Introduction

The food waste issue and policies

The amount of food production necessary to sustain global demands is increasing, due in part to the constant increase in the global population (Mak et al. 2020). Food waste generation has gained significant traction in the past 10 years. Globally, an estimated 1.3 billion tons of food are lost during the supply chain, leading to an overall economic impact of about 1 trillion dollars (FAO 2011). According to Statista (2021), 931 million tons of food waste is produced on a yearly basis, 561 million of which is household waste (74 kg of food waste per person), 244 million is from food services and 118 million is from retail. Particularly in the European Union (EU) alone, 130 kg of food waste was produced per capita in 2020. Among all 27 member states, Cyprus accounted for the highest level of food waste per person (400 kg/capita), along with Denmark (221 kg/capita) and Belgium (250 kg/capita). Croatia and Slovenia had the lowest waste outputs, at 71 kg/capita and 68 kg/capita, respectively (Eurostat 2023a, b). The EU produced an estimated 153 million tons of food waste in 2021, surpassing its yearly imports of 138 million tons. This quantity corresponds to approximately 6% of the EU's total greenhouse gas emissions and has a monetary value of 143 billion euros. Developing nations account for around 310 billion dollars of these losses, while industrialized countries face roughly 680 billion dollars in losses (FAO 2022).

However, food waste (i.e., from agriculture, food sector and households) is rich in valuable food sources for microorganisms (i.e., proteins, carbohydrates and lipids). In this regard, many researchers are putting tremendous effort into the utilization of this waste to produce high-added-value products like compost (Al-Rumaihi et al. 2020), energy (Xin et al. 2018) and textiles (ichael et al. 2023; Shirvanimoghaddam et al. 2020; Stylianou et al. 2023). The introduction of the bioeconomy and circular bioeconomy is thus a valuable way to decrease food waste and plays a primary role in reducing the effects of such waste on all three sustainability pillars (the environment, society and the economy) (Morone et al. 2021; Sharma et al. 2021).

The sustainable bioeconomy has the potential to convert biowaste, residues and discards into valuable resources as well as to develop innovations and incentives (Lekkas et al. 2021) to assist retailers and consumers in reducing food waste by 50% by 2030. This transition goal is part of a global strategy implemented under the auspices of the Sustainable Development Goals (SDGs) of the United Nations (UN) (United Nations 2015). In particular, sustainable and responsible consumption and production (SDG12) of energy, resources, infrastructure, etc., has gained increasing traction as a means of achieving sustainable development. At the same time, the utilization of food waste through the sustainable bioeconomy can lead to sustainable cities and communities (SDG11) with an emphasis on waste generation reduction and end-of-life treatment as well as economic growth (SDG8), technological advancements (SDG 9) and new job opportunities within city limits (SDG8). Such initiatives will thrive, ensuring the protection of both aquatic and terrestrial environments (SDG13, SDG14) through the transfer of knowledge and knowhow to the involved personnel (SDG4) (United Nations 2015).

To achieve such a sustainable rate of waste recycling, biowaste materials (i.e., sugarcanes, olive trees, food waste, etc.) can be viewed as being of substantial value in this process (Cheng et al. 2022; Nazir et al. 2022). Biodegradable waste (or biowaste) can be an important source of biomass, with several industries beginning to recognize its potential economic value (e.g., the agricultural, forest-based, chemical and energy sectors) (Chavan et al. 2022; Mishra et al. 2023; Yang et al. 2023). The use of by-products and waste to produce energy (i.e., biomethane, biodiesel, etc.) as well as high-value-added products (i.e., compost) in the framework of the bioeconomy reduces fossil fuel dependency. In particular, biomethane has been proposed as an important model in the circular bioeconomy, and its utilization and relevance are increasing in order to (among other reasons) ensure affordable energy (SDG7) (D’Adamo et al. 2023; Zulqarnain et al. 2021). The development of the bioeconomy is likely to increase demand for these secondary products, changing the economic conditions for production (European Commission 2018). According to Eurostat (2023a, b), 131 kg of household food waste was estimated to have been produced per EU inhabitant. This estimate includes both edible and inedible food. Recent methods for reducing food waste have focused on overcoming this problem by proposing the deployment of a new strategy based on valorization, promoting innovative practices and enhancing smart initiatives with a local impact (Omirou et al. 2023).

Composting constitutes one of the most widely used technologies for organic waste recycling (i.e., food waste, green waste). In fact, green waste and biowaste represent almost 40% of all municipal solid waste (MSW), while, according to many researchers (Chen et al. 2018; Loizia et al. 2021; Oviedo-Ocaña et al. 2023), organic waste represents more than half of the total MSW globally. The landfilling of such waste results in leachate generation, contributing to soil toxicity as well as greenhouse gas (GHGs) emissions, which have vast environmental and public health implications, including cancer, asthma, lung irregularities, etc. (Koul et al. 2022; Rodríguez-Espinosa et al. 2023b). Composting refers to the aerobic biological decomposition and stabilization of organic waste. The main composting parameters include, but are not limited to, the C/N ratio, moisture content, temperature, pH and nutrient availability. Controlling these parameters results in suitable high-added-value products (i.e., soil) for use in agriculture and erosion control. Its benefits lie in its enhanced carbon storage capacity in soils, the reduced need for fertilizers, pesticides and other chemicals, and its capacity to improve the soil structure and density (Rodríguez-Espinosa et al. 2023a, b).

