Changes in Main Parameters of Biological Stabilisation of Municipal Waste from a Full-Scale Mechanical-Biological Treatment (MBT) Installation

In 2021, the amount of municipal waste generated in the European Union (EU) was 530 kg/inhabitant on average and differed greatly from county to country, from 799 kg/inhabitant in Norway to 270 kg/inhabitant in Romania. These differences are due not only to the waste collection and management method but also to an increase in consumption and economic wealth [1].

According to data for 2021 published by GUS (Statistics Poland) [2], almost 121 million Mg of waste were produced in Poland, of which 11.3%, i.e. 13.7 million tonnes, were municipal waste. This means that the amount of municipal waste produced per inhabitant of Poland increased from 342 kg in 2020 to 362 kg in 2021. Many factors influence the variability and amount of waste generated in a given place. These can be, among others, the type of development, the frequency, and popularity of selective waste collection, the way of heating buildings, the number of commercial premises in a given area, population density, and wealth status of the inhabitants, as well as the season of the year. The quantity, type and composition of waste in urban and rural areas can differ greatly.

In Poland, municipal waste management, as in many other EU countries, is based on mechanical-biological treatment (MBT) technology. This technology aims to biologically stabilise the organic fraction (OF) of municipal waste before landfilling or preparing municipal solid waste (MSW) for energy recovery incineration processes [3]. In Poland, there are 174 installations for the MBT of unsorted municipal waste (mixed). There are also 163 facilities (data for October 2021) for the disposal of waste generated in the MBT of unsorted municipal waste (mixed) and municipal waste sorting residues (Figure 1).

Figure 1.

Around 570 MBT systems were in operation in Europe with a capacity of 55 million tonnes of MSW. The transformation of the waste management model from linear to circular has been underway for several years to achieve sustainability and increase Europe's global competitiveness. The circular economy aims to achieve MSW recycling at 55% in 2025, then 60% in 2030, and up to 65% in 2035 [5]. The implementation of the circular economy eliminates MBT facilities from the market. However, it is believed that their market position for MBT technologies will remain strong for many years to come, although with the irrevocable increase in the level of material and organic recycling of waste, they will adapt to the increasing supply of separately collected waste streams [6].

Of the EU countries using MBT processes (e.g. Austria and Germany), the main parameters allowing the stabilisers produced by these processes to be deposited in landfills are the setting of limit values: –

combustion heat;

In Poland, on 9 January 2023, the Minister of Climate and Environment published a decree on MBT, i.e., MBT of unsorted (mixed) municipal waste [8]. According to this Regulation, MBT consists of processes of mechanical and biological treatment of waste combined into one integrated process of treating mixed municipal waste to prepare it for recovery, including recycling, energy recovery, waste-to-energy process, or disposal. Sediments generated in the process of biological treatment are transferred, according to the waste handling hierarchy, for recovery or neutralisation. According to Section 4, the sub-screen fraction (separated on a screen of below 80 mm) requires applying the process of biological stabilisation of waste, which is an aerobic or anaerobic process with microorganisms, as a result of which the physical, chemical, and biological properties of this fraction are altered. MBT can be a one-stage process (at least 4 weeks) or a two-stage process (stage I – at least 2 weeks, stage II – 6 to 10 weeks). The product of biological stabilisation of waste (stabilised) must meet the requirements specified in 7 section 1: 1)

respiratory test AT₄ value below 10 mg O₂/g of dry matter and

Meeting one of the two requirements regarding the SW ignition loss and organic carbon content does not reflect changes occurring in the organic substance as a result of humification and its biological stabilisation, and consequently, the loss of its capacity for further biological decomposition. Only indicates the total loss of the organic substance as a result of mineralisation. Only determining the value of the AT₄ parameter (respiratory activity in mg O₂/g DM) for the SW allows unequivocally evaluating the loss of capacity of the organic part to further decompose in an aerobic or anaerobic process and qualifies these SW for disposal. The aim of this study was to monitor the changes that occur during the biological stabilisation of the mixed municipal waste sub-screen below the 80 mm fraction (MMWSF). This will determine the time during which the municipal waste sub-sit fraction can be landfilled.

