Goat Manure Potential as a Substrate for Biomethane Production—An Experiment for Photofermentation

Abstract

This article presents the current state of biogas (biomethane) production technology—an example of the use of goat manure in terms of photofermentation efficiency. The theoretical and experimental potential of biomethane using biodegradability for anaerobic fermentation of goat manure was indicated. Goat manure was tested for its elemental composition to determine the suitability of this raw material for biogas production. The quality of biogas produced under atmospheric conditions from goat manure placed in a reactor (photodigester) was assessed. An attempt was made to determine the process conditions for immobilization on a goat manure bed (depending on the research material collected), which allows for demonstrating the activity of the fermentation bacterial flora, thus influencing the amount of biogas (biomethane) produced in the reactor. A mechanism for the photofermentation process involving the production of biomethane was developed. The novelty of this article is the development of the use of goat manure in an innovative way, pointing to the development of the biomethane industry. When comparing goat manure, active group (compact bed), it should be noted that K 3.132%, Na 0.266%, Ca 1.909% and Mg 0.993% are lower values compared to the material with values of K 3.397%, Na 0.284%, Ca 1.813% and Mg 0.990% which are higher. This is undoubtedly due to the presence of nutrients in the deposit that support the biomethane production process. The active group (compact bed) material A shows a dynamic increase in biomethane production with lower nutrient values. However, material B, having a higher percentage of ingredients, shows stabilization of biomethane production after the sixth month of the process. Technological trends and future prospects for the biomethane sector were initiated.

1. Introduction

Agricultural, economic and human activities generate large amounts of various types of organic waste. Domestic activities include sewage sludge from the sewage treatment process in sewage treatment plants and municipal waste in landfills. Business activities include various types of waste from the food industry, such as the production of juices, jams and beer. However, agricultural activities generate many types of waste. These include, in particular, excrement from farm animals such as cows, pigs, horses, goats, chickens and geese. As a result of their metabolic processes, slurry and manure are produced. They constitute a good natural fertilizer.

However, a problem occurs when a farm has many animals and too small acreage of arable land on which this natural fertilizer could be used. Another problem in this situation is that methane (CH₄) is emitted from such fertilizers scattered on the field. CH₄ is a greenhouse gas whose emissions are required to be limited by the European Union due to climate change. In such a situation, the solution to both problems may be the production of biogas [1], as this raw material is well suited for this purpose. The resulting biogas can then be used to produce electricity [2] for the needs of a farm and a residential house. Farm animal excrement can be classified as Renewable Energy Sources (RES). These are sources that constitute a group of commonly available, non-fossil sources, created in natural, repeated natural processes. They are characterized by no negative impact on the natural environment. Generating energy from renewable energy sources offers many benefits. This favors environmental protection because it reduces greenhouse gas emissions, mainly CH₄ and carbon dioxide (CO₂). Biomass processing is also associated with reducing emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x). Thanks to the use of biomass, the area of landfills is also reduced [3].

Biogas is a gas produced during the processing of biomass, which is one of the basic renewable energy sources. Chemically, it consists mainly of CH₄ and CO₂. Biogas [4] is a product of the biological process of converting organic substances without access to oxygen (O₂) with the participation of microorganisms with the content (50–75%) of CH₄ in biogas, depending on the substrates. Ammonia (NH₃) and hydrogen sulfide (H₂S) are unfavorable due to their destructive effect on bacteria involved in the fermentation process. They also have a negative impact on the technical infrastructure used for the fermentation process. However, the composition of biogas depends largely on the type of raw material from which it is produced. Even if the raw material is the same, the composition of the biogas may also be different. This is especially the case when biogas is produced from animal excrement. Within the same animal species, the composition of their excrement will vary depending on the type of food provided to the animals, as well as their age and health status. Of course, other species will also have a different composition of excrement, which will translate into the composition of the biogas produced from them.

Biogas is produced in the methane fermentation process [5]. This process takes place in several basic phases [6]. The methane fermentation process takes place in anaerobic conditions.

The first phase is called hydrolysis. Under the influence of the action of enzymes produced by microorganisms, organic compounds found in organic matter, such as proteins, fats and carbohydrates, are broken down into simpler organic compounds. These include: amino acids, fatty acids and sugars. The organic matter introduced into the fermentation chamber changes into pulp in this phase. Complex organic compounds, such as proteins, fats and carbohydrates, are broken down into simple compounds, i.e., simple sugars, amino acids and fatty acids. The decomposition time depends on what compounds are subjected to the decomposition process. Hydrolysis of carbohydrates occurs the fastest, within a few hours, and hydrolysis of proteins and fats takes up to several days [7]. Biomass containing more hemicellulose than cellulose has a greater hydrolysis capacity.

The second phase of biogas formation is the acidogenesis phase, in which acids are produced. Acidogenesis is a process in which hydrolysis products are transformed into organic acids, e.g., acetic, formic, butyric and propionic acids, ethyl and methyl alcohols, amines, aldehydes, CO₂ and H₂. The transitional organic compounds formed in the hydrolysis phase are further decomposed by acid-forming bacteria into simpler organic acids. These include acetic acid (CH₃COOH), propionic acid (C₂H₅COOH) and butyric acid (C₃H₇COOH). Small amounts of CO₂, lactic acid (CH₃CHOHCOOH) and alcohols are also produced.

The third phase is referred to as the acetanogenesis phase. The organic acids formed in the previous phase are transformed into CH₃COOH, CO₂ and H₂. In octanogenesis, products of acidogenesis, ethanols and volatile fatty acids are converted to acetates, CO₂ and H₂.

The fourth and last phase in the biogas formation process is the methane phase. Methanogenesis is the last stage of the methane fermentation process, in which most of the methane is produced as a result of the transformation of acetates and alcohols [8]. During this stage, methanogenic bacteria produce CH₄ and CO₂ using acetic acid and H₂. Bacteria in this phase are most sensitive to environmental changes and disruptions in previous phases. Hence, efficient biogas production requires ensuring appropriate process conditions [9].

One of the most important factors is the activity of microorganisms that produce biogas. In order to obtain their high activity, many conditions must be met. These include the regularity of loading, maintaining the composition of substrates, the method of feeding substrates (continuous or periodic), appropriate temperature (most often mesophilic fermentation is carried out at 32–38 °C), ensuring the appropriate pH and the appropriate C to N₂ ratio. If this ratio is too high, complete conversion of carbon cannot occur. If it is too low, NH₃ may be produced, which even in low concentrations, inhibits the growth of bacteria and may even lead to the destruction of their entire population, which will result in stopping the fermentation process [10]. Factors influencing the course of subsequent stages of methane fermentation, optimal conditions and process parameters, include the following:

(1)

Temperature; optimum temperature for various types of bacteria: psychrophilic fermentation 20–25 °C, mesophilic 35–37 °C and thermophilic 55–60 °C [11]. The increase in temperature has a positive effect on the metabolic activity of microorganisms and the stability and efficiency of production CH₄ [12,13]. Thermophilic fermentation is more difficult to control the process conditions and is also more energy intensive. Most biogas installations in the world operate using mesophilic technologies [14], including in Poland [15].

(2)

pH; this is one of the most important factors influencing the fermentation process. The optimal pH range is different for each stage of CH₄ fermentation. For bacteria responsible for hydrolysis and acid-forming bacteria, the optimal pH range is 4.5–6.3 [4]. According to Kallistova et al. (2014) [16], these are values in the range of 5.5–6.5. The most favorable pH range for the development of bacteria responsible for acetogenesis and metanogenesis includes values from 6.8 to 7.5 [4].

(3)

The ratio of carbon (C) to nitrogen (N), as shown in

Table 1. Carbon (C): nitrogen (N) ratio values for selected substrates (own study based on [4]).

Substrats	C:N
Cow dung	16–25
Pig manure	6–14
Slaughterhouse waste	22–37
Fallen leaves	50–53
Algae	75–100
Poultry manure	5–15
Sheep dung	30–33
Goat manure	10–17
Wheat straw	50–150
Corn stalks/straw	50–56
Sugar beet/sugar foliage	35–40
Fruits and vegetable waste	7–35

; in order to ensure the proper growth and activity of the bacteria responsible for fermentation, it is necessary to provide them with the appropriate amount of nutrients, macro- and microelements [17]. When the C to N value is too high, microorganisms may not process the total amount of carbon, while a value that is too low causes the formation of NH₃ [18]. According to Puñal et al. (2000) [19], the optimal C:N value is 10–30, while according to Jadhav et al. (2023) [20], it is C:N 20–30.

To ensure optimal fermentation conditions, mixtures of individual substrates are often used to increase the efficiency of the process, selected in such a way as to achieve the most favorable C:N ratio in given conditions [21,22].

