Linking Water Treatment with Wetland Restoration - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 366

This is then modified through bacterial activity to ammonia and is further modified
and transformed to nitrites and nitrates by bacterial processes, all of which takes
place in an aerobic water column (hence the importance of adequate dissolved oxy-
gen). But the largest removal process is really denitrification. Wetlands aren't always
accruing nitrogen from the amount being added; they're also losing a lot to the atmo-
sphere as part of the natural cycle. And this is one of the sustainable processes that
make wetlands work for a large-scale nitrogen improvement (Bays 2004).
It is possible to make some broad conclusions about treatment wetland perfor-
mance (Bays 2004; France 2003). The general range of performance from wetlands
is in the order of 50 to 90 percent removal for BOD, total suspended solids, and the
nutrients nitrogen and phosphorus. But importantly, in terms of public health con-
cerns and considering wetlands as part of a treatment train for potable water supply
or for wastewater treatment, they're especially good at removing pathogenic bacteria
to background levels. Other constituents such as metals are also removed well by
wetlands, essentially down to detection limits.
One example to show how treatment wetlands work is a project by CH2MHILL
near Indio in southern California (Bays 2004). This is an area with an extremely low
level of rainfall of about 8 cm per year, and with evaporation rates in the order of
about 250 cm per year—in other words, essentially the same kind of climate as that
in southern Iraq (see chapter 21 for other CH2MHILL projects in California). This
project involved construction of a small wetland, 6 ha in size, that receives about
3,000 m 3 per day of flow. What is special about this case study is that the inflow to
the wetland is treated primary effluent, not secondary treatment quality effluent. As
a result, the inflowing water is characterized by very high BOD. And with a long
residence time, in the order of thirty days or so, there's plenty of opportunity for
water to evaporate from the system, something that needed to be factored into the
design (Bays 2004). A curve was fitted to the system which mapped the removal rate
of BOD through time and along the treatment train, which consisted of three sepa-
rate cells. In the first cell, there was an almost straight-line reduction, showing a very
high removal rate from about 140 to 40 mg/L BOD. In the second cell, the removal
rate leveled off, ending in about 30 mg/L, and in the last cell there was a bit more
removal, with a final outflow concentration of about 10 mg/L. This sort of analysis
gives insight into how the system actually works. This first treatment cell takes out
those constituents within BOD that are readily decomposable and degradable, and
as a result, does most of the “heavy lifting” (Bays 2004). By the time water moves
into the second cell, it has been conditioned somewhat. The BOD has changed and
been transformed and it takes longer for the organic compounds to decompose and
be degraded. And by the third cell, the BOD has been removed and transformed
so now it's even easier for the wetland to remove that organic load. For the case of
wetlands in Iraq that are receiving a high BOD load, they could easily be calibrated
to this model (Bays 2004). In this respect, removal rate constants could be developed
that would be specific to the climate and the amount and type of wastewater being
generated for each locale in the country.