In recent years, a global issue that has been a matter of concern for local communities and governments in the EU is the growing flow of immigrants. In Greece, after the refugee crisis in 2015, many asylum seekers ended up on the Aegean Islands, including Lesvos. Especially in Lesvos, two Reception and Identification Centers (RICs) were organized to serve asylum seekers: one in Moria and one in Kara Tepe. Those RICs were located a few kilometers from the city of Mytilene, the capital city of Lesvos Island (Asylum Information Database 2023a). It was evident that the facilities could not support the enormous number of immigrants arriving on the islands, resulting in significant environmental pressures, particularly in relation to waste generation and waste management. Boxed food was provided two times per day, together with bottled water, while there were cases where traditional meals were also prepared by the refugees.

The present research aimed to investigate alternative approaches to solid waste management and, specifically, the composting of the organic fraction of the MSW from the RICs. In the framework of the SDGs and the circular economy principles as well as the local waste management plan of Mytilini, biowaste management is considered a priority for efficiently diverting the organic flow from the landfill and increasing recycling (Stenmarck et al. 2012). Using two different composting units for household composting and for the co-composting of local agricultural byproducts and food waste (i.e., olive leaves, olive pomace), respectively, the cost of such an implementation given the characteristics of the area was explored and recorded. As the characteristics of insular communities differ from those of urban settings in terms of strategy development (Hameeda et al. 2019; Loizia et al. 2021), waste management, infrastructure, culture and innovation potential, such results could provide the foundation for similar insular communities to develop carefully crafted and customized strategies for food waste reduction and byproduct usage (i.e., composting) within the context of the circular economy.

Materials and methods

Study area and sampling periods

Lesvos is a Greek island in the North Aegean Sea. The research was conducted in the Kara Tepe refugee camp, which used to be inhabited by vulnerable migrant families. Two sampling sessions were carried out, in May and November 2019, to collect data from a sample group of refugee households and investigate possible seasonal trends. Each sampling session lasted 1 week, with daily samples analyzed and categorized into several categories: packaging materials, types of plastic, paper, food residues, etc.

Collection of solid waste generated in Kara Tepe RIC

The research was carried out in three phases: (i) data collection (quantity of waste), (ii) compositional analysis to categorize waste (paper, plastic, organic, etc.), and (iii) the interpretation of the results and characteristics. In this research, the first step was to calculate the daily amount of waste disposed of in the bins by the residents. Several accommodation units inside the Reception and Identification Center were selected and were considered a representative sample. Therefore, the number of units as well as the corresponding number of residents per unit were the characteristics that determined the sampling size and the distribution of each unit type. During sampling, the number of people per house was considered constant.

Three main waste streams (recyclable, biodegradable and residual) were monitored and measured using an analytical scale with 0.001-g resolution provided by the Waste Management Laboratory of the Department of Environment—University of the Aegean. Each sampling session lasted 7 days—a full week—and followed the methodology proposed by Arvanitis et al. (2018) and Mathioudakis et al. (2020), where all the bins were emptied the previous day. The analysis presented here focuses on the biodegradable fraction of the MSW produced in Kara Tepe RIC.

Experimental composting setup

Two composting trials were organized and realized using household composters (provided by the Municipality of Mytilene) with a tailored active aeration system that was developed by the Waste Management Laboratory. The first trial contained food residues from the RIC, olive leaves and branches as a bulking agent (Κ1), and the second trial contained food residues, olive leaves and branches together with two-phase olive pomace (K2). The olive mill waste—two-phase olive pomace—was added to the second pile to investigate the option of co-composting, as this biowaste is abundant on the island of Lesvos because of its high olive oil production (Loumou and Giourga 2003). The experimental period exceeded 120 days, and detailed monitoring allowed the observation of the evolution of physicochemical processes as well as the critical parameters in both trials.

The preparation of the composting bins involved the installation of perforated aeration pipes and a blower (Bulle 650W) that allowed variable rates of air flow in the biodegradable material (Fig. 1). The material for each trial (K1, K2) was mixed and placed in the home composting bins, while the food residues (cooked) that were used and mixed with bulking agents contained beans, potatoes and chicken. The olive leaves and branches were shredded using a Stihl GH 460 petrol garden shredder. After adding the mixtures to the composters, the active aeration system was activated and operated with a blow (ON–OFF) pattern that was adjusted according to the composting phases and the air requirements of the processes. During the first 2 months, the air blower was set to operate automatically for 2 min every 12 h; during months 3 and 4, it was set to operate automatically for 4 min every 6 h.

Fig. 1
figure 1

Representation of the operation of the active ventilation system (figure produced by the authors)

Full size image

Determination of the parameters affecting the composting process

For the composting trials, several physicochemical parameters, such as temperature, oxygen, humidity, pH, nitrogen and carbon, were measured during the whole composting process as described by Daskaloudis and Lekkas (2021) and Michalopoulos et al. (2018). Temperature was measured daily; pH once a week; and nitrogen, carbon and humidity once a month for the first 2 months and then at the end of the experimental period. The daily oxygen measurements started on the 15th day of the composting period.

