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23 October 2022

How Can Sewage Sludge Use in Sustainable Tunisian Agriculture Be Increased?

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1
LR Valorisation des Eaux Non Conventionnelles (LR16INRGREF02), Institut National de Recherches en Génie Rural, Eaux et Forêts, Université de Carthage, Ariana 2080, Tunisia
2
Laboratoire D’économie Rurale, Institut National des Recherches Agronomiques de Tunis, Université de Carthage, Ariana 2049, Tunisia
3
Laboratoire de Gestion et de Valorisation des Ressources Forestières (LR16INRGREF01), Institut National de Recherches en Génie Rural, Eaux et Forêts, Université de Carthage, Ariana 2080, Tunisia
4
Centre for Advanced Middle Eastern Studies & Division of Water Resources Engineering, Lund University, SE-22100 Lund, Sweden
Sustainability2022, 14(21), 13722;https://doi.org/10.3390/su142113722
This article belongs to the Special Issue Sustainable Use and Management of Nonconventional Water Resources for Agricultural Development
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Abstract

In recent years, farmers in Beja, an agricultural governorate in northwestern Tunisia, have expressed their willingness to use urban sewage sludge as agricultural fertilizer, especially with the unavailability of chemical fertilizers and the soil type of the region that is poor in organic matter. However, there is an imbalance between the important farmers’ demand versus the limited quantity of sludge produced by the Beja wastewater treatment plants (WWTPs). In the face of this, this study aims to identify the problems related to the agricultural reuse of sludge in Beja and propose solutions to solve them. The quality of the sludge produced by the five Beja WWTPs was assessed based on physicochemical and microbiological parameters. The data were collected using the Delphi method, with 15 experts representing different positions on the issue treated. The SWOT-AHP methodology was used to define the strategies promoting the sustainable use and management of urban sewage sludge for sustainable agricultural development in Beja. Results showed that there were no problems with compliance with the Tunisian standards NT 106.20 for the sludge produced. A set of twelve practical conclusions was identified, constituting the strategies of Strengths–Opportunities, Strengths–Threats, Weaknesses–Opportunities, and Weaknesses–Threats deduced from the SWOT-AHP.

