Influence of microalgal N and P composition on wastewater nutrient remediation.

Microalgae have demonstrated the ability to remediate wastewater nutrients efficiently, with methods to further enhance performance through species selection and biomass concentration. This work evaluates a freshwater species remediation characteristics through analysis of internal biomass N:P (nitrogen:phosphorus) and presents a relationship between composition and nutrient uptake ability to assist in species selection. Findings are then translated to an optimal biomass concentration, achieved through immobilisation enabling biomass intensification by modifying bead concentration, for wastewaters of differing nutrient concentrations at hydraulic retention times (HRT) from 3 h to 10 d. A HRT <20 h was found suitable for the remediation of secondary effluent by immobilised Scenedesmus obliquus and Chlorella vulgaris at bead concentrations as low as 3.2 and 4.4 bead·mL(-1). Increasing bead concentrations were required for shorter HRTs with 3 h possible at influent concentrations <5 mgP L(-1).

Impacts of microalgae pre-treatments for improved anaerobic digestion: thermal treatment, thermal hydrolysis, ultrasound and enzymatic hydrolysis.

Anaerobic digestion (AD) of microalgae is primarily inhibited by the chemical composition of their cell walls containing biopolymers able to resist bacterial degradation. Adoption of pre-treatments such as thermal, thermal hydrolysis, ultrasound and enzymatic hydrolysis have the potential to remove these inhibitory compounds and enhance biogas yields by degrading the cell wall, and releasing the intracellular algogenic organic matter (AOM). This work investigated the effect of four pre-treatments on three microalgae species, and their impact on the quantity of soluble biomass released in the media and thus on the digestion process yields. The analysis of the composition of the soluble COD released and of the TEM images of the cells showed two main degradation actions associated with the processes: (1) cell wall damage with the release of intracellular AOM (thermal, thermal hydrolysis and ultrasound) and (2) degradation of the cell wall constituents with the release of intracellular AOM and the solubilisation of the cell wall biopolymers (enzymatic hydrolysis). As a result of this, enzymatic hydrolysis showed the greatest biogas yield increments (>270%) followed by thermal hydrolysis (60-100%) and ultrasounds (30-60%).

The impacts of replacing air bubbles with microspheres for the clarification of algae from low cell-density culture.

Dissolved Air Flotation (DAF) is a well-known coagulation-flotation system applied at large scale for microalgae harvesting. Compared to conventional harvesting technologies DAF allows high cell recovery at lower energy demand. By replacing microbubbles with microspheres, the innovative Ballasted Dissolved Air Flotation (BDAF) technique has been reported to achieve the same algae cell removal efficiency, while saving up to 80% of the energy required for the conventional DAF unit. Using three different algae cultures (Scenedesmus obliquus, Chlorella vulgaris and Arthrospira maxima), the present work investigated the practical, economic and environmental advantages of the BDAF system compared to the DAF system. 99% cells separation was achieved with both systems, nevertheless, the BDAF technology allowed up to 95% coagulant reduction depending on the algae species and the pH conditions adopted. In terms of floc structure and strength, the inclusion of microspheres in the algae floc generated a looser aggregate, showing a more compact structure within single cell alga, than large and filamentous cells. Overall, BDAF appeared to be a more reliable and sustainable harvesting system than DAF, as it allowed equal cells recovery reducing energy inputs, coagulant demand and carbon emissions.