The lack of support for construction and operation of installation of biogas plants in Poland makes looking for cheap and efficient substrates. The most commonly used substrate is maize silage. This is related to the high biogas efficiency and the advanced technology of its extraction. The problem, however, is the cost of buying the silage by the biogas plants, which has grown considerably in recent years, due to its wide use. It finds food applications, is a rich nutrition source for animals but is also the most commonly used substrate in the production of renewable energy for production of bioethanol as well as in biogas fermentation. As a result, maize is a desirable substrate on the market. This paper indicates the possibility of using of post-slaughter waste in biogas plants. The methane fermentation of these waste materials allows them to be safely disposed of, so that these substrates can be obtained for a small fee or even for free. Slaughterhouse waste materials have also shown high biogas and methane efficiency and are therefore the substrates desired in the anaerobic biodegradation process. It should be noted, however, that these substrates require additional thermal treatment prior to application into the fermentation chamber, which reduces the potential income for the biogas plant..
In a biogas plant, the right choice of the substrates is a key element influencing the
kinetics of overcoming methane fermentation. The basic factor is the content of dry
matter and dry organic matter. An important issue is the purity of the substrate, i.e.
lack of foreign bodies in the form of stones or sand. The bacterial microflora plays
very important role in the methane fermentation process, so selected substrates
should be free from bactericidal and pathogenic organisms or chemicals that prevent
the functioning of anaerobic bacteria. An important issue in the selection of
substrates for biogas plants is the cost of their acquisition, availability and location of
the source of their acquisition. (Kozlowski et al., 20
Further, most commonly used substrates in biogas plants are the materials of plant origin. The most commonly used plant is maize, and its silage. In 2012, 241590.19 megagrammes of this substrate (ARR 2014) were used for biogas purposes. Szlachta and Fugol (2009) highlight the efficiency of methane fermentation process based on slurry and maize silage. In the fermentation process, vegetable and fruit residues and their pulp are also commonly used. The use of these vegetable products is due to the properties they exhibit during the anaerobic process and biogas and methane efficiency.
Methane fermentation also makes it possible to efficiently dispose the substrates that
are potentially hazardous to the environment. Many of the biodegradable byproducts
from industry and agri-food sector may be successfully used in biogas plants, which
will also allow for safe manure production
This article has compared the use of maize silage in biogas plant as the substrate most commonly used with the substrate that pose a threat to the environment.
Maize is an important and most productive crop in the world. Both in Europe and
Poland, the economic importance of maize is high and growing
It is possible to distinguish two main directions of cultivation of this plant: grain and
silage. Maize is used for consumption and industrial purposes and for the feeding of
farm animals. In recent decades, along with the development of renewable energy,
both grain (bioethanol) and silage (methane fermentation) have been used for
bioenergetic purposes
If maize silage is used as a substrate in biogas plants, the costs of maize growing are
significantly important. Maize is a plant resistant to temperature fluctuations, so it is
suitable for the climate in Poland. However, the decrease in yield during drought is
noted. The maize cultivation allows for a very high yield of green mass from 1 ha,
which is important due to the high and constant demand for filling the fermenting
chambers. This plant is easy to ensilage and is characterized by lower acquisition
costs compared to other cultivated plants. The techniques for maize growing and
harvesting are also widespread, due to the use of maize for food purposes and to
provide cattle feed
It is also important to achieve high levels of methane and biogas production from
maize silage during anaerobic biodegradation. This results from a high dry matter
content (on average 28%) and dry organic matter (on average 90%), so it is
considered as an energy crop
In order to use maize silage as a substrate for fermentation mix, the cost of obtaining this substrate should be taken into account. As highlighted, maize is a multidirectional-use crop, so that cultivation for a biogas substrate becomes competitive with respect to silage used for food and feed purposes. This leads, to the gradual increase of the price, which is noted with the development of renewable energy. Studies of Szlachta and Tupiek (2013) show that profitability of maize for silage as a substrate for biogas plants depends on the level of yield and the costs of cultivation, harvesting and silage. Moreover, the authors also highlight the need to ensure competitive prices both for the farmer and the biogas plant. Studies have proved a higher income for maize cultivated for grain.
The basic characteristic of slaughterhouse waste is the high content of organic
compounds, so that they are naturally biodegradable. These waste materials are the
source of carbohydrates, proteins, fats and biogenic substances essential for the
development of microorganisms. In view of the above, in case of lack of
management, these waste materials constitute a serious threat to the environment.
