Pretreatment of RO plant

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I am facing a problem. We have installed an RO plant on surface water.

When I get SDI it is on higher side while filter paper looks clear.

Note we have settlement tank where we are using coagulants. 

Please suggest what to do

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7 Answers

  1. the most important for the water is not just the SDI. if you have the detailed water analysis document, the suggestion will be made according to the detailed data. 

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  2. Hi Nadeem,

    My first piece of advice would be that you don't purchase any equipment or change your process until you have a good idea of what the issue is.

    SDI is a generic measurement, which will tell you that the water is likely to cause fouling of your RO membranes, but won't actually tell you what the foulant is. For example, high SDI can be associated with iron, manganese, calcium, silica, algae as well as many others.

    The fact that the high SDI does not appear to be associated with discolouration of the filter might help to narrow down the list of suspects but ultimately you will probably need to do a detailed analysis of the water.

    Let me know if you are able to provide system design details and water quality data.

    Daniel

  3. Dear Nadeem, 

    We can offer a self-cleaning water filter as a pre-filter. We have screens that range from 5 microns up to 4000 micron  www.forstafilters.com  Let me know if I can be of help. 

  4. Hi Nadeem

    Yes, it would be beneficial to know what is causing the SDI, which as Ian Pearson mentioned could not be visible with your eyes. Adding a anionic flocculant with the cationic flocculant could work. However, you have to make sure the flocculant/coagulant is safe for RO membranes. Yes, pretreatment filtration could be adjusted as well from sand to zeolite for more efficient filtration.

  5. Hi Nadeem, that is one of the challenging things about water treatment - you cant always visually see the impurities, but they are always there. 

    You say you are treating a surface water with coagulation and settling - I presume you also have a sand filter.  So if the surface water is from a fairly contaminated source, there could be a number of organic molecules which are not being entrapped through the coagulation process.  You may need to try alternative coagulants - perhaps an anionic or non-ionic polyelectrolyte in combination with a cationic coagulant.  You may also need to consider using a filter aid on the sand filters, or modifying the sand filters to multi-media filters.  Of course you will need to ensure that all your unit processes are optimised.  Worst case scenario is you may need to pre-oxidise the raw water, or alternatively install a microfilter before the RO.

    Best wishes - hope all comes right for you