Environmental Engineering Reference
In-Depth Information
Historical reports also exist of floating vegetated islands formed on travertine
rafts. A lake now called Lago della Regina, formerly known as Lacus Albuleus, La
Solfatra, or Lago delle Isole Natanti, near Tivoli, Italy, once had vegetated floating
islands formed on floating masses of travertine. These were famously described
by Athanasium Kircher [85] and Francesco Lana [86] in the seventeenth century,
and in more detail by Sir Humphry Davy [87] in the nineteenth century. Lana [86]
described these floating islands as follows: “I myself saw several of these islands in
a small lake of sulfurous water not far from the Tiber; they were mostly circular or
oval, and rose four or six inches above the water. Their surface is flat and grassy,
and at the edges of some of them a few larger plants grow, which act as sails, so that
even the slightest breeze pushes the islands from one part of the lake to another. The
largest of them are a few yards in diameter, yet nonetheless can sustain several men
standing upon them.”
9.3.2 Water Improvement Processes
Floating vegetated mats can improve water quality through several different pro-
cesses. The primary process by which floating vegetated mats improve water quality
is through nutrient removal by the plants. It is also possible that denitrification may
occur in anaerobic zones of the mat. Another benefit of use of the mats is shading
of the water. This is of benefit when the entire surface of the water body is covered
because it may reduce populations of undesireable algae which are dependent on
sunlight reaching the water. Although not yet researched, and not a water quality
improvement process, it can be hypothesized that for wastewater lagoons a com-
plete cover by floating vegetated mats might improve odor problems. This would
be somewhat analogous to manufactured lagoon covers which are used to trap both
undesirable odors and methane.
9.3.3 Requirements for Successful Use of Floating Vegetated Mats
There are several requirements that must be met for successful implementation of
floating vegetated mat systems. The first requirement for a floating vegetated system
is a platform for supporting the growing vegetation. The platform must allow plant
roots to attach, grow through, and reach into the water. The platform also must
be durable and long lasting in relation to the quality of the wastewater. Secondly,
appropriate vegetation must be selected for the body of water. Vegetation that grows
well in moderately impacted water may not do well at all in severely contaminated
wastewater. The reverse is also true. Vegetation that grows prolifically on a floating
mat in water with high concentrations of N and P may not survive in waters with
low levels of these nutrients. In addition to selection of floating island platform and
appropriate plant species, the platforms must be sized appropriately for harvesting
of biomass. One major difference between floating mats and constructed wetlands
