Use of filtrate water from fermentation for the cultivation of microalgae

Economic and sustainable algae production

For an economic and sustainable production of algae biomass for material and energy recovery, the steps along the entire process chain must be optimized. The challenges for sustainable microalgae production in detail are:

  • Energy-efficient microalgae production
    This requires a photobioreactor that ensures a high rate of photosynthesis even at high cell concentrations, and whose energy requirement for algae production is lower than the energy content of the algae biomass produced.
  • Product extraction
    Solvent and solvent quality must be matched to the products; extraction should be carried out from the wet biomass in order to avoid energy input from drying processes.
  • Integrating the use of residual biomass
    After the extraction of the valuable materials, the lignocellulose-free residual biomass is available for anaerobic mineralisation to biogas and thus for energy value creation.
  • Recycling of nutrients
    In addition to the use of exhaust gas CO2, the use of waste water with a high nitrogen and phosphate content contributes to cost reduction.
  • Water circulation system
    The recirculation of water can be realized both by reusing the cultivation media and by using nitrogen and phosphate containing waste water.

Using nutrients from wastewater

Recycling of nitrogen and phosphate by coupling anaerobic digestion and algae production.
Recycling of nitrogen and phosphate by coupling anaerobic digestion and algae production.
Microalgae Phaeodactylum tricornutum, magnified 1000 times.
Microalgae Phaeodactylum tricornutum, magnified 1000 times.

Within the project "More biogas from waste and micro-algae residues poor in lignocellulose by combined bio-/hydrothermal gasification - EtaMax" we succeeded in closing the nutrient cycles between algae biomass production and energy generation by anaerobic fermentation.

Wastewater streams from biogas plants for sewage sludge fermentation with high loading rates, so-called high-load digestions, are characterised by high ammonium and phosphate concentrations of up to 1300 mg NH4 per litre and 200 mg phosphate per litre respectively. In these high-load digestions, particle-free wastewater streams are obtained by ultrafiltration with rotating disk filters. Currently, ammonium is converted to nitrogen or precipitated together with phosphate in these wastewater streams in energy-intensive process steps. Reuse is therefore not possible. In the EtaMax project, these wastewater streams with high N and P content were therefore used for algae production.

In initial experiments with Phaeodactylum tricornutum, an algae containing the omega-3 fatty acid EPA (eicosapentaenoic acid), filtrate water from two different municipal biogas plants was successfully used as a culture medium. For continuous biomass production in flat plate airlift photobioreactors, only phosphate up to an optimal N-to-P ratio had to be added, depending on the origin of the filtrate water. The biomass productivities achieved with filtrate water were even higher than with synthetic medium.

Thus, synthetic media can be advantageously replaced by wastewater streams from anaerobic digestion. This is a further step towards the production of algae biomass for energy recovery (oils, biogas) with a sustainable recycling of water and nutrients, which also significantly reduces costs and energy requirements.