Environmental Engineering Reference
In-Depth Information
Along with inhabiting organic mud, worms also inhabit biological slimes
and are derived from sewage and wet soil. Nematodes multiply in wastewa-
ter treatment plants; strict aerobes, they have been found in activated sludge
and particularly in trickling filters and therefore appear in large concentra-
tions in treated domestic liquid waste. Nematodes range in length from 0.5
to 3 mm and in diameter from 0.01 to 0.05 mm. Most species have a similar
appearance. They have a body that is covered by cuticle, is cylindrical and
nonsegmented, and tapers at both ends.
Nematodes ingest bacterial pathogens that protect them from water sup-
ply disinfectants and enhance their chances of reaching the consumer.
Active motile nematode larvae can penetrate sand filters and survive chlori-
nation, but they are not normally expected to cause parasitic nematode infec-
tions (NAS, 1977). Free-living nematodes have a life cycle consisting of egg,
four larval stages, and one adult stage. Eggs are easily recognizable in in-
ished water, but raw water must have excessive microfaunal forms to allow
identification. Environmental conditions have an impact on the growth of
nematodes; for example, in anoxic conditions, their swimming and growth
are impaired. Temperature fluctuations directly affect their growth and sur-
vival; nematode populations decrease when temperatures increase.
Aquatic flatworms (improperly named because they are not all flat) feed
primarily on algae. Because of their aversion to light, they are found in the
lower depths of pools. Flatworms are very hardy and can survive in wide
variations in humidity and temperature. Surface waters that are grossly pol-
luted with organic matter (especially domestic sewage) have a fauna capable
of thriving in very low concentrations of oxygen. A few species of tubificid
worms dominate this environment. Pennak (1989) reported that the bottoms
of severely polluted streams can be literally covered with a “writhing mass”
of these tubificids.
For the drinking water practitioner interested in learning more about
aquatic worms, the current
Standard Methods and Procedures
has a section
covering nematological examinations that details sample collection and pro-
vides an illustrated key to freshwater nematodes.
Summary
Pathogenic parasites are not easily removed or eliminated completely by
conventional treatment and disinfection unit processes (De Zuane, 1997).
This is particularly true for
Giardia lamblia
,
Cryptosporidium
, and
Cyclospora
.
Filtration facilities can be adjusted in depth, prechlorination, filtration rate,
and backwashing to become more effective in the removal of cysts. The pre-
treatment of protected watershed raw water is a major factor in the elimina-
tion of pathogenic protozoa (see
Chapter 11
).
