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Article

Novel Specifications Regarding Biogas Production from Agriengineering Activities in Romania

by
Ioana-Ancuta Halmaciu
1,
Ioana Ionel
1,*,
Maria-Cristina Miutescu
2 and
Eugenia Grecu
3
1
Faculty of Mechanical Engineering, Politehnica University of Timisoara, Boulevard M. Viteazu 1, 300222 Timisoara, Romania
2
Faculty of Communications Sciences, Politehnica University of Timisoara, Str. Traian Lalescu, nr. 2A, 300223 Timisoara, Romania
3
Faculty of Management in Production and Transports, Politehnica University of Timisoara, Str. Remus 14, 300911 Timisoara, Romania
*
Author to whom correspondence should be addressed.
AgriEngineering 2024, 6(4), 3602-3617; https://doi.org/10.3390/agriengineering6040205
Submission received: 18 August 2024 / Revised: 19 September 2024 / Accepted: 24 September 2024 / Published: 30 September 2024
(This article belongs to the Special Issue Sustainable Development of Agroecosystems: Advances in Agricultural Engineering)
Browse Figures
Versions Notes

Abstract

:
This study centers on examining the carbon/nitrogen (C/N) ratio and metal levels in various batches of manure and their potential impact on biogas production through anaerobic fermentation. A novel aspect of this research involves the utilization of nine distinct batches sourced exclusively from livestock manure found in Romanian farms, without mixing with other potential substrates. At present, the farms are not harvesting manure for energy, but they are keen to invest in biogas production in the future as a necessary step towards renewable energy in a circular economy and a bio-waste management model. As a general conclusion that is resulting, it is shown that both the C/N ratio and the content of heavy metals in animal manure must be known when dealing with the animal manure fermentation process, especially when aiming for biogas production. The C/N ratio in the analyzed samples ranges from 6.7 to 30.2. While the ideal ratio is often considered 20–30, good methane production can occur outside this range, as seen in Sample B (small pig farm), with a C/N ratio of 13.8, proving the highest methane output. This shows that the C/N ratio is important but not the only factor influencing biogas generation. The metal content in the manure samples is similar to other studies, with potassium (K) ranging from 1.64% to 8.96%. Calcium (Ca) and K are the main metals found, posing little concern. The variation in values is linked to feed recipes. Monitoring heavy metals is crucial not only for biogas production but also for the safe use of animal manure as fertilizer, as soil contamination limits must be continuously supervised. The results are also valuable for the management of waste used as fertilizer in agricultural fields in accordance with EU law.

