Characteristics of organic wastes 57bacterial cells serve as food for protozoa and some small fishes, which are further
consumed by big fishes and finally by man.
The discharge of untreated wastes into a receiving water body will create a
biologically unbalanced condition. At high organic loading, the bacteria will
require more oxygen for their oxidation and synthesis (Equations 2.1 and 2.2),
resulting in the depletion of oxygen in the water body which is detrimental to all
aquatic life (Figure 1.3). Although algal photosynthesis produces some oxygen
(Equation 2.6), during night time when there is no sunlight algae respire or use
oxygen, causing a further depletion of oxygen in the water. Eventually, the water
will become anaerobic (without oxygen), dark in color, and only anaerobic
microorganisms and certain types of worms can live. Besides this organic
pollution, the water body would become very unaesthetic to the nearby inhabitants
and would lower the environmental quality of its surrounding areas.
However, knowledge of the pollution effects as cited above has, in turn, been
used for the design of technologies and facilities for waste treatment and recycling.
For example, the reactions of Equations 2.1 and 2.2 are basically used in the design
and operation of aerobic waste treatment facilities such as activated sludge,
trickling filtration, and composting. The algal-bacterial symbiosis is the basis for
the design of facultative waste stabilization ponds and high-rate algal ponds (for the
production of algal protein biomass). Equation 2.3 is the reaction where biogas
(CH 4 and CO 2 ) is produced; application of this basic information to waste recycling
is given in Chapters 3 to 9.
2.5 Diseases associated with human and animal wastes
There are various kinds of enteric microorganisms present in human excreta and
animal manure, some of which are pathogenic and some are not. They can be
classified into such major groups as bacteria, viruses, protozoa and helminths.
Some of the important enteric pathogens commonly found in human excreta and
wastewater, the diseases they cause, modes of transmission and geographical
distribution are listed in Table 2.22 Feachem et al. (1983) estimated the possible
outputs of some pathogens in the feces and wastewater of a tropical community as
shown in Table 2.23.