The conditions in which fermentation takes place and the type of substrate significantly affect the activity of microorganisms involved in the fermentation process. The balance of nutrients favors the occurrence of a diversity of microorganisms, thanks to which process disruptions can be tolerated to a greater extent, and the knowledge of these relationships enables the development of tools for designing, operating and controlling the process in order to make it run as efficiently as possible [23,24].

Depending on the raw material used for biogas production, the technological process of biogas production can be divided into dry and wet fermentation. Wet fermentation takes place when the dry matter content in the substrate does not exceed 12–15%, which allows it to be pumped. If the dry matter content exceeds 16%, it is dry fermentation. The product resulting from the fermentation of farm animal excrement is digestate, which is as good a natural fertilizer as manure or slurry.

In order to increase the productivity of methane fermentation, methods of pre-treatment of substrates (mainly lignocellulose) are used [25]. These methods are physical [26,27], chemical [28], biological [29] and hybrid [30]. The aim of this study was to increase biogas production from different substrates by applying mechanical treatment only to the undegraded digestate after the fermentation process in order to reintroduce it into the process. To evaluate this approach, the digestates were ball milled for four different treatment times (0, 2, 5, 10 min) and the effects on particle size, VOCs, methane yield and degradation kinetics were measured. No reduction in VFA was detected by this treatment. Mechanical treatment resulted in up to a three-fold increase in methane yield and up to a four-fold increase in daily methane production. This study of mechanical treatment of digestate residues showed no loss of VFA due to heating. Wet sieve analyses showed clear effects at different treatment intensities. Mechanical treatment increased methane production by 9% for two-stage corn silage digestate and by 17% for two-stage hay/straw digestate. A four-fold increase in daily peak methane production was achieved using a full-scale digestate [26].

The performance of mechanical and low-temperature (<100 °C) heat treatments was investigated to improve the current performance of anaerobic digestion performed on waste-activated sludge in the largest Italian WWTP. Heat treatment resulted in disintegration rates that were an order of magnitude higher than mechanical treatment (around 25% vs. 1.5%). CH₄ production increased by 21% and 31% compared to untreated samples, for treatment conditions of 70 and 90 °C, 3 h, respectively. Heat treatment also reduced the viscosity. Initial energetic and economic assessments showed that a final total solids content of 5% was sufficient to avoid the use of auxiliary methane for the 90 °C pre-treatment and subsequent processing, provided that all the heat generated was transferred by heat exchangers [27].

The pre-treatment of wheat straw with potassium hydroxide (KOH) at room temperature (20 °C) was investigated. The effect of pre-treatment on the chemical composition and physical structures as well as the subsequent enzymatic hydrolysis and anaerobic digestion was assessed. Wheat straw containing 10% total solids was treated with KOH solution for 24 h at a wide range of KOH loadings from 2% to 50% (w/w dry weight). Higher KOH loading resulted in a greater reduction in straw lignin and chemical oxygen demand in the resulting black liquor. A maximum lignin reduction of 54.7% was observed at a KOH loading of 50%. Compared to untreated straw, the specific hydrolysis efficiency reached 14.0–92.3% in the KOH loading range of 2–50%, and the methane yield increased by 16.7–77.5% for KOH loadings of 10–50% [28].

Correlations between temperature, moisture content (MC), volatile matter (VS), oxygen content (OC) and ambient air temperature and aeration strategies were predicted. The mathematical model was verified based on the correlation coefficients between measured and predicted results of more than 0.80 for OC, MC and VS and 0.72 for temperature. The simulation results showed that water reduction was enhanced when the average aeration rate (AR) increased to 15.37 m³ min⁻¹ (6/34 min/min, AR: 102.46 m³ min⁻¹) [29].

A factorial design experiment was conducted to investigate the effectiveness of Ca(OH)₂ pre-treatment, enzyme addition and particle size on mesophilic (35 °C) anaerobic fermentation of wheat straw. By combining particle size reduction and Ca(OH)₂ pre-treatment, the average methane potential increased by 315%. It was shown that alkali pre-treatment of a 3 mm diameter straw had the highest potential [30].

The most common substances (inhibitors) that are toxic to methane bacteria and reduce the rate and effectiveness of methane fermentation are NH₃, H₂S and heavy metals in the form of free ions.

Table 2. Acceptable concentrations of the main inhibitors of methane fermentation (own study based on [8,31]).

Reference	Inhibitor	Concentration mg∙(dm3)−1
[8] [31]	Ammonia	From 4000 From 1500
		Stimulation	Uninfluenced	Inhibition at Ph 7.4–7.6	Toxic
[31]	Ammonium nitrogen:	500–2000	200–1000	1500–3000	>3000
[8]	Hydrogen sulfide	From 50	-	-	-
[31]	Sulfur	From 50	100	160	1000
[31]	Heavy metals:	In free ionic form		In carbonate form
	Ni	From 10		-
	Cu	From 40		From 170
	Cr	From 130		From 530
	Pb	From 340		-
	Zn	From 400		From 160
	Cd	-		From 180
	Fe	-		From 1750
[31]	Sodium	6000–30,000
	Potassium	From 3000
	Calcium	From 2800
	Magnesium	From 2400
	Fatty acids	Isobutyric acid: inhibitory effect from 50

contains the concentrations of the main methane fermentation inhibitors.

Substrates for agricultural biogas plants are primarily the following:

(1)

Biomass in the form of animal excrements, straw, natural substances from agricultural or forestry production and other substrates that do not pose a threat to human health, animals or the environment, exclusively of biological origin, e.g., plant biomass from the maintenance of green areas, only obtained directly from the biomass producer and then directly transferred to the biogas producer, non-municipal waste, animal by-products, pulp, pomace, post-fermentation sludge, food that is not suitable for consumption or further processing.

(2)

Waste, including waste plant mass, animal excrements, waste animal tissue, waste from the sugar industry, waste from the dairy industry, waste from the baking and confectionery industry and waste from the production of alcoholic and non-alcoholic beverages [32]. Available technologies are being improved and new solutions are being sought [33,34]. Research is being carried out to increase the share of by-products and waste among these materials, which constitute a raw material for the production of biogas and energy in the process. These activities are consistent with the goals of a circular economy and contribute to reducing the share of fossil fuels in energy production and reduce the adverse impact of their use on the environment [22,35].

One of the substrates for biogas plants can be goat manure, Figure 1, and its characteristics are presented in

Table 3. Goat manure characteristic [22]; all values except nitrogen (N), carbon (C), hydrogen (H) and sulfur (S) are percentages of total fresh sample weight; average value for 3 samples.

Parameter	Goat Manure	Inoculum
Proximate analysis
Moisture [%]	37.7 ± 0.3	97.2 ± 0.3
TS [%]	62.3 ± 0.3	2.8 ± 0.3
VS [%]	52.8 ± 0.4	1.5 ± 0
VS [%-TS]	84.7 ± 0.2
Ash [%]	10.0 ± 0	1.5 ± 0.4
Ultimate analysis
N [%-TS]	2.8 ± 0.1
C [%-TS]	43.9 ± 0.3
H [%-TS]	1.5 ± 0.2
O [%-TS]	51.3 ± 0.2
S [%-TS]	0.6 ± 0
C:N	15.7 ± 0.7
Elemental formula	C365.9H123.0O320.4N20.2S1.9
Compositional analysis
Cellulose + Hemi-cellulose [%-TS]	72.4
Lignin [%-TS]	17.6
Chemical properties
pH	7.9 ± 0.1	7.5 ± 0.2
VFA [mg/L]	539.5 ± 75.7
Alkalinity [CaCO3 mg/L]	3965.0 ± 120.2
NO3−–N [mg/L]	12.7 ± 1.1
NH4+–N [mg/L]	398.0 ± 9.9
PO4- [mg/L]	1230 ± 0.4
Total N [mg/L]	429.5 ± 78.5
NO3− + NO2− [mg/L]	14.2 ± 0.1
TKN [mg/L]	415 ± 77.8

.

The theoretical and experimental potential of biomethane, biodegradability (BD) of anaerobic digestion of goat manure (GM) is presented in

Table 4. Theoretical and experimental biomethane potential, biodegradability (BD) of goat manure (GM) anaerobic digestions. (own study based on [22]).

Elemental Formula	C365.9H123.0O320.4N20.2S1.9
C:N	15.7 ± 0.7
Theoretical biomethane potential [mL/gvs] *	290.0
Experimental biomethane potential	274.1 ± 7.8 *
Biodegradability	94.5 ± 2.7 *

* value was corrected according to VS% after the initial calculation from the formula (Table 3).

.

