Abundance, biomass and viability of bacteria in wastewaters: impact of treatment in horizontal subsurface flow constructed wetlands.

The aims of this project were to assess bacterial populations (abundance, biomass and viability) in the influents and effluents of four constructed wetlands, and to analyse the effect of such biological treatment on these bacterial characteristics. Using the BacLight probe it was possible to determine the total abundance, the proportion of intact vs. damaged cells, and the lengths, widths and biovolumes, of bacteria in each of the samples. The reduction in bacterial concentration was higher (67%) in the wetland used for secondary treatment than in those used for tertiary treatment (15-39%). The proportion of damaged cells was higher in the influent (i.e. settled sewage) of the wetland used for secondary treatment (78%) than in the influents of those wetlands used for tertiary treatment (45-70%). This suggested that the majority of bacteria in the settled sewage were dead or damaged, and that these were removed from the wastewaters more effectively than were undamaged cells during conventional secondary treatment (in this case, using rotating biological contactors or RBCs). In each wetland, the proportion of damaged cells was higher in the influent than in the effluent, suggesting that, as with RBCs, damaged bacteria were removed more effectively within the wetland than undamaged bacteria. The majority of bacteria leaving the constructed wetlands used for tertiary treatment, and 50% of those leaving the secondary treatment wetland, were physically intact and therefore probably viable. Although there was a decrease in the abundance of total bacteria with treatment, bacterial biomass did not necessarily decrease with the treatment.