Filters made of concrete poured into steel moulds are layered with washed sand and gravel. Once water is poured from the top, it gets dissipated and flows through the layers and comes out clean and free of sediments, bacteria and some amount of cysts and worms.
The filter, which is about the size of an office water cooler, has a copper pipe fitted inside the filter wall running from the bottom of the filter to the discharge spout. The container, which can hold about 10 gallons (40-50 litres), is layered with specially selected, sifted and washed sand & gravel. A tin splash plate placed on top of the layers dissipates the force of the water poured into a filter while a tin lid protects the top of the filter opening.
The critical innovation is a 2-4 inch (5-10cm) layer of water that remains above the fine sand layer which permits ‘as-needed’ intermittent use and creates a bio-layer of microorganisms. This bio-layer innovation feeds on the pathogens in the dirty water and enhances the effectiveness of the traditional filter to eliminate sediments, bacteria, viruses, compounds, cysts, worms and other impurities by upto 95%. In addition, the bio-sand filter can be produced locally for low cost, as compared to ceramic, plastic or metal containers.
Advantages and disadvantages of Bio-Sand Filters
The bio-sand filter removes over 90% bacteria, 100% parasites besides removing turbidity, iron, manganese, arsenic and mercury. Durable and robust, the filters are fabricated from locally available materials and are small enough to fit into the smallest kitchens. The filter has no replaceable parts and no on-going costs and is easy to maintain. The quality of water improves over time and water tastes and looks good.
Manufacturing the bio-sand filters also provides local business and employment opportunities. At the same time, the disadvantage of the bio-sand filter is that the filter is heavy and difficult to be moved around. Its biological layer takes approximately one to two weeks to develop to maturity and in event of high turbidity (> 100 NTU), the filter gets clogged. The filter cannot remove colour or dissolved compounds and needs to be used on a regular basis.