Testing Manure before Using It for Biogas Production

Before using a given raw material, its composition is tested to determine its suitability for biogas production. It is also estimated how much biogas can be produced from it. The most important parameters that should be examined include the content of organic C and the amount of N₂. On this basis, the C to N₂ ratio is determined.

The most important determinations performed include the following [36]:

(1)

Determination of dry mass and dry organic matter.

(2)

Determination of total nitrogen and NH₃ using the Kjeldahl method.

(3)

Measurement of pH, conductivity and redox potential.

(4)

Elementary analysis.

Dry mass determination is performed using the gravimetric method. This involves drying a specific amount of sample at a temperature of 105 °C to a constant weight. After drying is completed, the sample is placed in a desiccator. After cooling, the percentage of dry matter is calculated from the difference in mass of the sample before and after drying. Weighing is performed on an analytical balance with an accuracy of at least 0.001 g. At least three series of determinations should be performed to ensure the repeatability of the results and their reliability. The determination of dry organic mass involves calcining the sample in a muffle furnace at a temperature of 550 °C to constant mass. After cooling the samples in a desiccator, the samples are weighed and the losses after ignition and the content of dry organic matter are calculated from the difference in masses before and after roasting. At least three series of determinations should also be performed to ensure the repeatability and reliability of the results.

The determination of total and ammonium N₂ is usually performed using the Kjeldahl method. This method involves the mineralization of organic nitrogen compounds contained in the tested sample using sulfuric acid in the presence of a catalyst—copper (II) sulfate (VI) with the addition of titanium dioxide. The sample is heated to a temperature of approximately 350 °C. Under these conditions, the organic N₂ compounds are converted into ammonium sulfate. Then, the samples mineralized in the above way are alkalinized with sodium hydroxide solution. The released ammonia is distilled off with steam and absorbed in a solution of boric acid with the addition of an indicator. In the last step, the boric acid solution is titrated with hydrochloric acid solution until the color of the solution changes from green to purple. In the case of determination of ammonium nitrogen alone, the entire research procedure is the same. Only the sample mineralization stage is omitted. The determinations should be performed at least three times to ensure repeatability of the results. The determination can be performed using a typical laboratory distillation set, and mineralization can be performed using a Kjeldahl flask. You can also use a mineralization kit and a steam distillation apparatus.

The pH value, electrical conductivity and redox potential are most easily determined using a multifunctional measuring device that allows the measurement of these parameters. The pH reaction is measured using an electrode adapted to perform measurements in suspensions. The pH electrode is calibrated using buffer solutions provided by the device manufacturer. After their expiry date, calibration is performed using pH standard solutions that can be purchased from the device manufacturer or from another manufacturer. These solutions should ensure a measurement accuracy of ± 0.01 units, and their manufacturer should meet the requirements of the PN-EN ISO 17034 standard [37].

The electrode for electrical conductivity measurements should be calibrated using, for example, a 0.1 M KCl solution (conductivity 12.9 mS/cm), which can be purchased from the device manufacturer. It should also meet the requirements of the standard [37].

The redox electrode can be calibrated using a 3 M KCl solution in which the potential of the Ag/AgCl half-cell is 220 mV. Various calibration fluids available on the market can also be used if they meet the requirements of the standard [37].

In elemental analysis, the percentage of elements, in particular C and N₂, in the dry mass of samples is determined. The CHNSO elemental analyzer is used for determinations. The determination involves incinerating the sample in a reduction–oxidation furnace with electronic temperature control. The gases resulting from this process are separated on a chromatographic column. The share of individual elements corresponds to the ratio of the amount of gases from the mineralization of the sample determined on the thermal conductivity detector.

In the absence of an elemental analyzer, the content of dissolved organic C can be analyzed using the Tiurin method [38]. An air-dry sample of the raw material is hot exposed to the action of the oxidant potassium dichromate (VI) in a strongly acidic environment (concentrated H₂SO₄). Under these conditions, in the presence of the HgSO₄ catalyst, approximately 95% of organic carbon is oxidized to CO₂. The excess oxidant remaining in the solution after oxidation of organic compounds is titrated with Mohr’s salt in the presence of an indicator (N-phenanthrolinic acid). Cooking must be slow and gentle enough to avoid excessive evaporation and decomposition of chromic acid (potassium dichromate decomposes at temperatures above 150 °C [39].

This is in accordance with the “Action program aimed at reducing water pollution with nitrates from agricultural sources and preventing further pollution”, which is an annex to the Regulation of the Council of Ministers of 12 February 2020 [40]. The annex contains average annual production volumes of natural fertilizers and the concentration of nitrogen contained in them depending on the species of farm animal, including goats, its age and efficiency, and the housing system.

Table 5. Goat manure production and nitrogen content (own study based on [40]).

Technological Group	Deep Litter Housing System
	Production [t∙year−1]	Nitrogen Content [kg N∙t−1]
Mother goats	1.2	8.4
Goat kids up to 3.5 months old	0.4	9.4
Goat kids over 3.5 months old up to 1.5 year old	0.8	6.9
Others	1.0	8.0

contains data on the amount of manure and nitrogen (N) content for goats whose housing system is deep litter.

Storing manure causes losses in tonnage due to the breakdown of organic matter over time—approximately 10–20% of the mass when stored for 1 to 6 months. During storage, goat manure loses its nutritional value, and the loss of ingredients depends on the storage time. At the same time, it may pose a threat to the environment. The amounts of N, phosphorus (P), potassium (K) and sulfur (S) in goat manure, depending on the storage time, are given in

Table 6. Nitrogen, phosphorus, potassium and sulfur content of goat manure depending on storage time (own study based on [41]).

Storage Time [Months]	N	P	K	S	Dry Matter [%]
	[kg∙t−1 sm]
0 to 1	11.8	2.5	9.1	5.8	34
1 to 6	8.2	2.7	8.8	1.5	33
6 to 12	5	2.2	8.2	6.6	37
>12	5.4	2.4	4	2.8	30

.

The amount of N₂ losses, in addition to the storage time, depends on the course of atmospheric conditions, namely, rainfall and temperature; losses are greater when storing manure during dry and warm weather. These losses are caused by NH₃ emissions into the atmosphere and nitrate leaching, which may cause groundwater contamination and runoff with surface waters into watercourses. In the case of phosphorus (P), these losses are relatively small because most of the P is chemically bound to organic matter and is not easily washed out. Storing manure in conditions of high air humidity and rainfall favor the leaching of soluble potassium (K) and sulfur (S) [41]. There are many possibilities of using various types of biomass, including goat manure. It can be used to fertilize crops and also be used to produce energy.

The use of animal manure as fertilizer is one of the oldest and most widespread methods of maintaining soil fertility in the world. Natural fertilizers are a valuable source of macro- and micronutrients for crops. They have a very beneficial effect, improve microclimatic conditions and increase the activity of soil microorganisms. Manure improves the physical and chemical properties of the soil, increases the capacity of the sorption complex, preventing the escape of ions—K⁺, Na⁺, Ca²⁺, Mg²⁺—due to its dark color and absorption of solar radiation, it improves thermal conditions, regulates water–air relations in the soil and has a beneficial effect on its structure. It increases the activity of soil microorganisms, stabilizes the soil reaction, which contributes to increasing its buffering capacity, binds and deactivates toxic residues of plant protection products, reduces the mobility of heavy metals in the soil and increases the content of organic matter in the soil.

In addition to the benefits, fertilization resulting from the impact of manure may cause undesirable biological effects on soil; its use may also be associated with certain types of threats, e.g., the transmission of diseases to humans during the distribution of fertilizer caused by Coxiella burnetii [42]. However, the results of studies by other authors [43,44] did not confirm this type of risk. The survival of pathogenic bacteria can be effectively minimized—through a combination of time and temperature—by composting [45]. Many studies show that methane fermentation and the conditions in which it takes place also significantly reduce the number of pathogenic bacteria present in materials constituting the substrate for biogas plants, or even eliminate them (sometimes even without the need for hygienization and sterilization), and the process residue in the form of digestate may constitute an organic fertilizer or an agent improving soil properties [46,47]. In their use in agriculture, including factors due to the content of Cu and Zn, phytotoxicity and hygienic properties, it may be associated with certain limitations and require the use of preliminary or post-treatment measures in order to increase the quality of the digestate and meet the requirements for this form of use. Therefore, in order to obtain maximum benefits, in the management of the process whose residues in the form of digestate are to be used as fertilizer, solutions should be taken into account to minimize or eliminate the costs associated with additional processing of the digestate [48,49], and also to avoid potential biological hazards [50]. There is still a lack of research on the impact of long-term fertilization with digestate on soil fauna and flora, and the resulting impact on soil quality. Improved knowledge on this subject could contribute to increasing the share of digestate and replacing mineral fertilizers with it [51].

