Viability assessment of algal wastewater treatment projects under outdoor conditions based on algal productivity and nutrient removal rate

In this work, a viability study of algal wastewater treatment projects under outdoor conditions is proposed. The viability analysis of solar plant projects based on annual exceedance probability, which is the probability that a certain energy value will be exceeded, is applied. The two outputs used (algal productivity and ammonium uptake rate) are obtained from literature models under different scenarios. The first scenario studies algal productivity of the microalgae Chlorella vulgaris cultivation in an open pond system. The second scenario considers the ammonium uptake rate of the microalgae Scenedesmus sp. Cultivation in a flat-plate photobioreactor. Both scenarios are set out under outdoor conditions in two stations with different climates. The work also aims at including in the viability study an analysis of the intermittency of productivity/ ammonium uptake rate to take into account intra-annual variability. The study of variability is usually limited to the estimation of statistics during each season. The methodology proposed, based on exceedance probabilities of variation, evaluates discontinuities in the production and/or nutrient removal processes. Finally, the work aims at evaluating the effects of storage on the discontinuities that can compromise project viability defining the Risk Factor Improvement Index. This index assesses the degree of improvement in viability due to discontinuity in productivity and ammonium removal processes provided by storage. According to this index, improvements of up to 31% for the productivity and up to 20% for the ammonium uptake rate can be reached. By including storage, the discontinuity effect and thus the intermittency analysis may be disregarded.