Influence of culture media composition on the rheology of microalgae concentrates on a large scale

The role of microalgae in the production of bioproducts and biofuels, along with their ability to provide a sustainable pathway for wastewater treatment, makes them promising alternatives to conventional processes. Nevertheless, large-scale downstream processing requires an understanding of biomass rheology that needs to be addressed further. This study aimed to characterize microalgal concentrates rheologically in different culture media. The presence of bacteria was quantified by photorespirometry and plate counting techniques. The culture medium was found to significantly influence viscosity, with primary wastewater exhibiting the highest viscosity and seawater plus pig slurry the lowest. The concentration of heterotrophic bacteria was directly related to the viscosity. Extracellular polysaccharides (EPS) in supernatant exhibited an inverse viscosity trend compared to biomass concentrates, with pig slurry cultures having higher concentrations. These findings emphasize the profound influence of culture medium and EPS on the rheology of microalgal biomass, underscoring the need for continued research aimed at facilitating and optimizing large-scale downstream processes within the framework of a circular economy and the attainment of the Sustainable Development Goals (6,8, and 12).

Assessing and modeling nitrite inhibition in microalgae-bacteria consortia for wastewater treatment by means of photo-respirometric and chlorophyll fluorescence techniques

Total nitrite (TNO₂ = HNO₂ + NO^-₂) accumulation due to the activity of ammonia-oxidizing bacteria (AOB) was monitored in microalgae-bacteria consortia, and the inhibitory effect of nitrite/free nitrous acid (NO₂-N/FNA) on microalgae photosynthesis and inhibition mechanism was studied. A culture of Scenedesmus was used to run two sets of batch reactors at different pH and TNO₂ concentrations to evaluate the toxic potential of NO₂-N and FNA. Photo-respirometric tests showed that NO₂-N accumulation has a negative impact on net oxygen production rate (OPR_NET). Chlorophyll a fluorescence analysis was used to examine the biochemical effects of NO₂-N stress and the mechanism of NO₂-N inhibition. The electron transport rate (ETR), non-photochemical quenching (NPQ), and JIP-test revealed that the electron transport chain between Photosystems II and I (PS II and PS I) was hindered at NO₂-N concentrations above 25 g N m^-3. Electron acceptor Q_A was not able to reoxidize and could not transfer electrons to the next electron acceptor, Q_B, accumulating P₆₈₀⁺ (excited PS II reaction center) and limiting oxygen production. A semi-continuous reactor containing a Scenedesmus culture was monitored by photo-respirometry tests and Chlorophyll a fluorescence to calibrate NO₂-N inhibition (5-35 g N m^-3). Non-competitive inhibition and Hill-type models were compared to select the best-fitting inhibition equations. Inhibition was correctly modeled by the Hill-type model and a half inhibition constant (K_I) for OPR_NET, NPQ, maximum photosynthetic rate (ETR_MAX) and the performance index PI_ABS was 23.7 ± 1.2, 26.36 ± 1.10, 39 ± 2 and 26.5 ± 0.4, respectively.

Metal-based flocculation to harvest microalgae: a look beyond separation efficiency

Metal-based flocculants are commonly used for biomass harvesting in microalgae-based bio-refineries. Besides the high separation efficiency, additional aspects should be considered, related to the toxicity of metals for the algal biomass. Partitioning tests for commonly used flocculants (i.e., FeCl₃ and Al₂(SO₄)₃) showed that metals were mostly transferred to the solid phase with more than 95% of dosed metal ending up into the biomass, and low metal concentrations in the liquid effluent (lower than 0.4 mg L^-1 for both metals), thus allowing for water reuse. Photosynthesis inhibition was tested on microalgae and microalgae-bacteria cultures, using a standardized photo-respirometry protocol in which typical concentrations used during coagulation-flocculation were assessed. Modelling dose-response curves, concentrations corresponding to 50% inhibition (IC₅₀) were obtained, describing short-term effects. The obtained IC₅₀ ranged from 13.7 to 28.3 mg Al L^-1 for Al, and from 127.9 to 195.8 mg Fe L^-1 for Fe, showing a higher toxicity for the Al-based flocculant. The recovery of photosynthesis inhibition was also quantified, to evaluate the possibility of reusing/recycling the harvested biomass. The results highlighted that the residual photosynthetic activities, evaluated after 1 h and 24 h of exposure to metals were partially recovered, especially for Al, passing from 67.3% to 94.6% activity, respectively, while long-term Fe effects were stronger (passing from 64.9% to 77.6% activity). A non-toxic flocculant (cationic starch) was finally tested, excluding potential effects due to biomass aggregation, as the reduction of photosynthetic activity only reached 3.4%, compared to control. Relevant modifications to the light availability and the optical properties of algal suspensions were assessed, identifying a strong effect of iron which caused an increase of the light absorbance up to approximately 40% at high Fe concentrations. Possible implications of dosing metallic flocculants in MBWWT processes are discussed, and suggestions are given to perform inhibition tests on flocculating chemicals.

Selection of photosynthesis and respiration models to assess the effect of environmental conditions on mixed microalgae consortia grown on wastewater

This study aimed at evaluating the effects of different environmental conditions (irradiance, temperature, pH and dissolved oxygen) on a microalgae-bacteria consortium cultivated in a pilot-scale open pond and fed on the liquid fraction of anaerobic digestate. A standardized photo-respirometry protocol was followed to evaluate the activity of microalgae under different conditions. Two datasets (specific photosynthetic oxygen production rates and respiratory oxygen consumption rates) were obtained for each environmental parameter, throughout the entire range of conditions found in the outdoor cultivation system. Different kinetic models available in literature were fitted to experimental data and the resulting outputs were compared through model selection estimators, in order to select the most appropriate equations. The proposed set of equations constitute a modelling tool for the prediction of algal growth rates in algae-bacteria systems, as a function of environmental conditions.