1. Introduction

Efforts are being made throughout the world in terms of sanitation and wastewater treatment with the aim of saving water and protecting the environment. These efforts lead to the production of increasing quantities of secondary residues called “sludge” [1]. Globally, sewage sludge production is around 200 million tons per year [2]. Europe, North America, and East Asia are the main sewage sludge producers in the world [3]. The first two continents produce 40 million tons of dewatered sludge (DS) per year. Germany is the leading producer of sewage sludge in Europe, with 1.85 million tons in 2012 (about 21% of the total tonnage in Europe). The United Kingdom comes next with the production of 1.14 million tons of DS, followed by France with more than 1.043 million tons of DS [4,5]. Over the past few decades, the traditional ways of sewage sludge disposing, such as incineration and landfilling, have been strongly questioned because of their potential negative impacts on human health and the environment due to their polluting load. Discharging sludge into the sea, still practiced in the United States and Europe during the 1990s [6,7], was finally banned. The EEC also prohibited the discharge of untreated waste into all waterways from any system serving over 15 thousand people [8]. This has caused a renewed interest in recycling waste in agriculture, a practice known and adopted for several decades in all regions of the world [2,9,10].
Wastewater and its treatment produce by-products that contain major nutrients essential for plant growth and human food production, such as nitrogen (N), phosphorus (P), and potassium (K) [11,12]. Modern agriculture is reliant on the massive use of NPK fertilizers. The world demand for these nutrients increased annually by 1.4, 2.2, and 2.6%, respectively, in 2014–2018 [13]. Their production is based on the Haber-Bosch process, the extraction of phosphate rocks and wood ashes, respectively [10,14]. Their global application reflects unsustainable strategies with subsidized and inefficient uses. Indeed, P-fertilizers use, e.g., in food production, is inefficient as it has been stated in Europe that it takes 4 kg of reactive P to produce 1 kg of food with 40% of surplus remaining in the soil, and 50% loss of the system including 17% in water bodies. Chinese studies have indicated that it takes 13 kg of reactive P to produce 1 kg of food [15,16]. Additionally, phosphate rock, which is the main P source, is non-renewable and exhaustible within the next 50 to 130 years due to expected population growth [17]. Therefore, it is important to minimize the loss of P and convert it into a closed cycle [13]. P management strategies should be envisaged because there is no P substitute in agriculture, and in many places, especially the tropics, access to P still limits agricultural productivity [10]. In addition, prices of inorganic P-fertilizers are unstable, which may cause potential spikes in food prices [18]. To remedy this situation and to achieve effective sustainability of agricultural production, it is necessary to consider environmental, economic, and social aspects [3].
The use of sewage sludge as agricultural fertilizer provides a better alternative than landfilling and incineration [19]. Agricultural reuse of sludge is considered worldwide as a way to reduce environmental pollution and contribute to the circular economy by recapturing “waste” as a resource [20]. The sludge’s significant organic matter content (approximately 50% of its solid fraction) can improve physical, chemical, and biological soil characteristics and lead to improved agricultural yields when applied as a fertilizer [12,21]. Organic matter plays an important role in soil aggregation. It improves the soil porosity and raises water retention and movement [22]. The addition of organic matter promotes the soil substances’ decomposition and establishes a microbial equilibrium [13]. However, the main constraint is the safety of reusing sewage sludge because of the potentially concentrated harmful contents of metallic trace elements (MTEs), emergent pollutants, and pathogens [21,23].
In Tunisia, a developing North African country, the strategy relating to sludge management by 2035 aims to design solutions for its recovery, especially in the agricultural sector [24]. The low soil organic matter content [25] and the high organic manure price in the country [26] encourage the use of sewage sludge as an amendment. Before 1998, agricultural sludge use was significant and exceeded 60% of the volume produced. However, this was performed in an uncontrolled manner, which led to the prohibition of any further spreading [27]. With the elaboration of the NT-106.20 standard, sludge reuse has restarted modestly on pilot plots [28]. At present, only 3260 tons are recovered out of a total of 130,000 m3/year of dried sludge through solar drying in beds from 123 wastewater treatment plants (WWTPs) in Tunisia [29],and despite the continuous increase in production, the amended areas with sludge remain insufficient, since the quantity of amended sludge did not exceed 18.6% of the quantity of dried sludge produced (2016) [30] and continued to decrease to 3% in 2020 [29]. In addition, the number of beneficiary farmers remains insufficient and has further decreased over the years [24,29,30,31,32,33].
Beja is among the leading Tunisian governorates in agricultural production. As agriculture is the region’s main economic activity, 91% of the land are dedicated to this sector [34]. During the 2019–2021 agricultural seasons and due to the unavailability of chemical fertilizers, such as ammonium nitrate and diammonium phosphate [35], seventeen farmers have expressed their interest in acquiring sludge from the regional services of the National Sanitation Utility (ONAS), and only fifteen of them received a part of the quantity requested [21]. Thus, the sludge demand by the Beja farmers is greater than the supply.
In view of the above, the theoretical objectives that this study aims to achieve are to identify the problems related to the sustainable agricultural reuse of sewage sludge in the governorate of Beja and propose solutions to solve them. These theoretical objectives lead to the practical objective of developing strategies that, based on the technical, socio-economic, environmental, and health aspects, promote the sustainable use and management of urban sewage sludge for sustainable agricultural development in Beja, a model that can be extrapolated to other regions of Tunisia and elsewhere.

