Biogas from wastewater’s sludge as potential resource for transportation. Experimental approach.

Considering growing energy requirements and the quest for self-sufficiency in energy sources, the utilization of waste-derived materials has surged in various fields of research in recent years. Within this framework, waste waters, sludge, or slurries, biodegradable substances, second-generation effluents, and the resulting by-products have emerged as crucial substrates of interest to produce biogas through anaerobic digestion (AD). The objective is to employ these materials as the foundation for generating renewable energy, ultimately establishing them as the primary energy source for households or industries. This paper endeavours to assess, both physically and chemically, the waste industrial and urban wastewaters sourced from west Banat region as a foundational substrate material for co-fermentation alongside residual biomass from cereals. The results obtained are presented.


Introduction
The continuing demand of energy, year by year and rising prices of fuel grows interest in green energy such as solar power, wind turbines but also biofuel.The versatility of biofuel gives a large scale to work with, from wastewater to animal manure and to even food waste.Biofuels are an interesting topic now because of their potential energy uses.
The fundamental concept behind a circular economy involves minimizing waste and resource utilization by preserving the value of materials, products, and resources for an extended period.When a product nears the end of its useful life, it can be reclaimed to extract value.Anaerobic digestion, a method that generates biogas from diverse sources such as agricultural residues, energy crops, organicrich wastewater, the organic fraction of municipal solid waste, and industrial organic waste, is a key process in achieving this objective.[1] Biomass refers to a biodegradable material derived from plants or animals, encompassing sources such as municipal waste management, forestry residues, garbage, agricultural production, and the agrifood industry.These materials can be utilized to generate heat or electricity.Various biomass sources include cereal straw, legumes, oilseeds, hay, wood (waste from forestry and wood processing), used wood packaging, energy crops, dehydrated sewage sludge, pellets, briquettes, bio-carbon, processing industry waste, agricultural waste (such as slurry, manure, and plant biomass), municipal landfill waste, and agro-food industry waste (such as beet pulp, bagasse, molasses, wine residues, and waste from dairy and oil products).Rotten and expired fruits and vegetables also serve as biomass sources for energy production.[2][3][4][5]  An example of a widely employed renewable energy source today is biogas.Biogas is generated through anaerobic decomposition, primarily consisting of carbon dioxide and methane.The methane content within biogas typically falls between 40% and 70%.The predominant source of biogas production is from dairy farms, leading to the establishment of a substantial industry.Biogas can serve as an alternative to propane and liquid petroleum gas for heat energy applications.
Wastewater stands out as a potential feedstock for anaerobic digestion, and the importance of sustainability is on the rise across various industries, including wastewater treatment.This heightened focus is driven by more frequent energy crises and the escalating global costs of energy [6,7].Conventional wastewater treatment technologies, particularly the sludge activation process with its substantial energy consumption (approximately 1 kWh/m3, constituting up to 60% of the system's energy budget), have spurred an increasing emphasis on developing sustainable wastewater treatment approaches tailored for situations with limited resources, especially energy.[9,10].
Preserving national public health hinges on the critical aspects of wastewater treatment and management.Stringent regulations have been implemented to guarantee the proper treatment of wastewater and the appropriate disposal of treated effluent [11].The annual production of livestock manure has exhibited fluctuations in recent years, ranging from 80 to 100 million tons.Slurries constitute approximately 20% of natural fertilizer, with the remaining portion being comprised of farmyard manure.Among all manures produced, dairy cow excrement holds the largest mass share [12].
There are three leading technologies for managing cow manure.The basic, but the most burdensome for the environment and neighborhood is storage manure in piles, and periodic (3-4 times a year) spreading manure on the fields for fertilizing purposes.In this technology, the energy contained in fresh manure goes strictly to the soil and is used by soil microorganisms [13] Manure composting is a more advanced technology using various machines (tractor aerator, manure spreader).Throughout the composting process, conducive aerobic conditions are established to facilitate the proliferation of bacteria.This prompts the rapid decomposition of organic matter in manure, leading to the substantial release of energy in the form of heat [14,15].During the thermophilic composting phase, the temperature of the manure pile can elevate to nearly 80 °C and persist for several weeks.While various concepts and technologies exist to utilize the heat generated in composting, practical challenges make it difficult to effectively harness the heat from compost heaps.
The primary advantage of the composting process lies in producing compost with a high fertilizer value and ecological significance [16].
Methane fermentation represents the third method currently employed for utilizing manure.In this process, manure undergoes anaerobic microbiological breakdown [17].This results in the liberation of energy from the organic content of the manure in the form of methane, serving as a chemical fuel.Additionally, digestate, a high-grade fertilizer suitable for agriculture, is produced as a byproduct [18,19].The biogas generated through anaerobic digestion can be directly utilized in cogeneration engines (CHP) to generate heat and electricity, or in trigeneration, to produce cold as well.Alternatively, the biogas can be refined to produce biomethane, which can be injected into the gas network for future use [20,21].
In the past, initial endeavors led to the production of first-generation biofuels, primarily derived from various feedstocks such as rapeseed [24], corn [23], and soybean [22].While economically viable and technically accessible, these biofuels come with notable drawbacks, including competition for water resources between food and fuel production, biodiversity loss, and constraints on feedstock availability [25].The expensive and intricate processes involved in producing these biofuels have considerably restricted their economic feasibility and profitability, despite their numerous advantages [26].

