Simultaneous recovery of calcium phosphate granules and methane in anaerobic treatment of black water: Effect of bicarbonate and calcium fluctuations

Calcium phosphate (CaP) granules were discovered in the anaerobic treatment of vacuum collected black water (BW), using upflow anaerobic sludge blanket (UASB) technology. This allows simultaneous recovery of CaP granules and methane in the UASB reactor. However, the role of BW composition on CaP granulation is not yet understood. Moreover, CaP granulation was not observed in previous research on anaerobic treatment of BW, although similar treatment conditions were applied. Therefore, this study shows specifically the influence of bicarbonate and calcium fluctuations in BW on the phosphorus accumulation in the UASB reactor, which directly affects CaP granulation. Without calcium addition, 5% of the total phosphorus (P) fed was found as CaP granules in the reactor (61 mgP g−1dried matter), after 260 days of operation. Simultaneously, 65% of the COD in BW was efficiently converted into methane at 25 °C. Variations of bicarbonate and calcium concentrations in raw BW showed a significant influence on phosphorus accumulation in the UASB reactor. Geochemical modelling showed that the increase of soluble calcium from 39 to 54 mg L−1 in BW triggers supersaturation for calcium phosphate precursors (Cax(PO4)y). Concurrently, bicarbonate decreased from 2.7 to 1.2 g L−1, increasing further the ionic activity of calcium. Formation and accumulation of seed particles possibly enhanced CaP granulation. Preliminary results showed that addition of calcium (Ca2+/PO43− molar ratio of 3) increased the accumulation of total P in the UASB reactor to more than 85%. This further increases the granulation rate and consequently, the process feasibility. •PO43− removal was observed for Ca2+ concentrations in black water above 54 mg L−1.•When HCO3− decreased from 2.7 to 1.2 g L-1, Ca2+ increased from 39 to 54 mg L−1.•A change from 39 to 54 mgCa2+ L−1 triggers supersaturation for Cax(PO4)y phases.•After 260 days, granules inside the reactor contained 10% of the total P fed.