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Article

Quantity and Material Composition of Foreign Bodies in Bio-Waste Collected in Towns from Single- and Multi-Family Housing and in Rural Areas

by
Wojciech Dronia
1,2,*,
Jacek Połomka
2 and
Andrzej Jędrczak
3
1
The Doctoral School of Exact and Technical Sciences, University of Zielona Góra, 65-417 Zielona Góra, Poland
2
Łużyckie Centrum Recyklingu, Municipal Waste Plant in Marszów, 68-200 Marszów, Poland
3
Institute of Environmental Engineering, University of Zielona Góra, 65-417 Zielona Góra, Poland
*
Author to whom correspondence should be addressed.
Energies 2024, 17(17), 4350; https://doi.org/10.3390/en17174350
Submission received: 21 July 2024 / Revised: 18 August 2024 / Accepted: 28 August 2024 / Published: 30 August 2024
(This article belongs to the Special Issue Waste-to-Energy and the Circular Economy: Towards Sustainable Resource Management)
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Abstract

:
There is a general consensus that bio-waste is a suitable material for valorization by means of the fermentation process with the production of biogas. The success of a bio-waste closed-loop economy will ultimately be determined by the demand for the products made from it. Poor-quality composts and fermentation products will not be allowed on the market in the long term. This means that not only final products but also bio-waste from separate collections must also meet the quality requirements. The aim of this 12-month study was a monthly analysis determining the level of contaminants in bio-waste collected from rural communities, single-family neighborhoods in urban areas, and multi-family neighborhoods in urban areas. The share of contaminants in bio-waste from rural areas and single-family urban housing averaged 8.2% and 7.2%, respectively, while multi-family urban housing had a significantly higher average of 16.6%. The primary contaminants identified were treated wood, plastics, mineral wastes, paper, and glass in rural areas and plastics, paper, treated wood, glass, and textiles in urban areas. The close positive correlation found between the total content of pollutants and, in particular, with plastics and kitchen waste in bio-waste collected in rural communities and from multi-family housing in cities indicates that they are likely the main source of the origin of these pollutants.