The study concerned the analysis of the following parameters: humidity, organic matter (OM) content measured as LOI and AT₄ coming from feed to reactor for biological treatment of subscreen fraction of municipal waste supplied from the installation localised in eastern Poland. Tests were conducted on feedstock placed in the bioreactor in May. The installation for biological treatment of waste enables oxygen stabilisation of 0–80 mm subscreen fraction coming from drum screening of supplied mixed waste in the quantity of 20,000 Mg/year. The waste was supplied using a wheel loader, placing it inside a bioreactor as loosely as possible, without compacting or driving on the previously delivered subscreen fractions. No more than 220.5 m³ of subscreen fraction was fed to each bioreactor at a time, with a filling level of a maximum of 3.0 m from the floor level. Waste placed in the reactor was turned over after a two-week process. The dimensions of the bioreactor used for aerobic stabilisation processes with an active volume of 220.5 m³ were the following: length (depth) – 15.0 m; width – 7.0 m; height – 5.5 m; fill height with stabilised material – 3.0 m.

The bioreactor was equipped with the following:

process air supply system: blower fan with a capacity of approx. 5.5 kW, static pressure of 5,000.0 Pa and efficiency of 200.0 m³/h; a system of 6 air supply ducts on the floor under the bioreactors with a length of 12.0 m each. The blowers were run once a day for half an hour;

MMWSF sample collection was carried out according to PN-Z-15011-1:1998 [9]. Three excavations were carried out and approximately 10 samples were taken from each excavation, with a total primary sample weight of 100 kg. A laboratory sample of 10 kg was then obtained using the quartering method. The samples were transported to the laboratory where they were prepared for further analysis. All analyses were performed in triplicate to ensure the reproducibility and representativeness of the sample. The humidity and OM content expressed as LOI was determined in dry ground samples (respectively at 105 and 550°C). Analyses were performed according to APHA [10]. AT₄ was measured by ÖNORM-Serie S 2027 standard [11] using WTW's OxiTop analysers. Data on the humidity and temperature inside the bioreactor were obtained from the installation. The OF in the waste was determined by separating organic and inorganic waste by weight. In the paper, this value is presented as a percentage.

The dependencies between the individual indicators of the process were analysed using Pearson's correlation. Differences in changes occurring during the municipal waste biological treatment process were measured using the ANOVA test – a system with repeated measurements, and then Bonferroni's post hoc test (Statistica 13.3).

Although MBT facilities have recently become common systems in waste management in many countries, studies on the transformations that occur during the biological stabilisation of MMWSF are not numerous, so the comparison of the results of this study with others is limited. The feed to the bioreactor contained 41.7% OF, LOI was 32.3% DM and AT₄ was 45.7 mg O₂/g DM.

In the course of the biological treatment process, a 22.6% loss of OF was registered in the total waste mass. According to [10], the overall efficiency of the reduction of OM of the MMWSF during stabilisation ranged from 40 to 60%. In the present study, the OF loss was only 20%. After 1 week of the duration of the process, there were statistically no significant differences compared to the feed to the bioreactor. According to the statistical analysis (ANOVA, Bonferroni's post hoc test), there were no significant differences in the content of organic parts in the waste between the bioreactor input and the OF content after the first week of the process, the second and third weeks and between the third and fourth weeks of the biological treatment process (Figure 2, Table 1). It means that we can speak of a reduction in OM in feed to the bioreactor only after 4 weeks of the process.

Figure 2.

We found that the final product of aerobic stabilisation of the MMWSF showed the presence of improper materials, mainly paper, plastic, and glass. Dias et al. [13] characterised residual material after aerobic stabilisation from 5 different MBT installations, and found that glass and stones predominated, constituting 32–67% and 10–26%, respectively. Some identified materials, such as plastic, metal, ceramic, and brick, were present in small percentages, constituting from 2 to 13% of the residual material. The share of materials that were difficult to identify was 12–49%. The authors considered it to be mainly organic matter. Supposedly, in the present study, such a low reduction of OF during biological stabilisation was due to the high proportion of residual material in MMWSF.

