Impact of the Spreading of Sludge from Wastewater Treatment Plants on the Transfer and Bio-Availability of Trace Metal Elements in the Soil-Plant System

Abstract

The spreading of sludge from sewage treatment plants increased the production of durum wheat and rapeseed. Their richness in nitrogen, phosphorus, and potassium gives them a beneficial effect on crops. However, the application of the sludge can induce increases in the concentration of metals in plant tissues. This increase can generate disturbances at the level of the cell and organelles, such as mitochondria and chloroplasts, which can be altered. Repeated applications of the sludge on the same site tend to increase the accumulation of heavy metals in the soil, so that an cause toxicities for soil microorganisms, animals, and humans, via the food chain. However, it is important to specify that these nuisances mainly concerned industrial sludge, but the use of this sludge is strictly prohibited. In addition, the high doses used in our field experiments are significantly higher than those authorized in agricultural practice. Finally, the risk assessment by calculating both the level of consumer exposure and the number of years for soil saturation shows that the use of urban sludge is safe, especially in the short and medium-term. Nevertheless, the quality of the sludge to be spread must be constantly monitored.

1. Introduction

The constant increase in the production of sludge from wastewater treatment plants presents a major environmental problem. Compared to traditional means such as landfill or incineration, agricultural sludge spreading appears to be the most cost-effective option for sludge disposal [1]. The use of sludge in agriculture appears among the most sustainable environmental solutions in their disposal. In fact, sludge potential fertilizer and the high cost of mineral fertilizers promote sludge use in agriculture. Nevertheless, their metallic trace elements content (ETM) presents a real disadvantage in their use. Actually, metallic elements retained by the sludge during wastewater treatment can cause high metallic charges accumulation in soil [2]. Metals can be found in the form of sulphites, oxides, hydroxides, silicates, phosphates, carbonates and insoluble salts. They can also be adsorbed or associated with the organic matter of the sludge. The amount of metals in the sludge depends on the origin of the wastewater and the treatments it has undergone [3, 4]. It is, therefore, necessary to try to understand the mechanisms and factors involved in the transfer of these elements into the soil and their effects on the plant following the addition of sludge. The behavior of heavy metals in soils and their absorption by plants depend on the quality of the sludge, the nature of the metal, the physico-chemical properties of the soils and the plant species. Plants differ in their ability to absorb and accumulate metals [2, 3]. From the perspective of an agricultural recovery of sludge, we have tried to contribute to the study of the impact of sludge on the transfer of metallic trace elements in the sludge-soil–plant system. Therefore, a field experiment was carried out in Oued Souhil (Tunisia). In this context, we propose to study the effect of two types of urban and industrial sludge on the distribution and compartmentalization of metallic trace elements in the different organs of two species (durum wheat and rapeseed) chosen according to their absorption capacity.

2. Materials and methods

The experimental protocol was installed in the field to the Agricultural Experiment Station of Oued Souhil - Nabeul, situated about 60 kilometers from Tunis and belonging to the National Institute for Research in Rural Engineering Water and Forest.

The urban mud used in this study is taken from the wastewater treatment plant in Korba with a treatment system at low load activated sludge followed by maturation. Sludge from this station underwent a stabilization in aerobic followed by drying on beds. The dry sludge is removed from the drying bed.

The industrial mud is provided from wastewater treatment plant Bou Argoub which hosts two big companies, the Tunisian beverage manufacturing company (SFBT) specialized in the food industry, and Assad company specialized in the electrical industry. Sludge from this station underwent a stabilization in aerobic followed by drying on beds. This sludge is loaded with heavy metals especially lead and chromium.

The plant materials that were used in this experiment are the rapeseed (Brassica napus), which is an annual plant with yellow flowers of the family Brassicaceae and durum wheat (Triticum turgidum) that can be defined as a species of wheat characterized by its hard and glassy kernel. Rapeseed was chosen for its ability to accumulate metals and also because it is one of the three main sources of edible vegetable oil with sunflower and olive.

Experimentation was carried out on two juxtaposed plots reserved for each crop (wheat or rapeseed). For each type of sludge, four doses (5, 25, 50 and 100 t ha⁻¹) were used. Results were compared to a control soil without any treatment.

Sludges were manually dug into the soil. Before utilization, the sludge was analyzed.

The soil was analyzed before the application of sludge and after the harvest. Sampling was conducted between the lines using an auger at four depths (0–10, 10–20, 20–40 and 40–60 cm).