1. Introduction

Bio-waste accounts for approximately one-third of the municipal waste mass generated in the EU [1]. The revised EU Waste Framework Directive (WFD) enforces the avoidance of bio-waste generation in EU Member States [2]. Furthermore, it specifies that any unavoidable bio-waste must be separately collected by Member States. This bio-waste should then undergo recycling processes, including biological treatments. These treatments can occur under aerobic conditions, known as composting, or anaerobic conditions, referred to as digesting. The goal is to ensure that bio-waste is managed in an environmentally friendly manner, minimizing its impact on the environment and maximizing resource recovery.
Both processes produce products—compost and digestate, respectively—that can be applied to agricultural land to improve its fertility [3,4]. Fermentation provides the added benefit of extracting useful energy from bio-waste in the form of biogas [5]. Doing so facilitates an economy with a more closed-loop use of resources.
The collected bio-waste, even if correctly sorted at the source, contains various contaminants (the term “contaminants” means physical impurities in bio-waste or in compost and other organic products derived from the recycling of bio-waste that are undesirable materials in the treatment process and/or impair the purity, potential use, and market value of these products) that can affect the quality of the end compost or digestate product [6]. The presence of pollutants in bio-waste generally results from the residents not always having a good understanding of the objectives of selective collection or conducting the collection process in a proper way [6]. Waste materials, which are generally inappropriately placed in the bio-waste stream, include glass, metals, and plastics (hard and soft/flexible) but can also include stones, debris, and others [7]. The contaminant content of the bio-waste studied by Lopes et al. [7] ranged from 6 to 10% dry matter (dm), (with plastics being the largest contributor). According to Silvestre et al. [8] and LUBW [9], the percentage of contaminants in source-separated bio-waste ranges from 2 to 18% wet weight (w/w). In Sweden, the contaminant content rate of separately collected kitchen waste from areas with multi-family housing was 1–5% w/w (excluding the weight of used bags for bio-waste collection) [10]. Household food waste and garden waste collected in Catalonia (Spain) contained between 0.13 and 30.2% of contaminants (average 10.7%) [11]. The authors showed a significant positive relationship between non-compostable materials entering the processing plant and the heavy metal content (zinc, copper, lead) in the compost produced.
Moretti [12] determined the content of undesirable components in four types of bio-waste generated in the city of Lyon (France) as potential raw materials for fermentation: garden bio-waste, restaurant bio-waste, household bio-waste, and supermarket bio-waste. Garden bio-waste contained by far the highest amount of inorganic inert materials—30.7% dm—while for other bio-waste, the share of inert materials was about 3% dm in bio-waste from households and supermarkets and nearly 5% dm in bio-waste from restaurants. The inorganic inert materials present in garden waste include particles of sand, gravel, or stones. Plastics were most abundant in bio-waste from supermarkets (39.0% dm). Household bio-waste also contained a large quantity of plastics (6.1% dm).
A study conducted for the New South Wales EPA in 2016–2017 (Australia) assessed the quality of kitchen and garden waste collected in a curbside system with bin sizes ranging from 120 to 240 dm3, collected weekly and fortnightly. It was found that the proportion of contaminants in the collected bio-waste varied over a wide range from 0.04 to 17.8% (average 2.6%) and that the size of the container had a significant impact on this proportion. The most common contaminants were plastics, metals, packaged food (including glass and plastic containers), other organics (e.g., leather, rubber, oils), and various bagged materials, as well as household items [13].
The reasons for the large differences in the contaminant content of bio-waste can be attributed to the different collection patterns of bio-waste. Bio-waste is collected either through a door-to-door system or via a “drop-off” method using communal containers. Garden waste is collected together with household kitchen waste or separately. The variability of the generation of particular types of waste, like seasonal (e.g., higher share of garden waste in summer) and regional (e.g., industrial, agricultural, or tourist regions) variability, is also an important factor [7,14].
Contaminants contained in bio-waste may pose problems in the treatment of bio-waste, especially in the case of fermentation. They can lead to the failure of mechanical equipment, the abrasion and clogging of pipes, and the formation of sludge at the bottom of the digester [15,16,17]. A reduction in digester volume due to sludge deposition negatively affects process efficiency and increases plant operating costs [18,19,20,21].
Contaminants present in bio-waste reduce the high quality of the end product produced (compost or digestate) and restrict further use and commercialization as a compost or liquid fertilizer [17,22]. This is determined by considerations such as:
  • Visual (aesthetic considerations, which are subjective and as such extremely difficult to quantify; for example, small but clearly visible pieces of plastic are a major problem when using organic products for horticultural purposes);
  • Health and safety (for example, the presence of sharp pieces of glass in final products).
In 2019, the EU introduced the Fertilizers Regulation, also focusing on recycled and organic materials [23]. In the Regulation, the sum of contaminants > 2 mm (e.g., glass, metals, plastics) must not exceed 0.5% dm. In addition, the allowable concentration of any impurity > 2 mm must not be higher than 0.3% dm. Stones were omitted from the standardized group of pollutants due to the fact that they pose no threat to the environment or human health.
Information from Germany suggests that a large proportion of the foreign matter contained in bio-waste can be removed by sieving the compost during final product preparation. As a rule, a yield of approximately 95% by weight means that if the collected bio-waste (kitchen waste from households and garden waste) contains 1% contaminants, the sieved and cleaned compost will contain about 0.15% (by weight) of contaminants(the calculations take into account, among other things, losses due to the decomposition of organics and the fact that foreign matter in bio-waste is given as a percentage of fresh matter and in the final product as a percentage of dry matter) [24].
The removal of contaminants from bio-waste is therefore essential, especially before the digestion process [25], but is costly and has its limitations. In addition to small mesh sieves, more advanced technologies such as air separators, magnetic separators, or hard material separators can be used.
The aim of this work was to evaluate the level of contamination of selectively collected biowaste depending on the source of the waste (rural areas, cities, type of housing) and the container used for collection. It was also to determine whether changes in the values of the parameters determined are related to seasonal variations in their composition. Correlations between the contents of individual pollutant components and kitchen and garden bio-waste per capita were also assessed to get a better understanding of the interrelationships and sources of some pollutants.
Knowing the composition of biowaste, including its content of contaminants, is needed to optimize the conditions for managing biowaste at the local, regional, and/or national levels and to plan, design, and optimize the service of collecting and treating the waste [26]. Furthermore, in any waste treatment process, the characteristics of the raw material are important in determining the design and operating parameters of the plant [27].

2. Materials and Methods

2.1. Categories of Waste Analyzed

The research covered selectively collected bio-waste delivered to the municipal facility of the Łużyckie Centrum Recyklingu (ŁCR) in Marszów. Samples of bio-waste were collected on a monthly cycle for 12 months. The study included:
  • Four rural municipalities with single-family housing: Żary (12,366 inhabitants), Żagań (7178), Trzebiel (5595), and Wymiarki (2070);
  • Four cities: Żary (35,411), Żagań (2546), Gozdnica (2780), and Łęknica (2300).
In urban areas, the study examined bio-waste collected from neighborhoods with single-family housing (T-SH) and multi-family housing (T-MH). A total of 12 bio-waste streams were analyzed. The bio-waste collection was conducted door-to-door. In single-family homes, whether in villages or towns, bio-waste and other selective fractions are collected in bags. In contrast, in multi-family districts of towns, bio-waste is collected in plastic containers with a capacity of 1100 dm3 or in bulk containers up to 7000 dm3, which are located in gazebos near the buildings.

