Bio-Waste from Urban and Rural Areas as a Source of Biogas and Methane—A Case Study from Poland

Abstract

The growing volume of household waste, especially bio-waste, poses a significant challenge to waste management systems. In Poland, bio-waste accounts for almost one third of total waste generation. To address this challenge, in the context of optimising the waste biomass value chain, we are investigating the potential of methane fermentation to convert bio-waste into valuable end products in the form of digestate (organic recycling) and biogas (a renewable energy source with a wide range of downstream applications). This paper presents the moisture content, loss on ignition and biogas and methane production efficiency for bio-waste and for the seven types of waste that are the main constituents of selectively collected bio-waste (meat, other edible waste (dairy), fruits and vegetables, grass, leaves, branches and the <10 mm fraction). Data on the technological properties of bio-waste and its constituents may be of interest to a range of stakeholders. The average moisture content ranged from 41.9% (<10 mm fraction and others) to 84.4% (fruits and vegetables), and the average organic matter content of the dry weight of the waste ranged from 37.8% (<10 mm fraction and others) to 88.7% (edible constituents other than meat and fruits and vegetables). The bio-waste had an average moisture content of 71.3 ± 1.7% and loss on ignition of 68.6 ± 1.7%. Biogas production from selectively collected bio-waste ranged from 285 to 404 Ndm³∙kg⁻¹ DM (mean: 347 ± 53 Ndm³∙kg⁻¹ DM), and methane production ranged from 191 to 271 Ndm³∙kg⁻¹ DM (mean: 215 ± 33 Ndm³∙kg⁻¹ DM).

1. Introduction

In 2017, the EU-28 (28 EU Member States) generated 249 × 10⁶ Mg of municipal solid waste, of which around 34% or 86 × 10⁶ Mg was bio-waste (168 kg per capita on average). This includes bio-waste collected separately and bio-waste collected together with mixed municipal waste (residual waste) but excludes bio-waste composted at home. Approximately 3.9 × 10⁶ Mg of bio-waste was generated in Poland, which corresponds to almost 101 kg of bio-waste per capita per year [1].

The most preferred option in the bio-waste hierarchy is to prevent the generation of bio-waste. However, if bio-waste cannot be avoided, it should be collected separately and subjected to biological treatment for the recovery of valuable substrates. This can be carried out under aerobic (composting) or anaerobic (digestion) conditions, with different products being produced (respectively, compost and biogas). However, it seems that, for bio-waste, methane fermentation should play an increasingly important role, as it has a number of favourable properties compared to composting [2]. The process has a limited environmental impact [3], includes a high potential for energy recovery in the form of biogas [4,5,6,7,8] and provides carrier material for the production of soil amendments, including compost [5,9]. In order to achieve optimal economic and environmental benefits, it is necessary to ensure demand for the compost produced from the digestate. A well-managed digestion process will allow energy recovery from the biogas produced, which, in turn, will reduce the need for fossil fuel energy.

The best results in recovering materials and energy from bio-waste are achieved by using digestion as a pretreatment followed by the aerobic stabilisation (composting) of the resulting digestate [10]. The integration of AD and composting processes is now recognised as an environmentally beneficial way of treating food waste. A study showed that only 5% of composting plants process anaerobic digestate in shared facilities [11]. In regions with low levels of organic matter in agricultural soils, composting may be the preferred option from an environmental perspective.

Food waste digestion is used as a waste treatment method in both developing and developed countries. This technology achieves good results in methane production [12]. The monofermentation of food waste often has some difficulties related to the stability of the system [13]. The fermentation of waste with a high content of easily biodegradable organic matter leads to the significant production of volatile fatty acids (VFAs), which can inhibit methanogenesis [14]. The co-digestion of food waste with lignocellulosic waste, such as garden waste (leaves and branches), avoids these problems. The combination of kitchen and garden waste improves the substrate properties (e.g., better C:N ratio) and, consequently, increases methane production [15,16,17]. The mixing ratio of these fractions is important to optimise the process [18,19]. Too high a proportion of garden waste reduces the energy efficiency of the process due to the presence of lignin, which does not degrade under anaerobic conditions. Replacing 20% of the organic loading rate (OLR) of food waste with lignocellulosic substrate improved both biogas production and methane production efficiency [17]. The general characteristics of the components of bio-waste are shown in

Table 1. Characteristics of bio-waste components [20,21,22,23,24,25,26,27,28,29,30,31].

