Effects of substrate shock on extracellular polymeric substance (EPS) excretion and characteristics of attached biofilm anammox granules

Environmental stresses are assumed to significantly impact the content and concentration of produced extracellular polymeric substances (EPS) and therefore influence the performance of an ananmmox attached film expanded bed (AAFEB) reactor. In this study, a transient high substrate concentration of 2500 mg N L^-1 (calculated as the sum of NN₄⁺-N and NO₂^--N) for 24 hours stimulated abundant EPS excretion as well as deterioration of anammox granules. The results indicated that a high EPS concentration of 89.6 ± 48.3 mg g^-1 VSS resulted in 35.0 ± 0.8% decrease in the granule settling properties and 30.5 ± 0.9% reduction in the total VSS amount. The production of EPS was reasonably attributed to the impact of utilization-associated and stress-associated effects by the substrate. The results of a series of batch experiments indicated that a rapid increase of loosely-bound EPS (LB-EPS) from 41.2 to 114.6 mg g^-1 VSS occurred when the substrate concentration steadily increased from 400 to 1000 mg N L^-1, in contrast, the tightly-bound EPS (TB-EPS) remained stable at 32.5 ± 2.8 mg g^-1 VSS. Thus, the LB-EPS was considered the key factor for the deterioration of granule stability and the substrate concentration should be controlled below 400 mg N L^-1 to avoid triggering the accumulation of LB-EPS. Furthermore, the formation and disintegration mechanisms of attached film anammox granules were also elucidated in this study.