Utilization of Sewage Sludge in Agricultural Soil as Fertilizer in the Republic of Benin (West Africa): What are the Risks of Heavy Metals Contamination and Spreading?

Laboratory of Physical Chemistry, University of Abomey-Calavi, BP: 4521 Cotonou Benin, Benin Laboratory of Inorganic Chemistry and Environment, University of Abomey-Calavi, BP: 4521 Cotonou Benin, Benin Laboratory of Applied Hydrology, University of Abomey-Calavi, Calavi, 01 BP: 526 Cotonou Benin, Benin Beninese Center for Scientifics Research and Technologies (CBRST), 03 BP 1665 Cotonou Benin, Benin


Introduction
Biosolids also known as sewage sludge are the treated organic residuals generated from domestic wastewater treatment plants. With development and population growth, the production of waste and sewage sludge steadily increases from a year to year across countries in the world. In advanced countries like China, over 11.2 million tons of dry sludge is generated (Chu et al., 2009); while Approximately 6×10 6 metric tons is reported in the U.S each year, of which about 60% is applied to agricultural soil and land application (WEF and NACWA, 2013). Sludge management has become a problem with growing importance due to it potentially environmental hazard and high cost for disposal. It has been reported to be an excellent fertilizer and a potential source of organic substances and essential nutrimental elements such as nitrogen and phosphorus for plants. As a consequence, sewage sludge can valuably replace synthetic N and P fertilizers (Kelter et al., 1997;Gao et al., 2008;Roca-Pérez et al., 2009). However, the potential of pathogen transmission and the presence of toxic and persistent organic chemicals and inorganic pollutants (heavy metals) in biosolids; have for the most part limited its valorization. Considerable amount of numerous biodegradable compounds is contained within sewage sludge which gives rise to secondary pollution (Lee and Han, 2013). Anaerobic digestion and aerobic composting technology are widely used for sludge stabilization. During both processes, some organic pollutants are well remediated; while common sludge treatment process such as anaerobic digestion or composting cannot remove heavy metals from sewage sludge (Chipasa, 2003). Metals can thus be accumulated in the sludge and its long-term application can affect the productivity of soils (McBride, 1995). Therefore it is critical to pay due attention to this matter for the sake of environment safety.
In the republic of Benin, the management of wastewater and sewage sludge remains a great challenge. Talking of wastewater treatment plant, there is only one well known WWTP existing in the republic of Benin, which is located in most populated town (Cotonou) of the country. Now, old of 25 years, the WWTP was supposed to treat the wastewater from 300,000 inhabitants; which is the equivalent of 180 m 3 sludge/day. Nowadays, the population has grown up and the WWTP has to deal with wastewater 3 times greater (600 m 3 sludge/day) than that it was supposed to normally deal with. Unfortunately, while the volume/quantity of wastewater knew a drastically increase, the capacity of the WWTP did not significantly increased. Such situation let put a doubt on the quality of the final effluent and produced sewage sludge. Besides, it is important to note that the WWTP was not built to treat and removed metals from sewage sludge. Yet, at the date, the sludge from the WWTP is already being used in agriculture as soil amendment. It would therefore be urgent to investigate the eco-toxicological risks that would be related to the use of sludge as agricultural fertilizer.
To our knowledge, no report on the risk of heavy metals contamination related to the use of sewage sludge from the WWTP in agricultural soils is reported in the republic of Benin these recent years. Therefore, the current work aims at first to investigate and quantify the of heavy metals including Cd, Co, Cu, Zn, Ni, Cr, Pb, Fe and Mn contents in the treated sewage sludge; and secondly, to proceed to their sequential fractionation in order to evaluate the eco-toxicological risks related to the use of sludge as fertilizer in agriculture.

Samples and Pretreatment
Dewatered sewage sludge collected from municipal WWTP was used in this study. Collected sludge was primary air dry, then under the oven at 105°C until constant mass and water content was determined. After drying, sludge was ground in a mortar and passed through a sieved size ≤2 mm. The sludge powder was stored in a plastic bag for further physicochemical characterization and heavy metals determination.

Physicochemical Characterization
Physical and chemical parameters included pH, Electrical Conductivity (EC), organic Matter Contents (OM), Total Phosphorus (TP) and metals contents. pH and EC were determined in the supernatant of dissolved sludge sample in reagent water ratio 1/10 (m/v) by the mean of a multi-parameters (HACH, HQ40d), OM was determined by weight ignition at 550°C for 5h. For TP determination, sludge sample was digested via aqua regia digestion according to ISO 11464, EN 12880 and quantify via Flow Injection Analysis (FIA).

Metal Total Concentration
Sludge sample was digested according to Dong et al. (2013). Briefly, a mixture (16 mL) of HCl (36%): HNO 3 (67%): HF (49%) ratio 4:1:1 (v: v: v) was added to 0.5 g of sludge sample. The mixture was brought to 110°C using a hot plat under hood and the digestion was performed until dryness. The residue was dissolved with 2% of HNO 3 , filtered and collected into 50 mL polyethylene centrifuge tube and finally brought up to the volume. Samples were stored at 4°C until analysis. Samples were in triplicate and the average values were used. Let's call C 0 the concentration of a given metal.