2. Materials and Methods

2.1. Study Area, Sludge Sampling, and Analysis

The study area is the governorate of Beja located in Northwestern Tunisia and subdivided into 9 delegations (Figure 1). It had 306,600 inhabitants in 2017 on an area of 3740 km2 [36]. There are five urban WWTPs in Beja located in the delegations of Beja-Nord, Medjez El Bab, Teboursouk, Testour, and Nefza, using the low load activated sludge process and drying beds for treatment of wastewater and sludge, respectively (Table 1). The dried sludge produced was from 49 to 540 m3 per bed in 2020 [37]. Sludge sampling was carried out in 2018, 2019, and 2020 from all plants in the region at a frequency of one sample per month (a total of sixty sludge samples collected per year). A quantity of 1000 g of dry sludge was taken from the drying beds of each WWTP in hermetic plastic bags. The physicochemical characterization of the sewage sludge studied was based on the parameters given in Table 2.
Table 1. Main features of wastewater treatment plants of Beja governorate [37].
Table 1. Main features of wastewater treatment plants of Beja governorate [37].
PlantDelegationSize in kg BOD5/DayFlow Rate (m3/day)Capacity E.I.Wastewater Treatment ProcessSludge TreatmentDried Sludge Production (m3)
Beja-NordBeja780014,000144,000Low load activated sludge (waterfall basins)Drying beds540
Medjez El BabMedjez El Bab2000450040,000Low load activated sludge (waterfall basins)Drying beds500
TeboursoukTeboursouk719128018,000Low load activated sludge (oxidation ditch)Drying beds345
TestourTestour720118019,000Low load activated sludge (oxidation ditch)Drying beds256
NefzaNefza680150017,000Low load activated sludge (oxidation ditch)Drying beds49
BOD5: biochemical oxygen demand during 5 days. E.I.: Equivalent inhabitants.
Sustainability 14 13722 g001
Figure 1. Location of wastewater treatment plants in Beja governorate, Northwestern Tunisia [38].
Table 2. Physicochemical parameters studied and analysis methods.
Table 2. Physicochemical parameters studied and analysis methods.
ParameterAnalysis MethodUnitSource
pHElectrochemical method-[39]
Dry matter (DM)Gravimetry%[40]
Organic matter (OM) Calcinationg/kg DM[41]
Total organic carbon (TOC)Colorimetryg/kg DM[42]
Total nitrogen (TN)Titrimetricg/kg DM[43]
Total phosphorus (TP)Atomic emission-ICPg/kg DM[44]
Cadmiummg/kg DM
Chromiummg/kg DM
Coppermg/kg DM
Mercurymg/kg DM
Leadmg/kg DM
Zincmg/kg DM
Nickel mg/kg DM
Nematode eggsMicroscopic observation (arithmetic mean)U/kg-
Faecal coliformsSolid/liquid extractionUFC/kg[45]

2.2. Collection of Information by the Delphi Method

The Delphi technique was used to collect information. The method relies on structuring a group of credible experts to deal with a complex issue [46], and through the consensus of their responses [47], inferences on subjective facts can be made. It starts with a delimitation of the problem, first questionnaire construction, and selection of experts [48]. As recommended by Skinner et al. [49], a panel of 15 experts representing different aspects regarding treatment and reuse of sludge was questioned with an initial bloc of 12 open questions relevant to economic, social, technical, environmental, and health dimensions (Table 3) followed the scheme presented in Figure 2. The selection of experts was based on the fact that (i) they are representative of the local people of Beja, who occupy or have occupied positions of interest in relation to our study subject within national bodies or organizations (Table 4); (ii) they are experts on wastewater treatment, operation of WWTPs, reuse of sludge, agricultural management and production, and social and rural economies; and (iii) the panel reflects a balance between women and men as shown in Table 4.
Table 3. Utilized Delphi questions based on economic, social, technical, environmental, and health dimensions.
Figure 2. Flowchart of the research methodology used. The arrows represent the relationships between the methodological stages.
Table 4. Expert panel compliance.
Based on their responses, a second bloc of questions with 28 closed options was developed and presented to all participants. Then, a third questionnaire was sent to each expert (Figure 2), including the average score assigned by the group to each question as well as individual qualifications of each expert so that he/she could maintain or modify his/her answer.

