Research On The Effects Of Mg²⁺ On Aerobic Granulation And Phosphorus Removal Mechanism Of Aerobic Granular Sludge

Aerobic granular sludge is a kind of self-immobilized sludge, which is found recently under the aerobic condition. In recent years, researchers have given more interests in it because of its advantages such as regular, dense and strong microbial structure, good settling ability, and high biomass retention, tolerance to high organic loading rate/toxic shocks and synchronous nitrogen and phosphorus removal.Based on the latest research findings on aerobic granular sludge in both domestic and abroad, three sequencing batch reactors were concurrently operated to examine the effect of Mg²⁺ augmentation on aerobic granulation. When simultaneous nitrogen and phosphorus removal was achieved by aerobic granular sludge, the characteristic and mechanism of nitrogen and phosphorus removal were explored.The experimental results demonstrated that Mg²⁺ enhanced the sludge granulation process in the sequencing batch reactor. Mg²⁺-fed granules had a higher biomass concentration and higher polysaccharide contents. It was found that augmentation with 10 mg/L Mg²⁺ benefited granular sludge formation and growth and significantly decreased the sludge granulation from 32 d to 18 d. The average size of granules with the addition of 10 mg/L Mg²⁺ were larger than that of the other two concentrations. However, the effect of Mg²⁺ at the concentration of 100 mg/L was not obvious as that of 10 mg/L Mg²⁺.Simultaneous phosphorus and nitrogen removal was achieved by aerobic granular sludge in single SBR system under the conditions of temperature 25℃, pH 7-8, anaerobic reaction time 90 min, and aerobic reaction 240 min. At steady operation, the anaerobic/aerobic single SBR system showed a very stable phosphorus, nitrogen and organic carbon removal performance. The average removal rate for total inorganic nitrogen, phosphorus and COD reached 91%, 94% and 90%, respectively at the influent concentration of NH₄⁺-N 25⁵0 mg/L, PO₄^3--P 5²5 mg/L and COD 370⁵00 mg/L. Significant amounts of phosphorus-accumulating organisms (PAOs) capable of denitrification can be accumulated and enriched in the aerobic granular sludge coexisting with nitrifiers under appropriate operational conditions, and denitrifying PAOs accounted for 83.3% of total PAOs. What is more important, the denitrifying PAOs take up and store phosphate using nitrate as electron acceptor, then the organic carbon substrate can be used simultaneously for both phosphorus and nitrogen removal. This is of significance since organic carbon content in most municipal wastewaters is too limited in order to achieve both phosphorus and nitrogen removal. Employing denitrifying PAOs in the biological nutrient processes also makes it possible to reduce sludge production and aeration demand and this will make a significant impact on the engineering practice of phosphorus and nitrogen removal process from wastewater.