2.2. Waste Collection Methods

Sampling and analysis of waste were performed according to the procedures of the Central Laboratory of the Institute of Environmental Engineering at the University of Zielona Góra. Until January 2027, the laboratory is accredited for waste sampling and testing [28].
The total bio-waste sample consisted of:
  • Waste contained in 10 randomly chosen bags from each of the 4 rural communes and 4 towns with single-family neighborhoods;
  • Waste taken in quantities of approximately 10–15 kg from each of 10 randomly selected bins in neighborhoods with multi-family housing in each of the 4 cities.
Samples of bio-waste from rural communities and single-family homes in urban areas were collected after they were delivered to the facility and vehicles were unloaded at the bio-waste drop zone. From the batch of bio-waste delivered from each of the 4 rural and 4 urban communities, the working group randomly selected 3 sets of 10 bags. The bio-waste from the set of 10 bags constituted the waste sample for analysis. The bio-waste bags were weighed, and their contents were emptied onto a sorting table for compositional analysis. Bio-waste samples from urban multi-family dwellings were collected at the collection site from three randomly selected sets of 10 containers with a capacity of 1100 dm3 in each of the 4 cities where the study was conducted. On the day of sampling, bio-waste weighing 10–15 kg was collected from the selected containers (the same as every month) into an empty 1100 dm3 container. The bio-waste collected from a set of 10 containers constituted the sample for analysis. Three containers of bio-waste of at least 100 kg were delivered from each city to the plant for compositional analysis.

2.3. Testing Frequency and Scope

Monthly bio-waste surveys were conducted from November 2021 to October 2022. The scope of these surveys included a morphological analysis of the bio-waste. The compositional analysis methodology involved manually sorting the mixture into specific bio-waste components until the maximum particle size of the remaining waste was less than 10 mm. The bio-waste was classified into three main categories: food waste, garden waste, and impurities. Food waste was further divided into edible and non-edible subcategories, with six distinct types of waste identified. In the garden waste category, there were four subcategories, and in the impurities category, there were four subcategories and eleven types of undesirable constituents. The contamination category excluded bags used for collecting bio-waste from single-family households, which were classified separately. In the bio-waste samples collected on 24 January, 14 April, 25 June, and 20 September 2022, moisture content and roasting losses of the constituents were measured, along with the morphological composition of the fraction smaller than 10 mm.
Approximately 2 kg of each sorted component was used to determine moisture content and roasting loss, and for components present in less than 2% of the bio-waste, the entire separated amount was analyzed. The analyses were conducted according to the procedures established by the accredited Central Laboratory of the Institute of Environmental Engineering.
The morphological composition was determined in the whole mass of the <10 mm fraction separated from the bio-waste if it was less than 2 kg or in a sample of approximately 2 kg separated from the <10 mm fraction by quartering. Organic biodegradable components, paper, plastics, metals, glass, textiles, processed wood, stones, and debris were sorted from the sample > 2 mm. Other waste > 2 mm and residue < 2 mm were treated as mineral waste.
The waste samples sent to the laboratory were dried at 105 °C and subsequently ground using a knife mill. The concentration of volatile solids (VS) was assessed by calculating the weight loss after the samples were incinerated at 550 °C for 12 h.

