Effect of Granule Sizes on Ammonia Oxidation Rates

Different aerobic granular sludge size fractions of a pilot plant were evaluated for their ammonia oxidation rates and community composition with batch tests and molecular tool (qPCR, genomic analysis). The pilot plant was fed with centrate and secondary effluent amended with acetate as the main carbon source in a well-mixed anaerobic phase. Small (0.33mm) granules had a 4.72 ± 0.09 times higher ammonia oxidation rate, and a 4.05 ± 0.17 higher abundace in ammonium oxidizers than large (2.23mm) granules, which aligned with modeling results showing a higher aerobic volume fraction for smaller size fractions. However, small granules had 1.43 ± 0.01 times lower ammonia oxidation rate and 1.66 ± 0.04 times lower amoA copy gene number per surface area than big granules, which was also confirmed with theoretical calculations. This work provided a roadmap to control nitrification capacity by two methods (a) blending larger granules into smaller fractions to increase the net aerobic surface area or (b) increasing the sludge bed volume and hence the mixed liquor suspended solid concentration to increase the total aerobic volume fraction.