Taking into account the above information presented on the basis of research carried out so far around the world, the article presents the associated properties of the substrate, i.e., goat manure, and was used for the direct production of biogas (bio-methane) using a photofermenter.

The research problem is to indicate an attempt to apply the method and assess the quality of biogas production via gas permeability—derived from the hydrodynamics of gas flow, for a goat manure deposit. The Institute of Technology and Life Sciences–National Research Institute in Poland, and specifically the Technological Plant in Poznań and Warsaw, became interested in this problem.

For this purpose, a laboratory photofermenter for biomethane production was developed.

The aim of the research presented in the article is to assess the quality of biogas produced under atmospheric conditions from goat manure placed in a reactor (photodigester). An attempt was made to establish process conditions for:

-

immobilization on the goat manure bed (depending on the research material collected), which allows for demonstrating the activity of the fermentation bacterial flora, thus influencing the amount of biogas (biomethane) produced in the reactor.

The following criteria were adopted to assess the production of biogas (biomethane):

(1)

The time of collecting research material;

(2)

The mineralization and elemental composition of research materials;

(3)

The % composition of individual biogas components for a given biogas flow;

(4)

The course of changes in individual biogas components depending on the temperature and time of biogas production.

2. Materials and Methods

Goat manure was collected directly from the storage place, which was located on a farm in an open space, as follows:

(a)

Material A stored for 1 month (fresh—wet sample);

(b)

Material B stored for 12 months (old—wet sample);

And in confined spaces:

(c)

Material C stored for 12 months (old sample—dry).

Goat manure tests were performed based on the following standards:

(1)

Determination of dry matter at 105 °C by gravimetric method—ISO 11465:1994 [52];

(2)

Mineralization of vegetation samples and natural fertilizers in concentrated mineral acids into general components (macro and micronutrients)—PB/31/12:2014* [53]; PB/31/14:2014*; [54]; PN-91/R-04014* [55];

(3)

Mineralization of vegetation samples and natural fertilizers in concentrated mineral acids for nitrogen—PB/31/09:2014* [56];

(4)

Determination of phosphorus and nitrogen—according to the SKALAR methodology [57];

(5)

Determination of Na, K, Mg, Ca (ASA)—PN-ISO 9964-1:1994 [58]; PN-ISO 9964-2/AK:1997 [59];

(6)

Determination of Fe, Mn, Zn, Cu (ASA)—PN-ISO 8288:2002 [60].

* research procedure Research Laboratory of Environmental Chemistry.

In accordance with the ISO 11465:1994 standard [52], dry matter (T = 378.15 K) was determined using the gravimetric method. In accordance with the standard PB/31/12:2014* [53], PB/31/14:2014* [54] and PN-91/R-04014* [55], samples were mineralized (applied to plant material and natural fertilizers) in concentrated mineral acids into general components (macro- and micro-nutrients). In accordance with the standard PB/31/09:2014* [56], the samples were mineralized (applied to plant material and natural fertilizers) in concentrated mineral acids to produce nitrogen. Nitrogen and phosphorus were determined according to the SKALAR methodology [57]. In accordance with the PN-ISO 9964-1:1994 [58] and PN-ISO 9964-2/AK:1997 [59] standards, sodium, potassium, magnesium and calcium were determined using atomic absorption spectrometry. In accordance with the PN-ISO 8288:2002 standard [60], iron, manganese, zinc and copper were determined using atomic absorption spectrometry.

The research procedure (*) Research Laboratory of Environmental Chemistry consisted of adapting routine research procedures to laboratory working conditions for samples with expected characteristics (composition), applicable to the determination of a given ingredient in the assumed concentration range.

2.1. Experimental Stand

The laboratory production of biogas in a goat manure bed is shown in the diagram—Figure 2. The method of testing the substrate in laboratory conditions for the production of raw biomethane is characterized by the fact that in a glass vessel–reactor (photofermenter with a capacity of 0.00075 m³), the bed is located in in the form of goat manure with a capacity of 0.000375 m³. After sealing the photofermenter, a flexible flow channel was led from the photofermenter to a closed tank constituting a biogas storage facility.

In this way, three reactors (photofermenters) were prepared for three test samples:

(1)

Material A;

(2)

Material B;

(3)

Material C.

The research was carried out at a laboratory station, the main element of which was a glass vessel (photofermenter) used for biogas production in atmospheric conditions. The photofermenter operated in conditions of heat exchange with the environment, and solar radiation caused production intensity due to the glass walls of the photofermenter. The laboratory station is equipped with the following:

(a)

Natural heat source (ambient heat from the Sun);

(b)

Thermometer for measuring heat;

(c)

Gas analyzer for checking the composition and physical parameters of naturally occurring gas mixtures, with a particular purpose for testing biogas.

The measurement was carried out in a biogas flow of 0.018 m³∙h⁻¹, indicating the % composition of individual components to be CH₄, CO₂, O₂, H₂ and H₂S in ppm.

2.2. Scope and Research Methodology

The aim of the work is to assess the quality of biogas produced in atmospheric conditions from a bed of goat manure placed in a reactor (photodigester), indicating the potential use of the substrate for the production of biomethane as part of renewable energy sources.

The research method concerned is as follows:

(1)

Determining the mineralization and elemental composition of research materials;

(2)

Establishing the conditions of the biogas (biomethane) production process depending on the adopted process criteria.

3. Results and Discussion

The starting point for interpreting the variability of the content of elements (Figure 3) of non-metals (N, P), light metals (Ca, K, Mg, Na) and heavy metals (Cu, Fe, Mn, Zn) considers the phenomenon of speciation [61]. It depends primarily on the sample storage conditions (humidity, environment, temperature), and secondarily on its age (susceptibility to weathering). The case of N should be considered primarily in the context of the influence of high pH and the age of the samples. Alkalinization of the environment causes the displacement of this element in the form of ammonia (NH₃). The moisture content, depending on the age of the sample, in turn affects the mobility of stable N forms; ammonium salts, NH₄⁺, are usually well soluble in polar environments, which facilitates their migration to the leachate during the drying of the tested material. Moreover, it is necessary to take into account the phenomenon of salt decomposition by anionic hydrolysis, which also leads to the deprotonation of the ammonium cation.

The similarity of the chemical properties of ammonium and alkali cations (K⁺, Na⁺) allows us to assume that the influence of moisture content and ionics of solutions (the issue of ionic strength and the salt effect determining the influence of foreign ions on the solubility of salts) will have a qualitatively similar effect.

The variability of Ca and Mg concentrations depending on the nature of the samples should be related primarily to the form of phosphorus (different susceptibility to dissolution of anions with different numbers of acidic protons).

Labile forms of Cu, Fe and Mn are (hydrated) cations in the oxidation state II, stable in acidic conditions. Amphoteric Zn also forms highly soluble complex salts (coordination number is 4) with inorganic ligands. A separate problem is the presence of organic substances (amines, amino acids and organosulfur compounds of aliphatic, alicyclic, aromatic and mixed nature), which have the ability to coordinate (chelate) bonds, especially Cu(II) and Fe(II).

Nitrogen (Figure 4a) occurs in manure in various forms. Available forms of nitrogen (ammonium, nitrate and uric acid ions) are nitrogen considered potentially available for uptake by plants cultivated in the season of application. Manure and poultry manure contain a relatively high content of mineral N (35–70% of total N). Free nitrogen from the atmosphere is not absorbed by plants because they require compounds, and in addition, virtually all nitrogen contained in the soil remains in organic compounds whose structure is too complex to be absorbed by most plant species [62,63].

Phosphorus (Figure 4b) occurs in animal excrement as a combination of non-organic and organic forms. Generally, 45 to 70 percent of P fertilizer is non-organic. Organic P makes up the rest of the total P. Essentially all inorganic P is in the form of orthophosphate, which is the form that growing plants take. Most organic P is readily broken down by soil microorganisms into its inorganic form. Factors such as temperature, soil moisture and soil pH influence the rate of P mineralization. P availability from manure ranges from 80 to 100 percent, compared to 100 percent availability in commercial fertilizers. When nutrient application is based on P, 90% availability is usually taken into account for application rate calculations. In other words, the total P content of manure should provide almost the same effect as an equal amount of P from commercial fertilizers in terms of crop response. Plants take up phosphorus primarily in the form of anions dissolved in water, especially H₂PO₄⁻ (pH < 7) and HPO₄²⁻ (pH > 7). Since the rhizosphere is usually kept in an acidic environment, the first form predominates. Rooted aquatic plants obtain phosphorus either from water or sediment. The uptake of phosphorus by plants varies over time; in the youngest phases, they take up little of it, then it increases from a maximum before reaching the maximum biomass, and then slows down [64,65,66].

