algae in water treatment

The question of using algae in water treatment is a recurrent topic which concerns:

• Firstly, the production of recoverable matter
• Secondly, assimilation capacity and therefore, the associated treatment.

Effectively, algae, and more particularly, micro and macro algae, are extremely diverse (and therefore offer a very vast potential) with a high rate of conversion (production of matter) based on a metabolism which may be fermentative (heterotrophic) or photosynthetic (autotrophic). Lastly, algal cells are without lignin which facilitates their recovery via methane fermentation.

Photosynthetic algae (assimilation of CO₂, N and P) can be used in water treatment which supposes the pre-treatment of water to eliminate suspended matter and degradeable carbon and consequently encourage a good diffusion of light radiation which is necessary for photosynthesis. To do this, the use of algae in water treatment is limited to tertiary treatment or to dehydration centrates from pre-treated anaerobically digested sludge with the main objective of producing a biomass which is easily biodegradeable and which generates energy.

The use of algae for the production of matter with a high added value for the agri-food, chemical, cosmetics and even pharmaceutical industries or for bio-fuels does not come within this scope insfofar as the main objective is not wastewater treatment.

It should also be noted that the benefit of algae production must be linked to international stakes on greenhouse gas emissions and the question of « carbon tax » via its role in fixing/ capturing CO₂.

In water treatment, two types of technologies are implemented for the growth and recovery of algae.

open reactors

The shallow lagoon type, with a controlled supply of nitrogen and phosphorous able to be placed inside a greenhouse to limit the impact of climate conditions (controlled temperature or even atmosphere with the addition of CO₂). In this case, algae develop spontaneously without specific seeding. Various technologies have been developed: the main characteristics being rather low Capex and Opex for a significant footprint and operating conditions which are quite difficult to control (sensitivity to external conditions, low efficiency of gaseous exchanges). In addition, the question of collecting excess algae is a topic which is little tackled in general, but which represents a veritable process operating and sustainability stake.

photobioreactors

Photobioreactors functioning with natural or controlled lighting placed in tubes, mini greenhouses or plastic envelopes. In this case, bioreactors are seeded and the development of algae is managed via nutritional inputs and controlled operating conditions. Various technologies have been developed whose main characteristics are a higher Capex and Opex with a quite complex design of reactors. On the other hand, the surface area covered is low, operating conditions are carefully monitored and are little dependent on location. Globally, the production yield is two times greater in a photobioreactor than in an open reactor.

Excess algae can be:

• Digested in anaerobic conditions to produce methane and generate energy
• Fermented to produce ethanol and other alcohols able to be used to produce bio-fuels.

Transformation technologies for algae are currently being developed but current production costs prevent this outlet from being competitive.

In the water treatment domain, the use of algae is not yet industrialised for the following reasons:

• Algae do not really perform a treatment but rather serve to fix nutriments (nitrogen and phosphorous). Consequently, in the case of anaerobic digestion, these nutriments are released into the centrate and therefore returned to the water line. The advantage of algae would therefore exclusively lie in the production of matter and any resulting energy generation (added to digestion).

• The collection and thickening of algae remains a point which is little industrialised and can account for almost 30% of total operating costs. Depending on the types of culture, physicochemical sedimentation, flotation or filtration can be used but little feedback on a full operating scale is available in order to permit a comprehensive assessment.