Graphical Abstract

1. Introduction

Climate change stems from various factors, including the extensive utilization of fossil fuels in energy production and transportation, energy inefficiency, and the underutilization of bio-energy despite available technologies [1]. Moreover, the substantial volumes of waste generated by the expanding population, coupled with inadequate management of this potential energy reservoir across diverse types, also play a role. Animals not only supply essential protein sources but also offer numerous other advantages. With the rise in population and food demands, the escalating quantities of livestock waste necessitate proper handling. Mitigating greenhouse gas emissions involves leveraging renewable energy sources like biogas. Biogas, derived from diverse raw materials such as animal manure, serves as a fundamental component. The growing agro-zoo technical sector has resulted in vast amounts of manure. Given the adverse environmental impacts of improperly managed waste, exploring effective waste management methods such as anaerobic digestion to convert waste into biogas becomes imperative. This process yields a renewable energy source, as well as a bio-waste management technology, contributing significantly to the decarbonization of animal husbandry, which is crucial for ensuring food security.
Climate change is due to several causes, such as the intensive use of fossil energy resources in energy generation and transport, energy waste [1], and less bio-energy compared to the existing possibilities and state-of-the-art clean technologies. However, the large quantities of waste generated by the growing population and the mismanagement of this potential energy resource of various types also have a contribution [2], as bio-energy could be generated based on correct bio-waste management. Animals are known to provide basic protein foods for many people, along with other benefits. The increasing quantities of livestock waste, correlated with the increase in population and food needs, require proper management [3]. Solutions to reduce greenhouse gas emissions also lie in the use of renewable energy sources, such as biogas. As a basic material, biogas can be produced from various raw materials, such as animal manure [4]. The gradual development and expansion of the agro-zoo technical sector have both generated huge amounts of manure. Considering that improperly managed waste can exert a negative impact on the environment, it is crucial to examine effective ways of waste management, such as the transformation of waste into biogas through anaerobic digestion, which generates a renewable source of energy. Producing and subsequently valorizing biogas is an effective way to contribute to the decarbonization of the animal husbandry field, which is actually intended to ensure food for the population.
Biogas is considered a feasible solution in the context of the energy crisis [5,6] and also a solution to reduce the pollution generated by agricultural waste [7]. It is worth noting that all agro-zoo-technical waste subjected to the anaerobic digestion process can become an important source of nutrients and methane production [8,9]. Another advantage of using waste is that it is widely distributed and ensures a low purchase price [10]. Following some studies, it was concluded that reactors fueled with pig manure could generate up to 6,597,520 MWh/year [11] of bio-energy.
Biogas is usually obtained through fermentation under special conditions, such as anaerobic digestion (AD), a method that transforms waste into bio-energy [12]. Anaerobic digestion is influenced by several factors, including waste quality [13], temperature, pH, C/N ratio [14], organic loading rate, and hydraulic retention time (HRT) [15].
In Table 1, important influencing factors upon biogas production are detailed, as stated in the literature, with the scope to outline the importance of the present research, namely to depict the C/N ratio and heavy metal content as main factors that potentially inhibit the anaerobic digestion process. Each of the factors listed earlier plays a crucial role in the anaerobic digestion process; the most important factors are the C/N ratio and the organic loading rate (OLR). The two factors indicate the performance of the process and the stability of the digester [16]. As such, it is extremely important to understand the properties of the raw material [17]. One can state that each of the listed factors is extremely important, which is why one should not pay too much attention to all of them [18].
The scope of the research is to identify, by applying a clean technology, if the composition of the animal waste manure originated from Romanian farms (in terms of the ratio C/N and heavy metal concentration) is affecting their anaerobic biodegradation process by forming biogas. The studied samples are collected from representative Romanian farms and diverse animals, as country-specific. Once the results are promising, conditions to use the waste manure to create a renewable energy source could be enhanced, also turning them into an energy producer, even only for their own purposes. Another scope is to compare the results to other similar ones, thus bringing an added value to the scientific literature flow.
Alternative methods or technologies that could improve biogas production could be further tested as specific to the local potential. Long-term studies are really necessary. The impact of metals in the soil is large, based on the food chain analyses; thus, using manure as fertilizers cannot be the only option for the use of manure. A scale-up plan and an assessment of potential obstacles would have been useful to see the wider applicability of the research.

2. Materials and Methods

For this study, a wide range of animal manure from several farms was used. Samples were collected from various sized private farms in Romania, including large, medium, and small ones. Small farms exhibit traditional care practices, particularly in feed supplementation with biodegradable waste. Conversely, large farms utilize automated processes with specific recipes for liquid feed production, closely monitored by specialists.
The size of the farm depends on the total number of animals raised for slaughter and/or breeding, i.e., milk and eggs. Thus, Sample A comes from a medium-sized farm consisting of about 700 pigs, Sample B was taken from a small farm with about 20 pigs, Sample C comes from a large farm with about 15,000 pigs, and Sample D comes from a small pig farm with about 100 heads. Samples E and F come from cow farms. Sample E was taken from a small cow farm with about 30 heads, while Sample F came from a large farm with 490 heads. In the case of Sample G, the farm is of medium size, having over 6000 chicken heads. The H Sample was taken from an ostrich farm (small size) with 100 heads. Sample I was taken from a small farm (10 horses).
We determined the C/N ratio for the following categories of animal manure: pig (Sample A), pig (Sample B), pig (Sample C), pig (Sample D), cattle (Sample E), cattle (Sample F), chicken (Sample G), ostrich (Sample H), and horse (Sample I). It is important to know the C/N ratio since it helps us to know the speed of decomposition of organic matter that takes place in the anaerobic digestion process. Then, with the help of the Niton XL3t portable analyzer, we identified the heavy metals present in the organic matter for the 9 samples. To find out the amount of biogas generated by each sample, we used a laboratory biogas production facility.
The values of the C/N ratio, as well as the rest of the experiments, were performed approximately 3 times on each sample until results with a representative average value were obtained. In order to obtain such results, as conclusive and correct, for each of the 9 characteristic samples for the 5 categories of animals, the following steps were followed:
  • At large farms, the samples were taken from the most populated animal sheds;
  • The mixture with the samples thus taken, specific to the farms, was performed mechanically;
  • At the end, the quantity corresponding to the purpose of the analysis was retained, the quantity being subsequently subjected to various analyses (drying at 105 °C, calcination at 550 °C, etc.).
Predictably, all the witness samples were kept in the freezer at −1/−2 °C in labeled plastic containers, which allowed the analysis to be repeated at a later date. The purpose of preservation was to be able to repeat the experiments. Generally speaking, three determinations of the same kind were performed on the same sample (batch). For the current study, the results were averaged. For differences greater than ±% between the experimental results, the analysis was repeated.