3. Results

Figure 1 illustrates the average annual material composition of separately collected bio-waste in urban and rural areas from November 2021 to October 2022. The share of kitchen waste (edible and inedible food waste) in the bio-waste collected from rural areas and from areas with multi-family housing in cities was similar (47.4 ± 17.1 and 47.6 ± 16.5%, respectively) and significantly higher than in the bio-waste from areas with single-family housing in cities (36.6 ± 18.4%). Edible food waste accounted for between 4.9 (T-SH) and 8.4% (RM) by weight of bio-waste (Figure 1).
The share of garden waste (grass, leaves, branches, other) in bio-waste from rural areas averaged 40.4 ± 17.5%, and that in bio-waste from urban areas averaged 35.8 ± 17.3% (multi-family housing) and 52.3 ± 18.4% (single-family housing) (Figure 1). The large proportion of garden waste in bio-waste from single-family housing in cities is understandable, as these homes typically have small gardens that are often used as lawns, flower gardens, orchards, and flowerbeds.
Contaminants made up an annual average of 8.2 ± 4.0% of the weight of bio-waste collected in rural areas, not including the bags used for collection (Figure 1). The weight of these bags, after being manually emptied, accounted for 4.0 ± 1.3% of the bio-waste weight. In rural bio-waste, the most significant fractions were particles smaller than 10 mm (3.0 ± 3.1% of its weight), processed wood (2.1 ± 3.0%), and plastics (excluding collection bags) at 1.3 ± 0.5%.
The proportion of impurities in bio-waste from single-family dwellings in urban areas averaged 7.2 ± 2.6% (excluding collection bags), while the bags themselves accounted for 3.9 ± 0.7%. Of the contaminants in bio-waste, the <10 mm fraction (2.9 ± 3.0% of bio-waste weight) and bagless plastics (1.5 ± 0.6%) were the most prevalent. In bio-waste from multi-family housing in cities, which is collected in 1100 dm3 plastic containers or bulk containers, the proportion of contaminants averaged 16.6 ± 6.0%. Plastics were the most prevalent contaminant, making up 6.3 ± 1.2% of the bio-waste by weight, despite the use of containers for collection.
The main component of the <10 mm fraction was biodegradable organic waste > 2 mm (T-SH: 42.7 ± 5.5%, RM: 42.5 ± 13.5% and T-MH: 58.3 ± 2.2%) (Figure 2). The proportion of contaminants > 2 mm in the <10 mm fraction was low. They represented, on average, 1.10 ± 0.62% of the mass fraction from bio-waste collected in cities from single-family housing, 2.20 ± 0.60% from multi-family housing, and 0.92 ± 0.21 of collected bio-waste in rural areas.
The Supplementary Materials include data on the moisture content and roasting losses of the <10 mm fraction. This fraction was separated from bio-waste collected in bags from rural municipalities and towns with single-family housing, as well as from containers in towns with multi-family housing.
The moisture content of the <10 mm fraction ranged from 24.3% (summer—T-SH) to 43.9% (summer—T-MH). The average moisture content of bio-waste collected from single-family housing in both rural and urban areas was similar, at 33% and 30%, respectively. This was approximately 10 percentage points lower than the moisture content of bio-waste collected from multi-family buildings in urban areas. The dry matter content of bio-waste components can be affected by the type of waste and the amount of rainfall the waste is exposed to during collection and sampling.
The roasting losses of the <10 mm fraction of bio-waste collected from single-family dwellings in rural and urban areas were, on average, 34 and 36% dm, respectively, and those of bio-waste collected from multi-family buildings in urban areas were around 41% dm (from 24 (spring) to 62% dm (winter)). The results show that mineral impurities in the <10 mm fraction account for between 60 and 66% of their dry matter.
The proportion and material composition of contaminants present in the bio-waste varied significantly from month to month between November 2021 and October 2022 (Figure 3). The observed trends in these changes reveal some interesting patterns:
  • Bio-waste from multi-family dwellings (gathered in 1100 dm3 containers or containers) was the most polluted; the share of contaminants in their weight was less than 10% in only one month (January) and as much as 20% in four months. The proportion of contaminants in bio-waste collected in rural areas >10% occurred in 4 months and in bio-waste from single-family housing in 2 months. The quality of bio-waste is a key issue: bio-waste containing more than 10% contaminants makes valorization unlikely;
  • Bio-waste collected in November and between April and June was clearly more contaminated than in the other months;
  • Mineral waste in greatest quantity occurred during the months of November and December in all areas;
  • In the group of components defined as bio-waste contaminants, plastics and the fraction < 10 mm were present in the largest quantities;
  • In bio-waste from single-family homes in urban and rural areas, the shares of paper, metals, glass (except for the sample taken from rural municipalities in March), debris, and stones >10 mm (except for the months of November and December) were less than 1% w/w.

