Phosphate removal in real anaerobic supernatants: Modelling and performance of a fluidized bed reactor

Phosphate removal in anaerobic supernatant coming from a centrifugation sludge station of an A(2)O process is studied. A fluidized bed reactor is employed to crystallize phosphate as hydroxyapatite or struvite using only air stripping to reach the supersaturation pH. The classic composition of supernatant (alkalinity 3550 mgCaCO(3)/l, PO4 139 mg/l, Mg 24 mg/l) does not require any addition of chemicals for phosphate removal. Seventeen runs are performed in a bench scale FBR obtaining very high conversion and removal efficiency and phosphate loss in the effluent less than or equal to 3.5%. The use of Ca or Mg enriched supernatant has no meaningful influence on efficiency, but it determines the prevalent salt formed between MAP or MAP. Efficiency can be related to pH and sand contact time in a double saturational model. The half efficiency constants: 0.075 h for t and 7.75 pH, have an important role in the process knowledge and optimization of plant design. Exhaust sand analysis indicates the same composition at the top, bottom and mean of the sand bed (39% mol MAP and 61% mol MAP). This result together with the high half efficiency constant for contact time indicate that the phosphate growth on the bed is not competitive. Finally, me phosphate release from the plant is studied. Results show a weak release rate, equivalent to 2.8-10% d(-1) phosphate as MAP, obtained at an operative pH range of 8.1-8.4. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.