Potassium ions (K⁺) (Figure 4c) are an essential component of plant nutrition and are found in most types of soil. They are used as fertilizer in agriculture, horticulture and hydroponic cultivation in the form of chloride (KCl), sulfate (K₂SO₄) or nitrate (V) (KNO₃). The potassium content in most plants ranges from 0.5% to 2% of the harvested crop weight (calculated as oxide—K₂O). Most agricultural fertilizers contain potassium chloride, while potassium sulfate is used on crops that are sensitive to chloride or require higher sulfur content. Sulphate is formed mainly as a result of the decomposition of minerals (double salts): kainite (MgSO₄ ∙ KCl ∙ 3H₂O) and langbeinite (MgSO₄ ∙ K₂SO₄) [67,68].

Sodium (Na⁺) (Figure 4d) supports plant metabolism, especially in the regeneration of phosphoenylpyruvate and chlorophyll synthesis. It replaces potassium in maintaining turgor pressure and supporting the opening and closing of stomata. Excess sodium in the soil contributes to limiting water uptake. At the cellular level (cytoplasm), high concentrations lead to enzyme inhibition, which in turn causes necrosis [69,70,71].

Calcium (Ca²⁺)(Figure 4e) plays a key role in plant life. From germination until the end of its cycle, interfering with many processes that take place there. On a functional level, calcium is essential for cell division and expansion. This element is involved in root growth, absorption of nutrients and the action of many enzymes. It is characterized by poor mobility in the plant, as a result of which its deficiency manifests itself in a delay in the development of younger parts. In this context, one of the most affected parts is the root system, which determines the absorption of other nutrients.

Calcium in soil is usually found in the form of organic and inorganic compounds. In the form of simple cations, it occurs in the soil solution and from where it is absorbed by the plant, and also plays the role of a flocculant. It is a component of soil minerals: calcite (calcium carbonate), dolomite (calcium-magnesium carbonate), gypsum [calcium sulfate(VI)] or apatite [calcium phosphate(V)] [72,73].

Magnesium (Mg²⁺) (Figure 4f) remains essential for plants primarily due to the synthesis of chlorophyll, the location of which in the center of the porphyrin ring plays a structural role similar to that of iron in heme. Magnesium deficiency in plants causes yellowing between the leaf veins at the end of the season [74,75].

Iron [Fe(II)] (Figure 4g) is a trace element necessary for plant growth. In most soils, Fe occurs in relatively high concentrations. The exception is soils rich in calcium, for which the bioavailability of Fe may be very limited. For soil applications, Fe fertilizers are based on chelate complexes with synthetic aminocarboxylate ligands, which effectively maintain Fe in the soil solution, even in alkaline soils, and thus increase its bioavailability for plants and absorption. However, these complexes have certain limitations related to their pH-dependent stability in the environment and susceptibility to ligand exchange reactions [76,77].

Manganese [Mn(II)] (Figure 4h) is an important trace element for plants and is required by plants to an extent comparable to iron. It is similar to iron in many respects, and manganese deficiency or toxicity is often confused with iron deficiency or toxicity. Manganese plays a role in photosynthesis (decomposition of H₂O), respiration and nitrogen assimilation. It also participates in pollen germination, pollen tube growth, root cell elongation and resistance to their pathogens [78,79].

Zinc [Zn(II)] (Figure 4i) is a microelement available to plants under acidic conditions. Plants growing in soils low in zinc are more susceptible to diseases. Zinc enters soils mainly as a result of weathering of rocks, but also with phosphorus fertilizers, manure and sewage sludge [80,81].

Copper (Figure 4j) is a key component of chlorophyllin (a chlorophyll analogue), playing an important role in photosynthesis and vitamin A production. Vitamin A deficiency can interfere with protein synthesis. Copper also helps form the lignin found in cell walls, which helps keep the plant upright and is important for seed set, stress resistance and pollen production. Thick, sandy soils are more susceptible to soil deficiencies, unlike acidic, highly leached and sandy soils. Copper binds to soils with a high organic matter content, which directly affects the availability of copper for plants. Soils with a pH > 7.5 and are highly weathered will experience greater copper deficiencies. The higher presence of oxides and carbonates will impact the availability of copper [82,83].

3.1. Evaluation of Goat Manure Research—Process Aspects

M. A. Haque et al. [84] from the University of Agriculture in Bangladesh examined the efficiency of biogas production from three types of manure: cattle, goat and poultry. The research was carried out in three reactors for each type of manure, for a total of nine reactors. The authors found that most biogas was produced from goat manure. The percentage of CH₄ in biogas from goat manure was also the highest. In all cases, the biogas production process was carried out in anaerobic conditions at a temperature of 37 °C for 60 days; 65 g of each type of manure was used. The determined parameters of the tested goat manure were as follows: dry matter content: 22.79%; dry organic matter content: 19.82%; ash content: 2.81%; organic C content: 13.25%; Kjeldahl general N₂: 1.18%; C/N ratio: 18.33; pH = 8.45. It was found that the production of biogas from goat manure was 0.3011 m³/kg of volatile solid matter. As a result of the research, the authors found that the amount of biogas produced from goat manure was significantly higher than from the other two types of manure: cattle and poultry.

Hanafiah M. M. et al. [85] studied biogas production from goat manure and chicken manure in Malaysia. In the first stage, the parameters of both types of manure were tested. Then, the efficiency of biogas production in mesophilic (temperature 37 °C) and anaerobic conditions was tested. It showed that more biogas was obtained from goat manure. The study was carried out in 1 L reactors, approximately 4 g of each type of manure was used. A biogas volume of 2141 mL was obtained from goat manure, and 1855.7 mL from chicken manure. Each tested sample of both types of manure was mixed with 500 mL of industrial inoculum as a catalyst. The process was carried out for 20 days and measurements were obtained. Goat manure had the following parameters: dry matter content: 63.85%; dry organic matter content: 87.07%; chemical oxygen demand (COD): 1.93 g/L; NH₃ content: 1.3 g/L; C/N ratio = 12:1.

Christian C. Opurum et al. [86] from Nigeria conducted a kinetic study of biogas production using a mixture of goat manure, poultry manure and plantain peels in various proportions. The process of producing biogas from goat manure itself was also investigated. The anaerobic fermentation process was carried out for 47 days in seven reactors with a capacity of 10 L each. Cow rumen fluid served as the source of inoculum. The process was carried out at a temperature of 25–36 °C; 520 g of goat manure or 520 g of goat manure mixed in various mass ratios with poultry excrement and plantain skins was introduced into each reactor. In each case, the pH was adjusted to 7.8 using NaOH or HCl solutions depending on the initial pH. The highest accumulated volume of biogas was found in the case of goat manure alone. It amounted to 23.36 dm³. In the case of mixtures of goat manure with the above-mentioned ingredients, regardless of the ratio, antagonistic effects were found, which resulted in lower biogas production than when goat manure alone was used as a substrate. The parameters of the tested goat manure were as follows: moisture content: 8.96%; ash content: 28.61%; fiber content: 12.65%; N₂: 1.7%; fat content: 2%; organic C: 24.94%; total solid matter content: 91.04%; volatile solid matter content: 62.43%; C/N ratio = 15:1; CHO: 37.18%.

Grimsby L.K. et al. [87] studied biogas production from dairy goat manure in Tanzania. Manure was obtained from small farms in the Uluguru Mountains. The substrate and inoculum were diluted to obtain 1% of volatile solid matter in a total mass of 700 g, which was introduced into 1 L reactors. The inoculum was fresh cow manure. The amount of goat and cow manure corresponded to 3.5 g of volatile solid matter each. The reactors were incubated at 35 °C and stirred at 60 rpm. The anaerobic fermentation process was carried out for 50 days. The production of biogas from pig manure, cow manure, Guatemalan grass and cellulose was also studied in a similar way. The best results in terms of the amount of methane in biogas were obtained for pig manure. It was 220 L/kg of volatile organic matter. The worst result was for goat manure and it amounted to 167 L/kg. Based on the observed annual amount of manure produced by 1 goat in the analyzed area (61 kg), the authors estimated the biogas production potential corresponding to one goat per 8.5 m³ per year. The authors explain the result obtained for goat manure by the fact that this manure contains, compared to other tested substrates, much more lignin (21% compared to 8–10% for the other tested substrates), which is difficult for microorganisms to decompose. Moreover, the solids content in the tested manure was on average only about 23%. COD accounted for 40% of the total solid matter.

