored water are drinking and domestic purposes such as cooking, bathing, and cleaning – provided the water has been adequately filtered and treated. Stored water may also be used for watering gardens, agriculture, flushing toilets, washing machines, and washing vehicles. The Texas Water Development Board estimates that a well-designed rooftop system on a 2,000 sq ft home can collect around 34,000 gallons of water annually in a region with average annual rainfall of 32 inches – underscoring the practical scale of what tank-based harvesting can achieve.

Sizing and maintenance 🔗

Selecting the right tank size requires balancing three factors: the amount of rainfall available in the area, the size of the catchment surface (usually the roof), and the household or farm’s daily water demand. If there are large variations in rainfall throughout the year, a larger tank may be necessary to store water during wet months for use during drier periods. Oversizing a tank is wasteful in terms of cost; undersizing means running out of stored water before the next rain event.

Regular maintenance is straightforward but non-negotiable. This includes cleaning inlet screens and first flush diverters after heavy rain events, inspecting tank walls and lids for cracks or damage, and periodically flushing sediment that accumulates at the base of the tank. Manufacturers typically offer warranties of 15 to 30 years for storage tanks, but only when they are properly maintained. Neglecting maintenance – particularly for tanks supplying drinking water – can result in contamination that poses a direct health risk to users.

What do you think? Given the wide range of materials available – plastic, concrete, metal, and fiberglass – what factors would weigh most heavily in your choice of tank for a household or small farm setting? And with water scarcity becoming increasingly common in many parts of the world, do you think tank-based rainwater harvesting could realistically serve as a primary water source for rural communities, or should it remain a supplementary option?

References

1. https://en.wikipedia.org/wiki/Rainwater_tank
2. https://en.wikipedia.org/wiki/Rainwater_harvesting
3. https://www.ntotank.com/blog/above-ground-vs-underground-rainwater-tanks
4. https://www.sintexonline.com/blog/rainwater-harvesting-why-underground-tanks-are-ideal-for-water-conservation/
5. https://www.twdb.texas.gov/innovativewater/rainwater/faq.asp
6. https://rainwaterharvesting.tamu.edu/tank-material/
7. https://rainwatermanagement.com/pages/rainwater-storage-options
8. https://blog.enduraplas.com/water-storage-rain-harvesting/what-type-of-water-tank-is-best-for-rainwater-harvesting
9. https://www.ntotank.com/blog/rainwater-harvesting-101
10. https://smartwateronline.com/news/common-mistakes-to-avoid-in-rainwater-harvesting
11. https://www.ntotank.com/blog/how-to-turn-rainwater-into-drinking-water
12. https://www.energy.gov/cmei/femp/rainwater-harvesting-systems-technology-review