Environmental Engineering Reference
In-Depth Information
CHAPTER 8
Sewage sludge disposal on land - impacts on
surface water quality
R. Hranova & M. Manjonjo
ABSTRACT: Basic principles of sewage treatment and sludge disposal
options, common for the region have been presented, together with
regulatory aspects for a safe practical application. A case study of digested
sludge and effluent mixture, disposed beneficially for pasture irrigation on
Crowborough farm in Harare, has been investigated during 1998-1999, in
terms of loading rates and the impacts of such type of disposal method on
surface water after 30 years of operation. Results show considerable
adverse impacts on natural streams, draining the pasture, with respect to
TP, TKN, nitrate and Ni, with pronounced seasonal variation. TKN and
TP impacts were pronounced during the dry season, while nitrates impacts
were pronounced throughout the year with higher impacts during the wet
season. Recommendations with respect to the future operation of the
pasture have been made.
1 INTRODUCTION
1.1
Reusing wastewater and sludge for irrigation
Urban population centers are a major source of wastewater generated by domestic,
commercial, institutional and industrial activities. In the vast majority of the cases
throughout the world, wastewater is collected transported and treated in centralized
wastewater systems, which discharge the effluents to natural water bodies and are
considered as point sources of pollution. In countries with limited water resources, these
effluents could be reused beneficially for different purposes. One of the most common
types of beneficial reuse is the application of wastewater in agriculture for irrigation
purposes. It could be applied after different levels of treatment of the raw wastewater.
During the treatment process, the main flow of wastewater is clarified and pollutant
constituents reduced at acceptable levels. As a result of this process, a considerable
amount of sewage sludge is generated, which contains in concentrated form the pollutant
constituents removed from the wastewater during the treatment process. Despite the fact
that the sludge could contain harmful contaminants, such as pathogens or toxic elements,
it could be considered as valuable product to be reused in agriculture due to the high level
of organic constituents and nutrients. If treated and applied properly, the sludge could be
reused as a fertilizer in different types of agricultural activities.
