19 Aug 06 15:02
I am trying to treat 50,000 gallons per day of cooling tower blow down waste water to use as water for irragation. My pH is 8.6,sodium 270 ppm,chloride190ppm and TDS 1568. Plants will not tolerate high pH and sodium levels. Any suggestions
quark (Mechanical) 20 Aug 06 04:10
RO can be a good option in your case. Neither sodium nor pH is a problem. Infact, the feed water pH of 8 to 9 is good. Just check silica.
quark (Mechanical) 20 Aug 06 04:13
PS: I am not knowledgeable about detrimental effects of sodium on any kind of water treatment system. Can you please explain? What is the composition of 1568 TDS?
BigInch (Petroleum) 20 Aug 06 05:54
Sodium salts aren't tolerated by most crops. TDS = Total dissolved solids
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quark (Mechanical) 20 Aug 06 06:21
Thanks, I mistook and by plants I presumed water treatment plants. I asked for constituent composition of TDS and not the full form.
LHA (Civil/Environmental) 21 Aug 06 08:23
Agree with quark's concern on TDS composition. It could be nothing, it could be just algae from the coils, which would actually be good for plants, although it might foul distribution hoses, nozzles, etc. But, my guess is you've used an algacide to prevent algae growth to increase cooling coil efficiency. If this is true, some component of the TDS is most likely the salts in the algacide. And if they are Cu salts - very common in algacides - those are toxic to plants. I think RO would remove most metals in any case though. But won't that be pretty expensive?
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bimr (Civil/Environmental) 21 Aug 06 09:58
This has been discussed on other posts, you might want to search the site. The blowdown TDS is too high for the crop irrigation as others have posted, so you will need desalination as treatment if you want to reuse the blowdown. A more practical approach is to convert your cooling tower system to a zero discharge cooling tower. Doing this will reduce your water supply needs (The extra water can be used for irrigation instead), and eliminate the blowdown altogether. You have not revealed what type of manufacturing plant that you are working on, but this approach has been used on many power plants, especially in water short areas. Evaporators are commonly used for desalination in these systems. Zero blowdown cooling systems cost more than conventional cooling tower arrangements. However, you will find that trying to desalinate the cooling tower blowdown (with RO especially) is a problematic and difficult proposition.
21 Aug 06 10:02
Thanks for replies. My Cu is only 3ppm and zinc .37ppm.Sulphates are 705ppm.The RO is obviously the way to go but we are trying to see if there is a way of precipitating the sodium etc. out through chemical or depth filters. I have heard of a solid "pill" put into the flow of water which reacts with the sodium giving nitrates but I am not a chemical eng so do not know the compositions of this reaction and what the pill would be made of.
bimr (Civil/Environmental) 21 Aug 06 11:08
An RO based system will work, but it will be problematic to operate because the RO system can not tolerate the suspended solids in the blowdown. Of course, you can filter out the solids. However, it is easier said than done to attempt to produce a low sdi water that is suitable for RO. There are no simple ways to get sodium sulfate out of solution. The solubility of sodium sulfate is approximately 2500 mg/l, so precipitation of sodium sulfate is not going to be of much help. You are looking at either evaporation or RO. There are no other options for desalination. You should start looking at other alternatives first. For example, 1. Is it possible to increase the blowdown so the concentration of salts is reduced? 2. Is it possible to go to a zero discharge cooling tower. etc.