Economic feasibility of microalgal bacterial floc production for wastewater treatment and biomass valorization: A detailed up-to-date analysis of up-scaled pilot results

How can we possibly resolve the planet's nitrogen dilemma?

Nitrogen is the most crucial element in the production of nutritious feeds and foods. The production of reactive nitrogen by means of fossil fuel has thus far been able to guarantee the protein supply for the world population. Yet, the production and massive use of fertilizer nitrogen constitute a major threat in terms of environmental health and sustainability. It is crucial to promote consumer acceptance and awareness towards proteins produced by highly effective microorganisms, and their potential to replace proteins obtained with poor nitrogen efficiencies from plants and animals. The fact that reactive fertilizer nitrogen, produced by the Haber Bosch process, consumes a significant amount of fossil fuel worldwide is of concern. Moreover, recently, the prices of fossil fuels have increased the cost of reactive nitrogen by a factor of 3 to 5 times, while international policies are fostering the transition towards a more sustainable agro-ecology by reducing mineral fertilizers inputs and increasing organic farming. The combination of these pressures and challenges opens opportunities to use the reactive nitrogen nutrient more carefully. Time has come to effectively recover used nitrogen from secondary resources and to upgrade it to a legal status of fertilizer. Organic nitrogen is a slow-release fertilizer, it has a factor of 2.5 or higher economic value per unit nitrogen as fertilizer and thus adequate technologies to produce it, for instance by implementing photobiological processes, are promising. Finally, it appears wise to start the integration in our overall feed and food supply chains of the exceptional potential of biological nitrogen fixation. Nitrogen produced by the nitrogenase enzyme, either in the soil or in novel biotechnology reactor systems, deserves to have a 'renaissance' in the context of planetary governance in general and the increasing number of people who desire to be fed in a sustainable way in particular.

Natural microalgal cultivation systems using primary effluent and excess sludge

Microalgae have been recently cultivated using resources from wastewater treatment plants, including nutrients, CO₂, and heat. In the present study, we focused on the natural cultivation of total microalgae rather than specific microalgae, for which culture conditions and the cultivation environment are difficult to prepare. Natural microalgal cultivation systems using 380-L raceway tanks were operated outdoors for 8 months and the effects of the culture substrate were investigated. The cultivation substrate was the primary effluent with or without excess sludge. The results showed that when diluted excess sludge was added to the substrate, microalgal biomass increased more than when the substrate contained only primary effluent. Additionally, the wastewater suspended matter and water quality were removed, reaching low levels. Microalgal culture systems using excess sludge showed higher organic acid content, higher biochemical methane potential, and higher efficiency in producing more microalgal biomass than wastewater treatment plant systems that did not use excess sludge.

Natural Pigments and Biogas Recovery from Microalgae Grown in Wastewater

This study assessed the recovery of natural pigments (phycobiliproteins) and bioenergy (biogas) from microalgae grown in wastewater. A consortium of microalgae, mainly composed by Nostoc, Phormidium, and Geitlerinema, known to have high phycobiliproteins content, was grown in photobioreactors. The growth medium was composed by secondary effluent from a high rate algal pond (HRAP) along with the anaerobic digestion centrate, which aimed to enhance the N/P ratio, given the lack of nutrients in the secondary effluent. Additionally, the centrate is still a challenging anaerobic digestion residue since the high nitrogen concentrations have to be removed before disposal. Removal efficiencies up to 52% of COD, 86% of NH₄ ⁺-N, and 100% of phosphorus were observed. The biomass composition was monitored over the experimental period in order to ensure stable cyanobacterial dominance in the mixed culture. Phycocyanin and phycoerythrin were extracted from harvested biomass, achieving maximum concentrations of 20.1 and 8.1 mg/g dry weight, respectively. The residual biomass from phycobiliproteins extraction was then used to produce biogas, with final methane yields ranging from 159 to 199 mL CH₄/g VS. According to the results, by combining the extraction of pigments and the production of biogas from residual biomass, we would not only obtain high-value compounds, but also more energy (around 5-10% higher), as compared to the single recovery of biogas. The proposed process poses an example of resource recovery from biomass grown in wastewater, moving toward a circular bioeconomy.

