Environmental Engineering Reference
In-Depth Information
then, for the species which performed well, grew rapidly during the spring months
with the result that harvesting was needed by late June of each year. After the first
cutting, sufficient growth occurred each summer for two additional cuttings. During
2005-2006 the platforms had pure stands of the planted species. By 2007 weediness
was a problem. Weeds included the wild millet and common bermuda grass along
with common agricultural weeds such as ragweed and thistle. This problem was
addressed by spraying with roundup. By spring 2008 all platforms were extremely
weedy. It was decided to continue the experiment during 2008 but use this data as a
measure of biomass and element uptake that occurred with a heterogeneous mixture
of the original planted species and many weeds. For many of the platforms the wild
millet, which was on the edge or in the wastewater lagoon prior to the start of the
study, became the dominant vegetation. The two cutting dates in 2008 were 5/28/08
and 8/11/08.
The 2005-2008 replicated study showed that St. Augustine grass was completely
unsuitable for producing biomass on highly contaminated wastewater. Only a small
patch on one platform survived past mid 2006. The giant reed initially appeared to
be an excellent candidate for producing biomass. However, after the second cutting
of 2006 it began dying and was completely dead by 2007. The common bermuda
grass, Tifton 85 bermuda grass, and wild millet all proved to be good producers
of biomass. Mean total biomass produced from 2005 to 2007 was 3213, 3559, and
3082 g m
-2
for the common bermuda grass, Tifton 85 bermuda grass, and wild
millet, respectively (Table 9.6). There were no significant differences among these
plant species.
The mean N content of the biomass harvested from the floating platforms ranged
from 2.78 to 4.72% (Fig. 9.5). There were few statistical differences among the
plant species on any of the harvesting dates. The greatest values were found in
the St. Augustine grass. Most of the mean N percentages for all species were in
excess of 3%. This was comparable to what was observed with cattail, soft rush,
and maidencane with full strength swine lagoon wastewater in the 2001-2002 float-
ing mat study. It would appear that given unlimited N availability from wastewater,
plant species which can survive the poor quality wastewater (including weeds) will
contain 3-4% N.
Mean P in biomass grown in the swine lagoon wastewater ranged from 0.46
to 3.16% (Fig. 9.6). Significant differences in mean P contents only occurred on
11/14/06 and 6/27/07. On 11/14/06 the St. Augustine grass had significantly more
P than the giant reed, while on 6/27/07 the wild millet had significantly more P than
the giant reed. Most of the cuttings had P percentages in the range of 0.5-1.0%.
The sprigged plants had greater P percentages on 11/14/06 than on any of the other
cutting dates. The weeds cut on 8/11/08 also had relatively high P percentages. In
general, the P concentrations found in the cuttings from this study were greater
than those observed in the 2001-2002 study, where the range for cattail, rush, and
maidencane was from 0.35 to 0.51%. Greater uptake of P by the 5 species tested for
this study than the three species tested in the earlier study may be related to species
differences.