Type of Waste	Water Content, %	Organic Matter Content, % DM	Nitrogen Content, % DM	C/N Ratio	Biogas Production, dm3·kg−1 DOM	Methane Content in Gas, %
Bio-waste	52–80	34–81	0.5–2.7	10–25	150–600	58–65
Kitchen waste	50–60	30–70	0.6–2.2	12–20	150–500	60–65
Pork (meat)	68	28	5.0	2–3	428	-
Beef (meat)	47	53	1.1	3–6	572	-
Poultry (meat)	62	44	6.8	5–10	266	-
Canned meat (minced beef)	87.3	93.8	-	-	675	-
Fruit waste and scraps	55	61.5	-	32	400	-
Waste and scraps of vegetables	86.4	80.2	-	12–13	370	-
Raw mixed vegetables	86.6	94.5	-	-	330	-
Garden waste	30–40	90	0.3–2.0	20–60	200–500	55–65
Green waste	20–75	83–92	0.3–3.0	20–60	200–500	-
Grass (cuttings from lawnmowers)	81.1–88.3	86.4–88	-	16–40	260–588
Spring grass	68	79.2	2.4	16–40	641	-
Summer grass	66.7	90.3	3.1	16–40	619	-
Leaves	20–72	82–93	0.2–1.0	20–60	100–330	58–62.6
Twigs and branches of trees (apple tree)	63.5	96.6	-	-	96	-

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Anaerobic digestion stands as a well-established biochemical process that plays a crucial role in transforming organic matter in oxygen-deprived environments. This controlled decomposition, orchestrated by diverse microorganisms, produces both stabilised organic sludge and biogas (primarily methane and carbon dioxide), a valuable renewable energy source. The process typically takes place in liquid phases, making it ideal for substrates with low solid concentrations and moisture contents (60–95%). Although anaerobic digestion operates at a slower rate than its aerobic counterpart due to reduced metabolic activity, it exhibits remarkable efficiency. The successful digestion of lignocellulosic materials, a key component of many organic feedstocks, depends heavily on factors such as material porosity, cellulose crystallinity and lignin content. Improving cellulose conversion efficiency often necessitates the application of pretreatment methods [32].

According to the European Compost Network [11], around 38 × 10⁶ Mg of municipal bio-waste (87 kg/(C·year)) was biologically treated in digestion and composting plants in Europe in 2019–2020. Almost 30% of this amount was digested. An estimated 6000 AD and composting plants will be needed to process the projected increase in municipal bio-waste required to achieve 35% MSW recycling [33]. In Poland, the need for new installations by the end of 2034 has been estimated at 28 digestion plants with a capacity of 30,000 Mg/year and 33–48 composting plants with a capacity of 15,000–10,000 Mg/year, as well as the retrofitting of some existing composting plants (approx. 37 plants) [33].

Deciding on an appropriate method of bio-waste disposal, designing bio-waste treatment facilities and providing a good estimation of biogas production (a renewable energy source with a wide range of further applications) require knowledge about the technological properties of bio-waste and its constituents. Improving this knowledge can be of interest to a number of stakeholders, such as national and local authorities, waste management companies and researchers.

The aim of this study is to determine the moisture and volatile solids content of the basic components of bio-waste, to assess their potential for methane production during the digestion process and to test whether the co-fermentation of these components increases methane production.

2. Materials and Methods

2.1. Types of Tested Waste

This study investigated seven types of waste, comprising the fundamental constituents of bio-waste that were selectively collected from rural communes and towns in Zary, Zagan, Gozdnica and Leknica (Figure 1). All of them are located within the scope of Luzyckie Centrum Recyklingu in Marszow. The waste types that were examined included meat, dairy waste, fruits and vegetables, grass, leaves, branches and <10 mm fraction, as well as bio-waste mixture [34].

