Environmental Engineering Reference
In-Depth Information
The objectives of effluent and sludge disposal are an extension of wastewater
treatment, which are to protect public health and the environment. Sludge disposal
techniques should be considered as the final stage of the sewage treatment process. It is
widely accepted that the volume and characteristics of the effluent and sludge affect the
choice of disposal options. USEPA (1995) specifies the concentrations of trace metals,
the level of pathogens and the attractiveness of the sludge to disease vectors, as the three
main sludge characteristics considered in disposal options. Lotter & Pitman (1997)
recommended several factors to be considered with respect to an optimal and safe choice
of sludge disposal techniques in South Africa, and recommended several disposal
options, appropriate for the conditions in the country:
• Factors, to be considered for an optimal choice of disposal method:
Environmental acceptability;
Cost effectiveness;
Operational feasibility;
Political acceptability.
• Sludge disposal options recommended:
Co-disposal with solid waste in landfills;
Remote farm disposal;
Composting and beneficial use as fertilizer;
Incineration.
USEPA (1995) recommends a variety of factors to be considered for an optimal choice of
a sludge disposal technique, which could be grouped into three major categories:
• Regulatory considerations,
• Technical aspects and;
• Economic factors.
Thus sludge disposal becomes a task that requires a multi-disciplinary approach, which
necessitates the participation of different types of specialists, such as environmentalists,
industrialists, water engineers, agricultural specialists, and water resources managers, to
work in a team with common objectives, in order to obtain optimal and cost effective
results.
USEPA (1995) defines surface disposal as the placing of sewage sludge in a mono-fill,
in a surface impoundment, on a waste pile, on a dedicated disposal site or in the form of
dedicated beneficial use. A surface disposal site is defined as one where sludge remains
on the ground for more than two years. Types of surface disposal sites are mono-fills,
surface impoundments and lagoons.
Mono-fills are trenches where sewage sludge with at least 15% solids content is
disposed and covered periodically. The application of cover distinguishes mono-fills
from piles and dedicated disposal sites, and from surface impoundments, which receive
cover only at closure of site. Mono-fills can be categorized into trenches where sludge is
applied into excavated areas, and area fills where sludge is applied on the surface.
Surface impoundments are located on the ground surface and use dikes to contain the
sludge. In cases where such impoundments are located below ground surface they are
