Compared to the control group, microscreens removed particles, reduced particulate organic mat-ter, and increased values. Particulate parameters reached steady-state in all treatment groups havingmicroscreens at the end of the trial. The time to reach equilibrium seemingly increased with increasing mesh size but the three treatment groups (100, 60 and 20 m) did not significantly differ at the end of the trial. Increased back washing frequency over a 24-h period had no further significant effects on the parameters measured. The results demonstrated the role and importance of a microscreen, and showed that mesh size, within the range tested, is less important at long operations under constant conditions