3.3 Organic Micropollutants
In addition to the easily quantifiable known elements found in treated WW or sludge,
the most problematic source of pollution is represented by persistent and emerging
micropollutants, among which are synthetic molecules such as plastics, hydrocarbons, certain drug molecules (hormones, antibiotics, analgesics, etc.) and endocrine
disruptors (e.g., pesticides, metabolites, toxins, plasticizers, surfactants) [9, 21, 34,
38, 44] (Fig. 2). The European Union has compiled a list of 143,000 industrial
chemicals, known as emerging, persistent, and ecotoxic chemicals, and criteria for
the agricultural use of biosolids have been established [32]. Among these molecules,
some are stable and recalcitrant to degradation and are characterized by a dose effect,
which would impose their control to avoid the related environmental problems
[7]. These contaminants were found in sludge and surface water at a concentration
between 1 μg and 1 mg L
À1 [9]. Motoyama et al. found that residues of 12 drugs
were found in recycled sludge in agricultural soil [39]. Recently, a fungicide
(carbendizine) was found in treated WW and soil [21], and there are interactions
between different pollutants and sludge formation conditions affecting their adsorption [23]. In addition, bisphenol A and irgasan were detected in untreated sludge
with removal of 60% for bisphenol A [46]. Recent studies have demonstrated that
conventional WW treatment using the activated sludge process is insufficient to
remove persistent micropollutants, and 32 pharmaceutical compounds were found in
sludge [47]. The fate of some of these molecules in soils is still unknown [38]. In the
long term, irrigation with treated WW could lead to the accumulation of organic
pollutants and micropollutants in soils such as phenolic compounds, surfactants,
polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pharmaceutical products, and MTEs with a disastrous impact on the soil microbiota and
soil physicochemical properties. The risk may then arise with their gradual accumulation in the soil over the years of sludge application or through the irrigation
process. For example, phenanthrene, a model compound for polyaromatic hydrocarbons, has negative effects on several bacterial groups, reducing soil richness and
Table 2 Heavy metal concentrations in sludge of various origins (mg kg
À1
)
Country
Zn
Cu
Cr
Pb
Ni
Cd
Hg
USA (1977) [32]
1,740
850
890
300
82
19.0
3.0
USA (2002) [41]
705
511
35
65
23
2.3
1.5
France (1999) [38]
761
286
4.5
107
35
4.5
2.1
China (2018) [43]
674
204
236
26
334
0.9
0.9
Concentration limit EPA [41] 7,500
4,300
ND
840
420
85
57
Sludge limit values, France
a
[44]
60–
200
20–
100
30100
70–
100
15–
70
0.5–
1.5
0.1–
1.0
ND not determined
a In ppm; the rest is expressed as a percentage of dry matter. Values likely to amend a soil
58
E. Ammar et al.
In addition to the easily quantifiable known elements found in treated WW or sludge,
the most problematic source of pollution is represented by persistent and emerging
micropollutants, among which are synthetic molecules such as plastics, hydrocarbons, certain drug molecules (hormones, antibiotics, analgesics, etc.) and endocrine
disruptors (e.g., pesticides, metabolites, toxins, plasticizers, surfactants) [9, 21, 34,
38, 44] (Fig. 2). The European Union has compiled a list of 143,000 industrial
chemicals, known as emerging, persistent, and ecotoxic chemicals, and criteria for
the agricultural use of biosolids have been established [32]. Among these molecules,
some are stable and recalcitrant to degradation and are characterized by a dose effect,
which would impose their control to avoid the related environmental problems
[7]. These contaminants were found in sludge and surface water at a concentration
between 1 μg and 1 mg L
À1 [9]. Motoyama et al. found that residues of 12 drugs
were found in recycled sludge in agricultural soil [39]. Recently, a fungicide
(carbendizine) was found in treated WW and soil [21], and there are interactions
between different pollutants and sludge formation conditions affecting their adsorption [23]. In addition, bisphenol A and irgasan were detected in untreated sludge
with removal of 60% for bisphenol A [46]. Recent studies have demonstrated that
conventional WW treatment using the activated sludge process is insufficient to
remove persistent micropollutants, and 32 pharmaceutical compounds were found in
sludge [47]. The fate of some of these molecules in soils is still unknown [38]. In the
long term, irrigation with treated WW could lead to the accumulation of organic
pollutants and micropollutants in soils such as phenolic compounds, surfactants,
polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pharmaceutical products, and MTEs with a disastrous impact on the soil microbiota and
soil physicochemical properties. The risk may then arise with their gradual accumulation in the soil over the years of sludge application or through the irrigation
process. For example, phenanthrene, a model compound for polyaromatic hydrocarbons, has negative effects on several bacterial groups, reducing soil richness and
Table 2 Heavy metal concentrations in sludge of various origins (mg kg
À1
)
Country
Zn
Cu
Cr
Pb
Ni
Cd
Hg
USA (1977) [32]
1,740
850
890
300
82
19.0
3.0
USA (2002) [41]
705
511
35
65
23
2.3
1.5
France (1999) [38]
761
286
4.5
107
35
4.5
2.1
China (2018) [43]
674
204
236
26
334
0.9
0.9
Concentration limit EPA [41] 7,500
4,300
ND
840
420
85
57
Sludge limit values, France
a
[44]
60–
200
20–
100
30100
70–
100
15–
70
0.5–
1.5
0.1–
1.0
ND not determined
a In ppm; the rest is expressed as a percentage of dry matter. Values likely to amend a soil
58
E. Ammar et al.