Environmental Engineering Reference
In-Depth Information
4 SLUDGE DISPOSAL ON PASTURES - IMPACTS ON SURFACE
WATER QUALITY
4.1
The study area
Crowborough Wastewater Treatment Works (CWTW) is the second largest sewage
treatment works of Harare, serving more than 400,000 people. The land use pattern of its
catchment area includes medium and high-density residential areas, commercial and
industrial enterprises. The location of CWTW and the dedicated farmland for sludge and
effluent disposal are shown in Fig.8.3. The Crowborough farm receives part of the pond's
effluent and the whole amount of sludge mixture (primary digested sludge and waste
activated sludge) daily throughout the year. The remaining part of the pond effluent is
pumped to another dedicated area - Ingwe Farm.
The Crowborough farm has a total area of 594 ha, but the active farmland comprises
of 17 paddocks with a total area of 427 ha covered mainly by star grass (
Cynodon
plectostachus
) and Kikuyu grass (
Pennisetum clandestinum)
. The farm is an example of
the beneficial reuse of sludge and effluent, where this mixture is applied to land, which is
used as a grazing pasture. It has been operated since the early 1970's. The terrain has a
gentle slope of about 0.008 from north to south. The topography of the farmland defines a
saddle like shape with a distinct watershed along the access road to CWTW. About 40%
of the farm area drains into the Little Marimba River and the rest into Marimba River.
The soils are variable in derivation, texture and depth, with clays in the north, sandy
clay and sandy clay loams in the center, sands and sandy loams in the south. Due to long
term irrigation with sludge and wastewater, a thick humus blanket covers the surface of
the farm. The lower southern part of the farm area and the areas along the two rivers are
wetlands with a high groundwater table, which do not dry out during the dry season and
some parts are inaccessible during the wet season. The farm is provided with night
storage ponds and an under-drain system for collection, transportation and discharge of
the seepage,
