Filter life and maximum capacity

[Guest]

#909

Hello,

I am looking at starting a mission trip and am looking at the different water filtration equipment around to determine which will be most effective.

I have a few questions about the biosand filter.

1) What is the life of the filter (how long does it last)?

2) How many litres of water can it filter per day?

3) What is the initial cost of these filters?

Thanks!

[Eric]

#910

Life of the filter should be however long the concrete lasts for. So decades potentially. The sand, if fine enough, should never clog deeper than a few centimetres, so from that point of view the sand also should never need to be completely changed.

Litres of water possible per day – that depends on the flow rate. Take a look on this website under ‘Flow rates’ page – the rate should not exceed 400 litres/m2/hour, or converted to the household filter surface area, something around 25 litres per hour maximum. Anything faster than this and you should try again with smaller sand size. Normally people will use the filter during the day, so then multiply by the assumed number of hours of use. But keep in mind that water that had remained in the sand during pause time, will have better quality than new flush water (water that has been put in and filters through immediately – so from that point of view, it would be better to promote filling the filter to the brim (usually 20 litres or thereabouts) and letting that filter through, then waiting for some hours before repeating… this gives more contact time for water that has been pushed through to interact with the sand/biological zone. So I’m just guessing here, but perhaps aim for 2 x per day and 1 x at night, so could say 60 litres per day with pause time periods?

Cost – depends on what type of filter. If the concrete filter, then you’d have to price up the filter depending on local costs of labour, cement, sand, gravel and PVC pipe. If you’re trying to promote a micro-enterprise to make these and sell them on for small profit (what we encourage, rather than doing them in handout fashion) then you also have to add some margin for the manufacturers, but in reality this final price will be determined by the market. In Kenya I believe they were selling for around $10, while in Afghanistan it was more like $23.