Otobrise C. et al. [88] from Nigeria studied the kinetics of biogas production from goat manure and Asimina triloba seeds. The anaerobic fermentation of goat manure and seeds of the Asimina triloba plant, as well as the rate and amount of biogas produced, were investigated. Fresh cow manure was used as the inoculum. Both substrates were first sun-dried to reduce the moisture content. Both substrates were then ground into a powder to increase their surface area and ensure a small particle size. Sludges of each substrate were obtained by mixing 1000 g of powder of a given type of substrate with 3500 mL of water, which constituted 22% of the total solid matter. The mixture of goat manure and Asimina triloba seeds was prepared in the same way, except that the substrate mixture was in a 1:1 molar ratio (goat manure and Asimina triloba seeds). In each case, 100 g of fresh cow manure was also added as an inoculum to accelerate the development of microorganisms. The experiment was carried out in 5 L glass reactors for 24 days. The temperature at which the process was carried out was 35 °C. The contents of the reactors were mixed daily, which prevented the sludge from caking and also uniformed the temperature and distribution of bacteria throughout the entire volume of sludge in the reactor. The volume of biogas produced in each of the three tested variants was measured every 24 h. The experimental results indicate that a larger volume of biogas was obtained from goat manure (4943 mL) than from Asimina triloba seeds (4329 mL). However, the best result was obtained for a mixture of goat manure and Asimina triloba seeds (5871 mL). The parameters of the tested goat manure before the experiment were as follows: pH: 7.21; BOD₅: 160 mg/L; COD: 490 mg/L; dry matter: 15.1%; volatile solid matter: 84%; total N2: 3.2 mg/L; total C: 28.3 mg/L, and after the experiment: pH: 7.18; BOD₅: 146 mg/L; COD: 312 mg/L; dry matter: 11.7%; volatile solid: 66%; total N₂: 2.7 mg/L; total C: 22 mg/L.

Gamma A. M. Osman et al. [89] from Sudan examined the effect of the concentration of fresh fluid from the cow’s rumen, added as a starter, on the efficiency of biogas production from goat manure. The biogas production process was carried out in anaerobic conditions using various amounts of fluid from the cow’s rumen, i.e., 0, 1, 2 and 3 g per 2.5 L of fermenter capacity. The experimental time was 45 days in all cases. The amount of biogas was measured daily. Solid goat manure was used and subjected to pre-treatment. It involved grinding and mixing. Manure samples for each test variant were prepared in such a way that 200 g of solid, pre-treated goat manure was mixed with one of the four amounts of fluid from the cow’s rumen given above and with tap water, obtaining a working volume of 2.5 L. Daily mixing was carried out manually by 30 s to ensure sample homogeneity. Each variant of the experiment was conducted in three repetitions. As a result of the experiment, it was found that after 45 days, there was no significant difference in the volume of biogas produced between manure alone and manure with three different amounts of cow rumen fluid added. A significant difference in the volume of biogas in the variant with 2 g of fluid from the cow’s rumen was observed only after the first 15 days of the experiment. By day 18 of the fermentation process in the goat manure-only variant, no biogas was produced at all. Only after 18 days did it begin to form and its growth was constant until the 45th day, when the process was completed. Based on the experiment, the authors conclude that goat manure is such a good raw material that biogas can be produced from it without the addition of inoculum. However, adding fluid from the cow’s rumen shortens the biogas production time and speeds up the reaction. The total volume of biogas obtained after 45 days of the process from goat manure alone was the lowest. However, the volume difference was not statistically significant. The parameters of the tested goat manure were as follows: total amount of solids: 97.1%; moisture content: 2.9%; volatile solid matter content: 63.8%; N₂ content: 2.5%; organic C content: 40.1%, C/N ratio: 16:1. The cow rumen fluid used had the following properties: solid substance: 36%; moisture content: 64.6%; volatile solids: 73.2%; N₂ content: 1.6%; organic C content: 54.3%; C/N ratio = 33:1.

Harjinder K. and Raghava R. Kommalapati [90] studied the biochemical potential of CH₄ production and the kinetic parameters of fermentation and biogas formation for different amounts of added inoculum. The following inoculum-to-substrate ratios (I/S) were tested: 0.0; 0.3; 0.5; 0.8; 1.1; 1.3 and 2.6; the accumulated volume of CH₄ formed was, respectively, 191.7; 214.3; 214.9; 225.9; 222.1; 222.8 and 229.9 mL per 1 g of volatile solid. Significantly less CH₄ was produced when the process was carried out only with manure. The inoculum came from a municipal sewage treatment plant. The process was carried out in mesophilic conditions in a 250 mL reactor. In each case, 1.98 g of goat manure was used and the appropriate volume of inoculum was added, then 100 mL of deionized water was added and mixed. Three replicates were used for each experimental variant to ensure repeatability of the results. Glass bottles serving as reactors were incubated at 36 °C. The study in each variant was conducted for 50 days. The volume of biogas produced was measured daily. It was found that 80–90% of the volume of CH₄ formed was obtained between 24 and 26 days and between 31 and 32 days of the experiment. The results of the experiment indicate that the production of biogas from goat manure is possible without the addition of inoculum, although it is slower and less biogas is produced than with the addition of inoculum. The authors explain the fact that biogas is produced from goat manure itself by the fact that goat intestines most likely contain bacteria that decompose organic matter and produce biogas. The authors noticed that the difference in the total volume of biogas produced in the variant with only goat manure and with different amounts of added inoculum was not significant. The parameters of the tested goat manure were as follows: moisture content: 35.9%; volatile solids: 52.8%, ash: 11.3%; N₂ content: 1.7%; C content: 35.5%; H₂ content: 6%; O₂ content: 56.2%; S content: 0.5%, C/N ratio = 20.9; cellulose and hemicellulose content: 72.4%; lignin content: 17.6%; pH: 7.8; alkalinity: 5265 mg CaCO₃/L; volatile fatty acids content: 2795 mg CH₃COOH/L; nitrate nitrogen: 9 mg/L; orthophosphates: 992 mg/L; ammonium nitrogen: 715.5 mg/L.

Zhang T. et al. [91] from China studied biogas production in the process of co-fermentation of goat manure with three different plant materials under anaerobic conditions. These included residues from three crops: wheat straw, corn stalks and rice straw. The substrates were mixed together (goat manure with one of the plant substrates) in different percentage ratios. Tests on the efficiency of biogas production in each variant were carried out in 1 L Erlenmayer flasks for 55 days. The process temperature was 35 °C. Manure from dairy animals was used as the inoculum. The total volume of the mixture subjected to fermentation in each case was 700 mL—including 140 g of inoculum and the appropriate amount of substrates. Deionized water was then added to the mixture so that the solids content was 8%. All reactors were gently stirred by hand for 1 min per day before measuring the volume of biogas produced. Biogas production from goat manure alone and residue alone from three crops was also studied for comparison. The results of the experiment indicate that the combination of goat manure with corn stalks or rice straw significantly improves the efficiency of biogas production regardless of the C/N ratio compared to goat manure alone. For a mixture of goat manure with corn stalks in the ratios of 30/70 and 70/30 and for a mixture of goat manure with rice straw in the ratios of 30/70 and 50/50, the following total volumes of biogas [mL] were obtained after 55 days of the process: 14,840, 16,023, 15,608 and 15,698. When goat manure was used as the only substrate, a total biogas volume of only 10,375 mL was obtained after 55 days. The biogas volume obtained for mixtures of goat manure with rice straw 30/70 (C/N ratio = 35.61), goat manure with corn stalks 70/30 (C/N ratio = 21.19) and goat manure with rice straw 50 /50 (C/N ratio = 26.23) was 1.62; 2.11 and 1.83 times higher than when only crop residues were used as a substrate. It was also observed that the volume of biogas produced when a mixture of goat manure and wheat straw was used was only slightly higher than when only goat manure was used for fermentation. The authors explain this by the high total content of carbon (35.63%) and lignin (24.34%) in wheat straw, which inhibits the biological decomposition of this substrate. The properties of the goat manure used in the tests were as follows: pH: 7.94 ± 0.15; total solids content: 33.65% ± 3.23%; volatile solids: 82.21 ± 8.93; total C content: 18.22% ± 1.4%; total N₂: 1.014% ± 0.11%; C/N ratio = 17.97 ± 0.84; no lignins were detected.