In the laboratory, soil samples were dried in open air and sieved to 2 mm or 0.2 mm depending on the type of analysis required. The main measured parameters were particle size, total calcium, conductivity, carbon, organic matter, total nitrogen and heavy metals concentration. For the particle size, we used the method of the International pipette Robinson, which is essentially based on the destruction of organic matter in the soil using H2O2 and the dispersion of clays by sodium hexametaphosphate. Clays and silts are measured in the suspension of land following the decay time that depends on particle diameter (NF X 31–107). The settling velocity was measured by the formula of Stokes. The Mud and soil samples were analyzed by XRF (X-Ray Fluorescence) and ICP-AES (Inductive Coupled Plasma Atomic Emission Spectrometry Activa–Horiba Jobin Yvon Spectrometer) in the Geosciences and environment Department of National School of Mines in Saint Etienne. The Soil pH was measured by using a 1:2 soil to water ratio. Plant samples were washed with tap water and rinsed three times with distilled water, then separated into leaves, stems and roots, dried at 40°C to constant weight, crushed and sieved at 2 mm. Moreover, the digestion of plant samples was performed using nitric concentrated acid, according to [5, 6, 7, 8]. The plant extracts were analyzed by ICP-AES.

The sowings were performed with 50 seeds m^2–1 for rapeseed and 350 seedsm^2–1 for wheat. The rapeseed harvest was performed after the formation of slices. We weighed the aerial part and the root. The same work was done to wheat. The samples were subsequently dried and crushed ore to determine the mix of metals in different parts of the plant. The different parts of the plant were dried at 80°C to constant weight and then crushed to a fine powder using a porcelain mortar to prevent metal contamination. Digestion is done at high temperature (70°C) with aqua regia. For histological analysis, preparing the samples carefully for transmission microscopy was essential for obtaining reliable results. Therefore, samples were set at 4°C with a solution of 20.5% glutaraldehyde, pH was maintained at 7.4 with a solution of sodium cacodylate (0.1 M). The samples were then washed with sodium cacodylate buffer (0.1 M) and post-fixed in a solution of 1% osmium tetroxide in veronal buffered (0.1 M) [9]. After several washes in distilled water, the samples were dehydrated with a graded ethanol series of increasing concentrations going from 30 to 100%. The final inclusions were made from a mixture of resin [10]. Only the sections with interference colors are gray or silver, that is to say (thickness of 600 to 900A° (1A° = 0.1 nm)) were collected and deposited on a copper grid with 3 mm diameter. The ultrathin sections were mixed using an alcoholic solution of uranyl acetate and 7 by 1% lead citrate [11]. On top of that, observations were made using a Hitatchi H800 electron microscope.

The data were subjected to analysis of variance. The comparison of means at 5% level of significance was performed by the Newman–Keuls test using the Statistica 7 software.

The amount of heavy metal in sludge is not a good indicator for metal availability for T. turgidum plant uptake; accumulation factors were calculated based on metal availability and its uptake by a particular plant. A calculation of biological concentration factor (BCF) was as in Eq. 1, biological accumulation factor (BAF) as in Eq. 2, and transfer factor (TF) as equation

BCF = Metal content (mg kg⁻¹) in root/metal content (mg kg⁻¹) in sludge (1)

BAF = Mean metal content (mg kg⁻¹) in shoot (root+straw+spike)/metal content (mg kg⁻¹) in sludge (2)

TF = Mean metal Content (mg kg⁻¹) in shoot (root+straw+spike) /metal content (mg kg⁻¹) in root (3)

3. Results and discussion

XRD analyzes have shown that industrial sludge has high levels of chromium and lead. These elements mainly exist as Daubreelite Cr₂FeS₄, Brezininaite Cr₃S₄, Wattersite Hg₅CrO₆, Crocoite PbC₂O₄, Pheonicochroite Pb₂O (CrO₄) and lead oxalate PbC₂O₄. As for urban sludge, we note the absence of chromium and the presence of lead in the form of Macphersonite Pb₄ (CO₃)₂(SO₄) and Lanarkite Pb₂O (SO₄).

The results of the XRD spectrum (Figure 1) were confirmed by those obtained by the chemical and mineralogical analysis (SEM) shown in Table 1. Thus, industrial sludge from BouArgoub has very high levels of chromium, lead and cadmium. These contents are higher than the limit values of the Tunisian standard NT-106, which is not the case of urban sludge of Korba. Both types of sludge are rich in organic matter and nutrients, especially nitrogen and phosphorus.

Figure 1.

