Wastewater Effluent Supports Algae Growth

This summer has been a busy one at NIVA’s Solbergstrand marine research station. The ALGECO team has been growing algae on VEAS municipal wastewater effluent. This effluent contains residual nutrients needed to grow algae and the ALGECO project aims to reimagine this waste product as a resource. The result would be sustainable algae production while simultaneously cleaning the water. What we need to understand is how much the algae can grow on this wastewater effluent and how long it all takes.

Algae growth curves in the raceway pond system

In the greenhouse, two raceways ponds were filled with wastewater effluent and algae 6 times for a total of 12 experiments. The growth of the algae was monitored for a 72-hour period and water samples were taken to measure the depletion of nutrients and pollutants. This establishes important baseline data, giving an indication of both yield, the amount of algae produced in the system, and productivity, the rate at which the algae are produced.

Algae productivity response to biomass density. Productivity (g/l/hr) is calculated over the previous daily sampling period and biomass density (CDW_t1 g/l)) is the algal dry weight at the start of the daily sampling period.

The results indicate that the system can produce more than 0,2 grams dry algal biomass per liter of effluent. Additionally, water chemistry analysis has demonstrated that total nitrogen is depleted to near detection limits within the 72-hour experimental period (data not shown). These are encouraging findings as we now aim to move from this batch cultivation system to a continuous treatment system.

In order to operate a continuous treatment system it is critical to understand the relationship between productivity and the amount of biomass in the system. When biomass is low, productivity is reduced because there are fewer algae in the system to reproduce. When the biomass is high, productivity is reduced because there are not enough resources, light and nutrients, for the algae to reproduce. As the algae biomass gets thicker and thicker it begins to shade itself, blocking the light needed for photosynthesis. At the same time the growing algae are depleting the available nutrients needed to support its growth. Further work is needed to untangle the effects of nutrient depletion and self-shading, yet these preliminary results provide an important starting point as we move towards a continuous treatment system.