# Pharmaceuticals Removal from Wastewater with Microalgae: A Pilot Study
## 3 Results and Discussion
### Occurrence and Removal of Pharmaceuticals
When calculating the total removal, a global value of 82.5% was achieved; additionally, Exp1 presented 87.9% and Exp2 65.7% for the total removal rates. This difference occurs due to the higher initial load in Exp1 and the higher removal in the pharmaceuticals with higher concentrations. Curiously, the exit load is similar in both Exp1 and Exp2 (4154 and 3792 ng/L, respectively). Therefore, we can observe that the microalgae system has good general removal rates but appears to have a limit in the residual pharmaceuticals left in the water. Additionally, Exp1 was performed in summer with higher temperatures and sunlight, which can increase not only algal growth and therefore removal by sorption but also algae metabolic activity, which could affect removal by absorption. Figure 2: Mean and standard deviation of pharmaceutical concentrations in both experiments (1 and 2) at the entry and exit of the microalgae system. As for the different therapeutic groups, the average removal ranged from 64.0% to 75.3%, for the others and antihypertensive groups, respectively, with the remaining groups (antibiotics and antipsychotics) with values near 74% (Figure 4). When analyzing the total average, antibiotics and antipsychotics presented higher values (88.8 and 86.4%, respectively), followed by others and antihypertensives (79.3 and 74.3%, respectively). Nonetheless, within each group, there was also great variability. In the antihypertensive group average removal values ranged from 54.6% (atenol) to 98.7% (propranolol). In the antipsychotics, the same pattern was observed with values ranging from 59.8% (ven-lafaxine) to 100% (sertraline). As for the antibiotics' removal averages, the lower value was 44.7% for ofloxacin, and the higher was 100% (azithromycin and sulfamethoxazole). Due to practical reasons, pharmaceuticals are grouped by therapeutic properties. However, within each group, they can have very different physicochemical properties. These variations  
Figure 4: Removal (minimum, Q1, median, Q3, and maximum) by therapeutic group. Figure 3: Box plot (minimum, Q1, median, Q3, and maximum) of removal in both experiments (1 and 2). imply that they will be impacted differently by the microalgae and other WWTP treatments, which explains the large variations observed. The pharmaceuticals with the lowest removal rates (lower than 60%) were the antibiotics ofloxacin (44.7%) and ciprofloxacin (53.2%), the antihyperstines atenol (54.6%) and losartan (56.2%), the antidepressant venlafaxine (59.8%), and diclofenac (52.7%). Some of them have also low removal rates in other types of wastewater treatment [7]. However, to increase their removal, we could conjugate this methodology with others, thereby complementing its efficacy. Other authors with similar works [19] reported removals ranging from not detected to higher than 90% in pilot high-rate algal ponds (HRAP) using HRTs of 4 to 8 days in different seasons. [PERSON] et al. (2017) also performed a pilot outdoor experiment using a closed photobioreactor that was able to remove 30-80% of pharmaceuticals from toilet water [20]. This system was operated using HRTs of 8 to 12 days. [PERSON] et al. (2020) also used HRAP operating at an HRT of 4.6 days, achieving moderate removal rates (40-60%) or higher, depending on the class of pharmaceuticals [21]. Remarkably, the pilot installed in this study was able to achieve high removal rates for several pharmaceuticals using an HRT of only 24 h. This lower HRT translates into smaller and cheaper equipment for algae treatment and lower occupation of WWTP space. An increase in the HRT can lead to higher biomass concentrations inside the PBRs, which theoretically would improve removal [22]. It is possible that the application of higher HRTs in our system could result in even better pharmaceutical removal.