For temperature measurements, a portable thermometer (Therma 1) was used, and the measurements were performed at two different horizontal points (at the center of the material and at its edge) and at three different depths (one near the surface, one in the middle and, finally, one near the bottom of the composter) (Fig. 2). The average temperature of the pile was obtained based on the three different depths.

Fig. 2
figure 2

Points (left) and depths (right) where temperatures were recorded (figure produced by the authors)

Full size image

Oxygen was measured with a portable device with a 50-cm stem (SW&WS, OA2). Calibration was carried out according to the instructions provided by the manufacturer. Moisture content (MC) was measured by drying samples in a Raypa digital drying oven at 70 °C to constant weight. The results were obtained as the averages of at least three replicates. The final moisture content was calculated using the following equation from Alifragis and Papamichos (2006):

$${\text{MC}}\% = \frac{{W_{1} - W_{2} }}{{W_{1} }} \times 100,$$
(1)

where MC% is the percentage of moisture with respect to the wet weight; W1 is the weight of the sample before drying; and W2 is the sample weight after drying.

pH measurements were carried out using the method of Alifragis and Papamichos (2006). More specifically, 60 ml of deionized water was added to 20 g of soil to create a 1:3 ratio and the determination was performed in triplicate. Before each measurement, the calibration of the pΗ meter (Hach sensION™ + pH 3 lab meter) was performed according to its user manual (www.hach.com).

The organic matter fraction was calculated by the loss of ignition (LOI) method. The dry samples, which were already in porcelain cups from the determination of the moisture content, were placed in a muffle furnace at 550 °C (Nabertherm, Germany) for 2 h (Komilis and Tziouvaras 2009). After the samples had burned, each porcelain cup with the remaining ash was weighed and the weight of the cup was subtracted to determine the weight of the ash, which allowed the percentage of the dry sample left in the form of ash to be determined. The following equation was used to calculate the organic matter (Kazamias et al. 2017):

$$\% {\text{ organic matter }} = { 1}00 \, - \, \% {\text{ ash,}}$$
(2)

where % ash is the percentage of the sample that remained as ash. The carbon content (%C) was calculated by dividing the organic matter by 1.83 (Adhikari et al. 2009; Jolanun and Towprayoon 2010; Rynk 1992).

Total Kjeldahl nitrogen (TKN) was determined using the Kjeldahl method. 0.5 g of ultrafine-grained material was placed in a Kjeldahl flask, where the organic nitrogen was converted to an ammoniacal form using heat, concentrated sulfuric acid (H2SO4) and catalysts. Ammonia was then distilled by adding water and sodium hydroxide (NaOH) and fixed in a 250-ml flask containing boric acid (Alifrangis and Papamichos 2006; Bazrafshan et al. 2016; Kazamias et al. 2017).

Determination of qualitative parameters of the final product

When the composting process was completed, several parameters were examined in the final product, including its phytotoxicity, microbial respiration and electrical conductivity.

Phytotoxicity

The determination of phytotoxicity was carried out using compost extracts which were introduced—sprayed—onto wet absorbent paper in petri dishes to grow cardamom seeds (Young et al. 2016). Two samples from each compost pile (Κ1, Κ2) were assessed on petri dishes in triplicate together with three blank samples. Twenty cardamom seeds were placed in each petri dish. They were then placed in an incubation chamber at constant temperature (20 °C) for 3 days. At the end, the number of germinated seeds was counted, and the lengths of their roots were measured with a caliper. The germination index (GI) was calculated using Eq. 3 (Tiquia et al. 1996; Kazamias et al. 2017):

$${\text{GI}}\% = \frac{{{\text{RG}} \times {\text{RL}}}}{100},$$
(3)

where GI is the germination index; RG is the average number of seeds that germinated; and RL is the average root length of the germinated seeds.

Microbial respiratory activity

The quantification of microbial respiration is often used for compost stability tests since there is a widely accepted index for it. For this study, the cumulative respiration index (CRI) (Komilis et al. 2011) was used for the quantification of oxygen consumption. It was expressed in g O2 dry kg−1 of the initial material, and it is described in Eq. 4:

$${\text{CRI}}_{T} { } = \mathop \sum \limits_{i = 1}^{n} \Delta {\rm O}_{2i} ,$$
(4)

where ΔO2i is the mass of oxygen consumed per time interval i per dry mass of material (g O2 dry kg−1).