4. Discussion

4.1. Amount and Material Composition of Bio-Waste and Its Impurities

Table 1 displays the average figures for annual quantities of bio-waste generated and impurities contained per capita in the controlled areas and the proportions of impurities in bio-waste, by weight.
From November 2021 to October 2022, rural residents collected an average of 2.0 ± 0.8 kg/(C∙month) of bio-waste. Urban residents in single-family neighborhoods collected 7.3 ± 2.8 kg/(C∙month), whereas those in multi-family neighborhoods collected 3.5 ± 1.4 kg/(C∙month). The total amount of impurities in the bio-waste collected was 0.24 ± 0.14 kg/(C∙month) in rural areas, 0.83 ± 0.48 kg/(C∙month) in urban, single-family neighborhoods, and 0.23 ± 0.44 kg/(C∙month) in multi-family neighborhoods. The quantity of contaminants in the bio-waste without the <10 mm fraction was 0.18 ± 0.08, 0.75 ± 0.39, and 0.47 ± 0.33 kg/(C∙month), respectively.
The percentage of impurities in bio-waste from rural areas and urban single-family neighborhoods as a percentage of dry matter was similar at 20.3 ± 10.6 and 20.4 ± 12.1% dm, and in towns with multi-family housing, the percentage was 34.8 ± 11.1% dm. The share of impurities without <10 mm fraction in rural and urban bio-waste in single-family neighborhoods as a % of dry matter was similar at 12.9 ± 11.1 and 14.5 ± 10.4% dm, and in towns with multi-family housing, it was 28.6 ± 11.1% dm.
The proportions of contaminants in the wet weight of bio-waste determined in this study are similar to those determined in household kitchen and garden waste collected in Catalonia (Spain), which contained up to 30.2% of contaminants (average 10.7%) [11]. Similar values have been reported by Silvestre [8] and LUBW [9]. In contrast, the shares of contaminants in the dry weight of biowaste are significantly higher than those reported by Lopes et al. [7] but similar to those determined by Moretti et al. [12] for biowaste generated in the city of Lyon.

4.2. Main Components of Impurities

In the bio-waste studied, all the types of contaminants analyzed were present, as in the bio-waste described in the works of Lopez [7], Silvestre [8], and LUBW [9]. For each controlled area, the five constituents considered to be contaminants with the highest contribution to bio-waste by weight were determined. Two methods were used to assess these components. The first method (M-I) consisted of determining, for the study period, the average value of the monthly mass fraction of constituents considered to be contaminants (% w/w) in the bio-waste and selecting the top five materials with the highest indicator values. The second method (M-II) consisted of determining, for the study period, the average monthly value of the per capita mass of impurities components (kg/(C∙month)) that appeared in the bio-waste and selecting the top five components with the highest indicator values.
Table 2 shows the average monthly quantities in bio-waste of all impurities, excluding the <10 mm fraction and the five main constituents in the controlled areas per capita (kg/(C∙month)) and their weight shares of impurities in bio-waste (% w/w). For each study area, the ranges of values (min. and max.) and the monthly mean value of the parameters are given.
The contaminants found in bio-waste in the highest quantities were:
  • In rural municipalities, the first four components regardless of the method used are: treated wood < plastics < mineral wastes and paper. The fifth component was glass by Method I or textiles by Method II; the five main contaminants accounted for 88% of all contaminants in the Method I assessment and 92% in the Method II assessment;
  • In cities in single-family neighborhoods where bio-waste was collected in plastic bags, the order was, according to Method I, plastics < paper < treated wood < glass < textiles, and according to Method II, plastics < treated wood < mineral wastes (stones, rubble) < paper < glass; the five main impurities accounted for more than 84% of all impurities in method-I and 82% in Method II assessments;
  • In cities in neighborhoods with multi-family housing, where bio-waste was collected in 1100 dm3 containers or in containers, the first two places, regardless of the method used, were taken by plastics < paper, followed by < treated wood < mineral wastes < glass for Method I and < mineral wastes < glass < treated wood for Method II; the five main impurities accounted for 86 and 79% of the total impurities, respectively.
The plastic bags used for collecting bio-waste in rural municipalities and single-family dwellings in urban areas were excluded from the above assessment. When bio-waste bags are added to plastics, they are the main impurities in all controlled study areas, regardless of the method used. In both rural and urban areas with single-family housing, the amount of treated wood was unexpectedly high. This was the result of the misclassification of the waste wood processed, probably due to low awareness among some residents. In urban, single-family, and multi-family neighborhoods (Method I assessment), the first two positions were occupied by plastics and paper, mainly plastic and paper bags. The high position of plastics is in agreement with the results of other authors. However, no large contribution of glass to contamination was found, as in the work of Lopes [7] and Rawtec [13]. In the Method I assessment, glass appeared fifth or fourth (T-SH) in the group of major contaminants. Conversely, glass was only found in the Method II assessment of bio-waste collected from multi-family housing in towns.
It is essential that plastics are removed from the raw material. If bio-waste is processed for use as fertilizer, there is a real risk that plastics will break down into small particles that will be dispersed into the environment and have a negative impact on ecosystems [29].