2.1. Laboratory Equipment

The C/N ratio was determined using the Leco CHN 828 laboratory equipment. With the help of the equipment, carbon, hydrogen, nitrogen, and proteins can be determined from organic matter [69]. The equipment has an ergonomic design and offers the possibility to insert 30 samples, potentially expanding the capacity up to 120 samples. The duration of the experiment was relatively short, 2.8 min, which boosts productivity [70]. Work was carried out applying standards, so the procedures used are actually those described by the standards. Drying the samples, further ongoing calcination in crucibles, and weighing are effectively standard processes. The analyses were conducted three times to calculate the arithmetic mean. The mentioned time represents the duration of a rapid analysis cycle for the model used to increase productivity, according to the indications and instructions provided by the device manufacturer.
At the beginning of the experiment, the original, raw sample is weighed in a tin capsule and inserted into the loader. Then the sample is transported to a sealed purge chamber. This chamber is designed to remove the environment/atmospheric air. From this chamber, the purged sample is automatically transferred into a reticulated ceramic crucible from the furnace. For efficient combustion of the sample, the furnace environment consists of pure oxygen. The released carbon and hydrogen gases are measured with infrared detectors. The gas sample is passed through hot copper, which helps to remove oxygen and transform nitrogen oxides (NOx) into N2 [71].
Two standards were used to determine the C/N ratio: DIN CEN/TS 15414-1: Solid recovered fuels: Determination of moisture content using the oven dry method; Part 1: Determination of total moisture by a reference method, and DIN EN 15407 Solid recovered fuels—Methods for the determination of carbon (C), hydrogen (H) and nitrogen (N) content. The indicated standards have certainly been used by other researchers, given that they are European standards, and the validation of the results requires working according to standardized procedures so that they can be validated and compared with other results. The DIN CEN/TS 15414-1:2010 standard deals with “Solid recovered fuels. Determination of moisture content using the oven dry method, Determination of total moisture by a reference method”. It is valid for solid fuels and for biofuels. It can be purchased or consulted from the standard provider, i.e., ISO: International Organization for Standardization [72], or from [73].
The other standard, DIN EN 15407, addresses Solid recovered fuels: Methods for the determination of carbon (C), hydrogen (H), and nitrogen (N) content. Generally speaking, by using standards, we increase productivity and quality while minimizing errors and waste [74]. The results are indicated in Table 2.
Considering that heavy metals can inhibit the anaerobic digestion process, the concentration of heavy metals was determined for all 9 samples. To detect heavy metals, each sample was heated to 550 °C. The identification of heavy metals in the nine samples was carried out using the Niton XL3t portable analyzer with X-ray fluorescence (XRF) [75]. This instrument assures a rapid and simultaneous identification of several different elements [76]). It has a high sensitivity and high measuring power. Moreover, the analyzer has a hot surface adapter, which allows testing in petrochemical refineries at temperatures up to 450 °C.
The analyzer can detect up to 30 elements starting from Mg to U [77]. It is suitable for analyzing batches having a temperature between 20 and ~50 °C, as it has a rechargeable battery that assures independence of several hours (up to 8 h) [78]. Such equipment can also be used for other diverse applications: fertilizers, feed, waste, and coal [69]. The results for all animal manure analyzed are comprised in Table 3.
Regarding the biogas production, the experiment was conducted using the Automatic Methane Potential Test System (AMPTS II) bio-processor. It consists of several digesters, allowing samples to be analyzed in parallel. To maintain a constant temperature (37 °C), digesters are kept in an 18 L thermostatic water bath. The installation is also provided with agitators, which have the role of mixing the digestate at the set time interval. The agitators are started by a control device, which provides power to the motors through the motor cables. The operating speed of the motors can be adjusted between 10 and 200 rpm. The equipment is also equipped with a device for measuring the volume of gases resulting (in this case, CH4). The data are issued automatically and processed by the sensors and the program serving the bio-processor, as used in the experiment.

