Food waste from the municipal waste stream is one of the priority challenges for the European Union’s (EU) 2020 circular economy action plan [1
]. First, it is important to define the nomenclature and systematics for the waste stream analyzed in this study. The focus of the article is the source-separated organic fraction of municipal solid waste (SS-OFMSW) from households and catering, collected in a bin or bag system for brown bins (according to the selective municipal waste collection system in force in Poland) [2
]. According to data on municipal solid waste (MSW) generated in Poland, the organic fraction of municipal solid waste (OFMSW) accounts for 26.68% [3
]. In this stream, the dominant share is plant-based food waste [4
]. In this context, it will be mainly food waste, as defined by Directive (EU) 2018/851 [5
]. The SS-OFMSW analyzed in the paper does not include green waste. Food waste is recognized as one of the leading global issues in sustainable development strategy [6
]. Reducing food waste and effectively utilizing the value from biowaste generated from food already wasted are some of the key challenges of the CE, which is confirmed by numerous scientific studies indicating the importance of the topic of CE-based food waste management. Although there are many reports and studies on the use of food waste in various conduct strategies [7
], it is impossible to determine clearly which scheme for handling SS-OFMSW consisting mainly of food waste will have the best application locally, since the properties of individual food products have a great influence in determining which waste management option is most favorable [10
The suitability and applicability of the value of food waste is closely dependent on the stage of the food chain at which it was produced [11
]. According to Commission Decision (EU) 2019/1597, food waste is identified separately for the following stages of the food supply chain: primary production, processing and manufacturing, retail and other food distribution, restaurants and catering services, and households [12
SS-OFMSW (household and catering waste) is categorized as post-consumer food waste, and due to its properties it is mainly used in traditional organic recycling processes such as composting and anaerobic treatment using methane fermentation. Value recovery in biorefineries, especially as a source of bioproducts, is still rather applicable for more heterogenous food waste from the pre-consumer stage of the food chain [13
]. Nevertheless, the fermentation process is a key technology in most microbial biorefineries due to its efficiency in converting organic waste into easily assimilable organic compounds [14
]. The stability of the physical and chemical properties of food waste is a key factor necessary for its effective use in recycling technologies.
The composition, and hence the properties, of food waste generated in the SS-OFMSW stream are determined mainly by seasonality and housing types [15
]. Food waste generally consists of carbohydrates (41–62%), protein (15–25%), and fats (13–30%). The exact composition of the waste depends on its blend, in particular the proportion of each food type in the waste. Studies by Wainaina et al. report that food waste consists mainly of fruit and vegetables (79%), meat and fish (8%), pasta and rice (5%) bread and bakery products (6%), and dairy (2%) [16
]. For example, meat, fish, and eggs are primarily sources of fats and protein, while vegetables, fruit, rice, and pasta are sources of carbohydrates [17
]. The main parameters characterizing the technological properties of food waste include: water content (74–90%), carbon-to-nitrogen ratio—C/N ratio (14.7–36.4), total solids content—TS (17–29%), and volatile solid content—VS (17–26%) [18
]. Another important technological parameter is the potential for methane production, which for food waste can vary widely, and Slopiecka et al. report a range from 216 to 1476 mL CH4
/g VS [17
As studies show, not only are the properties of waste and the choice of direct processing technology important for the efficiency of the organic recycling process, but a very important factor is also the time of storage of waste before it undergoes processing [21
]. In this context, the organization of a system for selective collection of biowaste is very important. A study by Dolci et al. showed that the type of waste bag used (plastic vs. paper bag) can result in significant differences in the properties of the SS-OFMSW when collected and at its processing stage [22
]. Contamination of OFMSW with undesirable fractions results in a reduction in the end result of the recycling process, affecting, among other things, the contamination and quality of the compost produced [23
], as well as worse parameters of the energy yield process during fermentation [17
]. Source separation of OFMSW is a key process as it reduces the content of inorganic substances and thus the share of impurities [25
]. The efficiency of a separate collection system can be expressed by the quality in container rate (QCR), which determines the proportion of SS-OFMSW, and impurities contained therein. The study by Gallardo et al. reports that these values reach levels of 79.95–90.00% (Spain—various regions), 89% (Italy—Calabria), 70–90% (Czech Republic—Usti nad Labem), and 97% (Belgium—Antwerp) [26
This study presents the results of qualitative research on municipal food waste from the selective collection system in Warsaw from three sources: single- and multi-family households and catering facilities. The purpose of the article is to identify the key technological and organizational parameters for conducting selective collection of SS-OFMSW in Warsaw conditioning the selection of the optimal path for its management. In particular, in this paper, the authors aim to fill considerable data gaps and explore the status of SS-OFMSW in urbanized areas with a developing system of source separation of food waste (up to 5 years of operation), using Warsaw as a case study. Such a study seems significant and important in the context of the organic fraction municipal solid waste separate collection mandatory for all EU member states by the end of 2023.
