Effects of high calcium concentrations on the development of methanogenic sludge in upflow anaerobic sludge bed (UASB) reactors

The feasibility of anaerobic treatment of wastewaters with a high calcium content (600–1200 mgCa2+.l−1) was studied. The influence of the amount of precipitation and the influence of differences in biomass yield were investigated on the development of anaerobic sludge. The experiments were performed in lab scale Upflow Anaerobic Sludge Blanket (UASB) reactors, inoculated with crushed granular sludge. Four reactors were operated during 180 days at a constant organic loading rate of 14 gCOD.l−1.d−1, treating completely and partly acidified wastewater with different calcium concentrations in the influent. Treatment of completely acidified wastewater with a calcium concentration of 1200 mgCa2+.l−1 resulted in an unstable COD removal efficiency (60–90%). Furthermore, cementation of the sludge bed occurred after 180 days of operation, leading to operational problems. With completely acidified wastewater and a lower calcium concentration in the influent (600 mgCa2+.l−1), less calcium precipitated and COD removal efficiency always exceeded 98%. The cultivated sludge had a high ash content, 80–90%, and methanogenic activity remained more or less constant at 0.7 gCOD.gVSS−1.d−1. However, scaling of reactor compartments occurred under the latter condition. The presence of a fraction of non-acidified substrate, i.e. glucose, decreased the risk of cementation, resulted in better granulation of the methanogenic sludge and seemed to prevent scaling of reactor compartments. Considerable amounts of phosphate were shown to be removed from the influent, most probably by coprecipitation, and incorporated in the anaerobic sludge. The results indicate that anaerobic treatment of high calcium wastewaters in UASB reactors is more feasible for partly than for completely acidified substrates.