hydroxy-butyrate, nitrate reductase, urease, and gelatinase and hydrolyzing benzoate
forming a gelatinous structure. Other gelatinous matrices produced by phytoplankton and associated bacteria can flocculate under stressful conditions [10].
In sludge, the EPS concentration ranges from 73 to 139 g L
À1 . These EPSs
consist of sugars, proteins, lipids, nucleic acids, glycoproteins, phospholipids, as
well as minerals and metals (Ca
2+ , Mg
2+ , Fe
2+ , Al
3+ , Cr
6+ , and Cd
2+ ), contributing to
the flocs formation [1, 10, 29]. EPSs allow bacterial adhesion to external surfaces,
forming a protective layer, and fill the space between prokaryotes and eukaryotes
inhabiting aggregates, where a variety of microorganisms develops and produces
more specific EPSs [7, 23, 24, 28, 30]. EPSs also play the role of scavenger, transport
and transform, store, and facilitate the degradation of nutrients in the environment,
help detoxify xenobiotics following their sorption, accumulate toxic metal ions, and
retain water [6, 10, 23].
For an efficient use of nutrients by microorganisms, the optimum C/N/P ratio
should be 100/5/1. The optimal production of sludge is obtained for a C/N ratio of
9 to 21; its deterioration occurs when the C/N ratio ranges between 21 and 43.
Moreover, under C and N limiting conditions, the biosolids’ hydrophobicity
decreases and dehydration becomes difficult [6]. The size and shape of the floc
vary according to the interactions between the types of particles (microorganisms,
organic and inorganic particles, and ions) and their affinities. These parameters
influence the growth and organization of flocs, resulting in hydrated clusters of
aggregates of living and nonliving organisms, interacting with the EPS. Nitrogen
limitation affects EPS production. In fact, for a high C/N rates, the microbial activity
decreases with the release of ammonia [4, 6]. However, floc size increases at high
C/N ratios. Sludge color may vary from white to brown, depending on the constituents and types of bacteria present, aggregated particle density, oxygen availability,
and age of aggregates [24, 31]. To facilitate sludge handling, its volume is reduced
by various processes including thickening, dehydration, and drying, which may
reduce its management costs [10, 24].
3 Chemical Composition of the WW Sludge
3.1 Macro- and Micropollutants as Plant Nutrients
The WW sludge microbiome partly assimilates carbon, nitrogen, phosphorus, and
residual nutrients; the excess quantities are found in sludge which, together with all
the non-metabolizable pollutants, make out of it a matrix rich in fertilizing material
(C, 32–38%; N, 2.9–5.2%; P, 1.5–2.7%) [29, 32]. This matrix contains also elements
at low concentrations more or less biodegradable [28, 33], which are at the origin of
the problem raised when it comes to the urban WW sludge valorization [23, 31,
34]. In fact, the fertilizing elements of the WW sludge are organic carbon in its
different forms, resulting from human activities, some of which is found in the form
of humic substances adsorbed on the EPS, which represent 20% of the sludge carbon
Environmental, Economic, and Ethical Assessment of the Treated Wastewater and. . .
55
forming a gelatinous structure. Other gelatinous matrices produced by phytoplankton and associated bacteria can flocculate under stressful conditions [10].
In sludge, the EPS concentration ranges from 73 to 139 g L
À1 . These EPSs
consist of sugars, proteins, lipids, nucleic acids, glycoproteins, phospholipids, as
well as minerals and metals (Ca
2+ , Mg
2+ , Fe
2+ , Al
3+ , Cr
6+ , and Cd
2+ ), contributing to
the flocs formation [1, 10, 29]. EPSs allow bacterial adhesion to external surfaces,
forming a protective layer, and fill the space between prokaryotes and eukaryotes
inhabiting aggregates, where a variety of microorganisms develops and produces
more specific EPSs [7, 23, 24, 28, 30]. EPSs also play the role of scavenger, transport
and transform, store, and facilitate the degradation of nutrients in the environment,
help detoxify xenobiotics following their sorption, accumulate toxic metal ions, and
retain water [6, 10, 23].
For an efficient use of nutrients by microorganisms, the optimum C/N/P ratio
should be 100/5/1. The optimal production of sludge is obtained for a C/N ratio of
9 to 21; its deterioration occurs when the C/N ratio ranges between 21 and 43.
Moreover, under C and N limiting conditions, the biosolids’ hydrophobicity
decreases and dehydration becomes difficult [6]. The size and shape of the floc
vary according to the interactions between the types of particles (microorganisms,
organic and inorganic particles, and ions) and their affinities. These parameters
influence the growth and organization of flocs, resulting in hydrated clusters of
aggregates of living and nonliving organisms, interacting with the EPS. Nitrogen
limitation affects EPS production. In fact, for a high C/N rates, the microbial activity
decreases with the release of ammonia [4, 6]. However, floc size increases at high
C/N ratios. Sludge color may vary from white to brown, depending on the constituents and types of bacteria present, aggregated particle density, oxygen availability,
and age of aggregates [24, 31]. To facilitate sludge handling, its volume is reduced
by various processes including thickening, dehydration, and drying, which may
reduce its management costs [10, 24].
3 Chemical Composition of the WW Sludge
3.1 Macro- and Micropollutants as Plant Nutrients
The WW sludge microbiome partly assimilates carbon, nitrogen, phosphorus, and
residual nutrients; the excess quantities are found in sludge which, together with all
the non-metabolizable pollutants, make out of it a matrix rich in fertilizing material
(C, 32–38%; N, 2.9–5.2%; P, 1.5–2.7%) [29, 32]. This matrix contains also elements
at low concentrations more or less biodegradable [28, 33], which are at the origin of
the problem raised when it comes to the urban WW sludge valorization [23, 31,
34]. In fact, the fertilizing elements of the WW sludge are organic carbon in its
different forms, resulting from human activities, some of which is found in the form
of humic substances adsorbed on the EPS, which represent 20% of the sludge carbon
Environmental, Economic, and Ethical Assessment of the Treated Wastewater and. . .
55