XRD spectrum of sludge from urban (a) and industrial (b) wastewater treatment plants.

Contents	Industrial sludge	Urban sludge	Tunisian standard
pH	6.3	6.7	<6
MO %	57.9	66.6	50–70%
N %	4.3	5.2	3–9%
C %	31.9	39.0	ND
C/N	7.5	7.4	5–12
Fe2O3%	1.02	1.88	ND
MnO %	0.03	0.02	<1%
MgO %	0.8	1.22	ND
CaO %	8.51	13.4	ND
P2O5%	2.18	3.24	4–5%
Cd mg kg^−1	11	3	20
Co mg kg^−1	18	28	ND
Cu mg kg^−1	68	158	1000
Fe mg kg^−1	8300	10,700	ND
Mn mg kg^−1	81	152	ND
Ni mg kg^−1	49	78	200
Pb mg kg^−1	577	63	800
Zn mg kg^−1	360	440	2000
Cr mg kg^−1	8030	155	500

Table 1.

Chemical contents (mg kg⁻¹) of industrial sewage sludge, urban sewage sludge and Tunisian standard values [12].

ND: not defined/Source ADEME.

According to [13, 14, 15, 16, 17, 18] sludge is a good source of nutrients for plant growth, it can improve the physical properties of the soil. Vlamis et al. [19, 20] reported that sludge can replace mineral fertilization. Phosphorus in sludge is as effective as phosphorus in fertilizers in increasing the extractable phosphorus in the soil to the level required for crop growth. Likewise, calcium carbonate in sludge increases soil pH more effectively than agricultural lime.

Our results made it possible to highlight the action of the sludge on the behavior of the plant in qualitative and quantitative terms. During the first year of experimentation, the productions obtained were generally higher than those obtained on the control plots. A dose-effect was very clear. The dose of 100 t ha⁻¹ records the highest production whatever the crop. Similar results are obtained by other authors. A clear improving action of sludge on English Ray Grass production [21, 22, 23]. Zaier et al. [24] showed that the sludge significantly stimulates the biomass production of B. napus. Similar results have been demonstrated by [25] who showed that the biomass production of durum wheat was significantly improved by 18% with the addition of 40 t ha⁻¹ of sewage sludge. Pasqualone et al. [26] found positive effects of increasing sludge doses on durum wheat productivity, 12 kg ha⁻¹ of sludge was demonstrated to can effectively replace mineral fertilization. Boudjabi et al. [27] found a significant increase in the number of tillers, ears and kernels per ear of barley in amended soils. This has been linked to an improvement in the physical and chemical properties of soils. This beneficial effect has been observed on several crops such as wheat, sorghum, maize, chili peppers, barley and potatoes [21, 28, 29, 30, 31].

For the two types of sludge, the results of the second application show a positive effect on rapeseed production at the doses of 5 t ha⁻¹ and 25 t ha⁻¹. However, in this second application, industrial sludge causes an increase in the concentration of metals in the soil, especially chromium, cadmium and lead. The cumulative application of industrial sludge generates an excessive accumulation of heavy metals [32, 33] which could be harmful to soil fertility, affecting the ecosystem and human health [34]. Marchiol et al. [35] observed that B. napus grown on multi-contaminated soil was tolerant to heavy metals. They concluded that this species could eventually be used successfully in polluted soils without its growth being affected and that heavy metal extraction can be maintained at a satisfactory level. In our case, the tolerance index of this plant exceeds 1 for all treatments but during the second year for the dose of 100 t ha⁻¹, industrial sludge gave lower indices than urban sludge. Nonetheless, the cumulative application of urban sludge increases the production of rapeseed regardless of the dose.

During the experimentation, the plants presented a normal appearance but some rapeseed leaves cultivated in plots having received 200 t ha⁻¹ of industrial sludge show spots of necrosis and a purplish color. In the literature, these symptoms are described as due to phosphate deficiency. Soil rich in iron or zinc can reduce the absorption capacity of phosphate ions, which could occur as a result of adding sludge to the soil [36]. In this context, [37] have also shown that the presence of high levels of lead can cause the formation of precipitate of lead phosphates, which cannot be assimilated by plants and consequently a phosphorus deficiency can occur. For rapeseed, we have seen a delay in germination in certain plots following a cumulative effect of industrial sludge. Also, germination appeared to be inhibited by the presence of urban sludge. Laboratory experiments confirmed this effect, but showed that germination was not permanently inhibited, but simply delayed. The latency period is dose-dependent, so it increases depending to the amount of sludge added. A similar effect was induced by heavy metals (Cu, Ni, Zn) in aqueous solution [38]. Also, germination is positively correlated with the degree of stability of the sludge and the organic matter contents [39]. The observed delay of germination was restored after one month and the development cycle resumed normally.

