Exposing more water limits penetration of sunlight during treatment due to scattering and absorption, depending on the turbidity of the water. Because of this, larger volumes are recirculated to receive sunlight in a solar reactor module, such as our CPC reactors. As the water is recirculated through the solar collector, light penetrates it completely, but when the spores in the water go through the tank and tubing, they are protected from the detrimental solar irradiation. As microorganisms that only receive sublethal UVA irradiation become more resistant to the UVA-induced oxidative stress (Hoerter et al., 2005), microorganisms may have a chance to partly recover and adapt more easily to the induced oxidative stress. This explains why longer treatment times and higher H2O2 concentrations were needed for CPC reactor disinfection.