## 4 Discussion
### Nutrient Removal  
P concentrations in dry biomass suggest that this nutrient was retained or only partially accumulated by the microalgae in both experiments, but it does not explain the increase in P concentration in the medium recorded, in particular in the indoor trial, where the highest concentration on P in the biomass was registered the day with one of the highest P concentrations in the medium. Since P adsorption in the biomass is necessary for the removal of this nutrient, biomass production and P accumulation are needed, with studies highlighting the importance of irradiance and photoperiod in the P uptake [29; 115; 116; 117]. Anincrease in the P concentration of the medium during phytoremediation tests in winter time has already been reported by other studies such as [56]. In their experiments, those authors reported that, in the winter season, an increase in P concentration in deeper and unlit tanks resulted and that this trend was linked to low biomass production and net phosphorous release from decaying biomass. However, no significant decrease in biomass concentration was recorded, suggesting that multiple phenomena, e.g., P release by bacteria [14], could be happening at the same time. In fact, under anaerobic conditions, PAOs break down polyphosphate and release orthophosphate, increasing its concentration in the medium [14]. During microalgae cultivation, low dissolved oxygen conditions could happen, and this could help explain the increase in P concentration in the medium even if no net release from the microalgae was recorded, so the contribution of the native bacteria cannot be excluded [49; 118]. In both experiments, N was largely removed from the medium, with better results achieved in the outdoor setup compared to the indoor setup, approximately 70% and 50%, respectively. This could be explained by the higher amount of light hours in the outdoor experiments (16 h) compared to the indoor experiment (9 h) [50; 88]. As demonstrated by [PERSON] and colleagues [37], different microalgae strains are affected differently by temperature and photoperiod. In their study, the authors reported that the growth of the employed strain of _Scenedesmus obliquus_ was more affected by the temperature than the photoperiod but still achieved 29% N removal even though no growth was recorded. On the contrary, the growth of the strain of _Chlorella vulgaris_ was more affected by reduced photoperiod, resulting in the absence of biomass production and limited N removal when cultured at 25 \({}^{\circ}\)C and 3:21 h L:D photoperiod [37]. As a result of microalgae and bacteria metabolism, nitrate concentration may rise, as previously reported in a study that used the same microalgae [67] or by other researchers [49]. Since both ammonia and nitrate are subject to limits imposed by Italian law [119], it is quite noticeable that, in the conditions proposed in this research, nitrate was largely depleted. Since the N percentage in biomass did not change in both indoor and outdoor experiments and limited growth was recorded only in the indoor experiment, only a fraction of N was assimilated by the microalgae, suggesting that most were removed from the medium through different mechanisms. Ammonia stripping is one of the phenomena usually implied in N reduction in phytoremediation. Some researchers report up to 80% of total N-NH\({}_{4}\)+ removal thanks to this phenomenon [47]. Unfortunately, the environmental conditions required for ammonia stripping, e.g., pH values higher than 10 and high temperature, have never been met in either outdoor or indoor setups [120; 121; 122]. Considering that unsterilized WW was employed, the effect of microorganisms naturally present in the medium can be considered. Indeed, microbial activities such as nitrification and denitrification could also be responsible for N depletion [12].