16.4 Urban Wastewater
16.4.1 Introduction
Water can be used very often because it gathers an outstanding collection of physical and chemical properties. Out of
any domestic activity, it can become a solvent, a thermal
fluid, or simply an easy- and handle-liquid. Sewage discharge
has considerably changed in quantity and quality together
with urbanization and industrialization, as well as with the
evolution of consumption patterns. Environmental protection and particularly water supply have become a vital and
strategic issue, which has passed from a quantitative to a
qualitative approach. Requirements for implementation
have been added to reach this challenge. Urban wastewater
(UWW) was discharged in fresh and oceanic waters without
prior treatment. The self-cleaning power of the natural
environment – particularly the activity of microorganisms –
was in most cases enough to eliminate the pollutants that they
contained. The significant increase in population involved
the discharge of ever larger important quantities of pollutants
in the receiving environments. The self-cleaning power has
rapidly proved inefficient, and the collection and treatment of
wastewater have become necessary to cope with the increasing discharges. Simple domestic discharge includes more
complex substances (pesticides pharmaceuticals, PCB, etc.)
and sewerage collected industrial, commercial, and craft
discharges with very different characteristics. As the need
for water increases, it is important to consider both the
quantity available and also its quality, which is often ignored.
It is therefore vital to treat wastewater. Several treatment
processes already exist, but the activated sludge* biological
treatment is the most widely used; lagoon treatment is
another process to treat wastewater (Box 16.1). Before
specifying the treatment to implement, it is first of all necessary to know the quantity and the quality of pollution in the
water as well as the characteristics of the site where the
wastewater treatment plant will operate. For example, lagoon
treatment requires very large areas.
16.4.2 Origin and Content of Wastewater
Usually there are two main categories of wastewater (also
called effluents):
(i) Urban wastewater (or domestic) resulting from household or commercial activities together with rainwater
(stormwater). It contains grease, soap, and detergents;
suspended and organic or mineral dissolved solids; and
a very large diversity of microorganisms. Household
wastewater consists of grey waters* and waters from
toilets, baths, and sinks.
Box 16.1: Extensive Water Treatments in Lagoons and
Planted Beds
Pierre Caumette
Wastewater treatment has to be objective to reach
water quality criteria in terms of suspended solids,
organic matter, and nutrients before its release into
the environment. Many systems use biological
activities, which, in wastewater treatment plants, are
optimized to achieve maximum efficiency. These
biological activities may also be used in extensive
treatment processes such as lagoon treatment plants
or macrophytic treatment plants. These treatments are
less efficient in terms of performance (degradation
rate, residence time) but not in the quality of
the treated water. In both cases, the role of
microorganisms is essential (Box Figs. 16.1 and 16.2).
The lagoon treatment process is a natural alternative to other treatment systems for the purification of
urban sewage and industrial effluents (e.g., food,
paper). The establishment of efficient lagoon treatment systems dates from the 1970s, and among the
most successful, those from the Ecosite of Meze (Box
Fig. 16.1) and the station of Rochefort (both in France)
are noteworthy. In lagoon treatment systems, raw sewage will stay in several basins (usually three), shallow
(maximum 1.5 m) and of great area. The biological
activities of aerobic and anaerobic degradation, as well
as primary production and grazing by zooplankton,
allow purification in the successive lagoons. The residence time in this case is very long (50–80 days), which
makes these systems having large buffer capacities with
respect to changes in load and effluent flow.
As in the large majority of treatment systems,
the first steps include a screening for the removal of
large size solids, a grit chamber to eliminate sand and
other solid deposits, and oil/water separation that
removes fat that may disrupt the treatment system.
The pre-treated effluent is then discharged into the
first and largest basin, which provides aerobic (in the
water) and anaerobic (sludge and sediment) bacterial
degradation of most of the organic load. The main
anaerobic metabolisms involved are fermentations,
nitrates respiration (including denitrification), and
P. Caumette
Institut des Sciences Analytiques et de Physico-chimie pour
l’Environnement et les Mate ´riaux (IPREM), UMR CNRS 5254,
Universite ´ de Pau et des Pays de l’Adour, B.P. 1155, 64013 Pau
Cedex, France
(continued)
664
J.-C. Bertrand et al.