2.2. Results

Table 2 shows the content of carbon (C) and nitrogen (N), expressed in grams, as well as the C/N ratio for each of the nine analyzed samples.
Although the vast majority of researchers conclude that the optimal C/N ratio must be between 20 and 30, there are researchers who demonstrated that good results can be obtained with a lower C/N ratio (10–20) [27,29,39]. Following a sequence of three repeated analyses on the same species (for which the arithmetic mean is representative), we found that the C/N ratio for pig manure ranged between 6.8 and 16.7, for cow manure between 16.5 and 30.2, for chicken manure it was 6.7, for ostrich manure 15.3, and for horse manure 22. Similar results are also presented in the existing literature. The ratio found was 13 for pig manure, 25 for cow manure [79], 3–10 for chicken manure, and 20–50 for horse manure [80]. In the case of ostriches, the ratio was 8.09 [81].
Analyzing the results obtained in this study, as well as in the other studies, one can conclude that the C/N ratio is important for biogas production, but not decisive in the process.
Table 3 indicates the content of carbon, nitrogen and hydrogen related to the original substance. By original substance, the initial (primary) batch, as it was sampled from the different farms, is meant.
High values in the case of sample D were recorded. For example, the total C concentration for this sample was 23 (% by mass) compared to the original substance. A relatively close value was also recorded in the case of sample G, where the concentration of total C was 22.6 (% by mass). Regarding total hydrogen, sample A recorded the highest value (11% by mass). Identical values were determined for samples E and F (10.4% by mass); close to samples E and F was sample I (10% by mass).
Animal manure is often used as organic fertilizers (amendments) to improve soil quality [82]. The quality of manure depends on several factors, such as the type of animal, age of animal, and feed administered [83]. Unfortunately, however, they often contain components that affect plant, animal, and human health [84].
Table 4 indicates the heavy metal concentration detected in the nine analyzed samples. The table shows that Sample A (pig manure), Sample G (chicken manure), and Sample H (ostrich manure) contained the highest accumulation of heavy metals. Pig manure (Sample A) contained significant amounts of Fe (767 ± 39 ppm), Zn (376 ± 14 ppm), and Mn (227 ± 36 ppm). In smaller quantities, we found metals such as Cu and Ca. In the case of Sample G (chicken manure), the Zn concentration prevailed (878 ± 21 ppm), followed by Cu (233 ± 15 ppm). Fe, Ca, and Mn were found in lower concentrations. Large amounts of Zn were also found in Sample H (ostrich manure). The concentration of Zn in the ostrich sample was 633 ± 17 ppm, followed by Mn (522 ± 46 ppm). Metals such as Fe, Cu, and Ca were found in lower concentrations. The other analyzed samples also contained amounts of metals, but their concentration was lower compared to the previously presented samples.
The results of the research indicated that the highest amount of Fe, 16,363.3 (mg∙kg−1), was found in pig manure, whilst the lowest was in chicken manure, 852.3 (mg∙kg−1). In addition to Fe, concentrations of Mn were also detected, e.g., 370.0 (mg∙kg−1) in cattle, and Zn 94.3 (mg∙kg−1) in horse manure [15]. In another study, amounts of Cu (642.1 mg Cu/kg dm) and As (8.6 mg As/kg dm) were identified in animals. Researche such as presented in [55,80], have shown that heavy metals such as iron (Fe), zinc (Zn), copper (Cu), arsenic (As), and manganese (Mn) are found in different concentrations in animal manure. Thus, in [85] one determined the concentration of heavy metals in compost obtained from several types of manure; Cu (65.6 mg Cu/kg dm and 31.1 mg Cu/kg dm) was detected in chicken and cow manure, respectively [55]. These results are also confirmed in this study. Furthermore, the information detailed in the article “Composting Manure” corroborates this study and those previously presented. For example, the presence of Cd (cadmium) in the soil can affect soil microbes and their activity, such as mineralization, which then changes the ecological balance [86].
The results obtained were referring to dry, raw batches. The literature indicates limits for other conditions. Still, the results are important in terms of indicating that the concentration of heavy metals must be definitely known, not only for the further utilization of the manure but also for the resulting residues after biogas production. In the cases analyzed, as much as we could afford (metals such as Zn, Cu, Fe, Mn, Ca, and K) according to available testing instruments, the results indicated that heavy metals were still present. It is not the scope of the article to analyze what the results for the soil could become after spreading such manure as fertilizer. Still, it is worth noting that all the mentioned species of heavy metals can have an influence on decreasing biogas production. The mentioned literature indicates that the influence could be attested by the presence of Hg < Cd < Cr, in this order (Cr the most reduced influence), or by the order of Cu (the most toxic) ˃ Zn ˃ Cr ˃ Pb (the least toxic) [64,65,66,67,68].
