Sanitation and Vector Control - Biological and Environmental Control of Disease Vectors - page 156

Table 8.1. Typical concentrations of key parameters found in urban
sewage.

Strength of wastewater (mg l −1 )

Parameter Weak Medium Strong

Biochemical oxygen demand <200 300 >500

Chloride 30 50 100

Dissolved solids 250 500 850

Total solids 350 700 1200

Suspended solids 100 200 350

Nitrogen 20 40 85

Phosphorus 6 10 20

Grease 50 100 150

same system. The strength of wastewater is
normally expressed in biochemical oxygen
demand (BOD), or the amount of oxygen needed
for bacteria to oxidize the organic matter in the
water. Table 8.1 presents typical concentrations
of key water quality parameters for urban
domestic sewage.
The quality of wastewater is key to larval
survival. Typically, Anopheles species are not able
to breed in water with low dissolved oxygen (DO)
concentrations or high salinity levels. However,
some studies have shown that An . gambiae s.s.
and An . stephensi can adapt and will breed in
water with DO concentrations below 3.0 mg l −1
and salinity levels over 3.5 dS m −1 (Mukhtar et
al ., 2006; Awolola et al ., 2007). Culicine species,
cockroaches and fl ies are usually less af ected by
the quality of wastewater and can breed in a
wider range of pollution levels.
Stagnating drains and manholes, which
are entry points into the sewerage system, often
have stagnant water and provide prolifi c
breeding sites for a variety of mosquito species
through shade, nutrient-rich water and stable
temperatures. In the southern USA, concerns
over West Nile virus transmission by Cx .
quinquefasciatus have led to extensive vector
monitoring programmes, which have identifi ed
manholes as among the main breeding sites for
Cx . quinquefasciatus (Strickman and Lang, 1986;
Su et al ., 2003). The removal of stagnant water
from manholes is nearly impossible given their
sheer numbers within a single sewerage system,
though this could potentially be achieved by
thoughtful design and careful construction. The
elimination of entry points through the insertion
of plastic dishes in the pick-holes of manhole

covers proved to be 100% ef ective in preventing
mosquito entry (Harbison et al ., 2009). The
impact of water-borne sewerage systems or
drains on vector-borne disease is not always
negative: the elimination of fi lariasis in
Charleston, South Carolina, USA in the 1930s
was largely attributable to the sewerage system
constructed in the city from 1890 that
dramatically reduced the foul water breeding
sites available to Cx . quinquefasciatus within the
city (Chernin, 1987). Manholes have also been
found to be home to cockroaches; a survey in
Texas, USA, investigated 670 sewer manholes
and found 40% to be positive for cockroaches:
17% of all manholes were found to contain
between 1 and 25 cockroaches, 9.6% between
25 and 100, and 13% more than 100
cockroaches (Eads et al ., 1954). The study found
no association between the socio-economic
status of the neighbourhood and the number of
cockroaches found per manhole.

Wastewater treatment plants
The centralized collection of urban sewage will
generate large amounts of wastewater, and with
it, the need for treatment to guarantee safe
disposal into the environment or use in
agriculture. Conventional wastewater treatment
technology in industrialized countries is
implemented to protect surface water ecology
and is designed to prevent eutrophication by
removing BOD, but it tends to be relatively poor
at removing pathogens. A wide variety of
wastewater treatment technologies exist that
could potentially remove all pathogens, heavy
metals and nutrients normally found in