Based on Austrian standards for respiration activity, a 4-day static respiration test was conducted with a sample from the final product of the compost piles (Komilis and Tziouvaras 2009; Binner et al. 2012). According to this method, 1-L vessels with manometric heads (Al-350, Aqualytic, Dortmund, Germany) were used. The vessels were filled with samples of 30–50 g (dry weight) with a moisture content of 50–60%. To collect the CO2 produced by the microorganisms’ respiration, a small beaker filled with 1 N KOH solution was placed above the sample in the vessel. Finally, the vessels were incubated in the dark at 20 °C. The consumption of oxygen (O2) was calculated by reducing the pressure recorded by the manometric heads using the ideal gas law, as shown in Eq. 5 (Komilis et al. 2011):

$$\Delta {\rm O}_{2i} = \frac{{\Delta P_{i} \times V_{{{\text{free}}}} \times 32}}{{R \times T \times W_{{{\text{cmp}}}} }},$$
(5)

where ΔO2i = mass of oxygen consumed per time interval i per dry mass of material (g O2 dry kg−1); ΔPi = pressure decrease over time interval i (mbar); Vfree = free air volume in the respirometer (L); 32 = molecular weight of oxygen (g mol−1); R = universal gas constant (83.14 L mbar K−1 mol−1); T = incubation temperature (K); and Wcmp = initial dry compost mass or initial OM mass (kg).

Electrical conductivity (EC)

The determination of electrical conductivity was performed with the SensION5 conductivity meter. Twenty milliliters of ultrafine-grained material and 20 ml of water were placed in a 50-ml beaker. The suspension was stirred periodically with a glass rod for 30 min and left to stand for 30 min before the measurement.

Results and discussion

Sampling

Population data

The sampling method was carried out proportionally in terms of the number of residents per accommodation unit. The units were categorized into different sizes according to the number of residents. Thus, the selection of accommodation for sampling was performed taking into consideration both factors. The same procedure was carried out for all sampled households, as shown in Tables 1 and 2.

Table 1 Characteristics of residents and accommodation units
Full size table
Table 2 Final data comparison for May 2019 and November 2019
Full size table

To sum up, the representative population size was estimated at 91 residents and 18 accommodation units in May 2019 and 156 residents and 30 accommodation units in November 2019.

Estimation of biomass production from food waste and food residues

From the analysis of the composition of MSW in the RIC after two sampling events (Tables 3 and 4), it appears that the largest percentage of the waste generated belongs to the category of food residues (61%). In May 2019, the population of the Kara Tepe RIC was 1355, and the average waste generation per resident was 0.60 kg, whereas the average biowaste generation per resident was 0.37 kg. Therefore, for the entire RIC, the quantities of MSW and biowaste can be calculated as follows:

  • 0.60 × 1355 = 813 kg/day = 0.81 tn/day of MSW

  • 0.37 × 1355 = 501.35 kg/day = 0.5 tn/day of biowaste.

Table 3 Average production of waste and biowaste per sample per capita in May 2019
Full size table
Table 4 Average production of waste and biowaste per sample per capita in November 2019
Full size table

In November 2019, the population of the Kara Tepe RIC was 1329, the average waste generation per resident was 0.51 kg, and the average biowaste generation per resident was 0.24 kg. Therefore, for the entire RIC, the quantities of MSW and biowaste can be calculated as follows:

  • 0.51 × 1329 = 677.79 kg/cap/day = 0.67 tn/day of total waste

  • 0.24 × 1329 = 31,896 kg/cap/day = 0.13 tn/day of bio-waste.

Katsouli and Stasinakis (2019) estimated that the MSW generated by refugees on the island of Lesvos was 0.55 kg per capita. Furthermore, as shown in Table 5, the composition of MSW in Greece included a significant amount of food waste. However, it is observed that the biowaste quantity in the RIC is high compared to the average for the country, which is mainly because of materials that are not found in the RIC (e.g., paper and glass), and thus it can be assumed that this is the case for all RICs around the Mediterranean region. Therefore, as food residues constitute the majority of the separated biowaste, an on-site treatment of the food residues to significantly reduce the environmental pressure caused by the operation of the RIC is proposed. The environmental pressure arises from the increase in the number of refugees, who create a new community without the main commodities yet have the same needs. According to the Asylum Information Database (2023b), 7363 refugees left the islands of Lesvos, Samos, Chios, Kos and Leros in Greece in 2022, and 558 more were transferred to the mainland. By the end of that year, 4371 refugees and asylum seekers were living within the designated facilities of the island (1709 in Lesvos, 1013 in Samos), placing immense environmental pressure on the islands. As presented, in-house composting can aid in the reduction of biowaste coming from the center and at the same time engage citizens in the treatment of food residues while producing a valuable end-product that can be used to grow fruits and vegetables, demonstrating the concepts of the circular economy (Feodorov et al. 2022). The implementation of in-house composting will also reduce the transportation and landfill disposal costs since it will significantly reduce the volume of waste driven to the landfill (Ayilara et al. 2020).

