Environmental Engineering Reference
In-Depth Information
The wastewater system of Mbabane City is centralized. There are two main trunk
sewers, located along the Mbabane and Pholinjane Rivers, which collect mainly domestic
and industrial wastewater from a gradually expanding sewer network. The wastewater is
discharged into the Gogholo treatment plant consisting of waste stabilization ponds.
Considerable part of the population relies on septic tanks and soakways, which, except
for Thembelihle district, is a fully acceptable and economical solution, given the
favourable soil characteristic and the large plot size (low density). In the low-income
areas and informal settlements, pit latrines and soak ways provide the services, the
facilities generally being in a poor condition.
Table 11.2 shows the past, present and projected population of the city to be connected
to the sewer system and represents the number of population, who have a connection to
the reticulation water supply system. Public tap users have not been included because of
the limited sanitary facilities that go with this level of service. It also shows the city's
population growth trend, which is high, as emphasized in previous chapters.
The existing wastewater treatment plant of Gobholo, situated near Mbabane River, has
been in operation since 1973. Some extensions and modifications were made in 1976
(Euro Consult 1994). The design capacity of the plant is approximately 3000 population
equivalent. Considering the population today and the connections to the ponds, the plant
is receiving over ten times the design load. The Gobholo Works comprises an inlet works
followed by seven waste stabilisation ponds in series. The inlet works comprises of
screens, grit channels and a flow measurement devise, which are not functional. All the
ponds are full of sludge. Actually, they are acting as settling basins and are providing
primary treatment only. The retention time is so short that there can be very little
reduction of organic pollution and pathogens through the works and these pollutants are
discharged into the stream, which discharges into Mbabane River as shown in Figure
11.7. Some 2 km down stream of the confluence of the stream into Mbabane River, the
water is darker in color and bears a noticeable smell of sewage. An estimate from SWSC
indicates that the discharge to the ponds is 1920 kg BOD/day, which corresponds to a
population equivalent of about 38,400.
3.5
Methodology
The study was performed during the period February to April 2001. Heavy rains
characterize this season of the year and the weather during this study was typical for the
season. The sample sites' location is shown in Figure 11.7. Site 1 was chosen as a control
point because it is located about 4 km upstream the Mbabane City. Along the upper
reaches of the river, no major anthropogenic activities are taking place. Site 2 was located
about 3 km downstream the industrial areas. At this location, the river and tributaries
have collected runoff from different types of urban land use patterns, including
residential areas, institutional/commercial areas, such as the city center, and several
industrial sites. Thus the water quality at this location represents the combined effect of
the urban runoff. Site 3 was located at a small stream, receiving the effluents from the
Gobholo treatment plant, which discharges into Mbabane River at a distance of about 1
km downstream the sampling point. The water quality at this point reflects the impact of
the treatment plant only. Site 4 was located at a distance of about 18 km downstream of
the city, thus reflecting the River water quality, which will be used by downstream users.
