UREOLYTIC PHOSPHATE PRECIPITATION FROM THE EFFLUENT OF AN ANAEROBIC DIGESTOR IN A POTATO COMPANY*

1. IN Selector (0,234 m³) in Effluent Anaerobic waste water treatment AIR OUT Reactor Precipur (1 m³) Influent barrel Struvite capture Decanter (0,3 m³) Effluent MgCl2.6aq Urea Effluentbarrel Sludge retour talent@work UREOLYTIC PHOSPHATE PRECIPITATION FROM THE EFFLUENT OF AN ANAEROBIC DIGESTOR IN A POTATO COMPANY* Evelyn Desmidt(1), Betty Maes(1), Karel Ghyselbrecht(1), Willy Verstraete(2), and Boudewijn Meesschaert(1)** (1) KATHOLIEKE HOGESCHOOL BRUGGE-OOSTENDE, Department of Industrial Sciences and Technology, Division of Chemistry, Zeedijk 101, 8400 Oostende and (2): UGent, Faculty of Bio-Sciences, LabMET, Coupure links, 9000 Gent Computer simulations Abstract An ureolytic phosphate precipitation technique was implemented on the effluent of the anaerobic pre-treatment. A removal efficiency of about 95 % was obtained. After drying, the precipitate was identified as dittmarite (MgNH4PO4.1H2O) by XRD and chromatographic analysis. Process The waste water of potato treatment plants contains high amounts of phosphate (about 100 ppm phosphate-P). This largely exceeds the discharge limits to surface water as stated by the EU (2 ppm P). An ureolytic phosphate precipitation technique which we previously developed on a lab scale (B. Meesschaert et al., 2007) was adopted to pilot scale ( V= 1 m3; F = 0.33 m3.h-1 ; FR = 0.33 m3.h-1) Feeding urea to an ureolytic sludge causes an alkalinisation of the micro-environment of the bacteria. In this alkaline conditions the addition of Mg2+-ions promotes the precipitation of phosphate as struvite and other magnesium salts, such as Mg3(PO4)2.8H2O). Computer simulations are helpful to calculate the amounts of ureum and Mg2+ that are necessary for the precipitation of the different magnesium salts. • The figure gives the SI for struvite at pH 9.2 • with start conditions between 10 and 100 ppm phosphate-P and a molar ratio [Mg2+]: [Phosphate-P] : [NH4+] of 3: 1: 4 • evolution of the SI when phosphate is precipitating as struvite starting from 93 ppm phosphate-P and starting from the same molar ratios as above • evolution of the SI calculated with the molar ratios as found in the effluent (30: 1 : 100) starting from [phosphate-P] of 8 ppm. Results • The bacteria have a double function: • Ureolytic decomposition of urea to NH4+ • 2) Nucleation site for the precipitation of Mg2+-salts Flow-chart The initial concentration of phosphate-P was about 90 ppm (3 mMol). The % removal is higher than 90%. After drying and analysis we found the precipitate to contain dittmarite (MgNH4PO4.1H2O), a dehydration product of struvite (MgNH4PO4.6H2O) and some (Mg)3(PO4)2.8H2O Conclusions Ureolytic phosphate precipitation effectively removed phosphate from the anaerobically treated waste water. Struvite was formed (beside some magnesium phosphate) which was identified as its dehydration product dittmarite. After an adaptation period the reactor behaved as a full continuous reactor. * Agristo N.V. Waterstraat 19, 8531 Harelbeke (Hulste) -- ** Corresponding author B. Meesschaert, E. Desmidt, J. Dick and W. Verstraete; Programme and abstract book 6th IWA Specialist conference on Wastewater Reclamation and Reuse for Sustainability (6th WRRS): Guiding the growth of water reuse, Antwerp, october 9-12, 2007; p 49. An enlarged abstract is available on a CD-ROM. Paper 072 in the section ‘Industrial recycling: Food industry’.