Nikosi S. M. et al. [92] from South Africa investigated the potential of renewable energy production from anaerobic individual fermentation and co-fermentation of chicken manure, goat manure, potato peelings and maize pulp. The amount of methane produced was tested in various variants of substrate mixtures and separately. In each case, the process was carried out under mesophilic conditions. Cow manure was used as the inoculum. Before starting the methane production test, the raw material was dried at 60 °C, the particle size was reduced to 2 mm and stored in a refrigerator at 4 °C until use. The inoculum was prepared by fermenting cow manure for a period of time during which no biogas was produced. This was conducted to ensure that the microorganisms digested all of the cow manure substrate before adding new substrate. In this way, it was ensured that the biogas produced came from a new substrate and not from the substrate contained in cow manure. The retention time of the inoculum preparation was 14 days. Chicken manure, goat manure, potato peelings, corn mush and cow manure inoculum were used as monosubstrate for monodigestion. Mixed fermentation in the following variants: chicken manure and potato peels, chicken manure and corn pulp, goat manure and potato peels, goat manure and corn pulp, corn pulp and potato peels was carried out at a quantitative ratio of 1:1 in each case. The temperature at which the fermentation process was carried out was 37 °C and the pH was 6.7–7.5—solutions of sodium hydroxide and sulfuric acid were used for correction. The bioreactors had a capacity of 500 mL, of which 100 mL of the upper space was left free. The retention time was 21 days. In the case of using single substrates, the most methane was obtained from corn slurry: 1650.8 mL CH₄/g of volatile solid. For goat manure alone, the result was 726.9 mL CH₄/g. The worst result was for chicken manure: 120.7 mL/g. In the case of various substrate mixtures in a 1:1 ratio, the best result was obtained for a mixture of goat manure and potato peelings. It was 1332.2 mL/g of volatile solid. As the authors write, such a mixture ensured the optimal level of nutrients and ensured the balance of microorganisms producing biogas. The characteristics of the tested goat manure were as follows: moisture content: 18%, volatile solids: 43.46%, ratio of volatile solids to total dry matter: 63.52%, C/N ratio = 20.47.

Alham N. R. et al. [93] from Indonesia studied the use of goat manure for a prototype biogas production installation in a simple way. The authors obtained approximately 0.49 m³ of CH₄ from 5 kg of goat manure as a result of the anaerobic fermentation process and approximately 0.767 m³ of biogas, which corresponds to 0.037 kg of LPG. This amount of biogas produced can provide electricity for 10 min for LED lighting. The average power generated was 0.033 W and the average voltage and current were 1.65 V and 0.033 A, respectively. A total of 5 kg of manure was mixed with 5 l of water, 300 mL of sugar cane juice and 40 mL of EM4 preparation, containing microorganisms carrying out the anaerobic fermentation process. All ingredients were mixed and placed in the reactor. The capacity of the tank in which fermentation was carried out was 19 l. Based on the dry matter content, 1.3 kg of dry matter of manure was fermented. The percentage composition of the obtained biogas was as follows: CH₄: 64.3%, CO₂: 12.5%, O₂: 6.2%, BAL: 17.0% The process duration was 20 days.

Mohammed H. H. and Morsy M. I. [94] from Egypt studied dry fermentation of goat manure in terms of optimizing biogas production and minimizing costs. The influence of temperature, type of fermentation (dry, wet) and hydraulic retention time on the optimization of the biogas production process and minimization of costs on a laboratory scale was investigated. The fermentation conditions were anaerobic. The fermentation of goat manure was tested at temperatures of 40 °C, 60 °C and ambient temperature. The reactors consisted of six glass bottles with a capacity of 5 L each. Samples with a total weight of 4 kg were placed in them. The retention time was 80 days. In the case of dry fermentation, the dry matter content was 30%, and for wet fermentation, 10%. All experiments were performed at pH 6–8. The accumulated volume of biogas for dry fermentation at temperatures of 40 °C, 60 °C and ambient was 58.95 L, 128.74 L and 82.07 L, respectively. For wet fermentation at the same temperatures, the accumulated volume of biogas was 92, 25 L, 61.72 L and 82.56 L. The highest accumulated biogas volume of 128.74 L was recorded for dry fermentation at 60 °C, and the lowest, amounting to 58.95 L, was recorded for dry fermentation at 40 °C. The temperature of the fermentation process is an important factor influencing the activity of microorganisms. The authors found that the amount and rate of biogas production in the case of dry fermentation was higher than in the case of wet fermentation due to the higher content of dry matter and organic matter decomposed by microorganisms. At a temperature of 40 °C, more biogas was produced in the case of wet fermentation. Very similar results were recorded for ambient temperature for both types of fermentation. The average percentage of methane in biogas in all cases was 61.89–69.35%. The characteristics of the tested goat manure were as follows: moisture content: 66.39%, dry matter content: 33.61%, volatile solids content: 82.21%, total organic carbon: 18.22%, total nitrogen: 1.014%, ratio C/N = 17.92, pH: 7.92; percentage of potassium (K₂O): 3.2%, percentage of phosphorus (P₂O₅): 0.53%.

3.2. Assessment of Biogas (Biomethane) Production in the Reactor (Photodigester)

The experimental research concerned a measurement system for assessing the quality of biogas (biomethane) in a compact bed under atmospheric conditions (mesophilic—conditions of the biogas production process). The basis for assessing biogas production is the course of changes depending on temperature. Interpreting Figure 5, it showed that for 6 months (from December to May) for the goat manure deposit—material A, with the increase in temperature from 18.0 °C to 20.5 °C (Figure 5a)—the share of percentage of CH₄ (3–9%) and CO₂ (3–9%). A classic phenomenon of gas permeability in a compact bed, characteristic of biogas production, was observed in the reactor (photofermenter). When heated, it causes methanogenic microorganisms to behave appropriately for the process. There is a clear trend of increasing biomethane production under photofermentation conditions.

A similar situation is in Figure 5b for the goat manure deposit—material B. As the temperature increases from 18.0 °C to 20.5 °C, the percentage of CH₄ (6–14%) and CO₂ (6–14%) increases evenly. The slight difference is a 1.5 times higher production of % CH₄ and CO₂, especially in May (sixth measurement).

However, interpreting Figure 5c, it indicated that for 6 months (from December to May) for the goat manure deposit—material C—with an increase in temperature from 18.0 °C to 20.5 °C, there was no CH₄ production percentage is 0. The percentage of CO₂ only begins to increase (0.5–1%) in the months from February to May. An anomaly was observed in the reactor (photofermenter)—the phenomenon of constant inhibition for a compact bed in the context of biogas production. At this stage of the process, there is much more O₂ (17.6–18.5%)—a clear trend of increasing oxygen was observed under photofermentation conditions.

In interpreting Figure 6, it was indicated that for 6 months (from December to May), depending on the deposit and its condition due to gas permeability, the functionality of the microorganisms contained in the compact deposit has a decisive influence. For goat manure—material A and material B—the production of biomethane occurs more so than in material C. This state of affairs is influenced by the elemental composition of the compacted bed in the form of goat manure (Figure 3). Taking into account the rounded values in the active group (compact bed) for material A and material B, it should be indicated that K 3.4%, Na 0.3%, Ca 1.9% and Mg 1.0% are lower values (Figure 3a,b) in relation to the inactive group (compact deposit) values of K 4.4%, Na 0.5%, Ca 1.0% and Mg 0.7% which are higher (Figure 3c)—thus causing inhibition of the process under photofermentation conditions.

Additionally, another scientific curiosity was observed, when comparing goat manure, active group (compact bed) material A, it should be noted that K 3.132%, Na 0.266%, Ca 1.909% and Mg 0.993% are lower values (Figure 3a) compared to material B, with values of K 3.397%, Na 0.284%, Ca 1.813% and Mg 0.990% which are higher (Figure 3b). This is undoubtedly due to the presence of nutrients in the bed that support the biomethane production process, Figure 6. For the active group (compact bed), material A shows a dynamic increase in biomethane production with lower nutrient values. However, material B, having a higher percentage of ingredients, shows stabilization of biomethane production after the sixth month of the process.

Microorganisms produce CH₄, CO₂, H₂ and H₂S in dark conditions (“dark fermentation”) at a temperature of 30–80 °C and pH 5–6 [95]. In contrast, photobiological [96] obtaining biomethane using purple non-sulfur bacteria [97] takes place in anaerobic conditions, in the presence of light and organic substrates. There are also mixed (hybrid) methods for a complex mixture of organic substances with the participation of visible radiation energy (photofermentation) [98]. Brown bacteria are used, which adsorb electromagnetic radiation to produce H₂ [99].

3.3. Technological Trends and Future Prospects for the Biogas Sector

Biogas is a clean and available source of energy from waste conversion with a smaller carbon footprint. The adopted policy of supporting RES [100] is still questioned in the technical and economic context, taking into account the UN Sustainable Development Goals [101], although the position of the European Union currently represents an increase in the development of the biogas sector.