The bio-waste components were carefully selected to ensure a minimum weight of 5 kg, manually separated from the selectively collected bio-waste. After collection at the treatment plant, the waste was transported to the laboratory within 24 h in plastic buckets for immediate testing. All types of selected waste were then shredded using a knife shredder to achieve a grain size below 5 mm. To determine the grain size of the waste, the waste fractions underwent a 5 mm sieve screening.

Bio-waste samples were acquired by blending bio-waste components in proportions established through studies of the morphological makeup of bio-waste samples collected in rural areas, urban settlements and single-family houses located in the towns of Zary, Zagan, Gozdnica and Leknica. The average proportion of components in the bio-waste was as follows: meat—0.58%, other edible waste (dairy)—0.14%, fruits and vegetables—43.85%, grass—17.10%, leaves—16.12%, branches—7.20% and fraction < 10 mm—15.00% [34].

The test waste samples were treated with digested sewage sludge obtained from the closed separate digestion chambers (CSD) at the Guben-Gubin (Poland) wastewater treatment plant.

2.2. Frequency and Scope of Testing

The bio-waste was sampled on two occasions during 2022, specifically on 14 September and 20 October. During these tests, the moisture content and roasting losses of the waste were determined, along with the unit production of biogas and methane during the mesophilic digestion process. The moisture content and roasting losses of the waste were tested following the guidelines and regulations of the Central Laboratory of the Institute of Environmental Engineering. The laboratory possesses current accreditation for waste sampling and testing, valid until 2 January 2027 [35], conforming to EN 15934:2012 and EN 15935:2021 standards [36,37]. The yield of biogas and methane production was determined in accordance with the laboratory’s existing procedure, as described below.

2.3. Analysis

2.3.1. Construction of the Test Bench

The determination of biogas production from the tested waste was carried out on a laboratory scale in a 30-station batch fermentation ‘digester’ (Figure 2).

The measuring station for one sample consisted of the following:

• A digester (reactor)—1 dm³ glass bottle, sealed with a rubber stopper, with two spigots: one for feeding the biogas to the gas burette, the other for removing air from the chamber;
• A gas burette with a capacity of 2.30 dm³ (tube 45.8 mm in diameter and 1.40 m high), 5 mL elementary division, with three nozzles: two at the bottom of the burette, one to feed biogas from the reactor into the burette, the other to connect the burette to the equalisation tank; and one at the top, to remove biogas from the burette;
• The equalising tank—a polypropylene container with a lower tube containing a saturated solution of sodium chloride (brine);
• A thermostat in which the reactors are placed.

The thermostat is a metal tub filled with water that is heated to the required temperature in the reactors, i.e., 36 °C for mesophilic fermentation. The arrangement of the heaters in the tank, the forced circulation of the water and the design of the control system for the heaters and the pump ensure an even temperature distribution throughout the tank and temperature fluctuations of less than 1 °C.

2.3.2. Sample Preparation for Test

The test samples comprised organic waste with a fineness of less than 5 mm, inoculation material and water. Three samples were prepared for each type of waste and introduced into three separate reactors. The endogenous activity of the inoculation material was determined in triplicate for each series of tests, including a control sample without waste.

In each reactor, 0.600 dm³ of inoculation matter (digested sewage sludge) and waste samples of a known moisture content were added to guarantee a 1:1 ratio of dry waste and sludge mass. Tap water was used to make up the volume to 0.700 dm³, and the pH of the mixture was measured; if the pH was between 6.8 and 7.8, it was adjusted with a solution of either NaOH or HCl. The rubber stopper was securely sealed over the bottle. To achieve anaerobic conditions, the air above the mixture in the reactor was eliminated by purging the reactor with nitrogen for two minutes. The reactor was then firmly connected to the gas burettes and placed in a thermostat.

