Wastewater treatment using microalgae: optimizing removal of P from wastewaters with a low total N:P ratio.

Phosphorus (P) is an essential element to sustain the global food and feed production. Large amounts of phosphate ores are extracted for the production of fertilizers. The recent global P crisis increased the awareness to use this natural resource in a sustainable way and to increase recycling. A significant part of the global annual P flux ends up in wastewater. Wastewaters are often treated using energy or chemical intensive technologies to decrease the P load and thus prevent eutrophication of receiving surface waters. The recycling of P removed from wastewaters using these technologies is limited. The application of new strategies to recover and reuse P is necessary. Microalgae based wastewater treatment systems hold the potential to remove and reuse the P, by integrating P into a biomass that can be valorised. Microalgae have been used for wastewater treatment since the 1960s, however the P recovery needs to be optimised. The wastewaters are often nitrogen (N) and not P-limited, thereby leaving large quantities of residual P in the water. The goal of this thesis was to evaluate the P recovery from wastewaters with a variable N:P ratio using microalgae. The flexibility of the microalgal P composition is determined under variable N and P supply and is compared among different microalgae species. The bioavailability of the wastewater dissolved P fraction is measured in bioassays. Indirect P removal, by microalgae mediated P precipitation, is studied and the resulting flocculation to harvest the cells is investigated.

The P uptake by microalgae under variable N and P supply was investigated in synthetic wastewaters to determine the influence of one nutrient on the removal of the other nutrient. Both Chlorella and Scenedesmus adjusted their biomass N and P content to the variable nutrient supply, while the influence on algal biomass produced was small. Chlorella had a higher biomass N content than Scenedesmus while the reverse was true for P. The concentrations of P in the biomass remained low and were relatively constant (0.6 – 0.8 % P) when the N supply and N concentration in the biomass was low. In contrast, at high initial N supply, the biomass P concentrations were more variable and increased to 1.7% P at high P supply. The underlying physiological mechanism may be related to the protein and rRNA content of the biomass. At higher N supply the protein synthesis increases and thus the rRNA synthesis may also be increased. As rRNA is a large constituent of the biomass P, this may explain the increased P accumulation under high N supply. Cyanobacteria are much less studied in the context of wastewater treatment compared to eukaryotic microalgae but have several potential advantages. A Pseudanabaena species was isolated from an eutrophic reservoir and had a growth rate similar to Chlorella grown under the same conditions. Under high N supply, this Pseudanabaena species has a high N content (up to 14%) due to the accumulation of protein (77%). The biomass N concentration is high compared to Chlorella (10%) and Scenedesmus (8%) grown under similar conditions. Under low N supply, up to 77% of carbohydrates were accumulated in the biomass. The biochemical biomass composition of this species is highly flexible and therefore this Pseudanabaena species is suitable for the production of animal feed (high protein content) or bio-energy (high carbohydrate content) by fine-tuning the N supply. The P removal potential of Chlorella, Scenedesmus and Pseudanabaena was compared. The fraction of P removed from all synthetic wastewaters was above 80% when the initial N:P ratio in water exceeded 20. In contrast, the P removal decreased when the N:P ratio was decreasing below 20. Wastewaters often have these low N:P ratios indicating that P removal by microalgae may be hampered. In such waters, removal of P was typically highest for Scenedemus and lowest for Chlorella because Scenedesmus can accumulate P to the largest extent.

The effective microalgal removal of P from real wastewaters requires that the P forms in wastewater are readily available for algal uptake. Wastewaters contain a mixture of different chemical P forms including inorganic and organic P, both in the dissolved and particulate phase. The bioavailability of these P forms (BAP) in wastewater may be variable and therefore the BAP of the dissolved P fraction of different raw wastewaters was evaluated. The bioassays were performed using Chlorella vulgaris, as a model species for wastewater treatment and used P-limiting conditions. Overall the BAP of the wastewater dissolved P fraction was high (> 70%). This corresponded with the high orthophosphate fraction measured in these wastewaters. Additionally the chemical fractionation showed that part of the colloidal P (e.g. humic-metal-phosphate) and organic P (e.g. phosphate esters) was also bioavailable, because the time for uptake under P-limited conditions was sufficiently large for effective use of the less available P species.

Microalgae can remove P from wastewater by direct uptake or indirectly by inducing precipitation of P. During microalgal growth, CO2 is taken up as HCO3- , thereby increasing the solution pH. As a consequence Ca-phosphates can more readily precipitate and the formed precipitate can induce flocculation of the microalgal cells due to charge neutralisation of the cell surface. This is attractive since low-cost harvesting of microalgae is a major challenge. In jar test experiments, flocculation by Ca-phosphate precipitation was induced at relatively low pH when calcium and P concentrations are sufficiently high. In real systems, however, this flocculation often fails probably due to the presence of inhibitory substances. The inhibition of flocculation in the presence of organic matter, including algal organic matter and model organic compounds, was evaluated. Addition of dissolved organic compounds showed that organic acids with a high molecular weight (e.g. humic acids, alginate) have a strong inhibitory effect on flocculation whereas glucose or acetate had no such effect. The inhibitory effects may be related to complexation of Ca2+ or effects of organic matter on growth of the Ca-phosphate crystals. Precipitation of Ca-phosphate in media with high organic matter content requires a high water hardness (> 0.50 mM Ca) and high PO4 concentrations (> 0.35 mM P). Taking these requirements into account, flocculation by Ca-phosphate precipitation is a promising cost-efficient harvesting technique which can remove surplus P from wastewater.