Table 5 Composition of MSW in Greece (source: ESDA 2018) compared to that in the Kara Tepe RIC
Full size table

Physicochemical characteristics of the biowaste

The K1 and K2 composter bins both had a volume of 0.3 m3. The measured physicochemical characteristics of the feedstock included the pH, moisture content, carbon (C), total Kjeldahl nitrogen (TKN) and carbon-to-nitrogen ratio (C/N), and the results are presented in Table 6. The initial C/N ratio was not adjusted as the scope of this research was to monitor the composting procedure without any intervention. However, the C/N ratio is crucial considering the dependency of nutrient acquisition by microorganisms on this ratio, and it thus affects the whole process. The optimal range of the ratio is 25–35, but composting can occur at lower or higher values, with nitrogen losses from volatilization or a slow pace of evolution, respectively (Daskaloudis et al. 2021). In this research, the C/N values of the initial mixed material of the composters were 23.8 and 25.94 for K1 and K2, respectively. This slightly higher value for K2 can be attributed to the addition of a two-phase olive pomace with a C/N value equal to 64.20. Siles-Castellano et al. (2020) also state that, during their composting trials, the C/N value of olive mill waste was higher than those of other types of agricultural waste, such as culled tomatoes and citric sludge. The same study concludes that the composting of materials with a C/N ratio as low as 17 can result in a stable and mature compost.

Table 6 Initial physicochemical characteristics of the feedstock materials and the K1 and K2 piles
Full size table

During the composting process, water facilitates the degradation of soluble substances and hydrolytic microorganisms, both of which are essential for the evolution of the process. On the other hand, excessive water can occupy air spaces, resulting in decreased oxygen availability for aerobic flora (Wang et al. 2015). Moisture levels of the initial materials in the present study were very low for olive branches, intermediate for chicken and two-phase olive pomace, and high for beans and potatoes (9.99%, 57.44, 51.42, 83.78% and 82.75%, respectively). Due to the high water content of food waste, the branches were included to adjust the moisture values in the composters. The initial moisture values in K1 and K2 were 72% and 75%, respectively, which are considered to be high for an effective composting process and resulted in a delay in the increase of the temperature. Similar findings were reported for another study, where composting cornstalks saturated with anaerobic digestate demonstrated a dynamic temperature rise when the moisture was 60% rather than 76%; nevertheless, the compost was not affected significantly (Xu et al. 2020).

Determination and monitoring of the parameters that affect the composting process

Temperature

The composting process is exothermic and comprises four phases. In the first phase (mesophilic), the temperature rises rapidly to 40 °C, indicating the beginning of decomposition. Then the temperature continues to rise to 70 °C; this is considered to be the second phase (thermophilic phase: 45–70 °C), where polysaccharides, proteins and fats are decomposed by thermophilic microorganisms. In the third phase (maturation), the temperature is maintained at 35–45 °C. Recalcitrant compounds are degraded by mesophilic microorganisms, which start to proliferate again. Lastly, during the fourth phase (curing), the produced compost is stabilized and the temperature value is near the ambient temperature value, given that the activity of the microorganisms has ceased (Finore et al. 2023; Onwosi et al. 2017).

In this study, the temperatures in both K1 and K2 rose sharply during the first few days and the process moved on to the thermophilic phase. However, temperatures in K2 were higher than those in K1. K1 reached the thermophilic phase after almost 1 month, and the maximum temperature, 50 °C, was observed after the first 34 days. The temperature remained between 45 and 50 °C for the next 20 days. The thermophilic stage was established in K2 during the first 3 days and ended almost 40 days later with a maximum temperature of 57 °C. During the composting of two-phase olive pomace, the temperatures reached over 70 °C (Cayuela et al. 2010), so higher temperatures were present in K2. However, the use of aeration in this experiment could have kept the temperature at low levels in both composters. The average temperature for the first 60 days of the experimental period was 43.5 °C and 46.4 for K1 and K2, respectively (Figs. 3, 4). According to Onwosi et al. (2017), optimum composting occurs in the range 40–65 °C. When temperature value decreased to near to 40 °C, water was added to the pile, resulting in a temperature rise within 24 h. Water was introduced into both piles during the experiment as moisture decreased in the piles, resulting in reduced microbial activity (Wang et al. 2015). The addition of the two-phase olive pomace to K2 increased the humidity and resulted in less of a need for water addition. Also, K2 was closer to the typical temperature profile (Sánchez et al. 2017), as it reached higher temperatures in the thermophilic stage than K1 did.

Fig. 3
figure 3

Variation in the temperature during composting in K1 (samples were taken from points at different depths)

Full size image
Fig. 4
figure 4

Variation in the temperature during composting in the K2 bin (samples were taken from points at different depths)

Full size image

Aeration

A deficiency of oxygen during composting can cause a low decomposition rate and low temperature values. Oxygen levels of 15–20% in composting piles are considered efficient (Vilela et al. 2022). In piles K1 and K2, the initial oxygen values recorded were low due to the high activity of the microorganisms in the initial stages (Ekinci et al. 2006; Onwosi et al. 2017). The moisture content of food waste is high and, as a result, spaces are occupied by water and the remaining oxygen is quickly depleted (Cerda et al. 2017). However, in the present experimental design, a bulking agent was introduced, which created voids in the mass of material, allowing oxygen to circulate (Chang and Chen 2010). After the first 20 days, when the oxygen levels were below 15%, oxygen remained relatively constant and in the optimal range during composting with the active aeration system; it only changed when the pile was stirred or water was added. Upon adding water, a temperature increase was noticed, while the oxygen content slightly decreased. The latter can be attributed to an increase in moisture, which promoted microbial activity (Zhang et al. 2019). Thus, the rise in temperature can be attributed to the decomposition of organic matter. Oxygen is consumed during the decomposition or/and water fills the free air spaces, decreasing the available oxygen (Onwosi et al. 2017; Haug 1993) (see Figs. 5, 6).