A study by [40] showed that sludge causes the late maturity of wheat. This was not the case in our experimentation where the wheat development cycle was not disrupted. An increase in crop yield resulted in an increase in the number of seed in our study. We also mentioned that urban sludge increases the oil yield of rapeseed after the 1st spreading. Similarly, [41] has shown that urban sludge considerably increases the productivity of sunflower oil. The use of urban sludge as a fertilizer has been considered for years given its richness in organic matter and nutrients [41, 42]. However, sludge contains phytotoxic metals which can cause certain problems at high levels [43]. The response of plants to these metals varies considerably from one species to another [44, 45, 46] and the results obtained are disparate. Other factors including edaphic can intervene. In our work, we have shown that whatever the site of culture (control, contaminated, heavily contaminated), wheat has a lower concentration of heavy metals than rapeseed. However, these differences are minimal on the control site and are accentuated with the addition of sludge, especially in the presence of industrial sludge. This finding is not the same for the seed since, on the control site, the wheat seems to concentrate more heavy metals than rapeseed, but this is only due to the high zinc contents of wheat seeds. If we disregard the zinc, we find the same result. In fact, Brassicaceae are generally considered to be metal accumulators that can tolerate high concentrations [47] unlike cereals [48]. However, the accumulating power of rapeseed remains low to consider phytoremediation [49]. It is important to note that both rapeseed and durum wheat were able to survive on a site treated with sludge loaded with heavy metals. This capacity may be due both to the existence of tolerance strategies in the plant making the assimilation of metal limited and also to the strong metal bonds in the sludge and the soil rich in matter organic. Most heavy metals can also be stored and detoxified in root tissues with minimal translocation to leaves whose cells are sensitive to phytotoxic effects [50, 51, 52, 53, 54].

Actually, the extent of metal contamination depends both on the concentration of the metal in the environment and on the intrinsic factors of the metal. The concentrations of the main tracemetalic elements are represented in Figure 2. In the control environments and those treated by urban sludge, the Zn contents are higher than the other metals while cadmium was the least abundant. The concentrations founded were as following: Zn > > Cr > Cu > Ni > Pb > Co > Cd. For the environments treated by industrial sludge, the amounts of trace elements were completely different, reflecting an increase in chromium and lead amounts. The concentrations founded in these environments were as following: Cr > > Zn > > Pb > Cu > Ni > Co > Cd.

Figure 2.

Effect of metal on some of metal trace element concentrations in the plant.

The presence of metals in the soil can influence the uptake of essential nutrients for plant growth [55, 56]. The essential divalent cations such as Ca²⁺, Mn²⁺, Zn²⁺, Mg²⁺ compete with toxic metals such as Cd. Therefore, the increase in the contents of these elements can reduce the absorption of metals such as Cd [57, 58, 59]. However, in our experimentation, for the rapeseed, we noted a synergy between a non-essential metal Cd and a trace element Zn. Also, the increase of Cd absorption decreases the absorption of iron and manganese by the root (Figure 3). These observations are explained by competitions between these different cations for surface complexation sites in the root and for the unspecific carriers of major cations or trace elements. Other works have shown that treatment with cadmium can cause deficiencies in iron, copper and manganese [57, 60].

Figure 3.

Influence of cadmium on the absorption of zinc, iron and manganese in rapeseed roots.

For wheat, the absorption of iron, manganese and zinc increases according to the doses (Figure 4) of the sludge and probably plays a non-negligible beneficial role on the Phyto availability of non-essential elements such as cadmium, an antagonistic effect could have taken place, which could explain the negligible Cd contents found in wheat [61].

Figure 4.

Effect of industrial sludge on the absorption of zinc, iron and manganese in the roots of durum wheat.

Sludge application causes a high accumulation of metallic trace elements in the soil, these metallic elements are then driven to the roots and finally to the aerial part. The addition of industrial sludge causes a significant contamination of the soil and the roots by heavy metals while the degree of contamination at the level of the aerial part is lower as shown in Figure 5. For urban sludge, the accumulation of heavy metals is high but much less than that of industrial sludge. The comparison between contaminated and uncontaminated environments shows that the more the environment is polluted with ETM (Trace Metal Elements), the higher the contents of these elements in the plant.