Converting tequila vinasse diluted with tequila process water into microalgae-yeast flocs and dischargeable effluent

During tequila production from agave, wastewaters are produced, such as dark-colored vinasse. To add value to this vinasse, microalgae-yeast biomass was produced on vinasse diluted with tequila process water (first rinsing water of agave syrup production). In batch experiments, a vinasse concentration of 10 %v/v resulted in the highest biomass productivity, pH and microalgae growth compared to 20 and 30 %v/v. To ease harvesting, microalgae-yeast flocs (MaY-flocs) were developed in a sequencing batch reactor (SBR). A MaY-floc SBR was run with diluted vinasse (10 %v/v) enriched to 76 mg N-TA L^-1, resulting in a doubled biomass productivity (49.5 ± 8.3 mg VSS L^-1 day^-1) of MaY-flocs compared to the best batch reactor performance. Based on response surface experiments, enrichment to 150 mg N-TA L^-1 and 5.9 %v/v vinasse are recommended. The MaY-floc SBR system is a promising, novel technology to treat tequila wastewaters while producing settleable MaY-floc biomass of interest to aquaculture.

Using microalgae in the circular economy to valorise anaerobic digestate: challenges and opportunities

Managing organic waste streams is a major challenge for the agricultural industry. Anaerobic digestion (AD) of organicwastes is a preferred option in the waste management hierarchy, as this processcangenerate renewableenergy, reduce emissions from wastestorage, andproduce fertiliser material.However, Nitrate Vulnerable Zone legislation and seasonal restrictions can limit the use of digestate on agricultural land. In this paper we demonstrate the potential of cultivating microalgae on digestate as a feedstock, either directlyafter dilution, or indirectlyfromeffluent remaining after biofertiliser extraction. Resultant microalgal biomass can then be used to produce livestock feed, biofuel or for higher value bio-products. The approach could mitigate for possible regional excesses, and substitute conventional high-impactproducts with bio-resources, enhancing sustainability withinacircular economy. Recycling nutrients from digestate with algal technology is at an early stage. We present and discuss challenges and opportunities associated with developing this new technology.

Life cycle assessment of high rate algal ponds for wastewater treatment and resource recovery

The aim of this study was to assess the potential environmental impacts associated with high rate algal ponds (HRAP) systems for wastewater treatment and resource recovery in small communities. To this aim, a Life Cycle Assessment (LCA) was carried out evaluating two alternatives: i) a HRAP system for wastewater treatment where microalgal biomass is valorized for energy recovery (biogas production); ii) a HRAP system for wastewater treatment where microalgal biomass is reused for nutrients recovery (biofertilizer production). Additionally, both alternatives were compared to a typical small-sized activated sludge system. An economic assessment was also performed. The results showed that HRAP system coupled with biogas production appeared to be more environmentally friendly than HRAP system coupled with biofertilizer production in the climate change, ozone layer depletion, photochemical oxidant formation, and fossil depletion impact categories. Different climatic conditions have strongly influenced the results obtained in the eutrophication and metal depletion impact categories. In fact, the HRAP system located where warm temperatures and high solar radiation are predominant (HRAP system coupled with biofertilizer production) showed lower impact in those categories. Additionally, the characteristics (e.g. nutrients and heavy metals concentration) of microalgal biomass recovered from wastewater appeared to be crucial when assessing the potential environmental impacts in the terrestrial acidification, particulate matter formation and toxicity impact categories. In terms of costs, HRAP systems seemed to be more economically feasible when combined with biofertilizer production instead of biogas. On the whole, implementing HRAPs instead of activated sludge systems might increase sustainability and cost-effectiveness of wastewater treatment in small communities, especially if implemented in warm climate regions and coupled with biofertilizer production.