4.3. Correlations between the Content of Impurities and the Content of Kitchen and Garden Waste in Bio-Waste

Correlations between the contents of individual impurity components and kitchen and garden bio-waste per capita were assessed in order to get a better assessment of the interrelationships and sources of some impurities (Table 2). They can be applied to other compositions and parameters of bio-waste to configure the treatment of bio-waste accordingly.
Kitchen waste, which is the main component of bio-waste, alongside garden waste, showed very high positive correlations with the total contaminant content of bio-waste collected in rural areas (R2 = 0.62) and in urban areas in neighborhoods with multi-family housing (R2 = 0.68) (Figure 4). No such correlation was found for bio-waste collected in urban areas with single-family housing. Garden waste, the other main component of bio-waste, showed a correlation with the total contaminant content of bio-waste collected in rural areas (R2 = 0.32) and in urban single-family neighborhoods (R2 = 0.35), but it was much weaker than that observed for kitchen waste (Figure 4). Figure 5 shows the correlations between the content of kitchen and garden waste in the bio-waste collected at the controlled sites and the impurity components that showed the highest value of the coefficient of determination (R2) (Table 3).
The best association with the content of kitchen waste in bio-waste from rural municipalities and towns with multi-family dwellings was shown for plastics (R2 = 0.56) and (R2 = 0.63), respectively. The likely reason for such a high correlation between these parameters is the interim collection of wet food waste in bags and plastic bags before it is put into the container (multi-family housing) or into the plastic bag for bio-waste (rural areas). In towns with single-family housing, the highest value of the coefficient of determination was found for paper (R2 = 0.24), while for plastics, it was only 0.056. This suggests that residents of single-family houses in urban areas usually dispose of their kitchen waste directly in the bio-waste bag due to its proximity to its location and that if they temporarily collect kitchen waste in bags, these are bags (pouches) made of paper and not plastic.
In rural areas, kitchen waste showed a high correlation, except for plastic content, with textiles (R2 = 0.39) and multi-component waste (R2 = 0.30). In cities in neighborhoods with multi-family housing, it showed a high correlation with paper (R2 = 0.64), metals (R2 = 0.77), and mineral waste (R2 = 0.57). No significant correlations with any impurity components were found for waste gathered from single-family neighborhoods in urban areas. Garden waste showed high correlations with plastics in bio-waste in rural areas (R2 = 0.29), with metals in bio-waste collected in bio-waste in cities in single-family housing districts (R2 = 0.46), and with glass (R2 = 0.40), textiles (R2 = 0.54), and multi-material waste (R2 = 0.32) in bio-waste in cities in multi-family housing districts.
The strong positive correlation observed between the total impurities, particularly plastics and kitchen waste, in bio-waste collected from rural municipalities and urban multi-family dwellings suggests that these are likely the primary sources of these contaminants. On the other hand, the observed lack of correlation of kitchen and garden waste with recycled wood, glass, metals, and mineral wastes in bio-waste collected in bags in rural municipalities and from single-family dwellings in cities may indicate that their appearance in bio-waste is not related to them. The presence of these contaminants in bio-waste is generally due to residents not always having a good understanding of the objectives of separate collection or carrying out collection in the right way [6].

5. Conclusions

Selectively collected bio-waste sent for recycling should only be of high quality. Only such raw materials can be used to manufacture high-quality products. The analyzed bio-waste showed an unsatisfactory level of purity due to the presence of foreign substances. The proportion of contaminants in bio-waste collected in bags from rural areas and towns with single-family homes did not exceed 10%, with averages of 8.2% and 7.2%, respectively. In urban areas with multi-family housing, the contaminant content was more than twice as high as that in single-family housing (both urban and rural), reaching 16.6%. This higher contamination level may be attributed to the use of bulk bins for collection. Such a system is characterized by the lack of responsibility for incorrect waste segregation by residents of a housing estate or tenement. The percentage of contaminants in bio-waste collected in rural areas ranged from 3.4–14.7%, and the percentage of contaminants in bio-waste collected in urban single-family dwellings ranged from 2.4–20.0% and 9.5–24.0%. Such a high proportion of contaminants may unfortunately disqualify this bio-waste stream for the production of high-quality compost.
The contaminants most commonly found in the highest amounts in the rural area using the % w/w as a criterion were treated wood, plastics < mineral wastes < paper < glass. In urban areas, the contaminants most commonly found in the highest’s amounts in neighborhoods with both single-family and multi-family housing were plastics > paper > treated wood, and the next two positions were glass < textiles (single-family housing) < mineral wastes < glass mineral (multi-family housing). The five main impurities accounted for more than 80% of all impurities in bio-waste without the <10 mm fraction. The close positive correlation found between the total content of impurities and, in particular, plastics and kitchen waste in bio-waste collected in rural municipalities and from multi-family dwellings in urban areas indicates that they are probably the main source of these impurities. Composts that are free or largely free of contaminants are difficult to produce from bio-waste with a foreign matter content of more than 3% by weight, even at great expense.
Foreign bodies are mainly introduced into the bio-waste sacks/containers as a result of incorrect decisions by individual residents or other users. Separate collection systems must be accompanied by continuous public relations activities by public waste management organizations and local authorities. Where these measures are insufficient, waste counseling should be intensified and focus on existing problems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en17174350/s1, Table S1: Scope of bio-waste material analysis title; Table S2: Moisture and roasting losses of fraction < 10 mm separated from bio-waste collected in controlled areas.