Figure 5.

Accumulation of metallic trace elements in the soil spread by sludges and in the different parts of the plant.

The difference in behavior between aerial organs and roots with respect to heavy metals is reflected in the ultrastructure. From a cytological point of view, the number of cells in apoptosis is higher in the roots. This suggests the existence of much more effective bio protection modalities in the root. The high doses of the sludge cause ultrastructural changes in the roots, but the nucleus retains its integrity and the chromatin is evenly distributed. As for the cytosol, it has a contracted appearance with grouping of ribosomes into polysomes. Mitochondria change their shape and have swollen ridges (Figure 6).

Figure 6.

(a) central cylinder of control roots in longitudinal section (Gr * 40). (b) central cylinder of a control root in cross section (c) Cross section of a root treated with 100 t ha⁻¹ BU (Gr * 40). (d, e) Central cylinder of a root of treatment 100BI (Gr * 40). Cp: Parenchymal cell; X: xylem; RL: Woody rays; CC: central cylinder; P: periderm.

Fragmentation of the vacuole into many small vacuoles is also observed (Figure 6). The plasma membranes appear damaged. The best-known strategy is to interfere with the entry of heavy metals into root cells by trapping them in the apoplasm where they associate with organic acids [62] or anionic groups in cell walls [63]. Once inside the plant, most heavy metals are held in deep cells, where they are detoxified by complexing with amino acids, organic acids or peptides and or they are sequestered in vacuoles [51]. This greatly limits translocation to aerial organs, thus protecting the leaf tissues, and in particular the metabolites of photosynthetic cells against possible damage. Another defense mechanism generally adopted by plants exposed to heavy metals is the improvement of cellular antioxidant systems which would limit oxidative stress [52, 53].

The export of metals to the aerial part is accompanied by physiological disturbances. A decrease in chlorophyll could be induced by excess zinc and cadmium [64]. Our results confirm this fact as shown in Figure 7. Likewise, a decrease in the level of carotenoids can be linked to an excess of cadmium [65] or to an excess of copper [66].

Figure 7.

(a) Variation of heavy metals in the leaf as a function of treatment. (b): Effect of cadmium on the biosynthesis of chlorophyll, proline, and MDA in the leaves of rapeseed.

Metals alter electron transport and inhibit the activity of Calvin cycle enzymes [67]. High doses of industrial sludge also weaken photosynthetic activity [68, 69] and cause a gradual decrease in photochemical quenching (qp) [70, 71] accompanied by a significant increase in non-photochemical quenching (NPQ).

Chromium directly inhibits one of the key enzymes in chlorophyll biosynthesis NADPH [72]. Likewise, several metals such as Cd, Pb, Cu, Zn, and Ni can replace Mg in chlorophylls, resulting in inactive molecules [73, 74, 75]. In the leaf, excess metals can also induce changes in membrane stiffness, permeability and stability [76]. At the cellular level, rearrangements were observed (Figures 8 and 9). The damage produced at mitochondria and chloroplasts is more severe than that of the nucleus.

Figure 8.

Observation under a transmission electron microscope of cross sections of root cortical cells of a control plant (2a and 2b) and of a plant treated with 100 t ha⁻¹ industrial sludge (2c to 2f). 2 a: Plasmalemma (pl) of a cortical cell of a control root 2 b: Cell organelles of a control root with homogeneous distribution of ribosomes (r). 2 c: Retraction of the cytoplasm with detachment of the plasmalemma and formation of a periplasmic space (ep). 2 d-e: Vesicular formations (vf) and membrane formations (fm). 2 f: Degradation of the primary (PI) and secondary (PII) wall.Pp.: Pectocellulosic wall, m: Mitochondria Ag: Golgie apparatus, cy: Cytoplasm, pr: Polyribosome.

Figure 9.

Observation under a transmission electron microscope of the cortical zone of a rapeseed stem from the control treatment (a and b) (a: General appearance of the cell and b: Appearance of the nucleus (N) and after a contribution of 100 t ha⁻¹ BI (c, d and e) (c: Detachment of the plasma membrane (pl) and formation of periplasmic space; d: Irregularly shaped nucleus divided into small nucleoli (nu); e: Vesicle surrounded by a single membrane and containing certain cellular organelles (nucleus, mitochondria and peroxisome (p) and in the presence of 100 t ha⁻¹ BU (f) with pairing of Golgian saccules forming a dictyosome (Ag) and releasing Golgian vesicles giving rise to lysosomes. (di): Cytoplasmic digitation.

