Environmental Engineering Reference
In-Depth Information
type were introduced during the 1950's and one typical treatment scheme is shown in
Figure 8.1. The influent (raw sewage) is treated in preliminary treatment facilities,
followed by primary sedimentation and biological treatment in trickling filters. After the
secondary sedimentation (humus tanks), the effluent is discharged to natural water
bodies. The secondary sludge is returned to the inlet of the primary sedimentation tank
and separated from the flow together with the primary sludge, which is stabilized in
anaerobic digesters, and after that is dried in sludge drying beds. The supernatant released
during the sludge stabilization process and from the drying beds is returned to the
primary sedimentation tank inlet and treated together with the sewage.
Conventional treatment systems are designed for wastewater of a specific quality and
hydraulic regime. Large variations in the hydraulic loading and the quality of the
wastewater disrupt the efficiency of conventional systems. When hydraulically
overloaded, conventional treatment systems fail to absorb the additional load. This results
in poor effluent quality. A balance between the biomass concentration and organic
loading is required for optimum treatment. High pollutant loads could disrupt the
functioning of the system and may lead to a breakdown of the biological process.
Conventional treatment systems cannot handle concentrated industrial wastewater
from specific industries, because such type of wastewater often contains toxic elements
and other constituents, which could have a negative impact on the biological process or
cause a total loss of the biological activity of the microorganisms. In addition,
conventional treatment systems require sludge treatment and handling facilities. Since the
treatment processes tend to concentrate the pollutants in the sludge, sludge treatment and
disposal is an important aspect of the plant design and operation.
In general, the main constituents removed by conventional treatment processes are
suspended solids and BOD 5 . Pathogen removal has rarely been considered as a special
objective in conventional wastewater treatment processes. In contrast, waste stabilization
pond (WSP) systems are efficient in pathogen removal, making the effluent suitable for
irrigation. A more detailed description of this alternative of the conventional sewage
treatment processes is given in Chapter 10.
2.1.2 Biological nutrient removal methods
Conventional sewage treatment systems do not remove nutrients at a high level and
consequently effluents from these plants, discharged into natural water bodies, could be
considered as a source of pollution with respect to nutrients, leading to enhanced
eutrophication. This is a serious problem for the countries in the region, given the high
average annual temperatures, the abundance of sunlight and the relatively limited
volumes of surface water resources. For this reason, during the 1970s, effluent discharge
regulations were introduced, which required a high standard of treatment of sewage with
respect to nutrients, before their discharge to natural water bodies. Alternatively, the
effluents from conventional treatment plants and WSP systems could be disposed on land
or reused for irrigation. As a result of these regulations, the new treatment plants were
designed to incorporate nutrients removal techniques and many conventional treatment
plants were upgraded to include such treatment methods.
Nutrients could be removed by chemical or biological treatment methods. In the
Southern African region, the biological nutrient removal (BNR) treatment was adopted. It
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