Sequential Fractionation
Sequential fractionation of metals was performed according to the modified method of Tessier et al. (1979) as reported by Aikpokpodion et al. (2013) (Table 1). Metals, in all extracted samples were determined using Atomic Adsorption Spectroscopy (AAS). Samples were in triplicate and the average values were used.

Sequential Extraction Efficiency (SEE) and Quality Control (QC)
The sequential extraction efficiency SEE (%) for a given metal was calculated as the ratio of the sum of the metal content in all extracted fractions (C 1 ) and it concentration obtained by three strong acids digestion (C 2 ) brought to 100 (Equation 1). The digestion of a certified reference material GBW07309 (GSD-9, grey sediment) (0.5 g) and blank were also performed for quality control. The recovery ratios (R) were calculated as the ratio of the extracted concentration (Cex) and its corresponding reference value (Cs) brought to 100 (Equation 2). R values were comprised between 88.5% and 105.5% (Table 3). The supplement concentration responsible of R values greater than 100% could be attributed to some slight errors and uncertainty during manipulation and the preparation of samples: (2)

Pollution Indicators
To better understand and assess the impact of pollutants in the environment, several parameters such as Mobility Factors (MF), Pollution Indexes (PI), Pollution Load Index (PLI) and Enrichment Factor (EF) were evaluated.

Mobility Factors (MF)
MF is defined as the ability of a metal to pass from the most stable form to unstable ones, where it is less energetically retained (Achiba et al., 2010). In order to access the mobility and bioavailable metals in soil, the MF was calculated as a ratio of metal concentration in the mobile fraction to the sum of all fractions: High MF values is interpreted as symptoms of relatively high mobility and biological availability of heavy metals in soils (Achiba et al., 2010):

Pollution Indexes (PI)
Pollution Index (PI) is used to provide a relative ranking of contamination levels. It is calculated according to (Lee et al., 2006) via the equation (Equation 4) below; where Cj is the concentration of a given element j in soil samples (mg kg −1 ) and Coj is its corresponding concentration (mg kg −1 ) in a reference soil or sludge. In this study, the reference sludge for the environment most protective European countries such as Netherlands, Sweden and Denmark (EC, 2001;Salado et al., 2008;Olofsson et al., 2012) (Table 2) was considered in the present study: F5 residue after drying transverse to conical Evaporation 170°C 6 h flask +10 mL of 7M HNO 3 on a hot plate for 6 hours. After evaporation at 170°C, 1 mL of 2M HNO 3 was added and the residue after dissolution was diluted to 10 mL. The residue was washed with 10 mL reagent water.

Enrichment Factor (EF)
Enrichment Factor (EF) is used to apportion the sources of trace metals in the sediment, soil and biosolids. EF of an element is obtained by comparing the abundance of a given trace element in the sludge relative to that same trace element in a reference material. Specifically, the EF of an element (X) is generally calculated relative to the average composition of crustal abundance (Taylor, 1964;Rudnick and Gao, 2003) using Al or Fe as the reference element (Y). In this study, EFs of selected elements relative to crustal abundance using Fe as the reference element were calculated according to the Equation 6 below:

Statistical Analysis
Origin 8 and SPSS 20.0 ANOVA one way were used for data analysis.

Physicochemical Characterization
The investigated dewatered sludge exhibit a slightly alkaline character (pH >6.5) and high electrical conductivity denoting high concentration of organic and inorganic ions such as Cl -, Na + , K + , NH 4 + , NO 3 -, SO 4 2-, HCO 3 in the sludge; which does not favor plant growth despite it is rich in organic matter and phosphorous (Table  3). Indeed, in previous study (Marchese et al., 2008), it has been reported that high EC values from 2.5-3.0 mS cm −1 is harmful for fruits and values between 4.5-5 mS cm −1 is harmful for the stems; which result into the low germination rate, withering of the plants.