2.3.3. Process Control

During this study, the following were measured: the daily gas volume, as well as the air pressure and the room air temperature. We used a GEOTECH GA5000 (QED Environmental Systems, Coventry, UK) analyser to measure the methane, carbon dioxide, hydrogen sulphide and ammonia content of the produced gas. The accuracy of the GA5000 is ±0.5% for methane and ±1.0% for carbon dioxide, and the accuracy of the hydrogen sulphide sensor is ±10 ppm and 20% for ammonia. The measurement interval for all parameters is 10 s. The tests were conducted for 21 days counting from the end of the lag phase. For each test, the preparation time (adaptation phase-lag phase) was subtracted, the value of which was determined according to the recommendations given in Siemiatkowski’s work [38]. The lag phase ended when the daily average gas production reached 25% of the maximum 3-day average value. The volume of gas produced during the lag phase was subtracted from the volume of gas produced during the entire test (lag phase + 21 days).

The measurement of the volume of biogas produced consisted of equilibrating the pressure in a gas burette with a saturated NaCl solution and reading the value from a scale on the burette. The pressure equalisation was achieved by closing the valve on the hose connecting the reactor to the burette and lifting the container with the NaCl solution until the liquid levels in the burette and container equalised. The volume of biogas was adjusted to standard conditions of temperature (273 K) and pressure (1013 hPa).

Periodically, after the biogas had accumulated in the burette in the amount necessary for the proper analysis of its composition, the measurement was performed and the column was refilled with the NaCl solution.

To conduct the test, the measuring device was linked with a hose to a nozzle with a valve at the top of each measuring tube. On a shelf above the measuring tubes was a container with the NaCl solution. Once the valve was unblocked, the weight of the saltwater forced the biogas to travel via the apparatus at a set rate of about 1 dm³ per minute. As soon as the measuring tube was packed with the NaCl solution, the test was finished.

The amount of biogas and methane produced from the waste samples was adjusted to account for the amount of gas produced in the control samples of the inoculation material (a sample without waste but with 2 g of crystalline cellulose).

The results obtained from laboratory tests were subjected to statistical analysis by calculating the mean and correlation coefficient [39].

3. Results

3.1. Water Content and Loss on Ignition

The moisture content and loss due to the roasting of the waste samples are presented in

Table 2. Moisture content and loss on ignition of selected bio-waste streams collected selectively during the autumn season.

Type of Waste	Water Content, %			Organic Matter Content, % DM
	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.
Meat	54.5–60.7	58.8	2.3	78.7–88.2	86.7	4.1
Fruits and vegetables	82.1–87.5	84.4	1.9	84.4–85.3	84.7	0.4
Edible others	62.7–66.6	66.1	1.6	85.1–95.9	88.7	4.2
Grass	74.7–77.7	77.0	1.7	71.4–80.4	75.9	3.2
Leaves	56.7–69.9	65.9	4.2	73.7–81.5	79.2	3.2
Branches	40.9–56.0	52.6	5.3	87.5–89.4	88.1	0.8
Fraction < 10 mm and others	30.8–51.7	41.9	7.3	38.1–42.9	37.8	3.8
Bio-waste	68.3–72.7	71.3	1.6	67.5–71.4	68.6	1.7

S.D.—standard deviation.

.

It was found that fruits and vegetables had the highest moisture content, which ranged from 82.1% to 87.5% with a mean value of 84.4%. The <10 mm and other fraction of the waste contained the least amount of water, with moisture levels ranging from 30.7% to 51.7% and a mean value of 41.9%. The bio-waste had moisture content ranging from 68.3% to 72.7%, with an average moisture content of 71.3%. The percentage of organic matter in the dry weight of waste ranged from 37.8% DM (fraction < 10 mm and others) to 88.7% (edible components, not including meat and fruits and vegetables). In terms of bio-waste, the proportion of organic matter in the dry weight ranged from 67.5 to 71.4% DM with a mean value of 68.6% DM, denoting its high potential for biogas production.

3.2. Biogas and Methane Production

The yield of biogas production from selectively collected bio-waste and the material components included in the composition are shown in

Table 3. Yield of biogas production of selected separately collected bio-waste streams.

