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Design of a crystalliser - Urine treatment for phosphoruous recovery via potassium struvite precipitation

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Crystalliser designStruvite

Optimising te flow patterns of the crystals while preventing wash out

We designed a crystalliser for treatment of human urine. Phosphorous was recovered in the form of potassium magnesium phosphate (K-struvite). Urine entered the reactor in the middle near the impeller. Different baffle configurations and impeller speeds were tested in order to find the optimal flow pattern (recirculation of crystals and no local crystal holdup or wash out).

Major findings

  • Some designs led to local recirculation zones at the top of the reactor, capturing crystals and preventing them from recirculating
  • The height of the middle shaft was critical (could give rise to crystal wash out)
  • An optimal impeller speed existed: optimal mixing of chemicals and creating an optimal upward velocity in the shaft

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