There is still not enough information about the influence of several important variables, which occur in natural systems, on the coagulation of humic substances. One of these rarely studied variables is type and concentration of algal exudates. Thi s paper presents the results of research of the influence of alga Scenedesmus quadricauda and its exudates on the results of coagulation experiments with model humic water. The arrangement of experiments should simulate one possible way how algae might influence natural water systems and water treatment.
The results demonstrate that both the residual concentration of humic substances and residual aluminium are influenced during the time which elapses after the addition of the washed algae to the model humic water. But the influence of algae on residual colour is partly different from their influence on residual aluminium. Early after the algae addition to the model humic water, they influence the system predominantly with their surfaces, because the concentration of their exudates is low. This increases the residual concentration of aluminium. On the other hand, longer “algal residence time” in model humic water decreases the residual aluminium concentrations. Systems with algae are sensitive to overdosing with respect to both residual humic substances and residual aluminium.