Research conducted from September 2021 to June 2022 indicates that in the SS-OFMSW stream, the QCR is 93.5%, 92.1%, and 97.1% for waste collected in multi-family, single-family, and catering developments, respectively. The research conducted shows a higher level of efficiency of selective food waste collection compared to the result obtained in 16 municipalities in southern Poland of 69.3% (including green waste collection) [33
] and during research conducted in Catalonia with efficiency of biowaste collection of 78% [34
]. The samples tested contained admixtures of waste of animal origin, which, in accordance with the requirements in force in Warsaw, should go to the stream of mixed MSW. The share of food waste of animal origin in SS-OFMSW from multi-family housing and catering is at a similar level (on average 0.6–0.7%), while in SS-OFMSW from single-family housing it is twice as much (on average 1.3%). In the pilot study conducted by the authors in 2019, this share was on average 1.44% for all types of developments [4
]. In research conducted by Seruga et al., this fraction was not shown. The share of paper in single-family housing in Warsaw (1.2%) corresponds to the share of this fraction in the SS-OFMSW tested by Seruga et al. (1.3%). SS-OFMSW from multi-family housing, and especially from catering, in Warsaw contains fewer impurities in the form of paper, 0.9% and 0.3%, respectively [33
]. In contrast, in a pilot study of SS-OFMSW cleanliness conducted in Spain, its paper contamination ranged from 2.18% to 3.48% [26
]. The share of plastic impurities in Warsaw remains within the range of 1.6–2.5%, obtaining the lowest value in the waste stream from catering, and the highest in that coming from single-family housing. These values are significantly lower than those presented in the work of Seruga et al. (5.4%) and Gallardo et al. (4.66–6.46%) [26
]. Other impurities found in SS-OFMSW in Warsaw (both in multi-family and single-family housing) have similar contents (within the range of 2.8–3.0%), and from catering they are seven times lower (0.4%). In the results of morphological research of food waste collected selectively in Warsaw, the presence of citrus fruits deserves attention. They constitute a share of 5.7–8.6%, the largest in single-family housing and the lowest in multi-family housing. In the paper [17
], the carbon-to-nitrogen ratio for this fraction has been determined. It is characterized by a low value (C/N < 10), hence their presence in biowaste being subject to biological processing is problematic.
Very high variability in the occurrence of impurities—both in terms of total content (CV = 59–74%) and the share of individual admixtures—was recorded in the SS-OFMSW tested. The highest variability was observed for the fractions present in the smallest amounts—glass (share 0.03–1.1%, CV = 112–195%) and metals (share 0.01–0.3%, CV = 112–170%)—as well as the animal origin fraction (CV = 125–145%). For paper and plastic admixtures, the variability is also high, within the ranges of CV = 82–136% and CV = 60–130%, respectively. In the case of paper and plastic admixtures, the highest variability is in biowaste from catering. On the other hand, in the case of glass and metals, higher variability was recorded for household waste. The high variability and diversity of contaminants found in SS-OFMSW is confirmed by the results of the research [33
], according to which even up to 30% plastics were found in biowaste from multi-family housing in the Lower Silesian province. The variability of food waste quality is also discussed in the paper [19
], indicating that 24% of these variations may be explained by the geographical origin, the type of collection source, and the season of the collection. The amount and type of impurities found in SS-OFMSW generated and collected in Warsaw clearly indicate the need to further improve the system for their selective collection.
According to the study [17
], food waste has general characteristics that can be extrapolated all over the world—it has a water content of 74–90% and a high percentage of VS (around 85 ± 5%). The food waste tested confirms these characteristics—its water content is 77–82% and VS are within the range of 84–88% (86 ± 1% for multi-family housing, 84 ± 7% for single-family housing, and 88 ± 4% for catering). In the paper [19
], a slightly higher value of VS of food waste from restaurants was indicated—91.6 ± 4.1%. On the other hand, the average TS value for food waste from catering and households in Warsaw was determined at the level of 18.3% and 21.9–23.1% (for single-family and multi-family developments, respectively), which is consistent with the data in the study [19
]—19.1% and 23.4%.
