Environmental Engineering Reference
In-Depth Information
efficiencies were affected little by the hydraulic loading trials. Phosphate removal of
32-71% occurred with the efficiencies being inversely related to hydraulic loading.
The FWS wetland removed most inorganic N whereas the SSF wetland removed
PO
4
-P at a rate equal to or even greater than the FWS. Removal of NH
4
-N and
NO
3
-N (effluent concentrations <0.3 mg NH
4
-N L
-1
and 0.01 mg NO
2
-N L
-1
)were
sufficient for recycle in the aquaculture system without danger of harming the fish.
In the same study Lin et al. [42] found that macrophyte density was a critical
factor affecting the reduction of SS and chlorophyll for the FWS wetland, but not
for the SF wetland. Suspended solids removals in both of the wetlands and the
combined system (47-86%) decreased significantly as the hydraulic loading rate
increased, strongly following the first-order mass-decrease equation. Phytoplankton
solids (biomass and detritus) were a primary source of SS in the aquaculture wastew-
ater. Both chlorophyll reduction (76-95%) and chemical oxygen demand (COD)
removal (25-55%) in the constructed wetland systems were apparently not affected
by hydraulic loading.
Maine et al. [64] used a free water surface wetland to treat wastewater contain-
ing metals (Cr, Ni and Zn) and nutrients from a tool factory in Santo Tome, Santa
Fe, Argentina. They found that water hyacinth (
Eichhornia crassipes
) was dominant
with a free water surface during the first year but then decreased as the water depth
was lowered. Cattail (
Typha domingensis
) then became dominant. While water
hyacinth was dominant, the wetland retained 62% of the incoming Cr and 48%
of the Ni. Nitrate and NO
2
-N were also removed (65 and 78%, respectively), while
dissolved inorganic PO
4
-P and NH
4
-N were not removed. When cattail became
dominant retention was 58% Cr, 48% Ni and 64% dissolved P, while 79% NO
3
-N,
84% NO
2
-N, and 13% NH
4
-N were removed. Maine et al. [64] also found that
NH
4
-N showed a different behaviour at different phases of vegetation development.
Several different studies have shown denitrification to be a major pathway in
wetlands. Mass balance in the Maine et al. [64] study suggested that N retained by
plants represented a minor fraction of the N removed from the incoming wastewater
in the small-scale wetland. They concluded that denitrification may have been the
major removal process. D'Angelo and Reddy [65] determined that most of the
15
N-
NO
3
(roughly 90%) applied to sediment-water cores was lost by denitrification.
Reddy et al. [62] measured large denitrification rates in the rhizosphere of emergent
macrophytes of deltaic marshes. Matheson et al. [66] performed
15
N balances in
wetland microcosms, and estimated that denitrification accounted for 61% of the
NO
3
-N load; 25% was retained in the soil, and 14% was stored in the vegetation
biomass.
Emergent macrophytes are known to release O
2
from the roots producing
a strong positive effect on nitrifying bacteria in the rhizosphere [67]. Sliekers
et al., [68] showed that anaerobic NH
3
oxidation is a qualitatively important path-
way in wastewater sediments. Maine et al. [64] concluded that in their system they
had simultaneous occurrence of partial NH
4
-N removal through the water hyacinth
decline period and the cattail dominance phases. They also concluded that dissolved
P might have been adsorbed onto Fe oxy hydroxides and later settled on the bottom
sediment. This occurred because enhanced phytoplanktonic and periphytic growth
