The aim of the paper is to compare two optical methods (UV spectrophotometry and laser granulometry) for a better knowledge of the behaviour of wastewater suspended solids. The first one, based on UV spectrophotometry, is used for the estimation of solids concentration and nature in water and wastewater with the help of an advanced exploitation method of UV spectra. But the interpretation of UV spectra, in presence of a very wide size distribution of solids is not always easy, because of the different phenomena responsible of light diffusion. The second method, more classical, allows to characterise the size distribution of particles.