We have conducted lab scale experiments to study the ability of microalgae to collect nutrients from domestic wastewater (black and grey water)

This work is based on a collaboration with the Faculty of Science and Technology at NMBU (Prof. Arve Heistad and Dr. Melesse Eshetu Moges), which has access to sewage from a vacuum toilet system of a student housing.

Figure 1 shows the overall set-up of the experiment from the toilet to the photobioreactor.

fig 2.jpg
Figure 1. Overall set-up of the experiment from the toilet to the photobioreactor. Figure: Melesse Eshetu Moges

The wastewater was pre-treated in a biogas reactor with subsequent filtration and hygienisation (UV). The resulting pre-treated wastewater with high concentrations of dissolved ammonium and phosphate was used as a nutrient source for the cultivation of green microalgae (Chlorella sorokiniana) in a continuous photobioreactor system.

When using a 10 % dilution and supplementing with phosphate, magnesium and a trace element solution, 99.8 and 99.2 % of N and P, respectively, could be removed from the water so that it could directly be discharged into the environment (Figure 2). 

Figure 2. Samples from different treatment steps of the wastewater. From left: 1. Black water after/before anaerobic treatment in a biogass reactor, 2. After filtration and UV treatment, 3. Grown culture of the green microalga Chlorella sorokiniana, 4. Biomass separated from liquid by precipitation, 5. Treated water ready for discharge. Photos: Melesse Eshetu Moges

When using the microalgal biomass as a fertiliser for growing plants, the achieved fertilisation effect was surprisingly high as compared with other organic fertilisers. Nitrogen in algae is rapidly released through decomposition after its addition to soil (Figure 3). 

Based on our studies we concluded that most of the nitrogen and phosphate was removed from domestic wastewater during the developed process, and stored in microalgal biomass, which could be used as a nutrient-dense fertiliser.

There are still some challenges:

  • Energy input in form of light, if solar radiation is rather low (e.g., Norwegian winter)
  • Necessary supplementation of nutrients (e.g., phosphorus, magnesium, trace elements) for a balanced nutrient composition
  • Large area consumption because of a low light penetration depth into the microalgal culture.



















Pot experiment.jpg
Figure 3. Growth experiments with barley to test the fertilising effect of microalgae. The control without additional nitrogen to the left, fertilised with 16 kg N/daa microalgae biomass in the middle, and the control with 16 kg N/daa of mineral fertiliser to the right. Photo: Trond Henriksen