Valorização de microalgas produzidas a partir do tratamento de residuos alimentares: biofertilizante e biogás por codigestão anaeróbia

Abstract

The current sanitation model is evolving toward a circular and self-sustaining operational concept. Integrated systems for food waste treatment coupled with microalgae production have emerged as a promising alternative for implementing this new approach, characterized by low energy demand and the generation of valuable by-products. However, knowledge gaps remain regarding optimal strategies for valorizing the microalgal biomass produced in such systems. Therefore, the objective of this study was to evaluate microalgae valorization through biogas production via anaerobic co-digestion with food waste, and as an agricultural biofertilizer. The study was structured into three parts: (i) investigation of mono- and co-anaerobic digestion of food waste and microalgae in semicontinuous reactors, assessing different organic loading rates and hydraulic retention times; (ii) evaluation of the potential use of microalgae as a soil biofertilizer in pot experiments using Brachiaria hybrid cv. Sabiá, with quantitative and qualitative assessments of soil and plant responses; and (iii) estimation and comparison of nutrient and energy recovery in a demonstration-scale reactor operated under mono- and co-digestion conditions, based on the best performance indicators from laboratory experiments. Results showed that the reactor operated with mono-digestion of food waste exhibited instability throughout the operational phases. The intermediate-to-partial alkalinity ratio (IA/PA) increased from 0.1 to 2.5, accompanied by a decrease in alkalinity to values below 0.9 gCaCO₃/L. The pH dropped below 5.0, and methane yield was significantly reduced from 0.22 LCH₄/gVS to 0.04 LCH₄/gVS. In contrast, co-digestion with microalgae maintained stable conditions, with neutral pH, IA/PA ratio below 0.3, and methane yield ranging from 0.2 to 0.3 LCH₄/gVS. Microalgae-based biofertilizers improved soil quality by supplying organic matter, macro- and micronutrients. Additionally, dry matter productivity of the forage crop increased by 12% and 19% when treated with fresh and co-digested microalgae, respectively. Co-digestion with microalgae also resulted in a positive energy gain, 60% higher than that observed in mono-digestion. Furthermore, the annual production of biofertilizer was equivalent to 1.2 tons of inorganic fertilizer. The results demonstrate that incorporating microalgae as a secondary substrate in the anaerobic digestion of food waste can be an effective strategy for maintaining stable process operation. Moreover, this approach promotes the circular reuse of resources in organic waste treatment systems, contributing to sustainable agriculture by enabling positive energy generation and optimizing nutrient cycling.