Microalgae offer a promising technology to remove and re-use the nutrients N and P from wastewater. For effective removal of both N and P, it is important that microalgae can adjust the N and P concentration in their biomass to the N and P supply in the wastewater. The aim of this study was to evaluate to what extent microalgae can adjust the N and P concentrations in their biomass to the N and P supply in the wastewater, and to what extent supply of one nutrient influences the removal of the other nutrient. Using Chlorella and Scenedesmus as model organisms, we quantified growth and biomass composition in medium with different initial N and P concentrations in all possible combinations. Nutrient supply marginally affected biomass yield of both microalgae but had a strong influence on the composition of the biomass. The nutrient concentrations in the biomass ranged 5.0-10.1 % for N and 0.5-1.3 % for P in Chlorella and 2.9-8.4 % for N and 0.5-1.7 % for P in Scenedesmus. The concentrations of P in the biomass remained low and were relatively constant (0.6-0.8 % P) when the N concentration in the biomass was low. As a result, removal of P from the wastewater was influenced by the concentration of N in the wastewater. When the initial N concentration in the wastewater was above 40 mg L(-1) the microalgae could remove up to 6 mg P L(-1), but this removal was only 2 mg P L(-1) when the initial N concentration was below 20 mg L(-1). A lower N supply increased the carbohydrate concentration to about 40% and lipid concentration to about 30% for both species, compared to around 15% and 10% respectively at high N supply. Our results show that sufficiently high N concentrations are needed to ensure effective P removal from wastewater due to the positive effect of N on the accumulation of P.

The feasibility of a new concept of wastewater treatment by combining a membrane bioreactor (MBR) and a microalgae membrane photobioreactor (MPBR) is assessed in this study. In this system, the organic carbon present in wastewater is expected to be fully oxidized in the MBR, while the nutrients are removed via the subsequent MPBR treatment. The effluent of a lab-scale MBR was fed into a PBR and a MPBR which served as growing medium for Chlorella vulgaris. The MPBRs demonstrated their superiority by limiting the algae wash-out, thus increasing the allowable optimum dilution rate (Dopt). At these corresponding Dopt values, 3.5 and 2 times higher biomass concentrations and volumetric productivities respectively were achieved by the MPBR. It is also possible to run the MPBR at still higher biomass concentration, thus enabling a smaller footprint and higher nutrient removal efficiency. However, reduced nutrient removal efficiencies were found to be one possible drawback.

Influence of organic matter on flocculation of Chlorella vulgaris by calcium phosphate precipitationBiomass and bioenergy, vol 54, 107-114.

23Flocculation of microalgae is a promising low-cost strategy to harvest microalgae for bulk 24 biomass production. However, residual flocculants can interfere in further downstream 25 processes or influence biomass quality. In this study, a new concept is demonstrated based on 26 reversible magnesium hydroxide flocculation, using Chlorella vulgaris and 27Phaeodactylum tricornutum as respectively a freshwater and a marine model species. We 28 show that flocculation was induced by precipitation of magnesium hydroxide at high pH (10 29 to 10.5). This resulted in a magnesium content of the microalgal biomass of 5% for Chlorella 30 and 18% for Phaeodactylum. After pre-concentration of the microalgal biomass by gravity 31 sedimentation, 95% of the precipitated magnesium hydroxide could be removed from the 32 biomass by mild acidification (pH 7 to 8). The pH fluctuations experienced by the microalgae 33 during flocculation/de-flocculation had no influence on biomass composition (FAME, total N 34 and P, carbohydrates, proteins, mineral content) and on the viability of microalgal cells. 35Magnesium can thus be used as pH-dependent reversible flocculant for harvesting microalgae 36 in both marine and freshwater medium. 37 38 39 3

Influence of magnesium concentration, biomass concentration and pH on flocculation of Chlorella vulgarisAlgal Research, vol 3, 24-29. Archived versionAuthor manuscript: the content is identical to the content of the published paper, but without the final typesetting by the publisher Autoflocculation is a promising low-cost method for harvesting microalgae for bulk 21 biomass production or wastewater treatment. Autoflocculation can be caused by 22 precipitation of calcium or magnesium at high pH. In this study, we investigated the 23 interactive effects of pH, magnesium concentration and microalgal biomass 24 concentration on flocculation of Chlorella vulgaris by magnesium hydroxide. The 25 minimum pH for inducing flocculation was lower when magnesium concentration in 26 the medium is higher. A higher pH and/or higher magnesium concentration are 27 required for flocculation when microalgal biomass concentration is increased. The 28 sludge volume formed during flocculation is highly variable and is influenced mainly 29 by the amount of magnesium hydroxide that precipitates during flocculation. The 30 sludge volume increases with pH and with magnesium concentration in the medium. 31There is an optimal pH where flocculation efficiency is maximized (> 95%) and 32 sludge volume is minimal (1 -2% of culture volume). Increasing the pH slightly 33 above this optimum results either in an increase in sludge volume and/or a decrease in 34 flocculation efficiency. We propose that autoflocculation by magnesium hydroxide 35 can be more easily controlled by the dosage of base rather than by targeting a specific 36 pH level.