Aquatic weeds and their utilization 341The wetlands environment needed for each specific case can be quite varied,
and therefore, a system designed to treat wastewater will be different from that
designed for any other purpose.
7.7.1 Wastewater contaminant removal mechanisms
The general mechanisms involved in the removal of contaminants by aquatic
weeds are discussed below.
BOD 5 removal
In aquatic systems, the BOD 5 associated with settleable solids in a wastewater is
removed by sedimentation and anaerobic decay at the pond bottom. The
colloidal/soluble BOD 5 remaining in solution is removed as a result of
metabolic activity by micro-organisms that are:
- suspended in the water column;
- attached to the sediments, and
- attached to the roots and stems of the aquatic plants.
The microbial activity at the roots and stems is the most significant for BOD 5
removal. Reduction of colloidal/soluble BOD 5 will be, at least in part,
depending on the design of the aquatic system. Direct uptake of BOD 5 by
aquatic plants is not significant. Factors affecting the BOD 5 removal rate and
efficiency of conventional trickling filters have similar effects on aquatic
systems.
The BOD 5 of effluents from aquatic systems will be primarily the result of:
- Extra-cellular organic compounds produced by plants during the
growing season, and - Organic compounds leached from decaying vegetation during
periods of vegetative die-off and dormancy.
These plant-related BOD 5 loads are part of the colloidal/soluble BOD load
and should be considered as such in aquatic system design. The release of BOD 5
by plants is species specific and affected by environmental factors. The BOD 5
leakage from plants is presently not well quantified in literature, although
releases upto 25% of the photosynthetically produced organic matter have been
reported (Stowell et al. 1980).
If aquatic systems are not overloaded, effluent BOD 5 concentrations are
primarily a function of the plant species grown, the growth phase of the plant,
and the wastewater temperature. In such systems, effluent BOD 5 concentrations
of 3-10 mg/L during the growing seasons and 5-20 mg/L during periods of
dormancy can be expected. An example of the seasonality of aquatic systems