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| Ammonium removal from the STP effluent increased with
an increase of hydraulic retention time (HRT) in the
wetland system. This was most pronounced in winter,
when ammonium concentrations in the STP effluent were
higher.
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| Nitrate concentrations of the STP effluent were higher
in autumn-winter than in spring-summer. Nitrate removal
increased with HRT during the whole year.
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| A higher phosphate release to the STP effluent with
an increase in HRT was observed in spring-summer, whereas
an increase in phosphate removal was observed with an
increase in HRT in autumn-winter. A release of phosphate
during the growing season may have been the result of
adaptation of the system to much lower phosphorus loadings
after instalment of the longer HRTs in early spring
(before that the HRT in the system was 2.4 days). Periphyton
on sediment and plant surfaces probably also played
a role in the phosphorus release to the water, because
periphyton biomass decreased in spring-summer due to
shading by emergent macrophytes.
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| Turbidity in the STP effluent was higher when it remained
longer in the wetland system. The water was nevertheless
very clear. The turbidity of the STP effluent was mainly
caused by sludge particles and ciliates at the inlet
of the wetland system, but at the outlet of the wetland
system by dead plant particles and algae.
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| A clear increase in reduction of faecal coliforms
with HRT was observed throughout the year. Disinfection
was less pronounced in winter. A HRT of approximately
5.5 days is needed to reach consistently low levels
of faecal coliforms.
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