346 Organic waste recycling: technology and management
Although plants can concentrate some metals by over three orders of
magnitude above the concentration in water (O'Keefe et al. 1984; Wolverton
and McDonald 1976b), the potential of this removal mechanism is small
compared to that of the other removal mechanisms.
The principal means for removing heavy metals during aquatic treatment is
by controlling the aquatic environment to optimize the precipitation and ion
exchange/adsorption removal mechanisms. The rate of removal and storage
capacity associated with these mechanisms are functions of redox potential, pH,
the presence of clay minerals and insoluble/particulate organic matter, and the
concentrations of co-precipitating elements and related compounds, such as
sulfur, phosphorus, iron, aluminium, manganese and carbonate. Unfortunately,
wetlands environments are sufficiently complex to preclude accurate theoretical
determination of what environmental conditions will result in maximum
removal of heavy metals in a given situation. The results of field observations
and experiments in this area have not been conclusive.
To some extent heavy metals will be removed in every aquatic system. The
extent of removal is affected by aquatic system design and management as well
as wastewater quality (including pretreatment). Resolubilization of sediment-
stored heavy metals (intentionally for discharge or accidentally as a result of
process upset) can occur. Ultimate disposal of heavy metals from aquatic
systems will be by the methods discussed for phosphorus in the previous
section.
Refractory organic removal
In aquatic systems refractory organic compounds are removed from solution by
adsorption to intrasystem surfaces and are altered chemically by physical,
chemical, and biological decay processes. During the residence time of a
wastewater in an aquatic system, chemically unstable compounds will
disintegrate and the diverse bacterial populations, characteristic of aquatic
systems, will metabolize the more biologically degradable compounds.
Refractory organics more resistant to chemical and/or biological decay will be
removed, at least partially, by adsorption to the bottom sediments, detrital layer,
or "ooze" layer enveloping the submerged parts of plants (once adsorbed, decay
as a result of bacterial or botanical metabolism and/or physical/chemical
processes takes place).
Aquatic systems are reported to remove phenolic compounds, chlorinated
hydrocarbons, petroleum compounds, and other refractory organics (Dinges
1979; Seidal 1976). Quantification of the removal of specific refractory organic
compounds from a specific wastewater by means of aquatic treatment methods
will require detailed studies (Stowell et al. 1980).