The authors regret that there were some errors in the text and Figure 5 of their original paper and apologise for any inconvenience caused. The corrected text is given below.
Figure 5 should be shown as
The initial headloss has been normalized with the volume of beads and K5-media and with the water rising velocity. It varies from 1 to 3.6 cm H2O/m/m.h−1 for the K5-media equipped biofilter (Figure 5(a)). For the reference biofilter, excluding two results under 1 cm H2O/m/m.h−1, the initial headloss ranged from 2.3 to 6.1 cm H2O/m/m.h−1 and is always above those of the K5-media equipped biofilter. Figure 5(b) shows the same results under boxplot form. This form, introduced by Tukey (1977) consists of a box extending from the first quartile (Q1) to the third quartile (Q3); a bar mark at the median and a cross mark at the mean; and whiskers. The schematic boxplot divides the data based on four invisible boundaries, namely, two inner fences and two outer fences. As usual, the interquartile range (IQR) is defined to be Q3–Q1. The inner fences are Q1–1.5 IQR and Q3+1.5 IQR, while the outer fences are Q1–3 IQR and Q3+3 IQR. The whiskers extend to the most extreme data within the inner fences. Data outside the outer fences are considered to be extreme outliers and are marked with a symbol. This statistical approach shows that values under 1 cm H2O/m/m.h−1 are outliers. Mean initial headloss values are 2.2 and 4.0 cm H2O/m/m.h−1 for the K5-media equipped and the reference biofilters, respectively. Moreover, the Mann Whitney tests confirm that initial headloss of the K5-media equipped biofilter is lower than the reference one. This result shows that the 0.70 m of K5-media does not introduce additional headloss into the biofilters under normal operating conditions while it plays a role of a usual fixed bed, as Biostyr® beads, allowing bacterial growth. Since a part of the pollution is removed before reaching the Biostyr® beads, the headloss within the Biostyr® beads is lower than in the reference biofilter. It can also be assumed that the K5- media may play a role on the air transfer inside the media, helping to reduce the total amount of air blown inside the biofilters. So, knowing that air blowing is one of the most expensive processes in a WWTP, K5-media may also help to reduce operation costs of the nitrification step.