2.3. SWOT-AHP Methodology

Assessment of the state-of-the-art of agricultural reuse of sewage sludge in the Beja region is based on the Strengths, Weaknesses, Opportunities, and Threats (SWOT) analysis matrix devised for strategic positioning and giving advice to implicated organizations such as the ONAS and the Regional Commissariat for Agricultural Development (CRDA). A conceptual framework was developed based on collected data to categorize the current situation into endogenous and exogenous factors. Endogenous factors are classified into strengths and weaknesses attributed to the organizations (ONAS and CRDA), while exogenous factors are classified into opportunities and threats attributed to the environment. However, as the SWOT analysis is a qualitative method not allowing direct strategical prioritization, it should be combined with another method, such as the analytical hierarchy process (AHP) that incorporates alternatives to make decisions based on multiple criteria [59]. The AHP considers qualitative and quantitative factors and leads to optimal solutions [60]. It follows a value measurement approach; the weight of each factor is obtained when comparing criteria pairs, which are subsequently given a score for their priority [48]. A comprehensive structure to combine intuitive and rational values is provided that verifies the consistency in decision making. By adopting the technique of Saaty [61], the reality perceived by the experts is transformed into a scale of reason determined through a peer-compared mathematical architecture. Thus, the best possible alternative or decision, facilitating strategic planning and decision making under multiple criteria, is found (Figure 2). The relevance of each factor is determined by comparing the alternatives two by two (A or B) and then evaluating them according to a significant scale (from 1 to 9) (Table 5). Finally, the internal consistency is assigned with the coefficient of reliability (Cronbach’s alpha) [62].
Table 5. Importance scale for peer comparison of decisions based on criteria and sub-criteria [63].
The objectives and the methodology were explained to all experts participating in the Delphi and the SWOT-AHP stages before the beginning of the consultation rounds.

2.4. Strategy Matrix Definition

The strategic matrix was performed according to Weihrich’s approach [64] and included four strategy groups emerging from crossing the results of external and internal factors in the SWOT analysis (Table 6): Weaknesses–Threats (WT), Weaknesses–Opportunities (WO), Strengths–Threats (ST), and Strengths–Opportunities (SO). It is a question of prioritizing the sub-factors that have become more relevant during the previous phase (SWOT analysis) by maximizing strengths and opportunities and minimizing weaknesses and threats.
Table 6. Matrix of strategies [64].

3. Results and Discussion

3.1. Sewage Sludge Quality in Beja

Results showed that the chemical and microbiological quality of the sewage sludge produced in all investigated WWTPs of the governorate of Beja is in accordance with the Tunisian standards NT 106.20 [65] (Table 7) for reuse in agriculture as a fertilizer. Beja sludge quality also complies with EU permissible limits of heavy metals in sludge for agriculture reuse, as well as in other Mediterranean countries such as Italy and France [66,67]. However, the zinc, cadmium, copper, and mercury contents exceed the limits applied in the Netherlands and chromium contents in Swedish legislation [68]. Typically, the use of sewage sludge is recommended to grow fodder and industrial crops. Food crops not in direct contact with the soil that are harvested at least 6 months later of application (e.g., wheat) may also be sludge fertilized [69]. Applying sludge to the soil in Beja, a cereal-growing region characterized by Mediterranean red soils, poor in organic matter, with a fine texture and calcareous accumulation [70], results in several benefits. These are providing valuable plant nutrients such as nitrogen and phosphorus, maintaining soil organic matter that plays a key role in strengthening soil structure and water-holding capacity [71], and stabilizing soil pH [21]. The fact that the sludge contains MTEs constitutes a disadvantage for agricultural use since an accumulation of these elements has negative consequences that can appear in the long-term depending on the physical and chemical conditions of the soil, especially in the case of uncontrolled application [72]. However, Ashraf et al. considered that the rational reuse of sludge contributes to the reduction of pollution risks because metal toxicity can be minimized by adding organic matter to the soil [73]. A proper balance ensuring appropriate sludge reuse, however, needs experimental investigations, including soil type, topography, and rainfall pattern [74,75], which can be assured by the agricultural research institutes in Tunisia. For example, Kchaou et al. [76] carried out a field experiment in the Beja area which made it possible to study the effect of the use of sludge on the growth, yield, and metal content of triticale (X Triticosecale Wittmack). They found that sludge application improved crop growth accumulated above-ground biomass and gave a significant part of the phosphorus, nitrogen, and other nutrient requirements of triticale crops. The biggest sludge rate used in this study (18 t/ha) increased straw yield by more than 123% compared to the control and about 57% compared to chemical fertilizer. In addition, sludge application did not increase the MTEs contents in the triticale crops.
Table 7. Characterization of urban sewage sludge from wastewater treatment plants (WWTPs) located in Beja governorate.