Material and Methods
Agricultural residues from barley, wheat, corn, and rye were subjected to drying in an oven and subsequently ground into particles smaller than 1 mm using a hammer mill.This procedure adhered to the necessary material documentation protocols.Following this mechanical processing, representative samples were meticulously collected and transported to the laboratory for further analysis.
To ensure the accuracy of laboratory analysis procedures, thorough consultations of equipment user manuals and adherence to biofuels standards were undertaken before initiating the laboratory determinations.Specific analytical standards designed for solid biofuels were employed to identify the primary parameters of interest for the agricultural waste samples.These parameters included the total content of carbon, hydrogen, nitrogen, sulfur, and chlorine (ISO 16948 & 16994), moisture content (ISO 1834), ash content (ISO 18122), calorific value (ISO 14918), and volatile matter (ISO 18123).
To assess the ash's melting behavior, we followed the CEN/TS 15370 requirements.All analyses were conducted using the ISO technique, and the measurements were performed on a dry basis.For solid substrates, this entailed determining the dry mass of the sample remaining after heating it to a temperature between 105 and 110 °C for an hour.This heating process was iterated until only the solid portion of the sample remained.
In the final analysis, we examined the concentrations of sulfur, nitrogen, hydrogen, carbon, and chlorine in randomly selected dry substrates.Figure 1 provides an overview of the experimental twin 5liter anaerobic digestion reactor system.After substrate analysis (on dry basis), the materials (3 liters of biogas factory substrate, 1 liter of wastewater from beer factory and 150 grams of degraded milled corn) were tested inside the 5-liter reactors (Figure 1) to determine the potential and the quality of biogas production.

Results and Discussions
Table 1 outlines the key characteristics of the degraded barley and degraded corn employed in anaerobic digestion.Both Tables 1 and 2 demonstrate that all substrates exhibit a favorable energetic potential, as evidenced by their net calorific values.The experimental substrate formed from a mixture of all 3 materials were subject of continuous anaerobic digestion for 20 days.The process parameters and results obtained are presented in the next figures.The results of the experiment indicate a relatively high potential for biogas production from corn wastes, particularly when combined with wastewater and biogas plant substrate.
There is substantial potential for producing biogas from organic waste and energy crops.Nonetheless, utilizing energy crops as the source material for biogas may pose competition with arable land and water resources used for food production, which depends on regional availability and the demand for these resources.Consequently, European Union member states enforce sustainability criteria for the application of biogas as a vehicle fuel.
Production of biogas from urban and/or industrial wastewaters has an enormous and sustainable potential in Romania, an energy resource that is, at this time, vastly ignored by national decision factors.Technologies to convert biogas to biomethane is well established and the biogas potential should be used for transportation fuel, in the beginning at least in urban public transport fleets.

Conclusions
An investigation was conducted to assess the anaerobic digestion of agricultural resources under mesophilic temperature conditions in a batch operation.The findings revealed that combining agricultural waste barley with wastewater from a beer factory proves to be an excellent substrate for anaerobic digestion due to its significant capacity for producing biogas enriched with methane.
Overall, the research findings indicate that the combination of agricultural waste and beer factory wastewater is an excellent resource for anaerobic digestion because it yields high-quality biogas, containing roughly 70% methane, suitable to be used for energy production.
The presented research, which focuses on the production of biogas from various substrate combinations, including beer industry effluent and degraded agricultural materials, and presents anaerobic digestion procedures without the requirement for additional inoculums, may be of interest to the scientific community.This technology is useful for lowering greenhouse gas emissions in addition to being energy-efficient.
Growth and advancement are possible because of the benefits of biogas, which include its contributions to waste management and nutrient recycling.Therefore, local biogas producers should think about combining a variety of feedstocks such agricultural leftovers, pulp and paper mill residues, and animal manure in addition to sewage sludge and food waste.[27] Methane can be used as a fuel source for cars that are intended to run on compressed natural gas (CNG) by enriching the gas and compressing it.When comparing biogas to natural gas and diesel, its lower emission levels make it a more appealing and eco-friendly option.
Biogas can power natural gas vehicles (NGVs) and dual-fuel vehicles in place of fossil fuel-based gas if it is cleaned to remove CO2 and H2S.
As a fact, in 2016, the leading countries in terms of biogas production for use as a vehicle fuel were Germany, Sweden, Switzerland, the United Kingdom, and the United States.[28] Globally, approximately 500 facilities generate biogas and enhance its quality to match that of natural gas, amounting to roughly 50 Petajoules (PJ) per year.

Figure 1 .
Figure 1.Front image of the test rig for 5-liter volume.

Table 1 .
Parameters of biomass used in anaerobic digestion.Part 1.

Table 2 .
Parameters of biomass used in anaerobic digestion.Part 2.