Escherichia coli removal in a treatment wetland - pond system: A mathematical modelling experience

Abstract

A full-scale treatment wetland (TW) (100 inhabitants, 14 m3·d−1), composed of two horizontal subsurface flow wetlands (TW1–400 m2 and TW2–200 m2) and a small pond (13 m2), has been evaluated for Escherichia coli (E. coli) removal. The results indicate a global removal from 1.74·106 to 685 MPN·100 mL−1 (3.41 log units), reducing E. coli sufficiently to reach values suitable for reuse purposes such as agricultural reuse, without energy and reagent consumption. The small pond at the end of the treatment train plays an important role in E. coli removal and biodiversity enhancement. Data from TW1 and TW2 have been fitted to the P-k-C* model, giving values of 134 and 100 m·yr−1 for the first-order kinetic reaction coefficient. For the pond, a process-based model using continuous stirred-tank reactor (CSTR) and a 3d-CFD model have been implemented and compared. The models indicate that solar disinfection and predation by daphnids are the most important mechanisms in the studied pond, representing 65% and 25% of the removal respectively. It can be concluded that CSTR can provide good results for small ponds and 3d-CFD model provides extra information, useful to enhance their design.