Model system for recovering nitrogen and phosphorus from yellow water to supply a microalgae culture on a building facade

One of the main aims of microalgae biofacades is to improve the sustainability of buildings by exchanging matter and energy between the building and the microalgae culture system installed on its facade. This study evaluates the potential for treating the yellow water from buildings (i.e. source-separated urine) as an added benefit to the biofacade concept of producing biomass from microalgae. To limit the impact on overall biomass productivity, only part of the biofacade was used for the urine treatment: urine was supplied to some of the PBRs while synthetic nutrients were supplied to the others. This chemical loop has the potential to reduce the environmental impact of both the microalgae culture process, by reducing the use of ammonium and phosphate salts, and the building itself, by recovering the urine before it is discharged as wastewater for further treatment.

However, evaluating and optimizing the process is not straightforward because algae growth is closely linked with nutrient and light availability and purification efficiency. The objective of this study is to ascertain the optimal operating parameters for the system: the yellow water dilution rate (dWW) and hydraulic retention time (HRT) of the culture through simulation, to maximize the proportion of the biofacade supplied with urine, the purification efficiency, and the biomass productivity, taking into account the dynamics of relevant parameters such as day–night cycles and urine production profile. The results reveal that by adapting the operating parameters (dWW and HRT) to the location of the building and the season, the system can recycle all the urine produced using about 50% of the biofacade, thus increasing the potential of microalgae biofacades to achieve yellow water recycling solution for buildings. With this process, over 90% of the nitrogen and phosphorus in yellow water is consumed by the microalgae, although the overall biomass production capacity is reduced by 40%. Using yellow water as a source of nutrients therefore has potential for reducing the environmental footprint of biomass production, with 50% less ${\text{CO}}_{2}eq$ emissions than biofacades supplied entirely with synthetic nutrients.