Anaerobic calcium phosphate bio granulation

Anaerobic calcium phosphate bio granulation was originally observed by Tervahauta et al. (2014c) during treatment of source separated black water. However, the required conditions and mechanism behind the granulation were unknown, which limited the phosphorus recovery efficiency to only 2%. Therefore, in this thesis, the anaerobic calcium phosphate bio granulation, to enhance the process feasibility for simultaneous recovery of phosphorus and methane in a single bioreactor, was extensively investigated. In Chapter 2, the results for the reproduction of the calcium phosphate bio granulation during anaerobic treatment of black water using an Upflow Anaerobic Sludge Blanket (UASB) reactor are presented. An increased bicarbonate concentration in raw black water reduces the ionic activity of soluble calcium significantly, decreasing the phosphorus accumulation in the UASB reactor. Thus, a low concentration of soluble calcium limits phosphorus accumulation, and consequently, affects calcium phosphate bio granulation. Without calcium addition, 5% of the total influent phosphorus was found as calcium phosphate granules in the reactor, after 260 days of operation. Simultaneously, 65% of the organic loading in black water was converted to methane at a process temperature of 25°C. Chapter 3 shows that addition of extra calcium increases the accumulation of phosphorus in the reactor from 51% to 89% and also stimulates the formation and growth of calcium phosphate granules. Moreover, calcium addition increases the phosphorus content in calcium phosphate granules (> 0.4 mm diameter) from 3.7 to 5.6 wt%. The phosphorus recovery efficiency as calcium phosphate granules increased to 31%, by dosing 250 mgCa2+ L-1 of black water. A characteristic outer biofilm around the calcium phosphate core was consistently observed, as presented in Chapter 2 and 3. Thus, the role of the outer biofilm on calcium phosphate bio granulation was described in Chapter 4. An increasing pH gradient from the edge (7.4) to the granule core (7.9) was measured. The pH gradient enhances internal supersaturation for calcium phosphate phases, creating conditions for preferable calcium phosphate enrichment of the granule over bulk calcium phosphate precipitation. The pH profile can be explained by measured bio-conversion of acetate and H2, HCO3-, and H+ into CH4 in the outer biofilm and eventually stripping of biogas (CO2 and CH4) from the granule. Consequently, H+ released from aqueous phosphate species during Cax(PO4)y crystallization are internally buffered, stimulating further calcium phosphate precipitation. In Chapter 5 results that demonstrate that the carbon source and bulk pH are crucial parameters for formation and growth of calcium phosphate granules in a UASB reactor are presented. Anaerobic calcium phosphate bio granulation was achieved using glucose as the sole carbon source while keeping the bulk pH at 7.5. Volatile fatty acids did not yield calcium phosphate granules, and higher bulk pH (8.0 to 8.2) enhanced calcium phosphate precipitation in the bulk of the reactor. Produced extracellular biopolymers stimulated agglomeration of biomass and inorganic calcium phosphate particles, promoting the formation of granules at relatively low upflow velocity ( In Chapter 6, a novel reactor design, to enhance the phosphorus content in the calcium phosphate granules and the concentration of granules at the harvesting location, is presented. The novel reactor consists of a combination of UASB and gas-lift technologies. The injection of N2 at the bottom of a concentric draft tube creates an internal loop with a concentric upflow velocity (18 m h-1), which lifts lighter particles from the bottom of the sludge bed. Consequently, at steady state, the bottom of the reactor became more compact (73 g of total solids L-1 of sludge), while at the top of the reactor a lighter sludge bed was obtained (31 g of total solids L-1 of sludge). Moreover, the phosphorus concentration at the bottom of the reactor (harvesting location) increased from 3.0 g L-1 in a conventional UASB reactor with calcium addition to 4.6 g L-1 in the novel reactor. Similarly, the phosphorus content in the granules increased from 6.7 wt% to 7.8 wt%. The higher shear obtained by the gas recirculation increases the phosphorus content by reducing the thickness of the outer biofilm, and consequently, decreases the organic content in calcium phosphate granules. Chapter 7 presents the results of the characterization of calcium phosphate granules, harvested in the novel reactor and subsequently fluidized at an upflow velocity of 76 m h-1. The calcium phosphate granules were characterized to study their potential use as replacement for phosphate rock. The higher shear applied, increased the phosphorus content of the granules to 10 wt% (or 23 wt%P2O5), from which 35% was dissolved within 5 min in citric acid and 85% in H2SO4. Heavy metals (Cu, Zn, Cr, Ni, Cd, As, and Pb) and organic micropollutants in calcium phosphate granules are below the Dutch and European regulatory limits for application as a direct fertilizer, but pathogens in raw granules are a concern. Therefore, calcium phosphate granules were glowed at 550ºC to remove pathogens and the remaining organic content (29 wt%), increasing the phosphorus content to 15 wt% (or 34 wt%P2O5). The glowing process enables the direct substitution of phosphate rock by calcium phosphate granules in the phosphate industry for the production of water-soluble phosphate.