Using agro-industrial wastes for the cultivation of microalgae and duckweeds: Contamination risks and biomass safety concerns

Abstract

Aquatic organisms, such as microalgae (Chlorella, Arthrospira (Spirulina), Tetrasselmis, Dunalliela etc.) and duckweed (Lemna spp., Wolffia spp. etc.) are a potential source for the production of protein-rich biomass and for numerous other high-value compounds (fatty acids, pigments, vitamins etc.). Their cultivation using agro-industrial wastes and wastewater (WaW) is of particular interest in the context of a circular economy, not only for recycling valuable nutrients but also for reducing the requirements for fresh water for the production of biomass. Recovery and recycling of nutrients is an unavoidable long-term approach for securing future food and feed production. Agro-industrial WaW are rich in nutrients and have been widely considered as a potential nutrient source for the cultivation of microalgae/duckweed. However, they commonly contain various hazardous contaminants, which could potentially taint the produced biomass, raising various concerns about the safety of their consumption. Herein, an overview of the most important contaminants, including heavy metals and metalloids, pathogens (bacteria, viruses, parasites etc.), and xenobiotics (hormones, antibiotics, parasiticides etc.) is given. It is concluded that pretreatment and processing of WaW is a requisite step for the removal of several contaminants. Among the various technologies, anaerobic digestion (AD) is widely used in practice and offers a technologically mature approach for WaW treatment. During AD, various organic and biological contaminants are significantly removed. Further removal of contaminants could be achieved by post-treatment and processing of digestates (solid/liquid separation, dilution etc.) to further decrease the concentration of contaminants. Moreover, during cultivation an additional removal may occur through various mechanisms, such as precipitation, degradation, and biotransformation. Since many jurisdictions regulate the presence of various contaminants in feed or food setting strict safety monitoring processes, it would be of particular interest to initiate a multi-disciplinary discussion whether agro-industrial WaW ought to be used to cultivate microalgae/duckweed for feed or food production and identify most feasible options for doing this safely. Based on the current body of knowledge it is estimated that AD and post-treatment of WaW can lower significantly the risks associated with heavy metals and pathogens, but it is yet unclear to what extent this is the case for certain persistent xenobiotics.

Utilization of Non-Living Microalgae Biomass from Two Different Strains for the Adsorptive Removal of Diclofenac from Water

“…Among the strategies to reduce costs associated with the culture of microalgae is the utilization of flue gases as CO 2 supply and wastewater as nutrients and freshwater source [3]. In this way, microalgae could be used for the biosequestration of CO 2 while accomplishing wastewater treatment [39]. In any case, during cultivation, waste microalgae biomass is generated and a use should be given to this biomass within the actual circular economy context.…”

Production of Microalgal Slow-Release Fertilizer by Valorizing Liquid Agricultural Digestate: Growth Experiments with Tomatoes

“…Indeed, microalgae can efficiently remove the nutrients from digestate while producing high-value biomass that can be used for the production of biochemical, biomaterials, fertilizers and/or biofuels [12,14,15]. Since digestates are rich in macronutrients (N, P, K, S, Ca and Mg) [16][17][18] and thanks to its ability to uptake nutrients from media, microalgae seem a potential solution to remediate nutrients present in liquid digestate from agricultural biogas plants. For instance, Zuliani et al (2016) have investigated the cultivation of Chlorella vulgaris on different anaerobic digestates deriving from municipal wastewater, sewage sludge or agro-waste treatment plants in closed photobioreactor [13].…”

“…The cultivation of microalgae using Anaerobic Liquid Digestate (ALD) is of particular interest because it provides a production scheme that falls inside the context of a circular bioeconomy, by recycling valuable nutrients and the simultaneously producing of valuable biomass that can be further utilized as a feedstock for a biorefinery platform [17]. However, microalgae cultivated in liquid digestate or wastewaters, which typically contain various hazardous contaminants, and could potentially contaminate the produced biomass, raise various concerns about the safety of their consumption restricting their use for human applications [17].…”

“…The cultivation of microalgae using Anaerobic Liquid Digestate (ALD) is of particular interest because it provides a production scheme that falls inside the context of a circular bioeconomy, by recycling valuable nutrients and the simultaneously producing of valuable biomass that can be further utilized as a feedstock for a biorefinery platform [17]. However, microalgae cultivated in liquid digestate or wastewaters, which typically contain various hazardous contaminants, and could potentially contaminate the produced biomass, raise various concerns about the safety of their consumption restricting their use for human applications [17]. For this purpose, microalgae used as slow-released bio-fertilizers or bio-stimulants have been recently investigated obtaining promising results and can be considered as an eco-friendly solution avoiding chemical pollution especially by leaching or evaporation of nutrients as it happens when synthetic fertilizers or liquid digestate are directly applied in land [22,23].…”

Effect of post-treatment process of microalgal hydrolysate on bioethanol production

“…Therefore, there is a need for a strategy that uses carbohydrate residue after producing biofuels 8 – 10 . However, the chemical treatment of these carbohydrates to produce edible value-added carbohydrate products may also produce undesirable contaminants due to the adverse effects of organic solvents 11 , 12 . Therefore, utilization of these microalgal carbohydrates as a source for microbial fermentation medium has been limited to ethanol production 13 , 14 , although there is a need for more progress in this area.…”