Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 154

wastewater is (partially) treated. Septic tanks
should normally consist of two tanks, separated
by bal ed walls. Settlement and digestion of
waste usually occurs in the fi rst tank, although
some suspended material might be carried
forward to the second tank. Treatment results
partly through retention when materials settle
down; normally a 24-72 h retention period is
considered sui cient. Organic matter in the
sludge is broken down by anaerobic bacteria
with organic matter being converted into water,
CO 2 and methane. Sludge at the bottom of the
tank is consolidated as a result of the weight of
the liquids and solids above. Following treatment
within the septic tank, liquids are drained into a
soakage pit or drain.
The required volume of a septic tank can be
calculated by multiplying the number of
household members with their per capita water
consumption, which is then multiplied by three,
for the retention time. The construction of septic
tanks requires a skilled mason, and septic tank
construction costs are often higher than those
for a pit latrine or pour fl ush latrine.
Septic tanks all over the world have been
associated with mosquito breeding, especially
of Cx . quinquefasciatus (Charlwood, 1994;
Chavasse et al ., 1995b; Burke et al ., 2010), but
also Stegomyia (formerly Aedes ) aegypti (Burke et
al ., 2010). In Puerto Rico, it was calculated that
a single septic tank could produce as many as
170,000 Cx . quinquefasciatus and more than
18,000 St . aegypti per day (Barrera et al ., 2008),
but such high densities could be reduced by
installing a mosquito mesh on ventilation holes
and through proper construction, eliminating
mosquito entrance. During de-sludging cover
slabs often crack. In these instances, the best
way to prevent mosquito breeding is to cover the
slab with an additional layer of soil.

Anopheles species, though An . stephensi , a
secondary vector of malaria in Pakistan, has
been found to breed in cesspits and septic tanks,
probably when there is an absence of more
suitable breeding sites (Herrel et al ., 2001). The
vectors most often associated with on-site
sanitation are Cx . quinquefasciatus , the vector of
Wuchereria bancrofti and a variety of arboviruses,
and St . aegypti (commonly found breeding in
water storage tanks), the vector of yellow fever
and dengue. The association between the
common housefl y, diarrhoeal disease and on-site
sanitation is strong, as was shown by a study in
southern Tanzania where a single traditional pit
latrine generated almost 1000 fl ies per day
(Knudson, 2011). Fly control studies in Pakistan
and The Gambia showed a 25% reduction in
diarrhoeal disease (Chavasse et al ., 1999;
Emerson et al ., 2004). However, few studies have
investigated the impact of control methods on
on-site sanitation and fl y breeding.
Chemical larvicides have been used in septic
tanks, often with great results, showing re-
ductions of 90-100% in larvae numbers over
14-21 day periods (Cetin et al ., 2006; Shin et al .,
2011). Concerns over resistance, together with
the need for frequent re-applications, led to the
use of polystyrene beads in the late 1970s
(Reiter, 1978). The application of the 2 mm
diameter beads in a 1 cm thick layer in pits and
septic tanks follows any rise and fall of the water
level within the pit, thereby maintaining an
ef ective sealing layer. The beads are non-
biodegradable when the pit falls dry, but will re-
submerge when the pit fi lls up. Curtis and Minjas
(1985) reported that even 2 years after
application, the beads were still found in pit
latrines in Dar es Salaam. A large trial in Dar es
Salaam showed 77% and 46% reductions in
adult Cx . quinquefasciatus trapped in two separate
parts of town where all wet-pit latrines, cesspits
and septic tanks were treated with polystyrene
beads (Chavasse et al ., 1995a). On the island of
Zanzibar, the impact of polystyrene beads in pit
latrines on the prevalence of Bancroftian
fi lariasis was assessed. As part of the study, 3844
cesspits and wet-pit latrines were treated with
the beads. The study found a 65% reduction in
the adult mosquito population in houses
(Maxwell et al ., 1999). The treatment of cess-
pits and wet-pit latrines was accompanied by
mass treatment for Bancroftian fi lariasis with

Vector and disease control in on-site
sanitation
Vector breeding in on-site sanitation systems is
often directly linked to the amount of water that
is available. Dry pits will not generate
mosquitoes, but they will still promote fl y
breeding, while wet pits, as was shown for septic
tanks, have the potential to produce huge
numbers of Culex mosquitoes. The water in on-
site sanitation systems is often too dirty for