Fig. 5
figure 5

Oxygen values for K1, along with temperature measurements, the change in aeration and the points at which watering was performed

Full size image
Fig. 6
figure 6

Oxygen values for K2, along with temperature measurements, the change in aeration and the points at which watering was performed

Full size image

Moisture content

The moisture contents of the food residues and two-phase olive pomace used in K1 and K2 were measured at initialization and found to be high (more than 70%). However, due to the extended aeration and stirring of the piles as well as the increased pile temperatures, the moisture evaporated (Adhikari et al. 2008). At the beginning (see Table 7), the moisture values in both piles were high, but the moisture decreased faster in K2, possibly due to the fact that higher temperature values were observed, resulting in higher degradation rates and higher water evaporation (Xu et al. 2020) than in K1. Moisture values in the K1 pile were maintained at satisfactory levels throughout the process and decreased in weeks 8–10, prolonging the whole process (Wang et al. 2015). The total volume of water added to the piles to maintain moisture levels during composting was 114 L for the K1 pile and 102 L for the K2 pile. However, the moisture values in K1 and K2 were rarely found to be in the optimum range of 50–60% (Agnew and Leonard 2003). The different volumes of water added to each composting bin are related to the difference in the compositions of K1 and K2. It was observed that K2 retained its moisture more efficiently, which can possibly be explained by the two-phase olive pomace that was used.

Table 7 Moisture contents (in %)
Full size table

pH

pH is among the most important parameters which affect the compost quality. The pH of K1 and K2 was 4.9 on day 1 of the experiment. After 20 days in the thermophilic phase, the pH in K1 had remained constant, whereas an increase was observed for K2, as indicated in Figs. 7 and 8. In both piles, the pH reached high values (in the range of 8–9), whereas the values for the final composts were 9.4 for K1 and 9.5 for K2 (Figs. 7 and. 8). There were similar findings for food waste composted with sawdust, where the initial food waste had a pH of 5.1, the sawdust had a pH of around 5.25, and the final compost had a pH of 8.9–9.1 (Lee et al. 2017). According to Tognetti et al. (2007), these values may be related to the extended thermophilic phase, which favors the ammonification of organic nitrogen. An alkaline pH has a positive effect in terms of limiting the availability of heavy metals (Tiquia et al. 1996), but it can cause micronutrient deficiencies (Rosen et al. 1993) and ammonia evaporation (Pagans et al. 2006). It also favors microbial activity and the decomposition of organic compounds (Zhang and Sun 2016). High pH values in the final product of the compositing process make it unsuitable for application in agriculture (crops) (Waqas et al. 2017).

Fig. 7
figure 7

pH and temperature variations for K1

Full size image
Fig. 8
figure 8

pH and temperature variations for K2

Full size image

C/N ratio

The C/N ratio values of the initial pile materials are presented in Fig. 9, and the highest ratio was observed for the two-phase olive pomace (64.20) due to its high lignocellulosic content (Podgornik et al. 2022; Rashid et al. 2021). C/N was measured at the beginning of the experimental period, at the end of the first and second months, and at the end of the experimental period. After the first month, the C/N ratio values were 14.51 and 17.41 for K1 and K2, respectively. The C/N ratio was higher in K2 since it contained two-phase olive pomace (Fig. 9). Lower values were observed in August, possibly because a greater percentage of the carbon was broken down through the thermophilic phase, when the microorganisms were most active. The values decreased further at the end of the process, as the material was stabilized and most of the carbon reserves had been depleted. The decreasing profile of the C/N ratio is similar to the findings of Awasthi et al. (2017), Cerda et al. (2017), El Fels et al. (2014), and Komilis and Tziouvaras (2009). On the other hand, it has been reported that when the degradation rate is low, the C/N ratio increases, and when the optimal conditions are met, it starts to decrease (Awasthi et al. 2017; Daskaloudis and Lekkas 2021). According to El Fels et al. (2014), the C/N ratio can be used as an indicator of maturity, while composts with a value of 10 or lower can be considered mature. However, the use of the C/N ratio as the sole indicator alone is not sufficient, since composts with a high C/N ratio use up soil nitrogen, making it unavailable. On the contrary, the produced composts can provide nitrogen to plants and are therefore considered soil amendments (Diaz and Bertoldi 2007).

Fig. 9
figure 9

C/N ratios for K1 and K2 from June to October 2019

Full size image

Phytotoxicity

The GI values in the analyzed samples from K1 and K2 were 75.7% and 102.5%, respectively. Tiquia et al. (1996) and Kazamias et al. (2017) consider samples with a phytotoxicity rate of greater than 80% to be non-phytotoxic. However, Komilis and Tziouvaras (2009) reported that materials with phytotoxicity values such as 50% can be used, as there is no widely accepted limit. If composts with GI values higher than 80% are considered non-phytotoxic, only the final product from pile K2 meets this criterion. Daskaloudis and Lekkas (2020) obtained similar findings when they composted food waste with olive mill waste, and they noted that composting olive mill waste enhances the fertilizing value of the derived compost. In addition, GI is characterized by both the percentage of germination and the speed with which the seeds germinate (Renáta Talská et al. 2020). Therefore, the >100% germination percentage for K2 indicates that more seeds germinated than in the blank sample that only used deionized water.