Slaughter waste is a source of serious health hazards, due to potential development of
pathogenic microflora. By disposing of decomposition they are responsible for the
burdensome odor emissions that arise during storage. Moreover, improper storage
may lead to leaching of leachate effluents consisting of biogenic compounds (nitrate,
phosphate and potassium) to soil and water, disrupting the ecosystems balance
The slaughter waste indicates high biogas and methane efficiency that result from increased lipid content compared to plant material. Fats are a source of high carbon and hydrogen content, which makes them a high potential for methane production. It should be noted, however, that the presence of fats also contributes to the reduction of methanogenic activity and biomass adsorption. As a result, lower levels of degradation and longer HRT are observed.
According to the EU law, three categories of slaughter waste are distinguished (Regulation of the European Parliament and the Council (EC) 2009). Category I requires unconditional disposal of waste at the incineration plant and is unsuitable for utilization in methane fermentation. Slaughter waste from category II and III can be processed by anaerobic fermentation in biogas plants, after prior thermal treatment under additional conditions. 2
The research material was maize silage. The slaughter waste was also used for the study. These were homogenized waste materials from category II and category III. In order to investigate the biogas and methane efficiency, a fermentation inoculum containing the desired groups of organisms was required. Inoculum, which is the separated liquid fraction of the fermentation pulp from one properly functioning agricultural biogas plant, was used in the performed experiment. The following methodology was used according to the Polish Standards: PN-90 C-04540/01 for pH determination, PN-EN 27888: 1999 for conductivity, PN-75 C-04616/01 for dry matter and PN- Z-15011-3 in case of organic dry matter.
The research on methane efficiency of the substrates in batch culture technology was carried out in the Laboratory of Ecotechnology at the Institute of Biosystems Engineering (PULS) on the basis of internal procedures, based on adapted standards: DIN 38 414-S8 and VDI 4630, commonly used in Europe. Detailed methodology of performed research was presented by Cieślik et al. (2016)
At the beginning of the experiment the basic physicochemical parameters of the
investigated substrates were defined in order to determine the proportion of
individual components in the fermentation mixtures. Table 1 shows the dry matter
content and dry matter content of the substrates. Both silage and slaughter waste
show a high dry matter content and thus require an application of additional dilution
substrate
Slaughtery waste, cathegory II Slaughtery waste, cathegory III In case of investigated substrates, the price of maize silage is 25-35 €/Mg, whereas slaughter waste materials are the substrates for disposal, so the biogas plant receives a recycling fee of 5-100 €/Mg (depending on the waste type). Consequently, the possibility of utilizing of slaughter waste in biogas plants is of mutual benefit. For a meat factory that transfers waste to environmentally safe disposal without incurring costs. And in turn, the biogas plant adopts a burdensome substrate, which is also a valuable source of energy production. The biogas and methane efficiency shown by slaughter waste represents the potential for high energy production in biogas plant. If maize silage is used which shows significantly lower biogas and methane efficiency, it should also be marked that the cost of this substrate purchasing should be taken into account, which considerably decreases the income of the planned investment. Therefore, it is proposed to use maize straw silage to maintain the viability of the plant (Cieslik et al., 2016).
In many biogas plants the primary substrate except the slurry is corn silage. This is due to the biogas and methane efficiency of the substrate and methane fermentation run, which is devoid of the inhibitor of the process. The development of biogas energetics has forced cultivation of maize with higher content of dry matter and reduced starch content. Moreover, maize is a high yielding plant with resistance to temperature changes, which makes it one of the main crops in Poland. The wide range of maize applications makes the growing and harvesting techniques also widespread. However, use of maize in food industry and the possibility of energetic support of biogas plants, causes that the purchase price of this substrate entails less and less interest in using it for energy purposes. As a result, other substrates are being sought that will allow the profitability of the biogas plant, especially in case of such low subsidies for investments in renewable energy installations in Poland. It is proposed to use biodegradable waste materials from industry and agriculture. This article proves very high biogas and methane efficiency of slaughterhouse waste. However, it should be highlighted that despite the free substrate acquisition by the biogas plants, they also require additional thermal treatment, resulting in higher investment and operating costs of biogas plants. Methane fermentation of the maize straw silage under meso- and thermophilic conditions. Energy, 115, p. 1495-1502.