3.2. The Delphi Method Application

For the analysis of the Delphi results, all expert answers were considered equivalent [79] without differentiating or weighing them. This follows Sackman [80], who confirmed the absence of a correlation between the experts’ intelligence and the accuracy of their estimates. The first round of expert consultation with the first block of 12 open questions resulted in 83.3% of items meeting the consensus criteria, i.e., responses could be clustered in three continuous values with 75% of all panel responses [48]. However, the experts proposed questionnaire modifications. For this reason, the second block of 34 questions was reworked to better reach the objectives of the second round of consultations. This resulted in a consensus corresponding to 91.2% of all items and 81.6% of those initially proposed. Evaluation of the responses to the questionnaire was based on a central trend and dispersion measures of the set of answers for each item through the median and interquartile distances [81]. Table 8 shows the median, lower (Q1), and upper (Q3) quartiles in addition to the variation of relative interquartile calculated as the difference between Q3 and Q1. This iterative process determined values comprised between −0.25 and 0.25 in the relative interquartile variation between round one and round two [82]. The results displayed 89% of the total items consulted and 100% participation of experts in round two. Thus, it was decided that another round was not necessary [48].
Table 8. Consultation round results.
Agreements reached in the consultation (Table 9) relating to the identification of problems associated with the reuse of sewage sludge in agriculture in Beja included (I) positive effects of sludge application on soil quality and fertility, crop productivity, production cost reduction for farmers, in addition to income stability and improved social security. (II) The fact that sludge reuse could replace chemical fertilizers with volatile costs exerts a significant impact on the sustainability of farmers’ income. (III) The need to train farmers in good sludge-spreading practices and warn about the risks of uncontrolled spreading on human and animal health and agroecosystems. (IV) The necessity to have a well-trained and well-prepared CRDA staff before organizing periodic training sessions for farmers and assisting them during spreading. (V) The wastewater treatment systems were able to produce sludge in accordance with the recommended national quality standards NT 106.20 [65]; however, there are necessary maintenance activities to be carried out by ONAS in the treatment plants of Beja to ensure adequate equipment and optimal operation of the treatment processes, in addition to controlling industrial discharge upstream and prepare sheds for the treatment and storage of sludge that should be analyzed frequently. (VI) The State bodies should provide new transport and sludge-spreading equipment for farmers, repair the existing ones, ensure periodic monitoring of the soils amended by the sludge, draw up specifications for using sludge in agriculture, and apply the traceability principles for food safety objectives of agricultural products grown on soils amended by sludge.
Table 9. Agreements attained through the Delphi consultation.
The Delphi method presented the advantages of offering a strong involvement of the experts within the surveys carried out. As such, this method takes full advantage of the information and communication network concept [83]. The information collected is rich and abundant thanks to the confrontation and exploitation of divergent points of view. Anonymity guarantees independence of opinion, and the iterative process leads to a strong consensus [84]. However, the method has certain limitations, in particular, the length of the implementation due to the consequent number of round trips (3 or 4) and the availability of the experts. The time required (4 to 5 months) and the rigorous logistics (collection, circulation, feedback) of information are aggravating cost factors. Nevertheless, the uses of e-mail and discussion forums reduce expenses, make procedures more flexible, and speeds up the process [83]. The Delphi method also has the disadvantage that reaching consensus does not necessarily mean consistency [85]. It is advisable to arbitrarily determine a rule for validating the results of the interquartile intervals. Only opinions diverging from the median are considered and/or justified. This argument can, however, be weighted by the fact that atypical ideas are often richer in information than the norm. However, obtaining a consensus does not in any way guarantee the relevance of the prediction: a group of experts can consensually be wrong.