The LOI of the subscreen fraction transferred for the biological stabilisation was 32.2% DM and, already at the beginning of the process, this value was lower than the requirements for SW specified in legal norms. After 1 week, this value was reduced to 9.64% and according to the statistical analysis of the data obtained, the results did not show statistically significant differences until the end of the process (Figure 3, Table 2).

Figure 3.

The OF content of MMWSF depends on the percentage of individual size fractions (>60 mm, 60–40 mm, 40–10 mm and < 10 mm) [14]. Residual municipal waste is a very heterogeneous material. Biodegradable waste in samples from the <80 mm fraction includes vegetable waste, other organic waste, animal food waste, paper and cardboard waste, and textile material. The presence of these wastes affects the LOI of the samples analysed. For example, Bernat et al. [15] found that in a small fraction of 0–20 mm size separated from MSW, the LOI, commonly indicating the OM content, was 31.4% DM. In comparison, the same study characterised the 20–80 mm MSW size fraction, the LOI was 61.3% TS. This means that the smaller fraction was less reactive than the larger fraction, but that the 0–20 mm fraction cannot be regarded as an entirely mineral fraction, as previously thought. Jedrczak et al. [16] also found that the <10 mm fraction separated from MSW showed an LOI of 29.8 ± 7.1% DM, confirming its unstable nature and indicating a relatively high amount of organic matter. However, in another study, Połomka and Jędrczak [15] indicated that the <10 mm fraction was mainly a mixture of ash and sand. This implies that the <10 mm fraction should be less reactive and more stable than this fraction of particle size from MSW.

As mentioned above, in the 2023 Regulation [8], the value of respiratory activity AT₄ in SW after the biological stabilisation process of MMWSF should be lower than 10 mg O₂/g DM. In the waste supplied to the bioreactor, AT₄ was 45.7 mg O₂/g DM on average. Local EU standards specify the required value of AT₄ in the final product, e.g. in Germany, it should be 5 mg O₂/g DM, while in Austria the value can be slightly higher (7 mg O₂/g DM) [18].

The analysis of the results obtained showed significant differences between the AT₄ value in the feed and the values obtained in subsequent weeks of the duration of the process. After 1 week of the duration of the process, the AT₄ value decreased to 32.33 mg O₂/g DM and in the following week increased to 37.33 mg O₂/g DM. It was only after 3 weeks of the duration of the process that it decreased to 23.67 mg O₂/g DM, reaching 6.5 mg O₂/g DM after 4 weeks. No statistically significant differences were found only between the AT₄ value in the waste after the first and second week of the stabilisation process (Figure 4, Table 3). The increase in AT₄ values can be explained by an increase in the biochemical activity of the microorganisms in the waste as a result of the process conditions (humidity provision and aeration).

Figure 4.

The humidity of the MMWSF during the biological stabilisation process was also analysed. Initially, the humidity of the waste was at the level of 51.48%. After two weeks of the process, this parameter reached a value of 64.1%, and after 4 weeks – 15.3% (Figure 5). The humidity of the waste decreased as a result of intensive aeration, high temperature, and relatively high porosity of the feedstock. The air not only provided oxygen for the microorganisms performing mineralisation but also dried the waste.

Figure 5.

Another important factor affecting the development of microorganisms, and thus the efficiency of the process of biological stabilisation of MMWSF, is the humidity content in the reactor. In the first week of the process, the humidity content in the bioreactor was maintained at a level of 49% and then reduced to levels of 43% and 40% in subsequent weeks, until it reached 25% in the fourth week (Figure 6). Within the study, the highest temperature in the bioreactor, that is, 64°C, was obtained in the second week of the process. At the beginning and after the completion of the process, the reactor temperature was 25°C and 24°C, respectively (Figure 6).

Figure 6.