16.4.1 Introduction
Water can be used very often because it gathers an outstanding collection of physical and chemical properties. Out of
any domestic activity, it can become a solvent, a thermal
fluid, or simply an easy- and handle-liquid. Sewage discharge
has considerably changed in quantity and quality together
with urbanization and industrialization, as well as with the
evolution of consumption patterns. Environmental protection and particularly water supply have become a vital and
strategic issue, which has passed from a quantitative to a
qualitative approach. Requirements for implementation
have been added to reach this challenge. Urban wastewater
(UWW) was discharged in fresh and oceanic waters without
prior treatment. The self-cleaning power of the natural
environment – particularly the activity of microorganisms –
was in most cases enough to eliminate the pollutants that they
contained. The significant increase in population involved
the discharge of ever larger important quantities of pollutants
in the receiving environments. The self-cleaning power has
rapidly proved inefficient, and the collection and treatment of
wastewater have become necessary to cope with the increasing discharges. Simple domestic discharge includes more
complex substances (pesticides pharmaceuticals, PCB, etc.)
and sewerage collected industrial, commercial, and craft
discharges with very different characteristics. As the need
for water increases, it is important to consider both the
quantity available and also its quality, which is often ignored.
It is therefore vital to treat wastewater. Several treatment
processes already exist, but the activated sludge* biological
treatment is the most widely used; lagoon treatment is
another process to treat wastewater (Box 16.1). Before
specifying the treatment to implement, it is first of all necessary to know the quantity and the quality of pollution in the
water as well as the characteristics of the site where the
wastewater treatment plant will operate. For example, lagoon
treatment requires very large areas.
16.4.2 Origin and Content of Wastewater
Usually there are two main categories of wastewater (also
called effluents):
(i) Urban wastewater (or domestic) resulting from household or commercial activities together with rainwater
(stormwater). It contains grease, soap, and detergents;
suspended and organic or mineral dissolved solids; and
a very large diversity of microorganisms. Household
wastewater consists of grey waters* and waters from
toilets, baths, and sinks.
Box 16.1: Extensive Water Treatments in Lagoons and
Planted Beds
Pierre Caumette
Wastewater treatment has to be objective to reach
water quality criteria in terms of suspended solids,
organic matter, and nutrients before its release into
the environment. Many systems use biological
activities, which, in wastewater treatment plants, are
optimized to achieve maximum efficiency. These
biological activities may also be used in extensive
treatment processes such as lagoon treatment plants
or macrophytic treatment plants. These treatments are
less efficient in terms of performance (degradation
rate, residence time) but not in the quality of
the treated water. In both cases, the role of
microorganisms is essential (Box Figs. 16.1 and 16.2).
The lagoon treatment process is a natural alternative to other treatment systems for the purification of
urban sewage and industrial effluents (e.g., food,
paper). The establishment of efficient lagoon treatment systems dates from the 1970s, and among the
most successful, those from the Ecosite of Meze (Box
Fig. 16.1) and the station of Rochefort (both in France)
are noteworthy. In lagoon treatment systems, raw sewage will stay in several basins (usually three), shallow
(maximum 1.5 m) and of great area. The biological
activities of aerobic and anaerobic degradation, as well
as primary production and grazing by zooplankton,
allow purification in the successive lagoons. The residence time in this case is very long (50–80 days), which
makes these systems having large buffer capacities with
respect to changes in load and effluent flow.
As in the large majority of treatment systems,
the first steps include a screening for the removal of
large size solids, a grit chamber to eliminate sand and
other solid deposits, and oil/water separation that
removes fat that may disrupt the treatment system.
The pre-treated effluent is then discharged into the
first and largest basin, which provides aerobic (in the
water) and anaerobic (sludge and sediment) bacterial
degradation of most of the organic load. The main
anaerobic metabolisms involved are fermentations,
nitrates respiration (including denitrification), and
P. Caumette
Institut des Sciences Analytiques et de Physico-chimie pour
l’Environnement et les Mate ´riaux (IPREM), UMR CNRS 5254,
Universite ´ de Pau et des Pays de l’Adour, B.P. 1155, 64013 Pau
Cedex, France
(continued)
664
J.-C. Bertrand et al.