1. Introduction
Mediterranean countries alone produce 97% of the world’s olives [
1]. In the EU, Spain, Italy, and Greece are three of the most important producers, while Tunisia is one of the largest producers in the Mediterranean Basin. Disposal and handling of large quantities of olive mill wastes (OMWs) remain a critical environmental protection issue, particularly in those countries which have not banned three-phase extraction systems (such as Spain), which leads to what is often uncontrolled disposal of immense amounts of olive mill wastewater (OMWW). The situation is exacerbated by the fact that olive oil production is seasonal, and thus requires huge storage ponds or facilities since processing cannot be spread over time. It was previously estimated that in the Mediterranean approximately 30 Mm
3 of OMWW must be handled each year [
2]. In all Mediterranean countries, the direct discharge of OMWW into rivers and lakes is strictly forbidden, but in fact, the illegal direct disposal of OMWW into nearby aquatic resources and ecosystems is known to be a common practice. There is still no common EU legislation for OMWW regulation and management; standards, physicochemical parameters, limit, and threshold values for safe disposal are left to individual countries [
3]. In Tunisia, the restriction related to the discharge of raw OMWW is defined according to the standard NT.106.002 [
4].
The main physicochemical characteristics of OMWW making them a pollutant are their dark color, their strong offensive smell, high acidity, and high values of chemical oxygen demand (COD) and biological oxygen demand (BOD
5), with a COD/BOD
5 ratio between 2.5 and 5, which correspond to recalcitrant organic materials [
5], high electrical conductivity [
6], high values of solid matter [
7], organic compounds (lignins and tannins, associated with dark color), and long-chain fatty acids and phenolic compounds which contribute to their low degradability and their high toxicity to most crops and microorganisms [
8,
9,
10].
Much research has been carried out on numerous physicochemical methods for treating OMWW, alone or combined, such as oxidation, filtration, centrifugation, flocculation, incineration, coagulation, ultrafiltration, reverse osmosis, ozonation, or photolysis. Many of these approaches are efficient for pollutant removal—namely monophenolic compounds and high organic charge—and consequently quite useful as pretreatment methods, but they are expensive and do not generate valuable sub-products [
11]. Furthermore, since conventional olive mill waste-treatment methods have proven to be inadequate and ineffective for removing OMWW pollutants, several studies have been carried out in order to use this waste as a renewable resource and to transform OMWW from a pollutant to organic fertilizer, agricultural water source, or green fuel [
12].
Direct application of olive mill waste to soil has been investigated for many years. While useful as an amendment in moderation, studies have also shown that OMWW, due to phenolic contents, can also have a high antimicrobial capacity, being useful for soil sanitation against certain pathogens [
13,
14]. The high concentrations of phenolic compounds of fresh OMWW lend to toxic and antimicrobial properties that lead to slow microbial degradation of the organic matter in the soil, especially of highly resistant lignocellulosic compounds [
15]. Moreover, its high mineral salt content, low pH, and the presence of phytotoxic compounds have a negative effect on soil productivity [
16]. Direct application to soil has been reported to cause inhibition of seed germination, genotoxicity effect, and a decrease in phosphatase and fluorescein diacetate hydrolase activities in the soil [
17], the latter of these being enzymes which are important indicators of soil microbial activity and a soil’s capacity for the mineralization of plant-available nutrients.
Olive mill solid waste (OMSW), otherwise known as olive pomace, contains high potassium concentrations and high organic matter (mainly fibers). Unlike other organic residues that have been proposed for agricultural purposes such as sewage sludge and municipal solid wastes, the concentration of heavy metals in OMSW is almost imperceptible [
18]—this is an important factor when considering thermal transformations since these processes concentrate non-volatile components. Nevertheless, several studies have shown that, although it is less phytotoxic than wastewaters, it causes great nutritional imbalances since it modifies the nitrogen cycle in soil due to its high C/N ratio [
19]. For this reason, it has been previously considered that the suitability of OMSW as a soil amendment is enhanced when adding mineral nitrogen, and results show that its mineralization largely depends on the type of soil, being temporarily inhibited in acidic soils [
20]. Thus, it has been demonstrated that nitrogen fertilizers should preferably be added along with OMSW, as this technique results in a large increase in soil-available K [
21].
In order to avoid negative effects often observed when directly applied to the soil, the recycling of OMSW and their transformation into fertilizer by composting technology has been widely recommended. Composts from OMSW have been shown to serve as adequate organic fertilizers for olive trees or as part of a substrate or growing media [
22]. However, the dense and sticky physical texture of OMSW makes it difficult to maintain aerobic conditions inside the material for composting [
22]. In addition, other properties like excessive moisture content and the presence of non-easily degradable compounds, fats and polyphenols, make OMSW a difficult substrate for composting [
23]. In light of the above, several researchers have developed co-composting procedures for olive mill by-products with many agricultural wastes [
24]. Although some of these co-composts showed a satisfying degree of humification and no phytotoxic effect, physical characteristics of OMSW make it difficult to compost by forced aeration systems since different flow paths are generated for the air that dries the material forming aggregates [
25]. Moreover, high acidity values reached during co-composting of OMSW may represent a limitation for its soil application [
26].
