Aquatic weeds and their utilization 347Removal of bacteria and viruses
In aquatic systems concentrations of pathogenic organisms are reduced by
prolonged exposure (days) to physical, chemical, and biological factors hostile
to these organisms. Physical factors include sedimentation and exposure to ultra
violet (UV) radiation. Chemical factors include oxidation, reduction, and
exposure to toxic chemicals, some of which may be excreted by plants (Seidal
1976). Biological factors include attack by other organisms (particularly
bacteria) and natural die-off as a result of being away from a suitable host
organism for a long period of time.
However, the extent and reliability of reductions in pathogen concentrations
in aquatic systems is unknown. In most wastewater treatment situations some
form of disinfection e.g. chlorination, is necessary to satisfy public health and
water quality requirements.
Summary
The principal wastewater contaminant removal mechanisms operative in aquatic
treatment systems are summarized in Table 7.7. The effect of each mechanism
depends on the design and management of the aquatic system, the quality of the
influent wastewater, and climatic and environmental factors.
7.7.2 Aquatic system design concepts
Design concepts for aquatic treatment systems differ somewhat from those for
conventional systems because aquatic systems have larger surface areas, longer
hydraulic detention times, and aquatic plants. The aquatic environment or series
of environments necessary to achieve the desired level of wastewater treatment
must be envisioned. The plants, and equipment, to maintain this environment
must be selected. The effects of climatic and environmental factors on the
aquatic treatment processes must be anticipated and mitigating measures taken
as necessary. These concepts and others are part of the rational design of an
aquatic treatment system.
Aquatic processing unit (APU)
An APU is an assemblage of plants (and possibly animals) grouped together to
achieve specific wastewater treatment objectives. One or more APU's constitute
an aquatic system. In this regard APU's are similar to conventional processes
making up a conventional system but direct comparisons between APU's and
specific types of conventional processes cannot be made in general. However,
this does not preclude the use of APU's in conjunction with conventional