Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 157

wastewater. However, most of these wastewater
treatment technologies are expensive or require
reliable electricity supply. As a result, the
recommended form of wastewater treatment
technology for a developing country setting is a
system of wastewater treatment ponds (WSPs),
which do not need electricity and are easy and
cheap to construct and maintain. In low-income
countries, wastewater might be used in
agriculture or disposed of in surface water
sources that are used downstream for drinking
water purposes. Therefore, added benefi ts of
WSPs are that they provide relatively stable and
high pathogen removal efficiency. WSPs con-
sist of shallow ponds through which wastewater
fl ows, and wastewater is treated through
sedimentation and biological processes. A well
designed system of WSPs consists of a series of
anaerobic, facultative and maturation ponds in
which wastewater gets treated for a period
between 12 and 16 days.
The anaerobic ponds are relatively deep
(3-4 m) and receive the untreated sewage. The
task of the anaerobic ponds is to reduce organic
loading (BOD concentrations), which happens
through anaerobic digestion and settling. The
normal retention time in an anaerobic pond
might vary from 6 h to 2 days, and a well-
performing anaerobic pond will be able to reduce
BOD concentrations by 60% (Mara, 1997).
Following the anaerobic pond, wastewater will
fl ow to the facultative pond where it undergoes
treatment through both aerobic and anaerobic
processes. The facultative pond has a much
larger surface area and is normally 1.5 m deep;
its function is also BOD removal, which occurs
through anaerobic processes at the bottom of
the pond and through symbiosis of algae and
bacteria (aerobic processes) in the top layer of
the pond. A well-performing facultative pond
should be bright green, because of large amounts
of algae present in the ponds. The fi nal, or
maturation, pond has a large surface area and is
at maximum 1 m deep. Its primary purpose is to
promote pathogen die-of . The shallowness of
the pond will allow maximum exposure to UV
radiation, which causes rapid photosynthesis,
thus increasing DO concentrations, temperature
and pH, which are all bactericidal. A well-
performing system of waste stabilization ponds
can achieve an 80-90% reduction in BOD and
nutrient concentrations (Mara, 1997).

From a vector control point of view, the
disadvantage of WSPs is that they create large
bodies of open water close to urban centres,
thereby creating possible breeding sites for
disease-transmitting mosquitoes. Limited
attention has been paid to the potential role of
WSPs in the generation of vector mosquitoes. In
Nigeria, living within a 300 m radius of a WSP
was found to be a signifi cant risk factor for
malaria (Agunwamba, 2001). A study in
Faisalabad, Pakistan, found very high numbers
of Cx . tritaeniorhynchus and Cx . quinquefasciatus
in the anaerobic ponds of the WSPs. The
facultative ponds contained much lower
numbers of both culicine and anopheline
species, while in the maturation ponds only low
numbers of malaria vectors An . subpictus , An .
stephensi and An . culicifacies were recorded
(Mukhtar et al ., 2006). Vector breeding within
the WSPs was strongly associated with emergent
vegetation and with fl oating garbage at the
edges of the ponds (Mukhtar et al ., 2006). An
intervention study, which removed vegetation
from the ponds, repaired cracks in the pond
embankments and reinstalled garbage grids,
resulted in a 100% reduction in vector breeding
in all ponds (Ensink et al ., 2007). The
intervention came at a one-of cost of US$50,
but was highly successful.
In order to improve city living through the
provision of green spaces, an increasing
emphasis is placed on turning large open urban
drainage canals into look-alike natural water-
ways. This has the added advantage that it could
help to strip nutrients and organic material from
wastewater before it reaches the wastewater
treatment facilities. However, this trend could
also create productive and diverse mosquito
breeding habitats right through cities.

8.4 Sanitation and Vector Control
Promotion

Vector control is often considered an expensive
alternative to regular drug administration.
However, vector control programmes are usually
only evaluated for the reduction of particular
diseases. In the case of sanitation, nuisance
mosquitoes, fl ies and other arthropod pests in or
around sanitation facilities can be an important
deterrent for the use and maintenance of them,