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Desalination and hardness removal methods for use with solar power

To anyone who may be able to help,

I'm working on implementing a solar powered water purification system for installation in a slum here in Bangalore. The community currently sources their water from one of many possible borewells in the surrounding area that have been deemed unsafe to drink. Possible groundwater that the community could source from has been sampled and indicated high TDS (900-1,200 mg/l), salinity (chlorides approaching 400mg/l) and hardness (Mg up to 80mg/l, Ca up to 200mg/l, hardness as CaCO3 up to 800mg/l).

An ultrafiltration unit has been proposed that would provide medically safe water, but the high TDS, salinity and hardness would remain.

Because our economic model relies on the community buying safe drinking water from a local operator of the purification plant, it has been deemed that the taste of the water is important - if the water doesn't taste different then people may be less willing to accept it's benefits. It is then important to reduce hardness and salinity mainly in order to improve taste characteristics.

For this, an RO unit would work, but has only been found at 4-5 times the cost when the attached solar circuit is taken into account. It would also provide fully demineralised water, which could also have negative health consequences. Remineralisation is possible but again at extra cost. The RO unit then becomes a very expensive way of changing the taste of the water produced.

I know that the problem of low-cost, low-energy desalination is a fairly global issue. But if anyone is able to give some ideas or suggestions as to water treatment methods that could reduce the TDS and/or salinity of the water at low energy, it would be a great help. The main requirement at this stage is something cheaper than RO.


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Your "fully demineralised" concern leads me to point out that you don't need to desalinate the entire flow, just enough to bring your chloride down to a level that the local population find a blend of desalinated and non-desalinated water acceptable.

In an area like Bangalore where land is at a premium, RO is currently likely to be the most cost effective means of desalination at significant scale.

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Thanks for your help.

Wouldn't blending the flow require two treatment units (one for the non-desalinated water and an RO)? Or is it usually possible to adjust the mixtures in commercially available RO units?

JackH gravatar imageJackH ( 2014-10-21 00:19:24 -0500 )edit
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