[18]. For this, many techniques were deployed to manage the sewage sludge
including ocean disposal, incineration, landfilling, and agricultural valorization. In
fact, there are many strategies to use sewage sludge but also many restrictions on the
use of the given management method [11]. However, these routes of waste disposal
may have numerous drawbacks and environmental hazards. To preserve marine
ecology, ocean sewage sludge disposal was prohibited by the London Convention
97 protocol [19]. The presence of heavy metals, organic pollutants, pathogens, and
other trace elements confines the sewage sludge use as agricultural fertilizer. For the
best suitable option, some basic criteria have been described in the circular economy
“from waste to resources” sense. The importance of sewage sludge as a valuable
source of matter and energy and a potential risk related to the application of those
strategies have been appreciated [11]. Moreover, to reduce freshwater consumption,
as well as the discharge of effluents into freshwater ecosystems, treated WW (re)use
may be a valid option [20]. Consequently, WW and its sludge become a valued
resource rather than a waste product in line with circular economy rules. However to
the best of our knowledge, the applied guidelines do not cover the impact and
consequences of the introduction of known or unknown biological contaminants
such as microbial pathogens or microorganisms resistant to antibiotics, heavy
metals, or persistent and emerging micropollutants such as endocrine disruptors on
the soil microbiome and its metabolic activity [9, 21, 22]. Consequently, the
complex network of telluric microbial communities and mesofauna established
based on tight interrelations between soil abiotic and biotic parameters may be
affected [7, 23]. The introduction of nonindigenous potential invasive species may
cause adverse effects at several levels of biological organization, inducing the
elimination of indigenous microorganisms by competition, parasitism, or following
changes in soil physicochemical properties [9, 16, 23, 24]. At the current state, to fill
these gaps of knowledge, such studies remain to be achieved.
2 Biological Sewage Sludge Formation Within the WWTP
Biological treatment of WW converts dissolved OM into biosolids that consist of
highly hydrated flocs. Oxygenation allows the dissolved OM conversion into microbial cells that eventually settle in the WWTP basins, following a physical collision
and flocculation, based on the aggregation of colloidal particles. Consequently, the
flocs characteristics (shape, size, density, and porosity) affect its sedimentation rate
[6, 22]. WW sludge may be in liquid, solid, or pasty form and contain OM (carbon,
nitrogen, and phosphorus), mineral elements, and heavy metals [10] (Fig. 2).
The sludge structure is closely related to that of the flocs. Microorganisms in the
purifying biomass excrete complex mixtures of high molecular weight polymers
[6, 12, 23]. Microbial EPS are an abundant and important group of compounds that
can be secreted by Archaea, Bacteria, Fungi, and algae [25–27]. Cultivable bacteria
of the genera Bacillus, Pseudomonas, and Klebsiella count among the hyperproducers of EPS [28]. In addition to EPS, Zoogloea is producing poly-beta54
E. Ammar et al.
including ocean disposal, incineration, landfilling, and agricultural valorization. In
fact, there are many strategies to use sewage sludge but also many restrictions on the
use of the given management method [11]. However, these routes of waste disposal
may have numerous drawbacks and environmental hazards. To preserve marine
ecology, ocean sewage sludge disposal was prohibited by the London Convention
97 protocol [19]. The presence of heavy metals, organic pollutants, pathogens, and
other trace elements confines the sewage sludge use as agricultural fertilizer. For the
best suitable option, some basic criteria have been described in the circular economy
“from waste to resources” sense. The importance of sewage sludge as a valuable
source of matter and energy and a potential risk related to the application of those
strategies have been appreciated [11]. Moreover, to reduce freshwater consumption,
as well as the discharge of effluents into freshwater ecosystems, treated WW (re)use
may be a valid option [20]. Consequently, WW and its sludge become a valued
resource rather than a waste product in line with circular economy rules. However to
the best of our knowledge, the applied guidelines do not cover the impact and
consequences of the introduction of known or unknown biological contaminants
such as microbial pathogens or microorganisms resistant to antibiotics, heavy
metals, or persistent and emerging micropollutants such as endocrine disruptors on
the soil microbiome and its metabolic activity [9, 21, 22]. Consequently, the
complex network of telluric microbial communities and mesofauna established
based on tight interrelations between soil abiotic and biotic parameters may be
affected [7, 23]. The introduction of nonindigenous potential invasive species may
cause adverse effects at several levels of biological organization, inducing the
elimination of indigenous microorganisms by competition, parasitism, or following
changes in soil physicochemical properties [9, 16, 23, 24]. At the current state, to fill
these gaps of knowledge, such studies remain to be achieved.
2 Biological Sewage Sludge Formation Within the WWTP
Biological treatment of WW converts dissolved OM into biosolids that consist of
highly hydrated flocs. Oxygenation allows the dissolved OM conversion into microbial cells that eventually settle in the WWTP basins, following a physical collision
and flocculation, based on the aggregation of colloidal particles. Consequently, the
flocs characteristics (shape, size, density, and porosity) affect its sedimentation rate
[6, 22]. WW sludge may be in liquid, solid, or pasty form and contain OM (carbon,
nitrogen, and phosphorus), mineral elements, and heavy metals [10] (Fig. 2).
The sludge structure is closely related to that of the flocs. Microorganisms in the
purifying biomass excrete complex mixtures of high molecular weight polymers
[6, 12, 23]. Microbial EPS are an abundant and important group of compounds that
can be secreted by Archaea, Bacteria, Fungi, and algae [25–27]. Cultivable bacteria
of the genera Bacillus, Pseudomonas, and Klebsiella count among the hyperproducers of EPS [28]. In addition to EPS, Zoogloea is producing poly-beta54
E. Ammar et al.