Suspended matter filtration causes a counterintuitive increase in UV-absorption

In water treatment, filtration is often a first step to avoid interference of chemical or UV-disinfection with sus-pended matter (SPM). Surprisingly, in testing a ballast water filter with 25 and 40 mu m mesh screens, UV -absorption (A, 254 nm) of filtered water increased with the largest increase in the finest screen. The hypothe-sis that filtration partly removes large particles and partly replaces them with small unfiltered ones, leading to an overall increase in absorption, was tested by measuring particle counts, particle-size distributions (PSD) and by modeling the Mass Normalized Beam Attenuation Coefficient (A/SPM) before and after filtration. An indepen-dent model verification was made by measuring and modeling A/SPM of three differently sized Arizona test dust suspensions. It is concluded that filtration is a good pretreatment for chemical disinfection systems because it removes the suspended matter mass, but that the production of smaller particles increases UV-absorption and hence may reduce disinfection performance.

Automated summary

Filtration is an important step in water treatment to remove suspended matter. However, it also generates smaller particles that increase UV absorption and may reduce disinfection performance.