• Energy Research

Waste, especially biomass in general, is a large reservoir of nutrients that can be recovered through different technologies and used to produce biofertilizers. In the present study, environmental impacts of the production of microalgae biomass-based phosphate biofertilizer compared to triple superphosphate through life-cycle assessment conducted in the Simapro® software were investigated. The functional unit of the analysis was 163 g of P for both fertilizers. Phosphorus was recovered from a meat processing industry effluent in a high-rate algal pond. Impacts related to the entire biofertilizer chain impacted mainly on climate changes (3.17 kg CO2eq). Microalgae biofertilizer had higher environmental impact than conventional fertilizer in all impact categories, highlighting climate change and terrestrial ecotoxicity. An ideal scenario was created considering that: all energy used comes from photovoltaic panels; in the separation step a physical method will be used, without energy expenditure (i.e. gravimetric sedimentation) and; biomass will be dried in a drying bed instead of the thermal drying. In this scenario, the impact of biofertilizer approached considerably those of triple superphosphate. When impacts of biomass cultivation and concentration stages were disregarded, drying step was of great relevance, contributing to increase biofertilizer impacts. More research is needed to optimize the algae production chain and determine the possibility of obtaining higher added value products more environmental attractive.

Microalgae-based treatment can be applied to the bioremediation of agro-industrial wastewater, aiming at a circular economy approach. The present work compared the technical-environmental feasibility of operating a bubble column photobioreactor (PBR) and a high rate pond (HRP) for microalgae biomass production and wastewater treatment of a meat processing facility. The comparison was made regarding biomass productivity, phytoplankton composition, treatment efficiency, life cycle assessment, and energy balance. The daily yields of total biomass and the maximum specific growth rates were 483.33 mg L-1 d-1 and 0.23 d-1 for PBR and 95.00 mg L-1·d-1 and 0.193 d-1 for HRP, respectively, with a predominance of the species Scenedesmus acutus. The treatment efficiency of COD (~50%) and phosphorus (100%) were similar in the two reactors. However, the PBR showed greater assimilation of ammoniacal nitrogen (100% removal) due to the higher microalgal biomass productivity. Environmental impacts were assessed through the ReCiPe methodology for midpoint and endpoint levels. Results revealed that CO2 supply was the most impactful process for both systems (>60%), but HRP reached lower environmental burdens (-105.90 mPt) than PBR (60.74 mPt). Energy balance through the Net Energy Ratio also resulted in the HPR advantage over the PBR (NER = 14.23 and 1.09, respectively). Still, both reactors present advantages when applied to different valorization routes. At the same time, both present room for improvement in the light of bioeconomy and biorefineries, aiming at sustainable wastewater treatment plants.

Hydrothermal carbonization is a thermochemical treatment whose objective is to convert carbohydrate components of a given biomass into carbon-rich material in an aqueous medium. Biomass of wastewater grown microalgae is among the various potential biomasses for this route. However, operational parameters of hydrothermal carbonization for different types of biomass are still being investigated. In general, larger temperature ranges (180-260 °C) are applied to woody biomasses, which have fibrous and/or ligneous structures and, therefore, are more thermally stable than algae biomass. This study presents the hydrothermal carbonization of microalgae biomass cultivated in an agro-industrial effluent. For this purpose, a Parr reactor was operated at different temperatures (130, 150 and 170 °C) and retention times (10, 30 and 50 min). Results showed improvements in the properties of the hydrochar, mainly energy yield and carbon concentration, after the thermochemical treatment. Energy recovery was improved, as well as hydrophobicity of the carbonized material. It was observed that in the retention time of 10 min, the increase in temperature provided an increase of 7.53% in the yield of solids. On the other hand, in the retention times of 30 and 50 min, when the temperature was increased, the solid yield decreased 6.70% and 0.92%, respectively. Thus, the highest yield of solids (77.72%) and energy (78.21%) was obtained at the temperature of 170 °C and retention time of 10 min. There was a high ash content in the raw biomass (32.99%) and an increase of approximately 3% in the carbonized material, regardless of the applied treatment. With the exception of potassium and sodium, the other macro and micronutrients were concentrated in the hydrochar after thermochemical treatment, indicating the potential of the material for agriculture application, in addition to energy use. Results showed that the retention time was the most significant operational parameter of the process.

In order to ease one of the main challenges of biomass production in wastewater, the harvest stage, this study proposes as main innovations: the comparison of technical and environmental performance of different methods of harvesting biomass which have not been addressed in the literature and the projection of an optimal environmental scenario for biomass harvesting. For this, three harvesting methods were evaluated and compared, namely the gravitational sedimentation (GS) via settling tank, coagulation with tannin followed by gravitational sedimentation (TC/GS), and a biofilm reactor operated in parallel with a settling tank (BR/GS). TC/GS required less time to concentrate the biomass (121.13 g/day); however, the biomass had a higher moisture content (99.02%), which may compromise its direct application for production of most bioproducts and bioenergy, only a dewatering step is recommended. The harvesting methods interfered in biomass characterisation, mainly in carbohydrate content, which was higher in biomass concentrated over time (28-37%) than biomass concentrated in a single day by coagulation (13.8%). The results of the life cycle assessment revealed that in scenarios which included TC/GS methods and the BR/GS presented less environmental impact in relation to only GS. Additionally, the combination of these two methods comprises the best scenario and promises to optimise the harvest of biomass grown in wastewater.