Environmental Engineering Reference
In-Depth Information
CHAPTER 9
Sewage sludge disposal on land - impacts on
soils and groundwater quality
R. Hranova, D. Love & A. Amos
ABSTRACT: Pollutant transport mechanisms in soils and ground water,
together with regulatory aspects and design criteria for a safe application
of sewage sludge on land have been presented. The impacts of long-term
sludge land disposal on soils and groundwater quality have been assessed
by means of a specific case study of digested sludge and effluent mixture,
disposed beneficially for pasture irrigation on Crowborough farm in
Harare, performed during the period 2002-2003. Results show that soils
were acidified at selected portions of the pasture, subject to excess
irrigation loads, but in general, the soil's organic carbon and nutrients
content has been improved. The metals content of soils were far below
recommended maximum values. Adverse impacts on ground water were
found with respect to metals - Cr, Cd, Pb and Ni, which present a high
environmental risk, and restrict the use of this aquifer for any future
beneficial purpose. Recommendations to improve the present practice
have been made.
1 INTRODUCTION
The beneficial reuse of treated wastewater and sludge for different types of agricultural
activities is one of the most widely applied options in the world. Sewage has been applied
to fields since ancient times. During the ninetieth century and the beginning of the
twentieth century, cities throughout Europe and North America installed water-borne
sewage systems and many of them established sewage farms, adopt-ing crop irrigation as
their preferred means of sewage disposal. In a survey carried in 1987 in the United
Kingdom, it was found that agricultural use was the most economic option accounting for
40% of the 1,210 million tones dry sludge disposed annually on 1-2% of farmland in
England and Wales. In 1987, 30% of the 6 million tones dry sludge in the European
Community was used in agriculture, whereas in Hong Kong sewage sludge was utilized
for landscaping and topsoil regeneration (Lotter & Pitman 1997).
The agricultural use of sewage is an attractive method of diverting nutrients from
rivers (where they result in eutrophication) to agriculture where they are used as nutrients
by plants for biomass production. This option can be cost-effective if land is readily
available, near to the treatment plant, and also, if it is properly applied and managed.
Apart from the nutrients, the moisture content of treated wastewater and sludge is used to
replace the use of fresh natural water for the same purpose. It is well known that