Recently, the production of biochar has been studied as a conversion technology for olive mill wastes—both solid and liquid—to deal with their usual toxicity [
27]. Biochar is a C-rich organic material, which is produced during slow exothermic decomposition of biomass under low oxygen conditions and at temperatures ≤700 °C [
28]. Biochars can be used to improve the productive capacities of degraded and low-fertility soils [
29,
30]. Depending on soil mineralogy, pH, and organic matter, among others, their application can increase soil pH, base saturation, electrical conductivity, available P, exchangeable Ca, Mg, Na, K, and cation exchange capacity [
31,
32]. Other olive industry biomass sources including olive tree prunings and olive pits have been satisfactorily processed through pyrolysis in order to obtain biochar [
33]. However, biochar obtained from olive biomass—as is the case for most other lignocellulosic feedstocks—has low N contents, limiting its fertilizer value [
33,
34]. Biochar is a good strategy for increasing soil organic carbon contents with long residence times due to its recalcitrant nature. Following pyrolysis, a large proportion of non-volatile total plant nutrients are soluble [
35], but following this, mineralization and nutrient release is expected to be slow [
36]. In light of the above, recently, a new environmentally friendly strategy was implemented for the valorization and recovery of OMWW, based on the impregnation of wood sawdust with OMWW, followed by drying and slow pyrolysis and leading to the recovery of nutrients contained in OMWW [
12]. This strategy does in fact increase N contents of the final biochar in addition to enriching it with macro- and micro-nutrients. With a thermal treatment strategy such as this, the recycling of OMWW in agricultural systems may be improved, with multiple benefits for the environment and soil protection, while promoting the circular economy.
Although the production of biochar is a fairly established technology, at the moment it is not economically competitive compared to the production of fertilizers using other techniques because of its high cost of production and the skilled workforce and amounts of energy that are required [
37]. The economics of biochar depend on the availability of advanced technology to produce and optimize co-products based on management objectives. Moreover, the economic feasibility of biochar must contemplate the costs of processing, distribution of the product, and energy generated during the pyrolysis process [
38]. However, if long-term carbon sequestration is valued above renewable energy, then more biochar should be produced. Its economic feasibility improves when life cycle costs and environmental impacts are accounted for. This is especially the case when biochar is produced with reference to biomass availability and sustainability and is used as a soil amendment for greenhouse gas emissions reduction [
37]. Therefore, in order to maximize the economic outputs and beneficial outcomes, the supply chain including feedstock collection, transportation, pyrolysis plant design and operation, and product recovery need to be optimized [
39].
In this study, we highlight the potential of two thermal conversion technologies: on the one hand, conventional pyrolysis for solid fractions, and on the other, hydrothermal carbonization for liquid fractions. The choice between either technique depends generally on the desired quality and the physio-chemical characteristics of the produced chars. Slow pyrolysis (e.g., at 500 °C, a common temperature for high-quality chars) creates a stable carbonaceous product with a relatively high specific surface but with relatively low nutrient content. Chars produced from pyrolysis are known for their carbon sequestration value when applied as a soil amendment [
40]. On the other hand, hydrothermal carbonization (HTC), which is conducted at lower temperatures, typically between 140–350 °C, has larger final yields, i.e., more of the mass is conserved as compared to pyrolysis chars (typically less than 60%) [
41]. HTC also produces liquid phase liquor which to date has not received much attention as a fertilizing substance but concentrates some nutrients such as potassium (e.g., [
42]). Furthermore, it has been shown that resulting carbonaceous materials have high mineral contents which are valuable in agriculture for their slow carbon mineralization and other complementary effects to fertilizer [
43]. Moreover, the surface of hydrochars have low pH
zpc and high densities of phenolic, carboxylic, and aliphatic groups, contrasting greatly with the properties of pyrolysis chars, which have few aliphatic groups and are more aromatic [
35,
44].
In the present study, the objectives were to conduct an inventory of olive mill wastes generated in relevant Mediterranean countries and determine the potential for thermal conversion and agricultural applications. To this end, olive mill waste samples were analyzed to estimate the flows of matter and nutrients contained in this immense waste stream. Subsequently, assays were carried out to produce biochar with two thermal treatment technologies—pyrolysis and hydrothermal carbonization—in order to characterize the volumes of chars and HTC liquors which could possibly be generated from this resource, as well as the matter and nutrients also contained therein. Then, we upscaled these values to the country level in order to provide a basis for calculating the flows of matter and nutrients contained in the untreated wastes and thermally treated wastes, which may aid relevant policies and strategies for dealing with the problem of OMW disposal and valorization on the EU and country levels.