In the leaves of plants cultivated in the presence of high doses of industrial sludge loaded with metals, the number of chloroplasts has decreased. Similar results have been reported by [77]. Other structural damage is also frequently observed such as swelling of chloroplasts, rupture of the envelope, deformation of thylakoids. The thylakoid membranes lose their parallel arrangement, the grana become disorganized and the thylakoid surface is reduced. These ultra-structural changes are accompanied by an increase in the degree of membrane lipid peroxidation, appreciated by the production of malondialdehyde. We have also observed an enlargement of the mitochondria, the disintegration of the membranes, the disappearance of the ridges and a clear vacuolation. The intensity of toxicity actually varies from cell to cell.

The attenuation of toxicity could be due, for example, to the retention of metals on the cell wall [78] or their sequestration in the vacuole [79] or their storage in inactivated on specific proteins, amino amines or peptides.

The study of the risks of ETM (Trace Metal Elements) associated with the spreading of sludge requires not only knowledge of the total metal content, but also of the metal content in the various compartments that make up the soil. Nevertheless, assessing the total stock of an element is a good approach to study the degree and extent of soil contamination by a metallic element. Our results have shown that in general, heavy metals are preferentially localized in the surface horizon and this for different media (Figure 10). However, on the control medium, the difference in accumulation between the 3 horizons is minimal and the contents are more or less comparable. The more the environment is polluted, the greater the difference in accumulation between the horizons. In fact, the level of accumulation of heavy metals depending on the pollution of the environment is much greater at the surface horizon than for the underlying horizons.

Figure 10.

Accumulation of heavy metals in the different horizons of the soil.4.

The second addition of sludge increases the ETM (Trace Metal Elements) content in the soil of the two crops. The surface layer (0-10 cm) appears the richest in cadmium, chromium and lead. After the second harvest, the cultivated wheat soil has higher ETM (Trace Metal Elements) contents than those of the rapeseed plots (Figure 11). This is due to the low extracting power of wheat compared to rapeseed. In a study on Brassica napus, [80] reported that in the presence of sewage sludge the extraction of heavy metals by this species increases significantly due to its hyperaccumulation power, but its use in phytoremediation is a very long process.

Figure 11.

Comparison of heavy metals in the different horizons of the soil (between 0 and 10 cm and 10 and 20 cm) after the first and second harvest.

To assess the risk associated with heavy metals in the case of spreading waste products (sludge, compost, wastewater, etc.), one of the most widely used methods is to prevent the accumulation of trace elements in the soil [81, 82]. Only the soil and sludge contents, as well as the quantities of sludge added are taken into account. Taking into account all our data, we tried to make a balance in order to assess the risk of saturation of the soil by heavy metals provided by the different types of sludge in the case of our experiment. The results obtained are shown in Table 2. It emerges from this table that for all the heavy metals, the soil reaches saturation much more quickly with the input of industrial sludge than for urban sludge and this is easily explained by the respective quality of sludge. For urban sludge, the probable duration of saturation varies between 361 and more than 160,000 years depending on the sequence: Cd >> > Ni > Pb > Cr > > Cu > Zn. This is in favor of the use of urban sludge given that the risk of soil contamination is low, in particular for toxic metals (Cadmium, Chromium and Lead) and that the elements which arrive first at the thresholds are copper and zinc which are trace elements essential to the plant. The problem arises differently with the spreading of industrial sludge highly loaded with heavy metals. Indeed, in this case the risk of contamination is present since the saturation time drops to very low levels for toxic heavy metals to reach less than 10 years for cadmium and chromium and 30 years for lead. The sequence found is: Ni > Cu > Zn > Pb > Cd > Cr.

	Cd	Cu	Cr	Ni	Pb	Zn
Heavy metals soil standard (ppm)	3	140	150	75	150	300
Wheat and rapeseed soil (ppm)	0,66	28,03	27,84	6,35	19,06	56,84
Average annual contribution per BU of 5 t ha ^−1 year^−1 (g ha^−1 year^−1)	0.05	998	439,25	92,75	295,5	2425,25
Increase due to sludge	1.38 10^−5	0,277	0,122	0,026	0,082	0,674
Number of years before saturation for Urban sludge	168,480	404	1001	2640	1597	361
Average annual contribution per Industrial Sludge 5 t Ha^−1(g ha^−1 year^−1)	909,5	766	63,380	312,5	15,075	3800,75
Increase due to sludge	0,253	0,213	17,606	0,087	4188	1056
Number of years before saturation for Industrial Sludge	10	526	7	789	31	230

Table 2.

