Effects of ciliates and rotifers on fungal degradation of natural tannins

Tannery wastewater presents high concentrations of pollutant, recalcitrant molecules (e.g. tannins). Such molecules reduce the efficiency of the biological treatment processes. Recent studies showed that several fungal strains, including Aspergillus tubingensis MUT 990, were effective in the degradation of these recalcitrant molecules, due to their ability to produce the enzyme tannase. However, the growth of A. tubingensis, and consequently its degradation performance, decreased when the bacterial load was high. The aim of the study was to evaluate whether the introduction of bacterivorous organisms (such as protist ciliates and/or rotifers) in batch scale experiments using fungi to remove natural tannins could: 1) reduce the bacterial load, 2) favor fungal growth and 3) enhance the degradation of selected recalcitrant compounds. To achieve this goal, rotifers and different functional categories of ciliates were isolated. Cultures that were able to survive both an acid pH (5-5.7 range) and the presence of certain recalcitrant compounds, were established. The grazers Paramecium sp., Tetrahymena sp., Vorticella sp. and a culture of rotifers were selected due to their capability to survive and grow in two tannin solutions: Tara (tannin extract of Caesalpina spinosa) and Quebracho (tannin extract of Schinopsis lorenztii lorenztii). Three batch tests were performed and the following parameters were monitored: bacterial and grazers (ciliates and rotifers) counts, Soluble Chemical Oxygen Demand (sCOD), Total Organic Carbon (TOC), pH and fungal dry weight inside Polyurethane Foams Cubes (PUFs). In the three batch tests, and in all conditions where grazers were present, the bacterial load was reduced. In the First Batch in Tara tannin, the single grazers individually tested showed a higher bacterial reduction than the controls without grazers. The highest bacterial reduction was achieved in presence of activated sludge. In the Second Batch Tara (7 days), the highest bacterial reduction was achieved with Tetrahymena sp. 41. Without Tara tannin (Treatment 1), the grazer reduced the bacterial load to a greater extent than Treatment 3 (cerophyll, Tara, fungus and ciliate), which simulated, on batch-scale, the natural condition of tannery wastewater. In the Batch Quebracho, the ciliate Paramecium sp. 39, and the activated sludge without fungal strain, showed the greatest reduction in terms of bacterial load. It was interesting to notice that the decrease on bacterial load was significantly greater in the treatment with both the Paramecium sp. 39 and the activated sludge without fungal strain, compared with the Control 2, which contained only the fungal strain. Comparing the PUFs Dry Weight results of the three batch tests, it was possible to hypothesize that the fungal biomass was not able to grow during the first week. However, an increase in fungal biomass was observed after 18 days, in some trials of the First Batch Tara and in all conditions of the Batch Quebracho. sCOD results showed that the faster kinetics of degradation occurred before 6 days in the First Batch Tara in all conditions with grazers. Although the highest sCOD removal percentage was achieved when using activated sludge, the use of grazers led to sCOD reduction in less days than the use of fungi without grazers. In the Second Batch Tara, the combined presence of fungi and Tetrahymena sp. supported the higher Tara tannin removal. This removal increase was close to significance if compared to the treatmewnt with only Tetrahymena sp.. In contrast, the Batch Quebracho (18 days) showed a slight difference, in terms of sCOD, between the beginning and the end of the experiment. These results could be useful in biotechnological approaches to enhance the degradation of such recalcitrant compounds in tannery wastewater