3.3. SWOT-AHP Method Application

The construction of the SWOT matrix, shown in Table 10, was based on the responses collected in the Delphi consultations and presented as Strengths (S), Weaknesses (W), Opportunities (O), and Threats (T), according to the economic, social, technical, environmental, and health analysis’ dimensions of Table 3.
Table 10. SWOT matrix related to the state-of-the-art of agricultural reuse of sewage sludge in Beja.

3.3.1. Developing the Hierarchy of Decisions

We aimed to identify problems related to the agricultural reuse of sewage sludge in Beja and propose solutions that solve them. The hierarchical dependency between decision criteria and sub-criteria with strategic decisions was identified based on four levels (Figure 3). The first level is to identify strategies for improving conditions and propelling the agricultural reuse of urban sewage sludge in Beja. The second is the criteria of decisions, in this case, opportunities, threats, strengths, and weaknesses. The third level is comprised of sub-criteria of decisions in accordance with elements in the SWOT analysis outcome. Finally, the fourth level concerns the strategic decisions emanating from the SWOT analysis.
Figure 3. The hierarchical decision model for establishing the strategies to increase sludge use in agriculture.

3.3.2. Relevant Criteria and Sub-Criteria through the AHP Methodology

To assess the relevance of the criteria, another consultation of experts was launched, creating specific data necessary for the applied AHP method. The experts answered in a peer comparison SWOT structure, presenting relevant sub-criteria. The method does not involve direct contact and interaction between the experts, which avoids bias. Each expert was invited to supply a direct estimation of his or her approach, and all answers were processed mathematically. The algorithmic processing of the values proposed by the experts and the computation of each reliability coefficient (α) was carried out using the AHP Excel template with multiple inputs [87].
For the expert consultation, 15 invitations were sent to experts in the field of wastewater treatment and the operation of treatment equipment in WWTPs, as well as to academics and officials related to agricultural management, agricultural production, and social and rural economics. In total, seven responses gave results according to Table 11. To estimate the consistency of the consultation results, we calculated the reliability coefficient (αT) to 0.743, which indicates adequate results [88,89] (Table 11).
Table 11. Relevance for all decision criteria and sub-criteria.
At level two, related to criteria assessment, the consulted experts considered that strengths (35.0%) and opportunities (27.9%) had a greater weight than weaknesses (21.2%) and threats (15.9%) when analyzing the state-of-the-art related to agricultural reuse of sewage sludge in the governorate of Beja (Table 11). In addition, at level three, concerning sub-criteria assessment, the experts highlighted the S1 indicating that sewage sludge is a fertilizer essential for agricultural production that can improve crop productivity and O1 concerning wastewater treatment systems adapted to produce sludge that meets quality standards. However, they call attention to W1 concerning the fact that the demand for sludge by farmers in Beja is greater than the quantities available, and T2 for the impact of climate change on sewage sludge quality. Indeed, the treatment, disposal, distribution, and reuse of wastewater and, therefore, sludge as by-products of wastewater treatment are subject to climate change effects through high energy costs and by increased volumes of wastewater entering treatment plants in areas subject to increases in rainfall, and by increased needs for reuse where droughts become more frequent [90]. The main processes affected by climate change in activated sludge WWTPs, such as those of Beja governorate (Table 1), are sedimentation, biological aeration of warm wastewater, processing of sludge, stabilization ponds, and chlorination [91].