Author Contributions

Conceptualization, A.J., W.D. and J.P.; methodology, A.J., W.D. and J.P.; validation, A.J. and W.D.; formal analysis, W.D.; investigation, A.J., W.D. and J.P.; resources, W.D. and J.P.; data curation, A.J.; writing—original draft preparation, W.D. and A.J.; writing—review and editing, A.J. and W.D.; visualization, A.J. and W.D.; supervision, A.J.; project administration, A.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data supporting this study are available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. The average annual material composition of separately collected bio-waste in rural municipalities (RM) and urban households from single-family (T-SH) and multi-family (T-MH) developments, along with the material composition of the impurities they contain.
Figure 1. The average annual material composition of separately collected bio-waste in rural municipalities (RM) and urban households from single-family (T-SH) and multi-family (T-MH) developments, along with the material composition of the impurities they contain.
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Figure 2. Average annual material composition of the <10 mm fraction separated from separately collected bio-waste in rural municipalities (RM) and from urban households from single-family (T-SH) and multi-family (T-MH) developments and the composition of the impurities they contain.
Figure 2. Average annual material composition of the <10 mm fraction separated from separately collected bio-waste in rural municipalities (RM) and from urban households from single-family (T-SH) and multi-family (T-MH) developments and the composition of the impurities they contain.
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Figure 3. Monthly variation in the percentage of components classified as impurities in separately collected biowaste from rural municipalities and urban single-family and multi-family dwellings. Plastics: bags, sacks+ other plastics; Paper: sacks, bags + other paper; Metals: ferrous+ non-ferrous metals.
Figure 3. Monthly variation in the percentage of components classified as impurities in separately collected biowaste from rural municipalities and urban single-family and multi-family dwellings. Plastics: bags, sacks+ other plastics; Paper: sacks, bags + other paper; Metals: ferrous+ non-ferrous metals.
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Figure 4. Correlations between the content of kitchen and garden waste in the bio-waste collected at the controlled sites and the total amount of impurities contained therein.
Figure 4. Correlations between the content of kitchen and garden waste in the bio-waste collected at the controlled sites and the total amount of impurities contained therein.
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Energies 17 04350 g005aEnergies 17 04350 g005b
Figure 5. Correlations between the content of kitchen and garden waste in bio-waste collected at controlled sites and the impurity components showing the highest strength of association.
Figure 5. Correlations between the content of kitchen and garden waste in bio-waste collected at controlled sites and the impurity components showing the highest strength of association.
Energies 17 04350 g005aEnergies 17 04350 g005b
Table 1. Monthly average quantities of bio-waste generated, impurities contained per capita, and share of impurities in bio-waste by weight in controlled areas.
Table 1. Monthly average quantities of bio-waste generated, impurities contained per capita, and share of impurities in bio-waste by weight in controlled areas.
ParameterUMT-SHT-MH
Value RangeAverageStandard DeviationValue RangeAverageStandard DeviationValue RangeAverageStandard Deviation
Quantity of bio-waste generated [kg/(C∙month)]0.74–3.02.00.83.4–11.47.32.81.6–5.83.51.4
Quantity of impurities in bio-waste [kg/(C∙month)]0.091–0.590.240.140.18–2.000.830.480.16–1.50.630.44
Quantity of impurities in bio-waste without <10 mm fraction [kg/(C∙month)]0.065–0.310.180.0780.17–1.50.750.390.12–1.30.470.33
