and groundwater contamination. The first stage of the treatment consists in physical
separation of the pollutants of large size using mechanical tools to retain and harvest
solid and bulky waste such as floating grease, plastic bags, leaves, sheets, condoms,
etc. carried by raw WW. These materials are insensitive to biological treatment and
harmful for downstream installations (pumps and pipes). Decanted materials are
referred to as primary sludge, which are then collected [2, 4]. After the decantation
step, the water loaded with dissolved organic matter (OM) and suspended colloidal
particles (diameter < 200 μm) flow through to the biological treatment processes
where it will be assimilated and metabolized by a complex and diversified
microbiota including aerobic, anoxic, and anaerobic processes [2, 4, 5] (Fig. 1).
WW’s microbiota assembles into aggregated structures called flocs, where
exopolymeric substances (EPS) are establishing the bonds between microorganisms.
Aggregation and accumulation of mineral and organic compounds to and within the
flocs make their weight increase, allowing them to settle into the bottom of the WW
basins [2, 4, 6]. Free-living microorganisms, which do not aggregate to the flocs,
grow at the expense of dissolved OM and constitute suspended matter (SM) [7]. At
the end of the biological treatment, after assimilation or elimination of carbon,
nitrogen, and phosphorus, as well as other biodegradable elements, the WW
undergoes a second decantation in the clarifying basin. Following sludge decantation, purified water is discharged into the natural environment or reused mainly for
irrigation [8]. After this treatment, the secondary or activated sludge is collected
from the clarifier, which mainly consists of microbial cells and other SM, with the
eventual chelation of sewage compounds leading to the formation of so-called
“biological” or secondary sludge (Fig. 1).
It was reported that the United States is generating 40 Mt of sewage annually,
while in the European Union, the figure is 50 Mt, representing about 7.5 Mt of dry
matter [3, 9, 10]. In the countries located near the Baltic Sea watershed, the generated
sewage sludge is about 3.5 Mt of dry solids annually. Germany is the highest sludge
producer, followed by the United Kingdom and France. It is estimated that these
countries with Spain and Italy generate altogether nearly 75% of the European
sewage sludge [11]. In France, this production is of the order of 1.8 Mt SM
year
À1 , which represents 15–19 kg inhabitant
À1 year
À1 of SM [3]. Considering the
organic waste recovery, French policy predicts an increase of 55 to 65% over the
period going from 2020 to 2025 [10, 12].
Treated WW and sludge quality are specified by national and international
standards and guidelines according to the receiving environment or their use for
different purposes [13, 14]. Physicochemical parameters including organic pollutants or micropollutants and minerals, among which are metal trace elements (MTE),
and pathogenic microorganisms indicating fecal contamination and nematode eggs
are analyzed (Fig. 2). The analyses are achieved to ensure that there will be no
negative effect on the quality of the receiving environments, such as streams, rivers,
or soil and groundwater, when spreading the sludge or following agricultural fields’
irrigation with treated WW [15–17].
Based on the economic point of view, recent cost analysis data estimate that half
of the total operating cost of a WWTP accounts for sewage sludge management
Environmental, Economic, and Ethical Assessment of the Treated Wastewater and. . .
51
separation of the pollutants of large size using mechanical tools to retain and harvest
solid and bulky waste such as floating grease, plastic bags, leaves, sheets, condoms,
etc. carried by raw WW. These materials are insensitive to biological treatment and
harmful for downstream installations (pumps and pipes). Decanted materials are
referred to as primary sludge, which are then collected [2, 4]. After the decantation
step, the water loaded with dissolved organic matter (OM) and suspended colloidal
particles (diameter < 200 μm) flow through to the biological treatment processes
where it will be assimilated and metabolized by a complex and diversified
microbiota including aerobic, anoxic, and anaerobic processes [2, 4, 5] (Fig. 1).
WW’s microbiota assembles into aggregated structures called flocs, where
exopolymeric substances (EPS) are establishing the bonds between microorganisms.
Aggregation and accumulation of mineral and organic compounds to and within the
flocs make their weight increase, allowing them to settle into the bottom of the WW
basins [2, 4, 6]. Free-living microorganisms, which do not aggregate to the flocs,
grow at the expense of dissolved OM and constitute suspended matter (SM) [7]. At
the end of the biological treatment, after assimilation or elimination of carbon,
nitrogen, and phosphorus, as well as other biodegradable elements, the WW
undergoes a second decantation in the clarifying basin. Following sludge decantation, purified water is discharged into the natural environment or reused mainly for
irrigation [8]. After this treatment, the secondary or activated sludge is collected
from the clarifier, which mainly consists of microbial cells and other SM, with the
eventual chelation of sewage compounds leading to the formation of so-called
“biological” or secondary sludge (Fig. 1).
It was reported that the United States is generating 40 Mt of sewage annually,
while in the European Union, the figure is 50 Mt, representing about 7.5 Mt of dry
matter [3, 9, 10]. In the countries located near the Baltic Sea watershed, the generated
sewage sludge is about 3.5 Mt of dry solids annually. Germany is the highest sludge
producer, followed by the United Kingdom and France. It is estimated that these
countries with Spain and Italy generate altogether nearly 75% of the European
sewage sludge [11]. In France, this production is of the order of 1.8 Mt SM
year
À1 , which represents 15–19 kg inhabitant
À1 year
À1 of SM [3]. Considering the
organic waste recovery, French policy predicts an increase of 55 to 65% over the
period going from 2020 to 2025 [10, 12].
Treated WW and sludge quality are specified by national and international
standards and guidelines according to the receiving environment or their use for
different purposes [13, 14]. Physicochemical parameters including organic pollutants or micropollutants and minerals, among which are metal trace elements (MTE),
and pathogenic microorganisms indicating fecal contamination and nematode eggs
are analyzed (Fig. 2). The analyses are achieved to ensure that there will be no
negative effect on the quality of the receiving environments, such as streams, rivers,
or soil and groundwater, when spreading the sludge or following agricultural fields’
irrigation with treated WW [15–17].
Based on the economic point of view, recent cost analysis data estimate that half
of the total operating cost of a WWTP accounts for sewage sludge management
Environmental, Economic, and Ethical Assessment of the Treated Wastewater and. . .
51