All the SS-OFMSW tested had very good fertilizing properties, confirming its suitability for organic recycling. Similar levels of VS (indicated above) and TOC (33–36%) were reported for all three types of biowaste. However, other studies [35
] indicate a higher organic carbon content in food waste from restaurants (47.35%) compared to biowaste from catering in the capital city of Warsaw (33.99%). The VS content within the SS-OFMSW generated in Warsaw is characterized by a slight variation, at a level of CV = 3–8% (the highest for food waste from single-family housing). The variability of TOC content in the tested waste is in a slightly higher range—CV = 12–20% (highest for single-family housing and catering). Carbon-to-nitrogen ratio of food waste from the capital city of Warsaw was determined at the level of C/N = 18 ± 2 (for food waste from multi-family housing) and 17 ± 5 (for food waste from single-family housing and catering). A similar level for this parameter (defined as the average value for food waste from residential developments and restaurants) was indicated in the study of Fisgativa et al.—18.5 ± 5.9 [19
The water content of the tested SS-OFMSW indicates its greater suitability for processing in anaerobic processes. Additionally, the C/N parameter confirms this suitability for processing in anaerobic decomposition processes, for which a C/N level in the range of 10–25 is recommended, while for aerobic processes the requirement for C/N = 25–35 is indicated [35
]. According to the study [36
], plant origin waste is more compostable when its moisture decreases from 76% to 60% or C/N ratio increases from 12 to 24. Too low a C/N ratio can result in the formation of large amounts of ammonia, which contributes to odor nuisance and inhibits the aerobic decomposition process. The results of the study [37
] also indicate that the optimal moisture content for co-composting of food waste and green waste is 60%. They also indicate that the substrate at a C/N ratio of 19.6 can be decomposed effectively. A C/N ratio between 25 and 30 is usually considered the optimum ratio for composting; however, recent studies have shown that composting can be carried out effectively at a lower C/N of 15 [38
]. Thanks to composting at low C/N ratios, the requirement of bulking agent for adjusting the initial C/N ratio of a food waste composting mixture can be reduced. Based on the results of the research presented in the aforementioned scientific literature, SS-OFMSW from Warsaw can be organically recycled under aerobic conditions after appropriate pretreatment to correct water content and porosity.
From the point of view of anaerobic decomposition processes, the organic matter content of the digester feedstock (determined by VS and TS) is also an important parameter which is commonly used to determine the suitability of waste for the AD process [17
]. For much of the food waste tested in this study, the average VS/TS value was over 80%. This parameter for SS-OFMSW coming from Warsaw also exceeds 80% and is 84% for single-family housing, 86% for multi-family housing, and 88% in the case of catering. In the research conducted by Moretti et al., a VS/TS value of 90.8% was obtained for SS-OFMSW from households (without distinguishing the type of development) and 95% for catering [40
]. The high VS/TS value means that more feedstock can be consumed by the bacteria during the fermentation process. In addition, the raw material with a higher VS/TS ratio can produce more biogas and also less digestate after the digestion process.
From the point of view of anaerobic digestion processes, the C/N ratio is also an important element in process control. The optimal value of carbon-to-nitrogen ratio is usually maintained in the range of 20–30 [41
]. If this value is exceeded, the nitrogen will be completely consumed by the bacteria and this will reduce the amount of biogas produced. If the ratio drops too much below, nitrogen will be released in the form of ammonia, and it will increase the pH of the environment. This condition may disturb the nitrogen balance and have a toxic effect on methanogenic bacteria. The problem of the toxic effects of ammonia occurs in the fermentation of raw materials with high protein content [17
]. In turn, the study [18
] indicates the minimum recommended C/N ratio (15) for AD. Based on these data, it can be concluded that SS-OFMSW generated in the capital city of Warsaw is characterized by the C/N parameter being at the right level for proper anaerobic treatment.
In addition, the interaction between the C/N ratio and water content was noticed in the study [35
], and the best results in terms of biological treatment of waste were recorded under the conditions of low water contents and high C/N ratios, or high water contents and low C/N ratios. Food waste from restaurants tested in this study is characterized by about half the C/N ratio (8.85) compared to biowaste from catering in the capital city of Warsaw (17.20) and slightly lower water content, 70–80% and 77–86%, respectively. Even different C/N values for food service waste are reported by Moretti et al. in their study, the C/N is 13.1 [40
On the other hand, the optimal values of the C/P parameter for biological decomposition processing of waste are indicated in the range of 70–240 [35
]. SS-OFMSW coming from Warsaw is characterized by C/P = 42–59, which indicates too high content of total phosphorus (especially in the case of catering waste). It is therefore advisable to process it in organic recycling processes together with other organic carbon-rich biowaste.
