Proceedings

A full scale struvite crystallization system is in operation in the Guillarei municipal WWTP (influent flow 32500 m3/d, Spain) for the treatment of the centrate obtained after anaerobic digestion. The plant is composed of a two-phase fluidized bed reactor connected to a settler (Fig 1). Industrial grade Mg(OH)₂ is added as magnesium and alkali source.

In period I the centrate was directly fed to the crystallization plant; while in period II was pretreated by an autotrophic nitrogen removal process (ELAN® process), the combination of the biological processes of partial nitrification and anammox. This step removes part of the ammonium and bicarbonate ions, reducing the needs of alkali addition for struvite crystallization. Phosphate concentration in the liquid effluent was around 15 mg P-PO₄^3-/L. Struvite pellets with a size range of 0.5–5.0 mm, suitable to be used as agricultural fertilizer, were obtained.