Alperujo storage times impact final compost yields

The UCO studied, for the first time, how alperujo storage times affect the subsequent composting process, taking into account both the quality of the product and the emission of greenhouse gases and microbiological activity

Alperujo is the main by-product of olive oil extraction. This waste presents a significant environmental challenge due to its high organic load and presence of phenolic compounds, which can contaminate soils and surface water. However, when the circular economy comes into play, alperujo can have a second life as a fertilizer, allowing the waste to be converted into a new product of value.

In order to better understand the composting process in which alperujo becomes a safe fertilizer for fields, the Molecular Biology of Stress Response Mechanisms and Waste Bioengineering: Green Engineering at the University of Córdoba groups joined forces to verify how a previously unstudied variable affects the composting process: alperujo storage times.

After oil extraction, the alperujo is stored in holding ponds until it undergoes treatment; in this case, composting to produce an organic fertilizer. "We studied how two different storage periods (3 and 6 months) affected the parameters during subsequent composting, such as yield, greenhouse gas emissions, the degradation of phenolic compounds, and the microbial community," explained Francisco Javier Ruiz, lead author of the study, together with Marina Barbudo.

After studying what happens on a real scale, the results showed that short-term storage (3 months) improves compost yields; that is, a greater amount of fertilizer is obtained. In addition, it generates lower greenhouse gas emissions during composting. "In both periods, phenolic compounds are efficiently removed, which is very important for the final product (the fertilizer) because these compounds are phytotoxic and can be harmful when applied in fields," said José Alhama, another of the authors. "The reason for the elimination mostly lies in the high temperature reached by the mass of compostable material, reducing the concentration of phenolic compounds and sanitizing the product," added researcher Maria del Carmen Gutiérrez.

Another innovation is the study of the microbial community during the process. "An innovation was analyzing the microbial community, identifying it taxonomically, and seeing its functional capacity. Linking this analysis to storage times was something that hadn't been evaluated," says researcher Maria Ángeles Martín Santos.

Metagenomic analysis revealed differences in the bacteriome of the raw material depending on its storage time, variations that also became evident during composting. "The thermophilic stage (the second phase of composting in which there is a great increase in temperature) prompted the selection of thermophilic bacteria (those that withstand high temperatures), which manage to degrade organic matter," continued Marina Barbudo.

According to Carmen Michán "this knowledge allows modifications to be made. If we know that a microorganism favors the degradation of certain compounds, if at any given time we don't want the final result to have, for example, a lot of phenols, it can help to add certain microorganisms." In this way, composting can be optimized. This study characterizes what happens at each juncture, allowing us to take advantage of or change conditions to obtain maximum yields with less environmental impact.

Reference:

Ruiz-Castilla FJ, Barbudo-Lunar M, Gutiérrez MC, Michán C, Martín MÁ, Alhama J. Storage of Alperujo influences composting performance: Insights into gaseous emissions and functional metagenomics. J Environ Manage. 2025 Aug 25;393:127015. doi: 10.1016/j.jenvman.2025.127015