This paper examines the effect of humic acids (HA) and other solution constituents (Ca2+, Mg2+, Na+), frequently encountered in potable water sources, on the removal of a well-known herbicide, atrazine (ATR), by a laboratory-scale pilot Photocatalytic Membrane Reactor (PMR) system. Experimental results with different HA concentrations, in the presence (or not) of background cations, demonstrate the attainment of steady-state operation with constant degradation efficiencies and controlled membrane fouling phenomena. The excellent performance of the system is attributed to the efficient photocatalytic degradation of the dissolved organics, which is enhanced by steric interactions between the HA-ATR pseudo-complexes and the membrane. The successful laboratory tests hold promise for practical water treatment applications of the proposed PMR system.