Metal Contents
Data (Table 3) showed varied range of metal contents in the sludge. Metals contents are as follow: 27.1, 18.6, 777.1, 1271.7, 165.5, 297.0, 131.2, 3184.0 and 394.2 mg kg -1 DM for Cd, Co, Cu, Zn, Ni, Cr, Pb, Fe and Mn, respectively. It can thus been noted that Cd and Co are at the lowest concentration and Cu, Zn and Fe at the highest concentration. The high concentration of Fe could be explained by the fact that Fe is known to be the earth crust's one of most abundant elements on one hand; and its wide utilization in our daily life on the other hand. The approach of EF is commonly to apportion the sources of trace metals in the sediment, soil and biosolids. As shown in the Table 4, no element shows consistently an EF <1; enrichment values are greater than 10 except for Mn (5318.0,19.1,490.7,335.6,62.3,57.1,136.5 and 7.3,respectively for Cd,Co,Cu,Zn,Ni,Cr,Pb and Mn). This suggests anthropogenic sources of elements than crustal source (Upadhyay et al., 2011). Co is commonly used in ceramic and glass materials. It can also be found in rechargeable batteries and electrical and electronic appliances. To promote growth rate, Zn and Cu are used as macronutrient elements in foodstuff additives , which resulted to their accumulation in the sludge via excreta. In addition, other sources such as galvanized materials and car washes were reported for Zn and leaching from the newer plumbing systems was reported as the source for Cu (Sörme and Lagerkvist, 2002;Rule et al., 2006). Ni, Cr, Pb and Mn are detected at moderate concentration. The used of Cd, Ni and Cr, as microfunction elements in additives to enhance muscle productivity and immunity to diseases ; could explained their accumulation in the sludge. To better understand and appreciate the level of the metals in the sludge, comparison of their concentration in the sewage sludge and their corresponding value in the recommended reference biosolids (Table 2) is needed. Thus, although Cd, owns the lowest concentration in the biosolids compared to others elements, it is at the very high concentration in the sludge as it value is fixed at <2 mg kg −1 DM in the recommended reference biosolids (EC, 2001;Salado et al., 2008;Olofsson et al., 2012). Cu, Zn, Ni, Cr and Pb concentrations are also beyond the recommended values. As a consequence the use of such sludge in agricultural lands would lead to metals spreading and accumulation in the soil. The sequential chemical fractionation of metals carried out in the sludge (discussed below), provided more information and permitted to better appreciate and access the environmental eco-toxicological risk; as the bioavailability and toxicity of heavy metals in the environment depends on their chemical forms (Filgueiras et al., 2004). ANOVA one way statistical analysis revealed significant variation between metals concentration in the sludge (p<0.05). This suggests that metals in the sewage sludge may have some sources in common. Only Cd and Co did not show any dependency (p = 0.858) suggesting the possibility of not having a common source.

Sequential Extraction and Eco-Toxicological Risks
The results of the sequential fractionation of metals in term of mg kg −1 DM is shown in the Table 2. However, for better and easier appreciation of metals' level in the sludge, we are going to talk in the term of percentage (Fig. 1). Data showed that chemical forms distribution of metals varies from one to another. Cd, Cu, Ni and Pb are predominant in carbonates and organic bonds forms (F3 and F5), their concentration reached (34 and 27%), (46% and 23%), (18 and 39%) and (29 and 42%), respectively. It is widely said that organic bonds metals are relatively stable; however, under oxidizing condition which lead into the degradation of organic maters, metals could be released (Tessier et al., 1979) and easily be available for plants uptake. As it was predictable, Zn, Fe and Mn are predominately bonded to iron and manganese (F4): 37%, 51 and 40%, respectively. F4 is known as reducible fraction (Álvarez-Valero et al., 2009) can easily be reduced and converted to exchangeable fraction; especially in anaerobic condition as it use to be in agricultural soil. As a consequence, due attention should be paid to metals stabilization in the sewage sludge to enhance and promote its agricultural use. Co and Cr are strongly bonded to silicates particles (F6): 44.6% and 42%, respectively. It can also be noted that remarkable proportion, up to 23, 29 and 24.5% of Cd, Zn and Ni, respectively are bonded to silicates particles. Otherwise, F1 and F2 fractions of metals are the lowest; nevertheless, their sum reached up to 5% (Cd) to 20%. Such varied distribution of metals in the sludge has a very strong influence on the pollution indicators. Indeed, MFs PI and PLI widely vary from one metal to another (Table 4). Except Zn, all others metals MF is beyond 20%, this reveals the instability of metals and their potential mobility and migration in the sludge. Cu is the most mobile metal in the sludge (MF = 58%) followed by Cd (41%), Pb (40.8%) and Ni (32.7%). Co, Cr, Fe and Mn tend to exhibit similar mobility with MF ranging 26-29%. In the soil, F1, F2 and F3 fractions of metals are very sensitive to the environmental conditions (pH and Temperature) (Achiba et al., 2010;Mingot et al., 1995). As a consequence these elements can easily be released to the environment and uptake by plants. Moreover, the level of pollution's risk by metals is highlighted by the Pollution Index (PI). PI data showed that the application of such sludge in agricultural soil, would lead to a heavy Cd and Pb pollution (Lee et al., 2006) due to their high PI value (19.3 and 28.9, respectively). Repeated land application of such sludge would cause severe heavy metals pollution; as the PLI value (3.4) already indicated moderate heavy metals pollution of the sludge (Zhang et al., 2011).

Conclusion
Treated sewage sludge's physicochemical characterization, heavy metals contents determination and chemical fractionation have been investigated. The main conclusions are as follow: • Sewage sludge is slightly alkaline and exhibited high electrical conductivity (EC >4 mS cm −1 ) • Heavy metals concentration are at high level concentration beyond the recommended values in the sewage sludge • Metals exhibit very high mobility with MFs ranging 20 to 41%, mainly for Cd, Cu, Ni and Pb • Land application of such sludge will lead to a severe secondary environmental pollution, with its various negative impacts on the ecosystem The improvement of the treatment process is highly recommended for better metals' stabilization within sludge before agricultural use or land application.