How to Form Aggregates of Free Anammox Bacteria to Improve Sedimentation Performance

Abstract

Free anammox bacteria are superior in growth rate, but poor sedimentation performance limits their application in sewage treatment. In this study, we investigated how to form aggregates of free anammox bacteria to improve sedimentation performance. Calcium addition tests proved that free anammox bacteria could aggregate and form a larger entirety with better sedimentation performance through calcium cross-linking with alginate-like exopolysaccharides (ALEs). This was indicated by the particle size increasing by 411.45% and sedimentation performance (measured with supernatant transmittance) increasing by 195.35% after adding calcium for 12 h. Soluble extracellular polymeric substance (S-EPS) extraction and freeze–thaw testing elucidated that providing more cross-linking sites can strengthen the cross-linking, as indicated by the sedimentation performance increasing by 158.57% and 394.80%, respectively. Static experiments showed that cross-linking time was equally important. The sedimentation performance improved with longer static times under no severe external disturbances, with a 324.61% improvement after 84 h. However, the bacteria burst and the anammox activity disappeared after freeze–thaw treatment. Based on the above test results, a potential method for forming aggregates of free anammox bacteria to improve sedimentation performance was proposed: extract S-EPS with centrifugation first, add calcium, and keep the sludge free from external hydraulic interference.