Vinasse and livestock waste yields biogas

Use of sugarcane manure and residues can mitigate problems in the supply chain during the off-season of the sugar and alcohol industry

Vinasse and livestock waste yields biogas

Use of sugarcane manure and residues can mitigate problems in the supply chain during the off-season of the sugar and alcohol industry

Brazil has great potential for the production of biogas, a type of clean and renewable energy. However, it is estimated that only 1,5% of this capacity is used. Part of the biogas produced in the country comes from vinasse, a residue derived from sugarcane after the production of ethanol. During the sugarcane off-season, however, it is difficult to maintain energy generation. Taking this problem into account, Ana Beatriz Aguiar's doctoral research in bioenergy, carried out at the School of Food Engineering (FEA) at Unicamp, shows that livestock waste, such as manure, can be used in an integrated manner with vinasse.

Brazil, the world's second largest producer of ethanol and beef, according to data from the International Energy Agency and the Brazilian Association of Meat Exporting Industries, needs to better manage these sectors so that their waste generates less environmental impact. However, agriculture, although responsible for a large amount of waste, accounts for only 19% of the country's biogas production. The leading sector in this regard, sanitation, originates the solid waste and urban effluents responsible for 63% of the biogas produced in the country, according to data from the Panorama do Biogás no Brasil 2023 survey. “The research, then, combines the energy, economic and environmental aspects”, summarizes Aguiar, who is currently doing a postdoctoral fellowship at the São Paulo Center for Studies in Biogás e Bioprodutos (CP2B).

Integration between sectors

In the case of the sugar and alcohol industry, there is great potential for biogas, since each liter of fuel yields an average of 10 liters of vinasse. “This is a process that generates more waste than product, and today this waste is used in sugarcane cultivation as a fertigation system [application of fertilizers in irrigation water], which is beneficial for the plant,” explains Bruna Moraes, thesis advisor and coordinator of the Interdisciplinary Center for Energy Planning (Nipe) and CP2B.

The decomposition of vinasse in the soil, however, emits greenhouse gases. The problem is reduced if the organic part of the substrate is removed to produce biogas. “This biodigestion process does not negatively affect the nutritional part of the substrate. The organic matter is removed, and the other components, such as nitrogen, potassium and phosphorus, which are the main reason why the sugar mill owner dumps vinasse on the soil, remain there.” Thus, the advisor summarizes, it becomes possible to use vinasse more efficiently. 

Regarding livestock waste, the author decided to focus her thesis on the waste from this activity. Aguiar points out that cattle can produce between 5% and 6% of their body weight in manure per day. A 500-kilo cow, for example, generates an average of 25 kilos of waste. The thesis sought to evaluate anaerobic co-digestion (without oxygen) using vinasse and cattle waste during the sugarcane harvest, in addition to seeking alternatives for the off-season, when vinasse is not available and biogas production is interrupted.

 “The insertion of manure seeks to optimize this off-season period because what actually happens today in most plants is the interruption of the process. So, if we can align it with another substrate from another sector of the economy, we can optimize production during these months without impacting the microbial consortium [group of microorganisms that degrade organic matter] in the biodigester, as there will be continuity in the system’s feeding. This integration is the project’s greatest innovation,” explains Aguiar.

In addition to manure, the researcher conducted studies with so-called blood water, wastewater from cattle slaughter, as a co-substrate. “The red line [a term that refers to the blood channeling from slaughter] is a liquid residue with traces of blood. This is interesting because it is liquid and can replace vinasse in this sense. These are completely different residues, but since we need a liquid residue to make up the mixture, for example, with manure, a substrate with this property is interesting.”

Co-advisor of the thesis, Renata Rodriguez, professor at the Federal University of Alfenas (Unifal) and vice-coordinator of CP2B, points out the advantages of integrating production systems. “We need better management and better handling of waste because it is often very expensive for a producer to implement an energy recovery system for biogas. The work proposes integrating livestock farming with the sugar and alcohol industry, trying to bring these sectors closer together so that the waste from one becomes raw material for the other and the energy produced by one is used by the other.”

The results

Aguiar's research used two types of biodigesters: the anaerobic structured bed reactor (ASTBR) and the semi-continuous stirred tank reactor (s-CSTR), which were operated for more than 400 days.

Aguiar explains that each piece of equipment has its own operational characteristics and potential. The s-CSTR reactor is more robust and can operate with substrates that have higher concentrations of total solids, such as manure. The fixed bed reactor (ASTBR) requires a feed with lower levels of total solids, and is classified as a high-rate reactor, capable of operating with high volumetric organic loads. “This means that it can treat larger volumes of substrate in a shorter period of time, when compared to other models, such as the CSTR reactor,” he states.

In the case of ASTBR, there was an assessment of the performance operational with vinasse and cattle manure. In the s-CSTR, simulations of biogas production were carried out during harvest and off-season periods, incorporating blood water

The best results in producing methane were obtained with the mixtures of vinasse and manure, which proved more efficient than biogas production with vinasse alone. In the case of co-digestion with both residues, the researcher indicates, there was an increase in biogas production and greater system stability.

In the simulation during the off-season, the researcher confirmed that the operation with blood water and manure allows the biogas production process to continue. 

Challenges 

Biogas production, which is viable in Brazil, is one of the challenges set by the country at the 2024 United Nations Climate Change Conference (COP29), held in Azerbaijan, with regard to reducing net greenhouse gas emissions by up to 67% by 2035. “We believe that Brazil could lead this race to replace energy sources, but it is important to remember that those who want energy want energy security,” says Rodriguez. Therefore, it is necessary to “ensure that we will not be dependent on harvests, for example. To ensure that there will not be a lack of substrate due to climate issues.”

For a significant increase in biogas production, the co-advisor recommends massive investments from the government and the business sector. Another issue concerns the training and technological development needed to build the biogas production infrastructure. In the case of the biodigesters used by Aguiar, development was handled by startup MLima Engenharia, incubated at the Unicamp Science and Technology Park. This equipment was assembled by Brenno Lima, a PhD student in energy systems planning, who needed “benchtop” biodigesters (equipment for laboratory research). However, on an industrial scale, in the case of these biodigesters, there is still a great dependence on European technologies.