A team of researchers from the Institute for Animal Science and Technology of the Universitat Politècnica de València (Spain) has developed a project that combines pig slurry and agricultural by-products to optimize biogas production. Thus, it manages to add value to farms' excess slurry and offers a sustainable use to some of the by-products from the fruit and vegetable processing industry.
The project's main researchers María Cambra-López, Verónica Moset and Pablo Ferrer are agronomists coordinated by Prof. Antonio Torres. They explain that pig farms generate large amounts of slurry, consisting mainly of animal excreta, cleaning water and feed residues, the management of which normally consists of storing it in pools and then using it as fertilizer in agricultural fields.
However, because of the manure's properties, rich in nutrients -such as nitrogen and phosphorus- and organic matter, it can cause pollution to soil, water and atmosphere as a result of excessive accumulation of these nutrients in soil and water and the emissions of greenhouse gases and ammonia.
In areas such as the north of Castellón and inland Valencia where there is a high concentration of pig production, there is not enough agricultural land to absorb the large volume of slurry produced on local farms. Furthermore, the transport of this slurry to other areas involves extra costs -because of its high water content- that farmers are not willing to accept, says María Cambra-López.
Therefore, the team of Spanish researchers has studied the combined processing of pig slurry and agricultural by-products to produce biogas, in order to provide a sustainable use for these products. Moreover, this combination can avoid undesirable environmental side-effects and the project offers to turn pig slurry into a valuable product: energy.
Verónica Moset explains that slurry on its own does not produce much energy, and therefore a biogas plant is not a profitable business for farmers. However, if we combine it with certain fruit and vegetables from the region that are not good enough to sell, we can increase the methane level and this way produce biogas cost-effectively.
So far, researchers have tested in vitro the combination of pig slurry with peppers, tomatoes, peaches and kaki to study their potential to produce biogas and the optimal combination of both substrates. The engineers found that peppers increased by 44% methane production compared with slurry-only; tomatoes, by 41%; peaches, by 28%, and they did not observe any difference in methane production using kaki.
With this encouraging data, Pablo Ferrer says that they will carry out trials in large-scale digesters and simulate the real biogas production process using peppers, tomatoes and peaches. Researchers believe that in another year they will be able to offer results and could transfer the technology to real-scale centralized biogas plants.
Thus, the benefit of this project is extensive and varied. On the one hand, it reduces the emission of methane during slurry storage, a highly polluting gas that has a higher greenhouse effect than CO2. On the other, it provides farmers with an alternative use for pig slurry as well as an additional income.
The researchers are also working closely with the Centre for Animal Research and Technology of the Valencian Institute for Agricultural Research (IVIA) to evaluate the effect of addition of agricultural by-products such as rapeseed oil, orange pulp or rice husk in pig feed, on methane emissions from manure which could therefore increase biogas production.
This project is funded by the Foundation Agroalimed of the Regional Department of Agriculture, Fisheries and Food.
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