Microbial respiratory activity

Oxygen consumption by microbial respiratory activity was quantified through the total oxygen consumption using the cumulative respiration index (CRI). Samples were taken from the final products, and the measurements presented the cumulative consumption for 4 days, as presented in Table 8. Lower oxygen consumption was observed in pile K1 compared to K2, which is expected due to their different compositions (Adani et al. 2004). Figure 10 presents the pressure fluctuation and the decrease in pressure observed due to oxygen consumption by microorganisms. The overall decrease in oxygen uptake is a result of the reduction in the available organic matter caused by the decomposition activity of microorganisms (Godley et al. 2004; Gómez et al. 2005).

Table 8 Oxygen consumption values after treatment
Full size table
Fig. 10
figure 10

Curves of the pressure (hPa) in the manometric cylinder for the K1 and K2 composter piles (values were recorded every 16 min)

Full size image

The stability of the compost is affected by the total consumption of oxygen. As discussed by Komilis and Tziouvaras (2009), in compost characterized as stable, the oxygen consumption should be lower than 10 g O2/kg after 4 days, and the compost from both piles (K1 and K2) meet this criterion (Table 8).

Electrical conductivity (EC)

Pile K1 was characterized by slight salinity, as its conductivity values ranged between 4 and 8 mS/cm, while the K2 pile was characterized by minimal salinity, with conductivity values between 2 and 4 mS/cm (see Tables 9 and 10). When a compost exhibits increased conductivity values during the composting process, there are two possible explanations related to biodegradation by microorganisms. A high concentration of inorganic salts such as phosphates and ammonium ions can cause this increase (Gao et al. 2009), as can the slow decomposition of organic matter, which causes the slow release of metal salts (Alburquerque et al. 2006; Barberis and Nappi 1996). On the contrary, decreased values are related to increases in the concentrations of nutrients such as nitrates and nitrites (Pathak et al. 2012).

Table 9 Soil characterization based on its electrical conductivity
Full size table
Table 10 Electrical conductivity of the final product
Full size table

The lower conductivity value of K2 can be explained by the higher temperature that occurred earlier during the experimental period and for a longer time than in K1. High temperatures enhance the activity of microorganisms and the decomposition of organic matter, resulting in soluble salt production, which could have been precipitated subsequently during the stabilization phase. Composts with EC values lower than 4 mS/cm are appropriate for soil application, as they have minor to no adverse effects on the plants, as described in Table 9 (Zhang and Sun 2016, Sangamithirai et al. 2015).

The ability of plants to tolerate environmental stressors such as high salt levels in the soil is known as salt tolerance. The preferred concentration of soluble salts varies depending on the species of plant, taking into consideration factors like irrigation water quality and soil type. For instance, salt-sensitive crops like strawberries and lettuce can be significantly affected and even killed by conductivity values ranging from 2 to 4 mS/cm. On the other hand, salt-tolerant crops like wheat and rye can withstand higher values, up to 7 mS/cm. When applying composts to the soil, it is important to consider their impact on the electrical conductivity, soluble salt composition, and characteristics of the soil. Different composts can have varying effects on these factors. Therefore, the ideal compost to use depends on the specific crop being grown and the characteristics of the soil it will be applied to (Gondek et al. 2020).

Understanding the mass balance and compost production

Mass and water balance

Composting has been a proven practice for several years now and has been applied in full-scale operational systems. Commercial composting facilities need to be financial sustainable (Xing et al. 2022), and they usually significantly rely on the gate fee for the material they receive. However, they work under predefined specifications and a predefined process time and cost in order to deliver a market-oriented end-product: the compost. Thus, we collected mass and water balance data in order to understand and estimate the operational cost of the composting process and gain insight into the overall procedure and its operational parameters. We were particularly interested in the water usage, which is considered an important issue with regard to climate change and water availability (United States Environmental Protection Agency (EPA) 2023).

The initial masses of the materials placed in the two composters K1 and K2 were 217.7 kg and 217.03 kg (the initial material: i.m.), respectively, while the end product (the final material: f.m.)—the produced compost—weighed 18.5 kg for K1 and 32.3 kg for K2. During the composting process, significant amounts of water were consumed; this water was either absorbed by the microorganisms or escaped from the composters through evaporation. The total water loss was 70% for both composters. In addition, as expected, dry matter was reduced through the degradation of microorganisms and consequently the presence of carbon in the compost was reduced. A systematic analysis of the mass balance was performed for both experiments and is presented in Table 11 and graphically in Fig. 11 for K1 and K2.

