Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Use of filtrate water from digestion for the cultivation of microalgae

For the economical and sustainable use of algae biomass for material and energy recovery it is necessary to optimize the individual steps of the process chain. The challenges for the sustainable production of microalgae are as follows:

  • Energy-efficient microalgae production This requires a photobioreactor which ensures a high photosynthesis rate even at high cell concentrations and whose energy requirements for algae production are lower than the energy content of the algae biomass produced.
  • Product extraction Both the solvents themselves and the quality of the solvents have to be adapted to the products; extraction should occur from the wet biomass to avoid energy input through drying processes.
  • Utilization of residual biomass After recovery of the valuable products the remaining lignocellulose-free biomass can be used for anaerobic digestion into biogas and thus for an energetic added value.
  • Recycling of nutrients On top of the use of exhaust CO2 gas, the utilization of wastewater containing high amounts of nitrogen and phosphate, adds to cost reduction.
  • Water recycling Water can be recycled via a renewed use of the cultivating media and through the utilization of nitrogen and phosphate contained in wastewater.

Using nutrients from wastewater

Recycling of nitrogen and phosphate through the coupling of anaerobic digestion and algae production.
Recycling of nitrogen and phosphate through the coupling of anaerobic digestion and algae production.

One aim of the project ”Mehr Biogas aus lignocellulosearmen Abfall- und Mikroalgenreststoffen durch kombinierte Bio-/ Hydrothermalvergasung” (More biogas from low-lignocellulose waste and microalgae residues through a combined bio/hydrothermal gasification) (EtaMax) is to close the nutrient cycles between algae biomass generation and energy generation with anaerobic digestion. Wastewater streams from biogas plants for sludge digestion with a high loading rate, so called high-load digestions, stand out due to high ammonium and phosphate concentrations of up to 1,300 mg NH4 per liter or 200 mg phosphate per liter, respectively. In these high-load digestion plants particle-free wastewater streams are recovered through ultrafiltration with rotating disk filters. Currently the ammonium in these wastewater streams is converted into nitrogen by means of energy-intensive process steps or it is precipitated together with the phosphate. Therefore, it is the intention of the EtaMax project to use these water streams with a high N and P content for algae production (Fig. 1).

Results

Microscopic image of Phaeodactylum tricornutum.
Microscopic image of Phaeodactylum tricornutum.

During initial tests with Phaeodactylum tricornutum, an alga containing the omega-3-fatty acid EPA (eicosapentaenoic acid), it was possible to successfully use filtrate water from two different municipal biogas plants as a culture medium. Depending on the origin of the filtrate water, it was only necessary to add phosphate to achieve an optimal N-to-P ratio for continuous biomass production in the flat panel airlift photobioreactors. The biomass productivities achieved with filtrate water were even higher than those generated with the synthetic medium (see chart). This means that synthetic media can favorably be replaced by wastewater streams from anaerobic digestion. This is another step towards the production of algae biomass for energetic utilization (oil, biogas) in combination with the sustainable recycling of water and nutrients which also considerably reduces costs and energy demand.

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