No literature found the concentrations of all the metals tested. Still, one concluded that, based on the results obtained, despite their presence, no effect was achieved on the biogas production as the concentration is weak. It is recommended to analyze, especially when the manure is also devoted to field fertilizer. From case to case, the results might differ.
They all depend on the receipt for feeding the animals, which varies very much from farm to farm and is specific to regional possibilities and habits. Resuming the experimental results in Figure 1, the concentrations of all heavy metals depicted in the nine manure batches (dry) are indicated. As could not be included due to its very low content value. The values were completed using data from Figure 2, highlighting a final picture of the found dependency between the NH3 produced and the metal content.
The most present metal was Ca, and this element is not as dangerous as others mentioned in the literature (As is not represented graphically because the quantities obtained are insignificant). The farms are not situated near industrial areas or intense traffic zones, in which case airborne pollution sources could be active. One can also conclude that the specificity detected in the range of values is for sure due to the feeding system used, as well as the medicine treatment that could be used for the animals.
In Romania, two important legislation texts are valid: the Romanian regulation “Technical Norms of 2004 regarding the protection of the environment and especially of soils, when sewage sludge is used in agriculture” and the European law “Council Directive 86/278/EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture” [87,88]. Both indicate limits for the HM if used as fertilizers for agricultural terrains. In Table 5, the limit values accepted in Romanian and European legislation are indicated. These values must be analyzed in the spirit of the mentioned laws. Therefore, before the manure or digestate is used as fertilizer, it is imminently necessary to analyze the soil regarding the pH value and the already existing concentration of heavy metals, in addition to the determination of the HM content in the spread material.
This experiment ran over 30 days, but starting from day 21 onward, no changes in measured values (specifically, the amount of methane produced) were observed. Consequently, only significant results were retained in the graphs.
Figure 2 shows the volume of methane accumulated by each of the nine analyzed substrates. Among all analyzed substrates, Samples B (2464.5 NmL), D (2262 NmL), and G (1911.8 NmL) recorded the highest values. At the opposite pole were Sample A (414.1 NmL) and Sample C (558.3 NmL).
The curves shown in Figure 3 indicate that Sample B recorded the highest biogas production. The value achieved by this substrate was 328.8 NmL/g DSo (DSo: dry matter basis). Among all the substrates analyzed, the lowest methane production was recorded in Sample C during the sixth day (1.7 NmL/gDSo). Although the values of this sample (Sample C) were recorded in daily increments, the recorded values were still small compared to the other substrates.
As shown in Figure 4, the daily rate of methane production varied from one day to another. The highest values were recorded in the first 8 days of the experiment, followed by several days in which biogas production was relatively constant. Towards the end of the experiment, the daily rate of methane production began to decrease. As in the case of the two graphs presented previously (Figure 2 and Figure 3), Samples B, D, and G generated the highest amount of methane/day. Samples A and C generated small amounts of biogas during the entire experiment.
To obtain the quantities of biogas shown in Figure 2, Figure 3 and Figure 4, the experimental input data presented in Table 6 were used.
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Figure 2. Accumulated methane volume [NmL] for the several samples analyzed.
Figure 2. Accumulated methane volume [NmL] for the several samples analyzed.
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Figure 3. Methane production [NmL/gDSo] for the several samples analyzed.
Figure 3. Methane production [NmL/gDSo] for the several samples analyzed.
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Table 6. Biogas experiment input boards.
Table 6. Biogas experiment input boards.
SampleInput Material [g]Inoculum [mL]
Sample A190210
Sample B24376
Sample C35365
Sample D13387
Sample E39361
Sample F42358
Sample G12388
Sample H49351
Sample I30370
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Figure 4. Daily methane production rate [NmL/days] for the analyzed samples.
Figure 4. Daily methane production rate [NmL/days] for the analyzed samples.
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According to the instructions for the use of the bio-digester, all experiments were conducted at a temperature of 37 °C. The operating time of the agitator was set to 300 s, while the rest time of the agitator was set to 3600 s [89].

