11.6. Filtration or Purification

The filter is used to remove suspended pollutants from rainwater collected over the roof. A filter unit is a chamber filled with filtering media such as fiber, coarse sand, and gravel layers to remove debris and dirt from water before it enters the storage tank or recharges structure. Charcoal can be added for additional filtration.

The primary aim of filtration in rainwater harvesting is to clean the collected water by eliminating unwanted particles such as dirt, leaves, and pollutants that can affect the water's quality. A filter unit, which acts like a chamber, is filled with different materials that trap these impurities. The basic layers often include fibers and coarse sand at the top, followed by gravel at the bottom, to ensure that all kinds of debris are caught before the water flows into a storage tank.

Think of a coffee filter. Just like how a coffee filter allows liquid to pass through while holding back the coffee grounds, a water filter does the same for rainwater, allowing clean water to flow into storage while trapping dirt and impurities.

(i) Charcoal water filter: A simple charcoal filter can be made in a drum or an earthen pot. The filter is made of gravel, sand, and charcoal, all of which are easily available.

A charcoal water filter is a simple, effective method for purifying rainwater. This filter consists of layers of gravel, sand, and charcoal placed in a container like a drum or pot. The charcoal, which has many tiny pores, attracts and traps chemical impurities and can eliminate odors, providing cleaner water. The easy availability of these materials makes this method both accessible and affordable for many households.

Imagine you are making a natural air freshener using charcoal. Just as charcoal absorbs odors in your home, in the charcoal water filter, it absorbs impurities and unwanted chemicals from the rainwater, making the water cleaner and more pleasant to use.

(ii) Sand filters: Sand filters have commonly available sand as filter media. Sand filters are easy and inexpensive to construct. These filters can be employed for treatment of water to effectively remove turbidity (suspended particles like silt and clay), colour, and microorganisms. In a simple sand filter that can be constructed domestically, the top layer comprises coarse sand followed by a 5-10 mm layer of gravel followed by another 5-25 cm layer of gravel and boulders.

Sand filters are another straightforward option for purifying rainwater. They leverage layers of sand and gravel that filter out not only visible dirt but also harmful microorganisms. The top layer is usually coarse sand, which acts as the first line of filtration, followed by smaller gravel layers that help trap even finer particles. The design ensures that the water passing through is as clean as possible before it enters storage.

Consider a multi-layer cake. Each layer represents a different aspect of filtration. When you pour a liquid over it, the top layers are like the coarse sand, filtering out larger particles, while the lower layers act as finer filters, similar to how the cake holds its shape and ensures a delightful flavor. Similarly, sand filters ensure that the layers work together to produce clear, clean water.

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