Recovery of calcium phosphate granules from black water using a hybrid upflow anaerobic sludge bed and gas-lift reactor

Adding calcium during anaerobic digestion of vacuum collected black water (BW) in an up-flow anaerobic sludge bed (UASB) reactor increased the retention of total phosphorus (P) in the reactor from 51% to 87%. However, the insufficient mixing in the reactor caused cementation and relatively high content of organics in the recovered calcium phosphate (CaP) granules, limiting the P recovery. In this study, the UASB reactor was mixed with an internal gas-lift (UASB-GL) to prevent cementation and to enhance the P content in CaP granules. The novel UASB-GL reactor operated for 300 days, treating concentrated BW. At steady state, the removal of total COD and P was 92% and 90%, respectively. The gas injection created a sludge bed with an average total suspended solids concentration of 73 +/- 16 g/L at the bottom and 31 +/- 5 g/L at the top of the reactor. The concentration of solid P at the bottom of the reactor was 4.58 +/- 1.34 gP/L, while at the top a much lower concentration was obtained (0.75 +/- 0.32 gP/L). 89% of the CaP granules was found at the bottom of the reactor. The harvested CaP granules (> 0.4 mm diameter) contained on average 7.8 +/- 0.6 wt% of P. A potential recovery of 57% of P in BW as CaP granules was calculated, considering actual application of the UASB-GL reactor in source separated sanitation.