Environmental Engineering Reference
In-Depth Information
nearly completely removed within the aerobic, unsaturated zone of medium-to-fine
textured soil, and within a depth of 0.6 to 1.5 m, and when a neutral pH of the soil is
maintained during the land treatment system operation (USEPA 1992). The limitations of
such practice are connected to:
• Potential groundwater pollution and transmission of pathogens;
• Dependence on soil conditions and climate;
• Large land area requirements.
In cases of inadequate design procedure or operational practice, the method of
wastewater/sludge land application might become a source of diffuse pollution to ground
and surface water bodies, and a potential public health hazard. This chapter and the
following two chapters present specific cases of sludge and wastewater land disposal
methods and their impacts on soils, surface and ground water.
2 SEWAGE TREATMENT AND SLUDGE DSPOSAL
2.1
Sewage treatment
The efficiency of any land application system is dependant on a reliable wastewater
treatment technology and the understanding of the different processes involved. This
section presents briefly some wastewater treatment technologies commonly applied in the
countries of the region.
2.1.1 Conventional treatment methods
They usually consist of two separate and consecutive treatment stages - primary and
secondary treatment. Primary treatment consists of preliminary treatment, which has the
purpose to remove course materials, sand and grit by screening and grit sedimentation.
The materials separated during this stage are disposed as solid waste and do not form part
of the sludge generated during the treatment process. After being preliminary treated, the
wastewater is directed to primary sedimentation tanks, which have the purpose of
separating suspended particles from the main flow by means of gravity sedimentation.
The separated material is retained at the bottom of the tank and constitutes the primary
sludge. It is taken out of the tank at prescribed time intervals. Primary sedimentation
reduces the suspended solids' content of the wastewater to about 50%-60% and the
organic matter to about 30-40%. Pathogen removal is limited to the extent of these
microorganisms, which are attached to suspended particles. About 90% of the heavier
pathogenic microorganisms, such as helminth eggs and protozoa cysts, are retained in the
primary sludge. Helminth eggs have a long survival period of several months and could
be a potential public health hazard, when the sludge is disposed on land.
The secondary treatment stage consists in biological treatment of the wastewater,
where engineered biological treatment processes are applied to reduce the organic
material remaining after the primary stage. Biological treatment processes applied during
the secondary stage are aerobic, which means that the complex culture of different
microorganisms used to consume the organic matter present in the wastewater, require