Assessment of the risk of soil saturation by heavy metals provided by the different types of sludge.

Once in the soil, some of these metals persist due to their immobile nature and the risk of crossing plants is low. Their in-depth migration is unlikely. On the other hand, the most mobile elements can transfer through the soil to the aquifer or be absorbed by the plant. Besides the intrinsic criteria of metals, their bioavailability and their transfer are more or less modified, in particular by edaphic factors such as pH, temperature and organic matter contents, hence the need to take into account all factors for success of a spreading project.

The decontamination of polluted soils can be considered but remains a rather long process requiring a lot of time [80] calculated that more than 1000 years would be needed to clean up a contaminated site. These results have been confirmed by [83]. Likewise, [84] studying the phytoextraction of Brassica juncea on a site multi-contaminated mainly by Zn, Cu and Pb showed that the time required for decontamination is between a minimum of 1150 years and a maximum of 360,000 years.

When metals migrate through plants and the food chain is involved, account must be taken of the amounts of the metals that can be transferred to consumers. As B. napus is an edible oil crop [85], we were concerned with the quality of the seeds and the oil extracted in terms of its fatty acid composition.

An increase in the levels of metallic trace elements was indeed mentioned at seed level during the second campaign. We also detected a decrease in the oil content with the addition of industrial sludge. The composition of total lipids in fatty acids shows an increase in the percentage of oleic acid (C18: 1) at the expense of linoleic (C18: 2) and linolenic (C18: 3) acids under the effect of heavy metals provided by industrial sludge while no difference is recorded with urban sludge regardless of the dose. No significant difference in heavy metal content was observed with the contribution of different doses of urban sludge, even after two years of spraying. On the other hand, we detected increases in most of the heavy metals, in particular for the high doses of industrial sludge.

The cadmium contents increase significantly with the contribution of 100 t ha⁻¹, it reaches 25 ppb following the cumulative contribution. For lead and chromium, the increase is especially visible at the 100 t ha⁻¹ dose. These increases become more important during the second year. For nickel, the levels are higher compared to the control from the addition of 50 t ha⁻¹ of industrial sludge for the two spreading operations. For wheat, the composition of the seeds in metallic trace elements is little or not affected by the contribution of sludge. The observed increases concerned only a few metals and for the excessive doses of industrial sludge. In order to assess the health risk linked to heavy metals via the consumption of products amended with sewage sludge, we tried to theoretically determine the daily exposure of consumers of these products to heavy metals (EJE) and to compare it with tolerable toxicological doses (TDI). The TDI is defined as the amount of contaminants that can be ingested daily without adverse health effects [86, 87]. The harmful potential of a product is the greater the lower the TDI value. In our calculations, we considered the seeds of wheat and rapeseed oil that are suitable for human consumption.

The daily exposure dose attributable to the consumption of these EJE products is calculated according to the formula [88]:

$$EJE\left(\mathit{\mu g}k{g}^{-1}da{y}^{-1}\right)=\left[\mathit{Cproduct}\right]xQty\mathit{of}\mathit{product}$$E1

With: [C product]: Concentration of the metal in wheat grains or rapeseed oil; Product qty.: consumption of the product at the 95th percentile (g person⁻¹ day⁻¹) which is equal to 382 g day⁻¹ for adult person weighing 60 kg for wheat [88] and 25 g day ⁻¹ for rapeseed oil [87]. For the trace and zero contents, we assimilated them to the detection limits of the dosing device.

The intake of sludge in its two forms has no effect on the theoretical exposure to Cd of high consumers of wheat, which is estimated at 3.8 μg person⁻¹ day⁻¹ for all the treatments (Table 3). These values are clearly lower than the TDI which is 60 μg person⁻¹ day⁻¹. The problem arises differently for Pb where the dose of sludge (urban and industrial) influences by increasing the level of exposure of consumers but the values obtained are all below the TDI (216 μg person⁻¹ day⁻¹). It should be noted that the higher the dose, the closer one gets to the TDI. For Ni, the intake of both types of sludge increases consumer exposure to this element but without a net dose effect. In addition, the values obtained remain below the TDI (720 μg person⁻¹ day⁻¹). Exposure to Cr is increased with the addition of industrial sludge. Nevertheless, the comparison with the TDI could not be made because the available TDI is fixed for Cr III and Cr IV while our data relate to total Cr.