3. Discussion

The novelty brought by the authors is connected to the diversity of batches representing manures from animals in Romanian farms.
Table 1 presents a list of valuable results from recent publications, thus enabling a correct judgment and evaluation of the obtained results presented in the article. A comparison of the obtained results with other opinions and tests is possible only by respecting the unicity composition of each analyzed sample, which is mostly linked to its origin. The values presented for the C/N ratio, as well as the concentrations in metals, highlight results that partially attest to other researchers’ work and also complete them in terms of new findings (for example, the C/N ratio outside the “traditional” interval of 20–30). Of the nine analyzed samples, the lowest C/N ratio was recorded in samples A (pig manure) and sample G (chicken manure). The other samples analyzed recorded a good C/N ratio.
Regarding the presence of metals and their influence, again, in comparison to existing publications and the results indicated in Table 1, one highlighted the fact they were all specific to the nature of farms from where the batches were collected, as well as the food used.
The possible use of animal manure as fertilizer was analyzed as well (Table 5) in the concept of applying two legislations (one EU and one national).

4. Conclusions

This study investigated how the C/N ratio and heavy metal concentrations in animal manure impact the biogas production process of these raw materials, thus applying a bio-waste management model and a clean technology.
The novelty consists of the results presented for nine batches, the results of the multitude of analysis runs, which are useful both for biogas producers using animal manure and for fertilizing agricultural fields. The main results indicate the following:
  • Related to the analyzed lots, the recorded C/N ratio values ranged from 6.7 (Sample G, medium-sized chicken farm) and 6.8 (Sample A, medium-sized pig farm) up to 30.2 (Sample F, medium-sized cow farm). Some literature indicates 20–30 as the best ratio. Other authors found that this interval is not strict, with good results also being attested for ratios that exceed the values in the mentioned interval. The present results attest to this latter idea. The diverse values obtained can be related to the feed recipes, stating that in the case of Sample B (small pig farm), the feed is also administered based on biodegradable household waste. The greatest methane production was the one achieved by Sample B (small family pig farm), despite the C/N ratio equaling 13.8. For pig manure coming from a small farm (Sample D), the average ratio determined was 11.2. It can thus be stated that detecting the C/N ratio is important, but not necessarily decisive in the amount of biogas generated;
  • Related to the content of heavy metals in the manure batches, the values were comparable to other bibliographic sources. In the case of potassium (K), the range covered by the determined values was narrower, varying between 1.64% (Sample E) and 8.96% (Sample G). For Sample D, it was found that the values of heavy metals were included in the variation range recorded for the other samples without approaching the extreme values recorded. Ca and K are the main metals depicted, a fact which does not raise great concern as these elements are not the most dangerous ones. An explanation for the dispersion of the values is related to the feed recipes used. Finally, we explained that the presence of heavy metals is important to know, not only for the potential to generate biogas but also for further use as fertilizer on agricultural fields, as the heavy metal content is limited in the soil.
This study investigated how the C/N ratio and metal concentrations in animal manure impact, separately, the biogas production process of these raw materials, thus applying a bio-waste management model to produce a clean fuel. The C/N ratio is more important for the analyzed bathes originating from Romanian farms in terms of influencing biogas production in comparison to the metal nature and content detected. The metal concentration is important mostly in the case of using the manure as fertilizer; no special influence upon the biogas production was identified.