One of the most urgent needs for commercial applications of biogas is transportation. This sector is believed to be responsible for 23% of global CO₂ emissions. Therefore, it is crucial to scale up the latest bioenergy solutions to obtain biofuel. It is clear that to achieve the expected goal, bioenergy must be more efficient and the use of biomass available in a sustainable way in the environment must be optimized and fully utilized. The need to purify biogas results from the fact that, in addition to CH₄, it also contains other gases, such as CO₂, H₂S, nitrogen, hydrogen, steam and oxygen. Due to the large amount of CO₂, the calorific value of the gas is reduced, and other pollutants cause corrosion of the engine and other equipment such as boiler pipes and steel components. Only biogas purified from impurities and consisting of high-quality methane can be used for fuel purposes. Researchers and technologists limited themselves to conventional solutions, proposing several strategies for modernizing biogas to biomethane, which can be used in transport.

Several pioneering biogas purification technologies have been widely researched in recent years. One of them is absorption, a chemical or physical phenomenon during which components present in the gas phase diffuse into the solvent in the liquid phase, which requires careful selection. A suitable solvent must be of a non-hazardous nature, be relatively volatile and inexpensive. In the context of biogas purification, absorption-based technology effectively separates contaminants such as H₂S and CO₂ when they are more soluble in the absorber (scrubber) than methane. This technique does not require much infrastructure beyond separation columns. In addition to the fact that this technology is already commercially available and widely distributed in biogas plants around the world, scientists are still looking for perfection in biomethane production [102].

One of the innovative approaches involves the use of ionic liquids. These salts are characterized by high CO₂ absorption, high thermal stability and low vapor pressure, which is beneficial in biogas applications. Although many promising results have been obtained on a laboratory scale [103,104,105], the main obstacles to commercialization are high costs as well as possible operational and technical difficulties. The proper design of systems and processes requires a large amount of solubility data, which is still too small to accelerate the development of this technology in biogas purification. Another approach working in absorption technology is the use of amines to remove CO₂ from biogas and is widely used due to the possibility of achieving complete methane recovery [106,107]. Recently, a more innovative approach to these compounds has been explored to overcome the problem of low-energy efficiency, which is caused by the high energy input needed to regenerate the amine solution. It was proposed to replace dissolved water with proto-nova sensors, which can be regenerated at a lower temperature, reducing energy consumption and limiting the problem of corrosion [108,109,110,111,112].

Another technology used to improve biogas is adsorption, i.e., a phenomenon occurring on the surface and consisting of selective adhesion or the binding of mixture components on a microporous solid surface. Depending on the force that acts, this process is called physisorption and chemisorption. Physisorption involves weak Vander Waal forces between the adsorbate and the adsorbent. Chemisorption involves strong chemical bonds and this process cannot be easily reversed.

Pressure swing adsorption is a technique that separates CO₂ from the produced biogas by adsorbing it on a surface under elevated pressure. The adsorption material used is usually activated carbon or zeolites, cheap and available, with high porosity. The process is regenerated by sequentially reducing the pressure. Since water vapor and hydrogen sulfide can affect the structure of the adsorbent, they must be removed from the biogas mixture before CO₂ adsorption. Even though it is widely used and has a very well-established technology, PSA still has some drawbacks to overcome. The main issue is that the two-step process necessary to remove CO₂ and H₂S requires high pressure, which consequently leads to a relatively high global warming potential.

Novel concepts using pressure swing adsorption involve the development of new adsorbers to improve the removal of interfering gases, thereby increasing efficiency and reducing process costs. Recently, studies on polymer resins [113,114,115], biomass-based adsorbents [113,114] or silica [116,117,118] have been evaluated.

Membrane separation involves passing the raw material (in this case biogas) through a membrane and selectively separating it. The basic approach of this separation technology is the difference in chemical affinity and particle size [102]. Gases separated by a membrane separation module operate on the principle of selective permeation. Small-sized molecules such as H₂, H₂S and CO₂ pass through the membrane faster than methane, enabling separation. The efficiency of the refining system depends on the selection of the membrane and its selectivity and permeability. The new membrane system is relatively simple, requires a very low energy input, and its maintenance is not complicated. Nevertheless, like all other techniques, this one also has some drawbacks. The cost of the membrane is still relatively high and its service life is limited. Currently, ongoing research in the field of membrane technology is focused on developing an economically feasible polymer with good technological properties. The goal is to achieve higher permeability of the membrane material while maintaining satisfactory selectivity. Currently, the most frequently chosen industrial membrane material for biogas upgrading is polyimide. Trend materials with great potential for use are carbon molecular sieves and mixed-matrix membranes made of polymers and zeolites, graphene oxides or other organometallic compounds.

In recent years, innovative cryogenic technology has gained popularity, and several commercial plants already operate it. The basic principle of operation of this technology is the difference in condensation temperatures of biogas components. First, the raw biogas is cooled and compressed, converting CO₂ into a liquid phase. Since condensation occurs at a very low temperature, it is necessary to eliminate H₂S and water first to avoid freezing. Other gases such as N₂ and O₂ can be further condensed during CH₄ separation [119]. Because the technology requires a large number of devices, but at the same time ensures very good quality and purity of methane, scientists are working to improve its profitability.

Supersonic separation is a method that has recently come closer to attention. The equipment used in this separation technique consists of a compact tubular device with a nozzle, used to expand the flowing biogas to supersonic speeds, which consequently causes a drop in temperature and pressure. This creates a mist of water droplets and hydrocarbons, which are further separated. However, there is not much published data on this technique because the technology has been used for other applications [120,121,122].

Research is being carried out to increase the share of by-products and waste, which are used to produce biomethane in the photofermentation process. These activities are consistent with the goals of a circular economy and contribute to reducing the share of fossil fuels in energy production and reduce the adverse impact of their use on the environment.

Biogas production is the most important and most promising alternative to replacing fossil fuels in an environmentally friendly way. In addition to the many available renewable energy sources, biogas production occupies an irreplaceable position due to the unquestionable availability of biomass and the need to manage agro-municipal waste. Therefore, scientists around the world are conducting extensive research to develop cheap and sustainable production of biogas that can be used in transport, electricity and heat generation. The success of this endeavor would benefit the environment, economy and sustainable development of countries around the world. Research has identified various resources as feedstocks for biogas production that have high energy potential, such as manure and slurry, energy crops and municipal solid waste.

When selecting substrates, special attention should be paid, in addition to the biogas potential, to the possibility of obtaining and using such materials to minimize the energy and financial expenditure on supplies (transport), preparation and feeding of feedstock to the biogas plant. Monosubstrate biogas plants located near large, specialized farms, e.g., goat farms, can play a significant role. The effectiveness of the methane fermentation process is influenced by the activity of microorganisms, the growth and development of which are inhibited by the presence of various types of inhibitors present in the waste, which also needs to be taken into account when selecting substrates and technology. In addition to the benefits in the form of energy production, biogas plants play an important role in solving problems related to the storage and storage of natural fertilizers, loss of nutrients during storage and environmental problems by limiting greenhouse gas emissions. Additionally, post-digestate, which is a waste of one process, becomes a valuable product for fertilizing crops, improving their quality and agricultural usefulness. These activities are consistent with the objectives of the circular economy.

4. Conclusions

The number of operating biogas plants increases year by year, which gives hope for achieving the goal set by governments. Interdisciplinary research is constantly being conducted and new approaches are emerging to find the perfect technological solution for economic and sustainable biogas production, its refining and, finally, exploitation. There is still a long way to go on the path to perfection, but the steps that have already been taken have certainly contributed to achieving the assumed goals:

(1)

It indicated that for 6 months, depending on the deposit and its condition due to gas permeability, the functionality of the microorganisms contained in the compact deposit has a decisive influence.

(2)

The elemental composition of the goat manure bed has a significant influence, thus causing inhibition of the process under photofermentation conditions.

(3)

A scientific curiosity was observed, when comparing goat manure, active group (compact bed), it should be noted that K 3.132%, Na 0.266%, Ca 1.909% and Mg 0.993% are lower values compared to the material with values K 3.397%, Na 0.284%, Ca 1.813% and Mg 0.990% which are higher. This is undoubtedly due to the presence of nutrients in the deposit that support the biomethane production process.

(4)

Active group (compact bed) material A shows a dynamic increase in biomethane production with lower nutrient values. However, material B, having a higher percentage of ingredients, shows a stabilization of biomethane production after the sixth month of the process.

(5)

It was indicated that for 6 months (from December to May), depending on the deposit and its condition due to gas permeability, the functionality of the microorganisms contained in the compact deposit has a decisive influence. For goat manure—material A and material B—biomethane production occurs more so than in material C.