		Estimated exposure μg person^−1 day^−1				EJE/DJT
wheat seed		Cd	Pb	Cr	Ni	Cd	Pb	Cr	Ni
	Control	3,8	19,1	525,3	261,7	0,064	0,088	—	0,363
	5BU	3,8	19,1	540,5	355,3	0,064	0,088	—	0,493
	25BU	3,8	19,1	498,5	395,4	0,064	0,088	—	0,549
	50BU	3,8	143,3	548,2	336,2	0,064	0,663	—	0,467
	100BU	3,8	191,0	531,0	269,3	0,064	0,884	—	0,374
	5BI	3,8	19,1	624,6	382,0	0,064	0,088	—	0,531
	25BI	3,8	59,2	618,8	368,6	0,064	0,274	—	0,512
	50BI	3,8	210,1	601,7	389,6	0,064	0,973	—	0,541
	100BI	3,8	194,8	685,7	475,6	0,064	0,902	—	0,661
Rapeseed oil	Control	0,003	0,005	0,0002	0,014	4,3E-05	2,52E-05	—	1,98E-05
	5BU	0,003	0,005	0,0001	0,016	4,3E-05	2,44E-05	—	2,27E-05
	25BU	0,003	0,006	0,0002	0,016	4,3E-05	2,60E-05	—	2,20E-05
	50BU	0,003	0,005	0,0002	0,016	4,3E-05	2,44E-05	—	2,22E-05
	100BU	0,003	0,005	0,0002	0,017	4,3E-05	2,52E-05	—	2,31E-05
	5BI	0,003	0,005	0,0002	0,014	4,5E-05	2,44E-05	—	1,91E-05
	25BI	0,005	0,006	0,0002	0,014	7,9E-05	2,60E-05	—	1,89E-05
	50BI	0,016	0,006	0,0003	0,017	2,6E-04	2,83E-05	—	2,38E-05
	100BI	0,055	0,014	0,0008	0,016	9,2E-04	6,61E-05	—	2,22E-05

Table 3.

Estimated exposure and calculated mean Hazard quotient.

The hazard quotient (QD) is defined by the ratio between the calculated EJE and the corresponding TDI, according to the formula of [89] i.e. QD = EJE/DJT If the hazard quotient is greater than 1, the occurrence of Adverse effects related to toxicants are potentially possible. Otherwise, the risk can be considered as theoretically non-existent. All the ratios calculated for wheat seed are less than 1, however it is important to note that the high doses, especially of industrial sludge, increase the QD which rapidly approaches 1 for Ni and in particular Pb. It is probable that ‘it is the same for Cr. Thus, it is imperative to note that whatever the quality of the sludge, the spreading must be done at suitable doses and must be controlled. For rapeseed oil, the estimated daily exposure is extremely low since the values are infinitely low and are much lower than the respective TDI. It should be noted that for Cd and Pb, the 100 t ha⁻¹ BI increase EJE. From a metals point of view, the addition of sludge, especially urban sludge and at low doses, does not generate a health risk, however, it should not be forgotten that the metals can be introduced by other products which must be taken into account in the process risk assessment hence the need to monitor these situations.

4. Conclusion

The spreading of sludge from wastewater treatment plants increased the production of durum wheat and rapeseed. Their richness in nitrogen, phosphorus and potassium gives them a beneficial effect on crops. However, the application of sludge can induce increases in the concentration of metals in plant tissues. This increase can generate disturbances at the level of the cell and organelles like mitochondria and chloroplasts which can be altered. Repeated applications of sludge at the same site tend to increase the accumulation of heavy metals in the soil which can cause toxicities to soil microorganisms, animals and humans, via the food chain. However, it is important to note that these harmful effects mainly concerned industrial sludge, but the use of this sludge is strictly prohibited. In addition, the high doses used in our field experiments are clearly higher than those authorized in agricultural practice. Finally, the risk assessment by calculating both the level of exposure for the consumer and the number of years for a soil to be saturated shows that the use of urban sludge is safe, particularly in the short and medium term. Nevertheless, the quality of the sludge to be spread must be constantly checked. Other metallic trace elements such as mercury, boron brought in by the sludge must be taken into account.

Acknowledgments

This work was supported by National Institute for Rural Engineering research, Water and Forestry, Tunis and Ecole Nationale Supérieure des Mines de Saint Etienne, France.

Conflict of interest

The authors of chapiter submitted for publication, we confirm that the results presented in this paper are real and original. The authors declare that they have no competing interests. The opinions expressed in this article are those of the authors and do not necessarily represent any agency determination or policy.