*3.3. Removal of Colour*

AL waste has a dark brown colour, so its decolourization is important to avoid negative environmental and visual effects. Highly coloured wastewater reduces the passage of light through the water, causing a reduction in photosynthetic activity and, therefore, altering the flora and fauna of the water [38]. The colour index profile is shown in Figure 5. Greater removals were obtained when the fungus was added.

**Figure 5.** Changes in colour index during biological treatment. The dashed lines shown the noninoculated tests C1 (-) and C2 (-), used as controls. The solid lines shown the inoculated tests E1 (•) and E2 (•). The standard deviation (SD) of the experimental data were in all cases less than 11.5% of mean value.

In the non-inoculate experiments (C1 and C2), the colour index was also reduced by endogenous microflora, especially during the first 24 h. Afterwards, the C1 slightly changed up and down, reaching final percentages of removal around 20%. When the fungus was inoculated (E1 and E2) the colour index decreased more abruptly especially during the first 24 h. Removals around 70% were obtained in both cases after 4 days. Then, the colour removal slightly increased and remained approximately stable, finally reaching a 60% reduction. The obtained results were in accordance with those found in the literature regarding the colour removals in recalcitrant wastewaters using white-rot fungi. Pakshirajan and Kheria [39] reported colour degradations of 64% after continuous fungal treatment with *P. chrysosporium* of industrial textile wastewaters. Ntougias et al. [25] reported colour removals around 60–65% in OMW using basidiomycetes fungus *Pleurotus*

spp. Similar reductions were reported for olive mill effluent treated by an adapted strain of *Trametes versicolor* [40].

Taking into account data reported by the literature and results obtained in this work, the use of white-rot fungi to treat AL could be considered to be a promising treatment technology. Although the removal efficiencies of sCOD, colour and phenolic compounds after treatment with fungi were slightly lower than those reported for OMW treated with physical–chemical treatments, it should be considered that these processes usually present serious drawbacks such as their high cost, bad odour, addition of chemicals, or fouling of the membrane [4,18]. Additionally, chemical oxidation treatments can produce more recalcitrant or toxic intermediate compounds, reducing the effectiveness of the treatment. In contrast, the use of white-rot fungus allows the degradation of a wide range of recalcitrant contaminants due to its ability to release extracellular enzymes, as well as lead to the detoxification of wastewater [25].