4. Discussion
This study provided up-to-date data on the flows of matter and nutrients contained in olive mill waste, a hugely important food waste stream in Mediterranean countries, where there is increasing political and economic incentives to properly treat and reutilize this resource. On the one hand, if raw olive mill wastes are to be promoted more extensively as a “soil improver,” falling under the new European Fertilizer Guidelines (EU 2019/1009) [
54], the OMWs were well within limits for Cu (limit 300 mg kg
−1) and Zn (800 mg kg
−1). Raw wastes also have high mineral contents. In the Spanish wastes analyzed, an average of 1.7% of the total mass was K (the highest of the wastes assessed in this study, whereas the other had K quantities in the range of 0.7%–0.9%). This property would probably qualify those particular wastes as “solid organic fertilizer,” according to the new EU Fertilizer regulations, which specify that these products must contain minimum 2% by mass total potassium oxide (K
2O; when applying the conversion factor, this is equivalent to 1.66% K). If considering the OMW as a “soil improver,” the OMWs were well within limits for Cu (limit 300 mg kg
−1) and Zn (800 mg kg
−1) [
54]. Furthermore, conversion to biochar is a management option which detoxifies organic contaminants present in the wastes, upgrading their agricultural value both in terms of nutrient availability and carbon sequestration. These OMSWs, when converted to chars, have their K contents concentrated (increased). When considering the large overall amounts of potassium in these waste streams (35,000 t annually, see
Table 5 above), it is seen that OMW could be targeted as a source for renewable minerals for agriculture.
Mediterranean soils are characterized by their low organic C contents, typically below 2%, and contents of as low as 0.5% are even common [
55]. In the countries considered, any contribution of organic matter is likely to improve soil quality and productivity. How much land surface could be amended with these materials, protecting soil quality and increasing soil available nutrients? To carry out this exercise, we can imagine the goal of increasing soil carbon by 0.4% (such as suggested by the 4p1000 initiative; [
56]) by way of land application of these oil milling sub-products. If we consider a standard soil density of 1.3 g cm
−3, enrichment of the plowed horizon (0–20 cm), this would equate to additions of approximately 17 t ha
−1 for OMSW (appx. 60% C) or 13.8 t ha
−1 for OMSW biochar (appx. 75% C). By using OMSW (unpyrolyzed dry mass basis), it would be possible to increase C by 0.4% on approximately 45,000 ha in Spain, 7000 ha in Tunisia, and 14,000 ha in Greece. For biochar solid fractions, it is more realistic to consider lower application rates since commonly cited application rates fall in the range of 2–10 t ha
−1. Assuming an application rate of 7 t per ha, biochar from these wastes would enrich soil carbon by 0.2% on approximately 50,000 ha in Spain, 6300 ha in Tunisia, and 14,000 ha in Greece. Therefore, due to the recalcitrant nature of biochar, with just two or three subsequent yearly additions it could be possible to reach the goal of 0.4%. Though conversion to biochar allows amendment of less land area, it is important to keep in mind the higher residence times of pyrolyzed materials, which can be on the scale of hundreds to thousands of years, whereas the maximum residence times of raw olive wastes will be at least an order of magnitude lower. To summarize, the potential is that each year more than 700 km
2 of arable soils in the three countries could be enriched in approximately 0.2% stable carbon in the form of biochar at an application rate of 7 t ha
−1.
Emerging research shows that processing liquid wastes with HTC, producing HTC liquor, reduces chemical oxygen demand, produces pyrolytic products with biostimulant properties, such as sorbitol, myo-inositol, glycerol, and galactitol [
44], and helps concentrate minerals in the solid fraction [
57,
58]. This is the alternative to traditional management—direct land application of both liquid and solid fractions—which can cause problems of high oxygen demand, phytotoxicity, and salinity. Countries still employing three-phase mills must manage huge quantities of wastewater, containing, among other elements, large quantities of P and K [
59]. Recovery of waste stream nutrients is a strategy for avoiding larger nutrient losses from the processing of limited, critical nutrients such as phosphorus [
60], and improving the nutrient status of Mediterranean soils. One result of this study is to see that thermal treatment will concentrate a portion of the soluble P in the solid phase following treatment [
58]. On the other hand, it is seen that following HTC the liquid phase is significantly enriched by potassium during the process, as shown in
Table 7 and also described in our related work [
44]. This fact must be taken into account when considering sub-products and fertigation uses with this resource, and it may aid in strategies for use of these mineral-rich wastewaters, avoiding consumption of mined sources (potash) which has its own notable environmental impact, costs, and losses in processing.
This study estimated total masses of olive mill sub-products to be valorized, their properties, and the agricultural land area which can be positively impacted by improved management of problematic wastes. The aim was to quantify this potential for measurable advancements in soil protection and carbon sequestration in agricultural soils through a circular economy approach. Future studies can build upon this work by considering a wider range of olive mill wastes and undertaking similar carbonization experiments with more materials and larger-scale plants in order to obtain improved estimates for these countries and the wider Mediterranean Basin.