362 Organic waste recycling: technology and management
The nature, magnitude, and duration of treatment process disruption will
depend, primarily, on the severity of the event causing the upset, which APU's
are affected, and the extent of damage/alteration to the aquatic environment.
Essentially nothing is known about the recovery times of aquatic processes
from various types and magnitudes of upsets. Recovery times are expected to be
longer in aquatic processes than conventional processes because environmental
conditions in the former are less conducive to rapid bacterial growth rates. If a
necessary function of the APU plants is damaged, the recovery time could be
several weeks or more if a new group of plants must be grown.
7.7.5 Review of existing aquatic treatment systems
There is a vast amount of literature available on aquatic treatment systems.
O'Brien (1981) summarized the design and performance characteristics of such
systems in the U.S., as shown in Tables 7.9 and 7.10. Most of these systems are
in the hotter parts of the U.S. and the climate there closely resembles the tropical
climates of developing countries. The wide differences in design parameters
shown in Table 7.9 are typical for an emerging technology. But sufficient
information is now available to suggest the criteria shown in Table 7.9 for the
design of water hyacinth systems intended to produce secondary or advanced
secondary effluents. Criteria similar to those given in Table 7.9 were also
recommended by Dinges (1979), who focused on the development of low
technology systems suitable for upgrading lagoon performance in small towns.
Suggestions by Dinges (1979) to supplement the design criteria in Table 7.9
include the use of:
- Perforated pipe to achieve uniform influent flow distribution;
- Filters around the effluent structure to prevent the escape of plants,
and - Circular galvanized wire-mesh enclosures in ponds to improve
production of the fish used for controlling mosquitoes.
The data in Table 7.10 suggests that secondary waste treatment standards can
be met if intensive management techniques are practiced. Ultimate disposal of
the harvested plants as surface mulch, compost, animal feed, or for generation of
biogas (see sections 7.5 and 7.6) ensures that resource recovery is also practiced.
But, in most wastewater treatment installations, these processes will not be
sufficiently profitable to offset the cost of solids disposal (because performances
of natural treatment systems are site specific and dependent on climatic and
related factors, wherever possible, pilot-scale experiments should be done to
confirm the selected design criteria).