5. Conclusions
In this paper, simulation results for a period of 3 years with
BIO_PORE model (Samsó and García, 2013) were presented to study
bacteria dynamics and distribution in a pilot constructed wetland.
At the start-up period heterotrophic bacteria (XH) were the first
group to develop and colonise the system. After day 80 and until the
end of the 3 years anaerobic bacteria groups dominated the system,
being sulphate reducing bacteria (XASRB) the most abundant group in
terms of overall biomass (47–79%) for most of the time. The high
sulphate reducing activity within the wetland caused toxicity by
dihydrogensulphide (SH2S) and delayed the growth of methanogenic
bacteria (XAMB). Nitrifying bacteria (XA) accounted for 1–2% of the
total biomass while sulphide oxidising bacteria (XSOB) grew mainly
under anoxic conditions andwere responsible for the complete denitrification
observed in the wetland (autotrophic denitrification).
Bacterial stability was achieved between 400 and 700 days after
starting operation. This time to stability is longer than the 75–100 days
reported by previous experimental works, although the criteria for bacterial
stabilisation is different from the ones used in this work.
After bacteria stability XH and XA occupied the first few centimetres
near the wetland's surface, where oxygen concentrations were higher
while XFB, XAMB and XASRB grew on the rest of the wetland and
thus had a much wider vertical distribution. XSOB grew in a very
restricted area near the outlet of the bed in which high SH2S and SNO
concentrations coexisted.
The driving force behind the displacement of bacteria after stability
was the progressive advance of accumulated inert particulate
solids (XIf) towards the outlet of the wetland, which pushed the active
bacteria zone in the same direction.
The results of this study coupled with previous field results will
give new insights and perspectives on wetland functioning. This
paper is just a stepping stone towards the end goal of establishing a
general conceptual framework of the functioning of constructed
wetlands based on modelling results.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.scitotenv.2013.04.073.