Storage mechanisms in constructed wetlands: Should we modify heterotrophic bacteria modelling?

This study investigated the occurrence of storage mechanisms in biofilm from constructed wetlands (CWs) through respirometric studies and calculated the corresponding heterotrophic growth yields. Respirometric tests were performed in biofilm extracted from horizontal sub-surface flow CWs batch loaded with three different COD mass loads: 7.0., 15.6 and 35.2 g COD/(m∙day). The bed removal efficiency remained above 96% for all mass loads and COD mass removal rates increased from 6.8 g COD/(m∙day) for the lowest load to 34.5 g COD/(m∙day) for the highest load. The percentage of tests with storage evidence decreased from 85% to 10% with increasing mass loads and the responses of the microbial community to the acetate pulse showed an adaptation to the feast-famine conditions, through storage mechanisms, for lower loads, and a metabolic shift to the use of COD for growth for higher loads. Heterotrophic biomass yield values varied from 0.54-0.56 g COD/g COD for low mass loads to 0.69-0.71 for higher mass loads, indicating that greater substrate availably triggers growth and reduces the occurrence of storage. Storage yield values supported this trend varying between 0.89 and 0.94 with increasing mass loads. Given the significant storage evidence obtained in the present study, it is suggested that a modified modelling architecture, which includes storage mechanisms, should be considered in future simulations of CW systems.