Author Contributions

Conceptualization, I.-A.H.; methodology, I.-A.H.; validation, I.I.; formal analysis, I.I. and E.G., investigation, I.-A.H.; writing—original draft preparation, I.-A.H. and I.I.; writing—review and editing, I.-A.H., M.-C.M., I.I. and E.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data contained within the article are presented as variable values and graphical representations.

Acknowledgments

The thanks of the authors go to the two distinguished professors: Eng. Negrea Petru from the Research Institute for Renewable Energies of the Politehnica University of Timisoara, and Assoc. Univ. Vintila Teodor from the “King Michael I “University of Life Sciences from Timisoara”, Department of Biotechnology.

Conflicts of Interest

The authors declare no conflict of interest.

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Agriengineering 06 00205 g001
Figure 1. Comparative results concerning the metal concentrations in nine manure batches of dry matter and the accumulated NH3 volume (NmL) marked by asterisks (one excluded ± variations and the low values for As).
Figure 1. Comparative results concerning the metal concentrations in nine manure batches of dry matter and the accumulated NH3 volume (NmL) marked by asterisks (one excluded ± variations and the low values for As).
Agriengineering 06 00205 g001
Table 1. Factors influencing biogas production, as mentioned in references.
Table 1. Factors influencing biogas production, as mentioned in references.
Factors Influencing Anaerobic DigestionDescriptionReferences
TemperatureIt influences microbial growth and the degradation of organic matter.
Low temperature reduces biogas production.
Variable temperature affects biogas production.
Higher temperature inhibits ammonia, and to avoid this, the C/N ratio must be higher.
Recommended: (i) it must not be higher than 1 °C/day
(ii) maintained at temperatures lower than 0.6 °C/day.		[19,20,21,22,23,24,25]
pH-ulThe pH indicates the concentration of hydrogen ions.
It contributes to the activity of microorganisms.
The range of 5.5 and 8.5 is optimal.
Recommended: to be higher in the second stage.
Adding an alkaline substance and ORL control can maintain the pH at the desired value.		[26,27,28,29,30,31]
C/N ratioThe C/N ratio contributes to the efficiency of anaerobic digestion (microbial growth, process stability).
It influences the production of biogas& the quality of the compost.
A high C/N ratio indicates a low nitrogen content.
A low ratio leads to the accumulation of ammonia (leads to inhibition of the process).
Recommended C/N ratio is 20–30.
Lower ratios of 10–20 have led to good results.
A ratio of 20 inhibits the process.
A ratio greater than 30 decreases the biogas yield.
The optimal ratio is influenced by the chemical composition and biodegradability of the substrate.
Good results were obtained with a C/N ratio of 20 or lower, 12.7.
In other studies, large amounts were obtained in the range of 15.5/1 to 19/1.
A 16/1 ratio ensures process stability.		[23,26,27,29,32,33,34,35,36,37,38,39,40,41,42,43]
Organic loading rate (ORL)The organic loading rate indicates the amount of organic matter that must be introduced daily into the digester (can positively or negatively influence the process of anaerobic digestion). The high loading rate of the digester leads to the accumulation of (VFA) and low pH, and the low rate affects the microorganisms through the lack of nutrients. In the organic material there are volatile solids (these can be digested) and fixed solids. Knowing ORL offers the possibility to monitor the methane yield.[44,45,46,47,48]
Hydraulic retention time (HRT)HRT is the average time that the soluble compound remains inside the digester.
For animal manure longer retention time (20–30 days), food waste has a shorter retention time (15 days).		[49,50,51]