3.4. Specification of Strategies

Four sets of strategies were deduced (Table 12) following the SWOT-AHP analysis, namely Strengths–Opportunities (SO), in-other-words maxi-maxi strategies, Strengths–Threats (ST), i.e., maxi-mini strategies, Weaknesses–Opportunities (WO), i.e., mini-maxi strategies, and Weaknesses–Threats (WT), i.e., mini-mini strategies [63].
Table 12. Specified strategies.
The criteria hierarchy results from the requirement to settle strategies based on reinforcing strengths (relevance: 35.0%) using opportunities (relevance: 27.9%); in particular, S1 (22nd), S2 (19th), S6 (17th), S4 (16th), S3 (13th) with opportunities O1 (21st), O5 (14th), O4 (15th), O2 (12th), O3 (9th); as well as those of turning W1 (19th), W3 (10th), and W2 (11th) into opportunities.
As indicated in Table 12, to reach the first hierarchy outcome, the strategy of the “SO” group was based on (I) promoting the reuse of sludge as organic fertilizer rich in nutrients, (II) providing equipment for sludge transport and spreading to farmers, and (III) bringing sludge produced in other regions where the demand for agricultural reuse is lower. Concerning the “ST” group, the strategy consisted of (I) providing farmers with standard-compliant sewage sludge independent of climatic change conditions, (II) creating secure sludge sheds for temporary storage, and (III) ensuring periodic monitoring of the quality and structure of soils amended by sewage sludge since sludge spreading on soil tends to increase MTEs accumulated in the soil [27]. However, several studies have shown that a significant accumulation of MTEs occurs mainly in the soil surface (0–10 cm depth) [92,93,94], and leaching is unlikely, provided a high soil organic content [27,95].
For the “WO” group, the strategy was (I) ensuring regular and effective maintenance of wastewater treatment systems, (II) strengthening the capacities of farmers for safe sludge reuse through periodic training on good sanitary practices, and (III) allowing farmers to acquire sludge, at the dose of 6 t/ha, repeatedly for three years [28,66]. Studies in Tunisia and elsewhere have shown that the cumulative effect resulting from the periodic addition of sludge over two or three consecutive years generates better impacts on the growth and production of crops and the quality of the soil than single applications [96,97,98,99]. About 18 t/ha divided over three years (6 + 6 + 6 t/ha) is considered efficient to meet the major nutrient needs for growth and to achieve optimal yields without having to resort to additional fertilizing [28,97].
Finally, the strategy of the “WT” group was (I) ensuring reliable control of the frequency and quality of product analyses, respecting the principles of traceability, (II) respecting the historical design thresholds of the WWTPs to produce sludge meeting the standards, and (III) create specifications regarding organizing agronomic recovery of sludge and encouraging farmers to respect the main limitations for agricultural sludge use, which are [65,99,100]:
-
Do not use sludge on agricultural land where vegetables and eaten-raw fruits grow,
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The sludge can be used after 8 months of natural drying,
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Grazing on land treated with sludge should not be permitted until two months after its application,
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Use mechanical burial methods for sludge and not traditional manual methods,
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Sludge produced should not be stored near drainage and irrigation canals and water resources,
-
Reduce the number of displacements of sludge so that the agitation of dust in the air is reduced to a minimum,
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Limit the application of the amount of sludge rich in heavy metals,
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During the 30 days following the application of sludge, limit access to agricultural land where it has been applied,
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Application is limited to areas with a 5% slope and no application near water supply plants, areas where the water table is 1 m deep, less than 150 m from a well, and less than 750 m from an intake surface water used for food,
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The sludge should undergo pathogen-reducing treatment (thermophilic process, composting, or humification) before any agricultural reuse.