Share of impurities in bio-waste [% w/w]3.4–14.78.23.92.4–20.07.34.89.5–26.316.66.0
[% dm]7.4–41.720.310.67.6–44.120.412.118.3–57.634.811.1
Contaminants share in bio-waste without fraction < 10 mm[% w/w]1.9–11.15.22.81.5–9.54.42.76.8–16.112.64.3
[% dm]1.0–34.512.911.11.4–36.314.510.417.1–54.028.611.1
Table 2. Monthly average amounts in bio-waste of all impurities without fractions <10 mm and five major components per capita [kg/(C∙month] and monthly average proportions of impurities in bio-waste as a percentage by weight [% w/w] in controlled areas.
Table 2. Monthly average amounts in bio-waste of all impurities without fractions <10 mm and five major components per capita [kg/(C∙month] and monthly average proportions of impurities in bio-waste as a percentage by weight [% w/w] in controlled areas.
Research AreaMethodQuantity of Contaminants in Bio-Waste without Fractions < 10 mmImpurities Present in Bio-Waste in Greatest Quantity, in Order of Greatest Content
AllMain 5
Average ValueValues RangeAverage ValueValues Range
RMM-I
[% w/w]		5.231.9–11.64.601.66–11.4Treated wood < Plastics (1) < Mineral wastes < Paper (2) < Glass
M-II
[kg/(C∙month)]		0.0980.023–0.2320.0900.020–0.224Treated wood < Plastics (1) < Mineral wastes < Paper (2) < Textiles
T-SHM-I
[% w/w]		5.231.9–11.64.381.53–9.51Plastics (1) < Paper (2) < Treated wood < Glass < Textiles
M-II [kg/(C∙month)].0.3250.055–1.470.2660.00–0.812Plastics (1) < Treated wood < Mineral wastes < Paper (2) < Textiles
T-MHM-I
[% w/w]		12.66.76–16.110.96.69–21.7Plastics (1) < Paper (2) < Treated wood < Mineral wastes < Glass
M-II [kg/(C∙month)]0.470.12–1.340.370.00–1.26Plastics (1) < Paper (2) < Mineral wastes < Glass < Treated wood
(1) Plastics: sacks+ other plastics; (2) Paper: sacks, bags + other paper. M-1: evaluation method I—average monthly proportions of pollutants in bio-waste as a percentage by weight of wet weight; M-2: assessment method II—monthly averages of all pollutants in bio-waste per capita.
Table 3. Determination coefficients (R2) of linear regressions between contents (kg/(C∙month)) of kitchen and garden waste and impurity components in bio-waste (kg/(C∙month)).
Table 3. Determination coefficients (R2) of linear regressions between contents (kg/(C∙month)) of kitchen and garden waste and impurity components in bio-waste (kg/(C∙month)).
Components of ImpuritiesPopulation-RuralSingle-Family Housing in TownsMulti-Family Housing in Cities
Food Waste Garden Waste Food Waste Garden Waste Food Waste Garden Waste
Total impurities0.6160.3200.0060.3520.6790.052
Plastics0.5600.2900.0560.1570.6290.085
Paper0.0770.2160.2410.1360.6350.015
Treated wood 0.1490.0250.0010.0070.1160.167
Glass0.0000.0620.0460.0550.0020.399
Textiles0.3890.0160.0030.0750.0060.536
Metals0.0380.1890.0480.4550.7730.008
Multi-material waste0.2950.0220.0000.3170.0300.321
Mineral wastes0.0420.2050.0910.1820.5720.068
Fraction ˂ 10 mm0.2450.0500.0050.0680.1160.088
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Dronia, W.; Połomka, J.; Jędrczak, A. Quantity and Material Composition of Foreign Bodies in Bio-Waste Collected in Towns from Single- and Multi-Family Housing and in Rural Areas. Energies 2024, 17, 4350. https://doi.org/10.3390/en17174350

AMA Style

Dronia W, Połomka J, Jędrczak A. Quantity and Material Composition of Foreign Bodies in Bio-Waste Collected in Towns from Single- and Multi-Family Housing and in Rural Areas. Energies. 2024; 17(17):4350. https://doi.org/10.3390/en17174350

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Dronia, Wojciech, Jacek Połomka, and Andrzej Jędrczak. 2024. "Quantity and Material Composition of Foreign Bodies in Bio-Waste Collected in Towns from Single- and Multi-Family Housing and in Rural Areas" Energies 17, no. 17: 4350. https://doi.org/10.3390/en17174350

APA Style

Dronia, W., Połomka, J., & Jędrczak, A. (2024). Quantity and Material Composition of Foreign Bodies in Bio-Waste Collected in Towns from Single- and Multi-Family Housing and in Rural Areas. Energies, 17(17), 4350. https://doi.org/10.3390/en17174350

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