Type of Waste	Biogas Production
	Ndm3∙kg−1 W/W			Ndm3∙kg−1 DM			Ndm3∙kg−1 DOM
	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.
Meat	232–322	270	33.3	591–707	656	43	752–821	756	39
Fruits and vegetables	31.4–64.2	53.3	12.9	252–358	342	58	295–424	404	67
Edible others	183–195	185	1.9	496–553	538	24	517–650	607	53
Grass	63.6–151	106	31.2	251–674	461	161	352–838	608	185
Leaves	39.6–106	70.9	24.8	91.5–354	208	103	124–434	263	120
Branches	52.4–70.5	59.4	6.9	88.7–160	125	26	99.1–183	142	30
Fraction < 10 mm and others	21.4–46.1	34.5	8.8	30.9–95.3	59.4	24	72.0–250	157	63
Bio-waste	77.5–128	99.3	20.3	285–404	347	53	422–566	505	57

S.D.—standard deviation.

. The yield of biogas production from selectively collected bio-waste ranged from 285 to 404 Ndm³∙kg⁻¹, with an average of 347 ± 53 Ndm³∙kg⁻¹, and for dry organic matter from 422 to 566 Ndm³∙kg⁻¹ DOM, with an average of 505 ± 57 Ndm³∙kg⁻¹ DOM.

The yield of methane production from selectively collected bio-waste ranged from 211 to 271 Ndm³∙kg⁻¹, with an average of 215 ± 33 Ndm³∙kg⁻¹, and for dry organic matter from 312 to 380 Ndm³∙kg⁻¹ DOM, with an average of 314 ± 39 Ndm³∙kg⁻¹ DOM. The values are shown in

Table 4. Yield of methane production of selected separately collected bio-waste streams.

Type of Waste	Methane Production									Methane Content of Biogas
	Ndm3∙kg−1 W/W			Ndm3∙kg−1 DM			Ndm3∙kg−1 DOM			%
	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.	Range of Values	Mean Value	S.D.
Meat	168–204	187	16.2	427–449	444	9	471–543	498	29	63.4–72.2	69.0	4.1
Fruits and vegetables	18.6–43.5	35.2	9.8	149–243	212	37	175–288	250	44	59.4–67.8	66.1	3.5
Edible others	103–114	113	7.5	276–342	334	28	288–402	370	48	55.6–61.9	61.7	3.5
Grass	38.6–85.3	54.0	18.3	152–382	236	88	213–474	310	99	56.6–60.6	51.0	7.5
Leaves	22.0–66.2	42.5	15.7	50.9–220	126	61	69.0–270	159	72	55.6–62.2	60.0	6.2
Branches	32.0–34.9	32.0	1.3	54.2–79.3	66.9	11	60.6–90.7	76.0	13	49.5–61.1	53.9	4.3
Fraction < 10 mm and others	12.8–25.4	19.6	4.5	18.5–52.6	33.7	13	43.0–138	88.7	34	55.1–59.7	56.8	2.0
Bio-waste	57.4–86.0	67.3	11.1	211–271	215	33	312–380	314	39	67.1–74.0	62.1	5.8

S.D.—standard deviation.

.

Figure 3 shows the contribution of components to the wet and dry weight of bio-waste and the amount of biogas and methane produced from it.

Table 5. Average yield production values for biogas and methane determined for the bio-waste in the studies and calculated based on the material composition of the bio-waste and the yield production values determined for the components of the bio-waste.

Type of Waste	Biogas Production				Methane Production
	Ndm3∙kg−1 W/W	Ndm3∙kg−1 DM	Ndm3∙kg−1 DOM	S.D.	Ndm3∙kg−1 W/W	Ndm3∙kg−1 DM	Ndm3∙kg−1 DOM	S.D.
Bio-waste—studies	99.3	347	505	57	61.7	215	314	39
Bio-waste—calculated values	64.2	285	363	90	38.0	166	210	51
Value difference (line 1 − line 2)	35.1	62	143	77	23.7	49	104	24
Value quotient (line 1/row 2)	1.55	1.22	1.39	0.33	1.62	1.29	1.49	0.28

S.D.—standard deviation.

shows the average yield production values of biogas and methane determined for the bio-waste in this study and the calculated values of YBP (yield of biogas production) and YMP (yield of methane production) determined for the bio-waste components, based on the material composition of the bio-waste, assuming that they are additive quantities.

