This study investigated the effect of plant surface
area (plant density) on the efficiency of organic carbon
removal in a bench-scale constructed wetland.
Constructed wetlands are commonly assumed to be biofilm
reactors in which organic carbon removal occurs primarily
through sedimentation and aerobic degradation by attached
microbial biofilms. In conventional biofilm reactors,
aerobic degradation of organic carbon is proportional to the
amount of surface area for microbial attachment, provided
that sufficient oxygen is available. In contrast, current
design equations for constructed wetlands assume that the
amount of surface area is not an important parameter.
A bench-scale simulation of a constructed wetland was
conducted, using bulrushes planted at varying plant
densities in soil with a free water surface depth of about
0.27 m. The carbon source was diluted ENSUR (TM). Total
organic carbon (TOC) removal was measured. Concentration of
TOC was correlated with biochemical oxygen demand (BOD).
Tests were conducted in conditions of light and dark, and
under two different carbon loadings. Performance of
bulrushes was compared with that of inert acrylic rods.
The rate of carbon removal by mature bulrushes was
found to increase with increasing plant density until oxygen
became depleted. Higher densities degraded carbon at rates
much faster than those predicted by current design
equations. Young bulrushes degraded carbon at faster rates
than mature bulrushes. Once oxygen was depleted, rates of
degradation were reduced to rates anticipated by current
models. When plant density was 15% or greater, oxygen
became depleted in less than 6 hours. Removal efficiency
was greater at higher loadings (70 mg/l BOD) than at lower
loadings (25 mg/l BOD).
Bulrushes performed significantly better than inert
rods, sometimes by a full order of magnitude. The microbial
community on the bulrushes appeared to be more complex and
robust than that on the rods. Also, the presence of light
did not significantly increase degradation rates for the
bulrushes but was significant for the rods. The microbial
community on the rods contained a larger proportion of
epiphytic algae. The presence of light did result is
greater overall efficiency of removal for both bulrush and
rods.
Currently, a major drawback of constructed wetlands in
wastewater treatment has been their demand for large areas
of land. This study suggests that it would be possible to
reduce the land area requirements for constructed wetlands
for both carbon removal and nitrification/denitrification
provided designs gave more consideration to oxygen supply.
Using current designs, a retention time of 4-8 days
typically results in 70% BOD removal. This experiment
suggests that wetlands with a retention time of about 1 day
could provide the same performance if additional oxygen were
supplied. / Graduation date: 1995
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/35230 |
| Date | 30 November 1994 |
| Creators | Kuehn, Elaine Jinx |
| Contributors | English, Marshall J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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