LIGHT PENETRATION IN WASTE STABILIZATION PONDS

The penetration of light into waste stabilization ponds was studied be cause of its importance in pathogen removal and algal productivity. Th e attenuation of light in ponds was dominated by light absorbtion by g ilvin (also called dissolved yellow matter or humic substances) and al gae, light scattering processes (turbidity) being of no importance. Gi lvin exerts a strong influence over the spectral variation and longer wavelengths penetrate much better than short wavelengths. Estimates of attenuation coefficients in the u.v. suggest that these wavelengths a re less penetrating than previously reported. Differences in algal con centrations cause the differences in light attenuation seen between po nds, though they also cause some spectral variation because short wave lengths are more affected by changes in algal biomass than long ones. In the absence of algae there appears to be a lower limit to the clari ty of ponds dictated by gilvin and other substances. Secchi disks were found to be reliable instruments for measuring light penetration.