At Vlako in Overloon, a manure processor, they have managed to double the levels in their mineral concentrate using membrane distillation.

Earlier this year, Vlako started a trial in which they were focussing on further thickening their mineral concentrate. At the end of this year, the thin fraction (liquid manure) of 6,000 tons of pig slurry was fully processed with the application of membrane distillation (MD) and the results of the analysis are now available. Indeed, Vlako succeeded in doubling the fertilization value of their mineral concentrate through membrane distillation, to 25 kilos of nitrogen per ton and 32 kilos of potassium per ton.

Concentrated fertilizer

Met MD is het mogelijk een mineralenconcentraat verder te ontwateren. Het mineralenconcentraat wordt initieel verkregen door RO (reverse osmosis). Bij MD worden dampdrukverschillen in vloeistoffen gebruikt. Aan de ene kant van het membraan stroomt opgewarmd concentraat en aan de andere kant koel permeaat (afvalwater van de RO) met een temperatuurverschil van 30 graden. Door de temperatuurverschillen ontstaat damp dat zich door het membraan werkt en het concentraat verder ontwaterd. Kalizout kan niet door het membraan, waterdamp en ammoniakgas wel. In de proefopstelling van Vlako verdampt ook ongeveer de helft van de ammoniakale stikstof door het membraan en komt dus in het permeaat terecht. Dit is desgewenst te voorkomen door de pH van het concentraat te verlagen met zwavelzuur. Vlako kiest ervoor om het permeaat nogmaals door de RO-installatie te doen en geen chemie te gebruiken. Het volume is beperkter en de eerste drie stappen van het RO-proces zijn overbodig vanwege de afwezigheid van het zout.

Convincing results

After a year of experimentation, they are convinced at Vlako's that MD works. The process is under control and the plant can therefore be scaled up to produce as much super concentrate as possible. The concentrate they now produce with RO alone contains 13 kilos of nitrogen per tonne, 16 kilos of potash and 0.13 kilos of phosphate. When scaling up with MD, these contents can therefore double.

Volume reduction

The dischargeable water volume increases from 60% to 76% of slurry through MD-processing. End products after RO treatment with 6.000 tons are: 1.200 tons of concentrate, 1.200 tons of thick fraction and 3.600 tons of water. Utilizing both RO and MD and the same volumes end product were: 240 tons of concentrate, 1.200 tons of thick fraction and 4.560 tons of water. The big difference here is in the volumes of concentrate in the different processes. After MD, 240 tons of super concentrate remains. The rest of the 960 tons is water.

Advantages of the super concentrated extract

• A larger sales area because there are twice as many minerals with each load.
• Less soil pressure thanks to the smaller mass of the super concentrate.
• The flexibility of the final product. When the outlet is not satisfactory it can be mixed again with thick fraction to export it as original product.

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