The experimentally determined YBP and YMP values of the bio-waste are significantly higher than the values calculated as the sum of the magnitudes of these parameters corresponding to the mass proportions of the components in the bio-waste mixture. The fermentation efficiency in relation to the dry mass of the bio-waste, expressed as YBP, increased by 28%, and as YMP, increased by 34%. The co-digestion of food waste with lignocellulosic waste such as leaves and branches leads to an increase in biogas production. This is in line with the observations of Borth et al. [40], who found that the combination of these components improves the material properties, including a better C:N ratio.

4. Discussion

The composition of the fermentation feedstock can significantly affect biogas production. Materials that are too concentrated or diluted can impede biogas and methane yields. Effective waste management solutions are crucial due to the increasing volume of bio-waste generation. The anaerobic digestion of food waste, a byproduct of food processing and consumption, presents a promising option because of its high organic matter and moisture content. The composition of bio-waste can vary widely (Table 1), and this variation can have a significant impact on the efficiency of anaerobic digestion. Household bio-waste is a waste stream rich in organic matter, which makes it possible to recover it through organic recycling. These theses have been confirmed in the works of various authors, e.g., Ding et al. [41] and Khan et al. [42]. Furthermore, household bio-waste represents a major fraction of MSW in Europe, and its sustainable management is crucial for resource recovery and environmental protection [43,44,45]. Our results show that the proportion of organic matter of bio-waste ranged from 67.5 to 71.4% DM with a mean value of 68.6 ± 1.7% DM. In all components of bio-waste, except for the <10 mm fraction, the loss on ignition exceeded 75% of dry matter. This demonstrates their high biogas potential.

Correlation analysis showed a highly significant correlation between the type of waste analysed and organic matter content (r = −0.5978) and biogas (r = −0.6682) and methane (r = −0.6898) production. A highly significant correlation was found between the organic matter content of the waste analysed and the biogas (r = 0.5235) and methane (r = 0.5117) produced.

The results show that bio-waste has a high potential for methane production. Biogas production from bio-waste ranged from 285 to 404 Ndm³∙kg⁻¹ DM (mean value 347 ± 53 Ndm³∙kg⁻¹ DM) and methane production from 282 to 380 Ndm³∙kg⁻¹ DM (mean value 214 ± 39 Ndm³∙kg⁻¹ DM). These values are in the middle of the range reported in the literature for bio-waste [20,46,47,48]. The co-digestion of food waste with lignocellulosic waste can increase methane production by about 28%.

The components of bio-waste, taking biogas production (in relation to dry matter) as a criterion, can be arranged in a series, as follows: meat, other edible components, grass, fruits and vegetables, leaves, branches and fraction < 10 mm and other. The order of the components in terms of yield of biogas production (in relation to organic dry matter) is as follows: meat, grass, other edible components, fruits and vegetables, leaves, fraction < 10 mm and other and branches.

The average methane content of biogas from bio-waste was 62.1 ± 5.8%. Both the limits we obtained and the average value are below the values found in the literature [46,49]. Despite the theoretically high potential of bio-waste components for biogas production, it is not an additive value. The value for a mixture of components is not the sum of the biogas production values corresponding to the components of the bio-waste.

The results show that fermenting bio-waste, which includes kitchen and garden waste, produces high levels of biogas. The high content of organic substances, including the presence of garden waste (structural materials), makes it possible to use the digestate as fertiliser. Fermenting a mixture of kitchen and garden waste is an example of a closed-loop bioeconomy.

Producing biogas from food waste offers significant environmental, economic and societal benefits, aligning with circular economy principles [50,51,52]. It is a crucial alternative to fossil fuels for sustainable energy generation. On the other hand, it is necessary to consider the shortcomings associated with the processing of bio-waste, including its seasonal variability and relatively high hydration and the seasonal variation in its composition [2,34].