Recovery of nutrients using ion exchange

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Although major efforts have been made in the past to close the nutrient water streams in soilless growing systems, still nutrient waste water flows are produced. Often these flows are discharged, leading to significant impact on the water quality of the surface and ground water bodies. The use of ion exchange can be a way to purify these water streams by removing the present nutrient elements. Alternatively, the sodium and/or chloride can be removed from the nutrient stream which will then enable the grower to re-use the nutrients in the greenhouse again.  When the nutrient solution is pumped through the ion exchange unit the positively charged nutrient elements or ions are absorbed onto a cation exchange resin while the negatively charged ions are absorbed onto an anion exchange resin. This means that the sodium, the element which in most cases is the reason for the discharge of the nutrient solution, is completely separated from the nitrate and phosphate, which cannot be discharged into the environment. When the resins are saturated, the anion exchange resin is regenerated with potassium hydroxide producing a potassium based solution containing the recovered nitrate, phosphate, and sulphate. (This solution will also contain the chloride that as present in the original solution. In many cases the chloride concentration is low enough to make reuse the generated solution) Before the cation exchange resin is regenerated with nitric acid, the sodium (Na) is removed from the resin by pumping a potassium chloride solution through the resin. It is not economical to remove 100% of the sodium because the excess potassium chloride (KCl) will be too much. An excess of KCl is required to remove most of the Na from the column and replace it with K. The specific circumstances at the greenhouse will determine how much excess KCl will be used. Alternatively, only a cation exchange resin can be used in case only Na has to be removed.  This process is then called a SRU (Sodium Removal Unit). With the SRU the nutrient rich water is pumped through the cation exchange resin removing the Na from the water. The solution is recovered for re-use after it has been pumped through the column. The resin is then treated as described above.

 

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Ion exchange 501.95 KB 62 downloads

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Ockie van Niekerk

Optima Agrik PTY LTD (OA), South Africa
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