Heavy metal (HM)concentrationHM induces toxicity and is non-biodegradable in nature.
Effect: inhibitory effect on methanogens.
HM end up in animal waste through animal feed.
HM has a higher density than water, and
HM scattered on the field affects the soil and water and can lead to the burning of plant roots.
The HM amount in organic matter should not be ignored,
It inhibits the anaerobic digestion process (affects methane production).
HM is not destroyed by anaerobic digestion.
HM concentrating must be known before spreading manure/digestate on the ground (fields).		[52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68]
Table 2. Carbon (C) content, nitrogen (N) content, and the C/N ratio.
Table 2. Carbon (C) content, nitrogen (N) content, and the C/N ratio.
Sample DesignationTotal Carbon (g)Total Nitrogen (g)C/N
Sample A23.43.46.8
Sample B47.13.413.8
Sample C40.12.416.7
Sample D45411.2
Sample E39.62.416.5
Sample F45.31.530.2
Sample G334.96.7
Sample H27.61.815.3
Sample I44222
Table 3. The content of carbon, nitrogen, and hydrogen related to the original substance.
Table 3. The content of carbon, nitrogen, and hydrogen related to the original substance.
SampleTotal
Carbon		Total HydrogenOrganic HydrogenTotal NitrogenUnit
Sample A0.8110.10.1% by mass
Sample B15.19.621.1% by mass
Sample C11.39.61.60.7% by mass
Sample D238.73.22% by mass
Sample E5.410.40.70.3% by mass
Sample F6.010.40.90.2% by mass
Sample G22.66.63.13.4% by mass
Sample H9.38.61.20.6% by mass
Sample I9.6101.20.4% by mass
Table 4. The heavy metal concentration detected in the nine samples.
Table 4. The heavy metal concentration detected in the nine samples.
SampleZinc (Zn)Copper (Cu)Iron (Fe)Manganese (Mn)Calcium (Ca)Potassium (K)Arsenic (As)
Sample A376 ± 24 ppm88 ± 11 ppm767 ± 39 ppm227 ± 36 ppm0.92 ± 37%7.51 ± 1.01%<2 ppm
Sample B363 ± 12 ppm39 ± 7 ppm0.27 ± 0.01%-1.12 ± 0.37%4.81 ± 0.59%<1 ppm
Sample C0.79 ± 0.01%581 ± 20 ppm0.62 ± 0.01%0.12 ± 0.01%10.66 ± 0.28%3.80 ± 0.24%<2 ppm
Sample D0.17 ± 0.01%237 ± 12 ppm0.19 ± 0.01%517 ± 37 ppm3.64 ± 1.08%3.54 ± 1.19%<1 ppm
Sample E85 ± 6 ppm16 ± 7 ppm0.79 ± 0.01%220 ± 30 ppm4.05 ± 0.07%1.64 ± 0.06%<2 ppm
Sample F106 ± 6 ppm-0.32 ± 0.01%186 ± 26 ppm--<1 ppm
Sample G878 ± 21 ppm233 ± 15 ppm0.17 ± 0.01%0.12 ± 0.01%14.47 ± 0.36%8.96 ± 0.36%<2 ppm
Sample H633 ± 17 ppm82 ± 11 ppm1.03 ± 0.01%522 ± 46 ppm8.94 ± 0.80%2.48 ± 0.64%<4 ppm
Sample I249 ± 10 ppm50 ± 8 ppm2.22 ± 0.01%-7.67 ± 0.10%6.79 ± 0.11%<2 ppm
Table 5. Comparative limit values accepted in Romanian and European legislation.
Table 5. Comparative limit values accepted in Romanian and European legislation.
Heavy Metals (HMs)Limit Values in Romanian Regulation (kg/ha/year)Limit Values in European Legislation (kg/ha/year)
Cadmium0.50.15
Copper1212
Nickel33
Lead1515
Zinc3030
Mercury0.10.1
Chromium12-
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Halmaciu, I.-A.; Ionel, I.; Miutescu, M.-C.; Grecu, E. Novel Specifications Regarding Biogas Production from Agriengineering Activities in Romania. AgriEngineering 2024, 6, 3602-3617. https://doi.org/10.3390/agriengineering6040205

AMA Style

Halmaciu I-A, Ionel I, Miutescu M-C, Grecu E. Novel Specifications Regarding Biogas Production from Agriengineering Activities in Romania. AgriEngineering. 2024; 6(4):3602-3617. https://doi.org/10.3390/agriengineering6040205

Chicago/Turabian Style

Halmaciu, Ioana-Ancuta, Ioana Ionel, Maria-Cristina Miutescu, and Eugenia Grecu. 2024. "Novel Specifications Regarding Biogas Production from Agriengineering Activities in Romania" AgriEngineering 6, no. 4: 3602-3617. https://doi.org/10.3390/agriengineering6040205

APA Style

Halmaciu, I. -A., Ionel, I., Miutescu, M. -C., & Grecu, E. (2024). Novel Specifications Regarding Biogas Production from Agriengineering Activities in Romania. AgriEngineering, 6(4), 3602-3617. https://doi.org/10.3390/agriengineering6040205

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