## 3 Results and Discussion  
### Tolerance of Duckweed to Raw Whey (RW) Wastewater  
A two-way ANOVA was performed to assess the influence of the proportion of RW in the culture medium (RW%) and surface density of the wet biomass of the duckweed inoculum (DEN) on \(Chl_{total}\). The results indicated that both RW% and DEN had a statistically significant effect on \(Chl_{total}\) (F (5, 36) = 1191, \(p<0.001\); F (2, 36) = 421, \(p<0.001\), respectively). Additionally, there was a significant interaction between RW% and DEN (F (10, 36) = 42, \(p<0.001\)), suggesting that the combined effects of these factors were not independent. The overall variability in \(Chl_{total}\) was partitioned into components attributed to the main effect of each factor and their interaction. The contribution of each factor and interaction to the variation in \(Chl_{total}\) was estimated as the percentage of their respective F-ratios relative to the sum of all F-ratios. Consequently, the contribution of the RW%-DEN interaction (2.5%) to the global variation in \(Chl_{total}\) was substantially lower than that of RW% (72%) and DEN (25%). Figure 2 presents the outcomes of the two-way ANOVA. As depicted in Figure 2A, the presence of RW resulted in a reduction in the mean \(Chl_{total}\) of duckweed compared to the control group (without RW). Conversely, Figure 2B illustrates that the mean \(Chl_{total}\) exhibited an upward trend with a higher DEN (Figure 2B). Duckweed demonstrated limited tolerance to the culture media containing the highest RW proportions (RW-40% and RW-50%), as evidenced by unhealthy visual appearances characterized by chlorophyll bleaching, which is indicative of stress. Consequently, these RW concentrations were excluded from the sequential culture tests in the photobioreactor (PBR) (see Section 3.3). Additionally, Figure 2C highlights that the effect of the RW proportion on \(Chl_{total}\) differed between the DEN levels of 5.00 and 2.50 or 3.75 g dm\({}^{-2}\), respectively. The decline in \(Chl_{total}\) at 5.00 g dm\({}^{-2}\) DEN from the control group (2.15 mg g\({}^{-1}\)) to RW-40% (0.71 mg g\({}^{-1}\)) was less pronounced compared to 2.50 and 3.75 g dm\({}^{-2}\). Given the high concentration of organic compounds present in RW (see COD and BOD\({}_{5}\) in Table 1), the results preliminarily suggest that duckweed had to be nourished mixotrophically to survive in water supplemented with RW. This is in line with other recent studies [31; 32], in which mixotrophic growth of duckweed species was observed to lead to a decrease in chlorophyll content compared to photoautotrophic growth. Mixotrophic duckweed cultures grew faster than phototrophic cultures but exhibited reduced chlorophyll levels. The addition of sucrose or glucose in mixotrophic conditions influenced the chlorophyll content of _Lemma_ species. Findings from [PERSON] et al. [31] suggested that the presence of an additional carbon source in mixotrophic conditions may impact the chlorophyll levels in _Lemma_ species compared to their growth solely under photoautotrophic conditions. While the main compound of RW is lactose, duckweed cannot assimilate lactose directly [33; 34]; however, it can utilize lactic acid as a carbon source [35]. Cheese RW often contains lactic acid bacteria (LAB) that convert lactose into lactic acid [36; 37]. The pasteurization process typically involves heating whey to a temperature around 70 \({}^{\circ}\)C for a specific duration to reduce the number of harmful bacteria, including LAB.