4. Conclusions

Due to the unavailability of chemical fertilizers during the period 2019–2021, farmers in Beja, one of the most important agricultural production areas in Tunisia, have expressed a great interest in reusing sewage sludge as an organic amendment in their fields. However, the application of sludge in agriculture needs to be regulated and controlled. Analysis of the chemical and microbiological quality of sludge produced by the five treatment plants located in Beja has shown that they comply with Tunisian standards NT 106.20 in terms of the content of metallic trace elements (cadmium, chromium, copper, mercury, lead, zinc, and nickel) and microorganisms (nematodes and fecal coliforms). This sludge can therefore be reused as organic fertilizer for fodder, industrial crops, and food crops with no contact with the soil that are harvested at least 6 months after sludge application. Sludge reuse for vegetables and direct eaten fruit is not recommended to avoid all health risks due to possible contamination by microorganisms.
To identify problems related to the sustainable agricultural reuse of sludge in Beja, investigations using the Delphi methodology with experts from different areas were performed to obtain information to feed a SWOT matrix. Strategies promoting sustainable use and management of urban sewage sludge for sustainable agricultural development in Beja were defined based on the SWOT-AHP methodology, which allowed to identify of practical conclusions summarized into three “SO” strategies of promoting the sludge reuse as organic fertilizer rich in nutrients (SO1), providing equipment for sludge transport and spreading to farmers (SO2), and bringing sludge produced in other regions where the demand for agricultural reuse is lower (SO3). Three “ST” strategies were identified, including the provision of farmers with standard-compliant sewage sludge independently of climatic change conditions (ST1), creating sludge-secure sheds for temporary storage (ST2), and ensuring periodic monitoring of the quality and structure of soils fertilized by sewage sludge (ST3). The three “WO” strategies recognized were ensuring regular and effective maintenance of wastewater treatment systems (WO1), strengthening the capacities of farmers for safe reuse of sludge through periodic training on the application of good sanitary practices (WO2), and allowing farmers to acquire sludge (6 t/ha) repeatedly for three years (WO3). In addition, the three “WT” strategies aimed to ensure reliable control of the frequency and quality of product analyses, respecting the principles of traceability (WT1), respecting the historical design thresholds of the WWTPs to produce sludge meeting the standards (WT2), and creating specifications regarding the agronomic value of sludge and limiting any uncontrolled reuse (WT3). Based on technical, socio-economic, environmental, and health aspects, these strategies promote sustainable use and management of urban sewage sludge for sustainable agricultural development in Beja. These results can be extrapolated to other Tunisian regions, and to other MENA countries, with similar climatic, soil, and socio-economic conditions.
This research was based on expert criteria, and it omitted, for design reasons, the vision of the population and particularly that of the farmers, knowing that the expert criterion risks can be affected by a particular interest or a personal experience. Thus, future lines of research should consider experiences drawn directly from farmers, individuals, and populations.

Author Contributions

Conceptualization, N.M. and N.O.; methodology, N.M. and N.O.; software, T.F.; validation, S.S. and R.B.; formal analysis, N.M.; investigation, S.M.; data curation, F.A.; writing—original draft preparation, N.M. and N.O.; writing—review and editing, N.M., S.S. and R.B.; supervision, W.O.; project administration, S.J.; funding acquisition, R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Ministry of Higher Education and Scientific Research of Tunisia, and the Ministry of Agriculture, Water Resource, and Fisheries (Tunisia) through the Contract program of the “Laboratory of Valorization of Non-Conventional Water” (LR11INRGREF02). The APC was funded by the European Union Horizon 2020 program FASTER project, grant agreement No. 810812, and the MECW (the Middle East in the Contemporary World) project at the Centre for Advanced Middle Eastern Studies, Lund University.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data can be supplied upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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