Table 11 Mass, water, dry mass and carbon balance for composter piles K1 and K2
Full size table
Fig. 11
figure 11

Mass and water balances for the K1 and K2 composters

Full size image

Energy consumption

In the composting process, active aeration was introduced (as described in the "Experimental composting setup" section) to maintain the oxygen level and provide oxygen to the aerobic microorganisms. The operational pattern of the aeration system was adjusted according to the composting phases and the requirements of the processes. Two periods were defined (Fig. 12): the first 60 days (months 1 and 2), when the blower ran for 2 min per 12 h and consumed 2.6 kWh; and the next 60 days (months 3 and 4), where its duration of operation was increased to 4 min per 6 h, consuming 10.4 kWh. In total, over the 120 days, the blower consumed 13 kWh, which means 0.0299 kWh per kg of initial material (i.m.) treated (for K1 and K2), as presented in Eq. 6:

$$\frac{{\text{Total kWhr}}}{{{\text{Total amount of i}}{\text{.m}}{.}}} = \frac{{13\;{\text{kWhr}}}}{{484.73 \;{\text{kg}}}} = 0.0299\frac{{{\text{kWhr}}}}{{{\text{kg}}}}.$$
(6)
Fig. 12
figure 12

Energy consumption of the air blower (Bulle 650W) for both K1 and K2

Full size image

Taking into account current electricity prices in Greece, approx. 0.16 €/kWh (June 2023), the electricity cost for the experiment was 2.08 € without fixed charges. If the required 0.0299 kWh/kg is considered, 4.7 € per ton of treated material or 0.0047 €/kg is needed.

Proposed waste management plan

The potential for real-scale application was studied, taking into consideration all the technical and operational parameters of the proposed practice. The daily biowaste production was calculated for May 2019 (the period with the highest population) and was found to be 0.5 tn/day; this amount requires three composters of size 0.3 m3 or two composters of size 0.4 m3 for treatment. Hence, a household composter—like those used in the experiments—can treat about 0.2 tn. Therefore, for the total annual biowaste production of 182.5 tn, approximately 913 home composters are needed. Since the composting period lasts 120 days, one composter can be used three times, so approximately 304 home composters are needed. According to the data presented in Table 13, the total cost of electricity and water for the on-site home composting of all food residues in a RIC hosting 1355 residences is no higher than 3.36 €/day.

Table 13 Cost estimation
Full size table

Conclusions

Food waste treatment is critical to meeting EU and national targets for diversion from landfills. This study investigates the feasibility of on-site food waste management in small communities, such as the RIC of Kara Tepe in Lesvos. It is well known that waste production has a strong correlation to seasonality and population accumulation, so a sampling methodology was designed for obtaining representative samples and therefore valid results. In addition, a tailored active aeration system was introduced in home composters to upgrade their performance and minimize treatment requirements. Subsequently, while the composting process evolved according to the typical profile, the required conditions for sanitization were not attained, as the temperature did not exceed 55 °C for the required period of sanitization. On the other hand, the respiration assays used to evaluate the stability of the compost produced from the piles tested demonstrated values near to 8 gO2/kg, indicating stable compost. Regarding phytotoxicity, both trials met the requirements set in the literature, especially the K2 setup with 102% GI, meaning that they could be used in agriculture as a fertilizer or soil conditioner. The mean values of EC and high pH values of the compost may pose a risk if applied in cultivations. Future research will focus on the achievement of higher temperature values and how this affects important compost parameters like stability, maturity, pH, EC, etc., in order to produce a high-value-added product and close the nutrient cycle in the food and olive oil production sector.

As there are significant differences between insular and urban areas with respect to both waste generation and waste management and treatment, the findings of the current research can serve as a stepping stone and as a foundational guide for islands, remote areas or displaced communities located in temporary settlements. The specific characteristics of the selected areas should always be considered when planning and developing optimal waste management that follows the principles of reduction, reuse and recycling while keeping the prerequisite of proximity—treating wastes as close as possible to the source (e.g., on-site composting). Therefore, the proposed methodology is feasible to exploit in similar-sized communities—villages or even a block of flats—for the on-site treatment and valorization of food residues, since it can provide a good-quality compost at a reasonable cost. Moreover, as migration numbers rise yearly, the findings of this research can also contribute to the development of global guidelines. These guidelines aim to ensure the protection and provision of essential resources like food, water and sanitation for migrants or displaced individuals in camps or even third-world countries. By incorporating a mechanism that considers the variations in food waste among different regions, national and international organizations responsible for managing these kind of communities can benefit from similar practices.

In addition, an analysis of the operational cost of the proposed treatment process was performed to estimate the energy and water consumption. The findings indicate that 304 home composters can cover the requirements of a population of this size (approximately 1300 people), at a cost of a mere 3.36 €/day in total. Therefore, we propose the on-site treatment of food residues, which constitute the majority of the separated biowaste both in the RIC and Greece. In-house composting can help to reduce the biowaste coming from the center and at the same time encourage people to treat food residues while producing a valuable end product that can be used to grow fruits and vegetables, demonstrating the concepts of circular economy. The implementation of in-house composting will also reduce transportation and landfill disposal costs, since it will significantly reduce the volume of waste driven to the landfill. By minimizing the amount of waste sent to landfills, we can also effectively decrease greenhouse gas emissions related to transportation and the methane emissions from landfills.