The suitability of the tested SS-OFMSW for treatment in anaerobic processes is confirmed by biomethane potential (BMP) research. All tested food waste has high biogas yields—at similar levels within the range of 384 m3
VS to 426 m3
VS (at reactor loads of 2.0660–3.0884 g VS·L−1
and ISR = 3–4). The average proportion of methane in biogas is 52% to 61%. In the study [16
], BMP was determined for many fractions of food waste. It is found in a very wide range from 232 to 1108 mL CH4
VS. The range of methane potential yield in carbohydrate-rich feedstock was determined to be between 226 mL CH4
VS and 599 mL CH4
VS. In addition, in the study [18
] a higher BMP of FW (460 mL CH4
VS) was indicated, but it is characterized by high variability (CV = 88%), while the studies conducted on biogas yield from municipal biowaste from Warsaw reported a variability of gas potential at the level of CV = 15–23%, less for SS-OFMSW from multi-family housing, and more for food waste from single-family housing and catering. Similar results of biogas yield from biowaste to the Warsaw results were obtained in the study [44
]—within the range of 449.6–453.3 L·kg−1
VS. On the other hand, in the paper [33
], the average biogas production rate of 120 L kg−1
of fresh SS-OFMSW was indicated (based on tests conducted on a full technical scale under thermophilic fermentation conditions) and the average share of methane in biogas was at a level of 58%. The usefulness of food waste for processing in anaerobic processes is confirmed by the study [19
], indicating that its physicochemical characteristics (especially volatile solids, chemical oxygen demand, and biomethane potential) show a good potential of FW for AD treatment, and that among the biological valorization processes, the AD of FW has been demonstrated to be one of the most advantageous technologies to maximize the substrate and energy recovery.
The results of SS-OFMSW research from Warsaw obtained confirm that the characteristics of FW present a very high CV. In the study [19
], high variations in FW characteristics were demonstrated—it was indicated that some of them exceed 100% and even 200%. This high changeability of food waste produced in urbanized areas makes it difficult to undertake waste management planning activities. According to the study [23
], a CV between 0% and 16% shows a low changeability, and data with such changeability can be treated as stable data.
The study [19
] showed that only two FW characteristics presented there have universal values with CV lower than or close to 16%: pH and BMP. In the SS-OFMSW research from the capital city of Warsaw, more such characteristics are shown. Table 5
presents the parameters of the examined food waste from Warsaw (along with their source of origin), which present a CV lower than 16% or close to it.
In Table 6
, selected SS-OFMSW properties tested for Warsaw in comparison with chosen European regions are summarized. Literature data availability did not allow BMP to be included in this comparative data set. This parameter is usually presented for specific types of biowaste fractions rather than distinguishing the region of their generation.
Reducing food waste and making efficient use of post-consumer food waste is one of the CE’s key challenges, particularly in highly urbanized areas. Waste from this stage of the supply chain is characterized by less homogeneity and a high amount of contamination compared to food waste from production and processing and is therefore mainly eligible for processing through traditional organic recycling processes (composting and anaerobic decomposition). The basis for its effective use is the knowledge of the physical and chemical properties specific to the given region and determined, among other things, by the characteristics of the waste generators. In this regard, its collection system is of great importance, and the effectiveness of this system is measured both quantitatively (as the amount of collected fraction in relation to potential opportunities) and qualitatively (as the share of impurities—quality in container rate). The stability of SS-OFMSW properties is also important.
This paper presents the results of a qualitative study of SS-OFMSW generated in Poland’s capital—Warsaw—from three sources (multi-family and single-family residential developments and catering). The collection efficiency of this waste, measured as QCR, is high and amounts to 92–97%. At the same time, Warsaw’s SS-OFMSW collection system is characterized by high variability from the point of view of impurities present in food waste, both in terms of quantity and quality. This variability is at the level of CV = 56–87%, which is characteristic of immature selective collection systems and can hinder waste management planning efforts.
All SS-OFMSW samples tested have very good fertilizing properties, especially VS and TOC. This confirms their suitability for organic recycling, especially under anaerobic conditions, for which their water content, C/N, and BMP are predisposed. All food waste tested has high biogas yields within the range of 384–426 m3·Mg−1 VS and an average methane content in biogas of 52–61%. Fertilizing properties, water content, and its gas potential show little variation, not exceeding 16%, which means that these data can be considered stable.
Further research should be carried out for developing collection solutions for SS-OFMSW in urbanized areas, also including food waste of animal origin and green waste, in order to choose the most efficient and sustainable processing path.