The process of biological stabilisation of MMWSF is conducted with the use of microorganisms. They are responsible for the decomposition of most organic substances. Microorganism populations are subject to changes during the process, depending on the physical and chemical conditions present and the type of waste. The most important factor affecting the growth of microorganisms and indirectly the efficiency of the process is temperature. Biological stabilisation of waste can be carried out at a temperature of 20–60°C. Both below 20°C and above 60°C, the activity of selected groups of microorganisms slows considerably. However, the microorganisms responsible for the process are significantly diversified, and biological stabilisation takes place even below and above these threshold temperatures, although it is less effective [19].

Irrigation is necessary because drying the composted material below 40% humidity content causes a decrease in microbial activity. This in turn leads to a drop in temperature, although mineralisation of the OM is not complete, and the material is not complete, and the material is still not stable. For this reason, it is important to monitor the humidity content of the SW. Otherwise, if the temperature drops due to the low humidity content, the operator may assume that the mineralisation of the waste has ended and transfer the material to open piles, for example. Then, if the humidity content in the open pile increases as a result of precipitation, the activity of microorganisms may increase. Unfortunately, because open piles are not commonly aerated and air only penetrates the outer layers of the to the outer layers of the pile, anaerobic zones can develop in which to produce methane, which is a greenhouse gas.

The correlation between individual MMWSF parameters and the humidity content and temperature present in the bioreactor was also analysed. Based on the results of Pearson's correlation, the content of OF in waste affects the LOI in the sample taken. The AT₄ value depends on the LOI and the humidity of the waste itself. Interestingly, no dependencies between these parameters and the humidity and temperature present in the bioreactor were found. The results are presented in Table 4 and Figure 7.

Figure 7.

At the time of the test, the SW parameters had to comply with the requirements under the integrated permit for the installation, that is, the LOI had to be lower than 35% of the dry matter and the AT₄ value had to be lower than 10 mg O₂/g DM. According to the analysis presented, the samples taken complied with these requirements. The main purpose of the biological stabilisation process of MMWSF is to stabilise the OM contained in the treated mass of waste. The effect is SW, which is waste characterised by reduced nuisance to the environment during its final disposal. The aerobic stabilisation process is classified according to the Waste Act [20] as D8 – biological treatment, not specified in any other section of Annex II, resulting in final compounds or mixtures which are neutralised using any of the processes described in items D1–D12. This means that the process of biological treatment of waste constitutes waste neutralisation. The stability of material is defined as its inability to putrefy [21], high degree of mineralisation of organic matter, or inability to emit odours [22]. Determining the degree of stability of the material is vital in terms of safe disposal of waste with low methane potential (< 20 Nm³/Mg) [19]. The legislation of many EU countries, including Poland, determines the completion of the intensive stabilisation process based on the respiratory activity parameter AT₄. The smaller the oxygen demand, the higher the stabilisation of the OF. Additionally, the total content of organic matter, expressed as LOI, is measured. However, it is the AT₄ measurement that is the most reliable indicator of waste stability, because its value depends on the activity of microorganisms responsible for the process.

Although MBT facilities have recently become common systems in waste management in many countries, studies on the transformations that occur during biological stabilisation of the MMWSF are not numerous, so the comparison of the results of this study with others is limited. Since the composition of municipal waste depends, among other things, on the season of the year, it would be worthwhile to carry out such studies for the other seasons in the future. During the biological treatment process of the mixed municipal waste subscreen fraction, a 22.6% loss of OF was found in the total mass of the waste in 4 weeks, the LOI decreased by almost 90% DM and the AT₄ value to 6.6 mg O₂/g DM. It means that after 4 weeks, the stabilised compost could be transferred to disposal according to the provisions of law. According to the hierarchy of waste handling methods, waste disposal is the last and least desirable method of waste handling. Unprocessed organic waste, which is the source of methane during the disposal at waste disposal sites, must be excluded from the disposal. The analysed installation for biological treatment of waste fulfilled its purpose – the waste produced is characterised by low nuisance to the environment, and its mass and volume were also reduced. The results of this study may help determine the technological conditions for the biological stabilisation of the MMWSF.