Unveiling significance of Ca2+ ion for start-up of aerobic granular sludge reactor by distinguishing its effects on physicochemical property and bioactivity of sludge

Almost all of the aerobic granular sludge (AGS) reactors were fed on certain amounts of Ca2+ ion, but whether and why it was necessary for reactor start-up remain unknown. Herein, this study conducted a set of comparative experiments in three AGS reactors, which were operated in parallel with Ca2+ addition in R3, hydroxyapatite (HAP) addition in R1, and without any forms of Ca addition in R2. Results showed that R3 not only achieved the complete granulation of sludge, but exhibited superior performance of COD and nutrient removal. In contrast, R1 had a slightly quicker granulation rate than R3 (R1: 0.07 day(-1); R3: 0.06 day(-1)), but the formed granules could not efficiently degrade pollutants. In R2, both sludge granulation and pollutants removal did not proceed normally. Further investigations found that the Ca2+ ion acted in three ways: (1) it increased inorganic composition of sludge to promote granulation; (2) the transformed HAP strengthened stability of granular structure; (3) it ensured bioactivity of granules by driving enrichment of functional microbes and synthesis of metabolism enzymes. Overall, this study systemically proved significance of Ca2+ ion for the start-up of AGS reactors and its influencing mechanisms on different properties of granules.