Sizing Guidelines for Household Rainwater Tanks

how big should my rainwater tank be

Apart from Australia, people in other parts of the world use rainwater tanks for many household chores. Depending on the tank capacity, installing a rainwater harvesting tank can lower your consumption from mains water systems and save money. However, calculating the size of any (cylindrical or rectangular) water storage tank depends on your use. To begin, let’s analyze some necessary design procedures before recommending a suitable rainwater harvesting system.

Primary Factors That Determine the Type of Storage Tanks

While maintaining an average consumption of 4,000L per week for your household, the minimum requirement for a four-week supply is a 16,000L tank. Apart from consumption rates, other factors like the number of persons living in a property, and average annual rainfall are necessary before purchasing household rainwater tanks. However, your household water storage requirements may vary according to the frequency of water scarcity in the area. When manufacturers design storage tanks, they also consider the area (type and size) of the catchment. Generally, these factors have impacts on water storage system designs.

determining how big your water tank should beCalculating the Capacity of Household Rainwater Tanks

The design for a water tank includes the runoff coefficient value. The total capacity (litres) that needs to be designed will be known after multiplying this runoff coefficient value with the catchment area and annual water harvesting potential. For example, a catchment area of 100 (sq. m) increased by an average annual rainfall of 0.5 (m), and runoff coefficient value of 0.5 will give a capacity of 25,000 litres. Don’t forget that 1m3 (cu. m) equals 1000 litres. This guideline for designing storage tanks provides a rough estimate of storage volume capacity. Usually, tank manufacturers often include a minimum safety allowance of between 10 to 20% space.

Calculating Your Rainwater Usage

You might need help when calculating your household’s rainwater runoff coefficient value. It’s essential to estimate the dimension of your roof area (sq. meters) before multiplying it with the annual rainfall (mm) value. Another vital factor to consider is how much rainwater the household consumes on the average every day. There are some basic after applications that can reduce water bills if you use rainwater. In some cases, single occupants use around 20 litres for flushing toilets and 150 litres of water for laundry. Apart from watering your lawn, someone who uses about 200 litres of rainwater daily might need a storage tank with a capacity of 7,000 litres.

Choosing a Rainwater Tank for Household Use

Instead of relying on main water supplies, people in Australia can enjoy rebate programs by using rainwater tanks. Naturally, rainwater is pure before it mixes with contaminants. So, plumbers should install screens (gutter mesh) around the circumference of collection pipes to prevent unwanted matter. Also, installing a pump and filtration device at the outlet pipe of rainwater tanks can ensure even distribution of clean water.

A rainwater tank model with corrugated steel material is often cheaper than other products. Usually, corrugated steel tanks are fire-resistant and durable. However, the drawback of using corrugated steel rainwater tanks is corrosion. The lightweight of polyethylene (plastic) rainwater tanks make it easy for plumbers to install or reposition them. There are zero risks of corrosion because rubber materials do not have metallic properties. However, wear and tear of these plastic rainwater tanks increase with the intensity of sunlight.

It’s the rigid structure of steel-reinforced concrete rainwater tanks that make them very durable. Since these materials are rust-proof, they can be installed as underground tanks for harvesting rainwater where there’s little ground space.

Fibreglass tanks are more expensive options because of the quality of their light and non-corrosive material. However, the drawback with these fibreglass tanks is their fragile nature. Since they also allow light to pass through their thin membranes, bacteria and algae might grow inside the fibreglass rainwater tank. So, owners of these must brace up for regular cleaning.


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