SLOW SAND FILTRATION FOR WATER TREATMENT OF A COASTAL LAKE
Slow sand filtration. Protozoa. Giardia. Cryptosporidium. Particle analysis. Surrogate parameters
The dissemination of pathogenic protozoa by drinking water has long been known and consolidated in the technical and scientific community. It stands out in resistance to the usual processes of disinfection and persistence for long periods in the environment. With the urbanization and growth of agricultural areas, surface water supply sources have become increasingly subject to various forms of pollution, contributing to the prevalence of protozoa in these water bodies. Several studies have pointed the persistence of protozoa after disinfection, which infers that treatments including methods such as pre-ozonization, flocculation, sedimentation, rapid filtration and chlorination do not necessarily mean such effective barriers against species such as Cryptosporidium. Slow sand filtration is a low-cost, high-benefit technology. It depends on little or no addition of chemicals and has simple operation. It is possible to find studies that showed high efficacy when removing large concentrations of persistent microorganisms such as Cryptosporidium and Giardia (oo) cysts, with removals over 5 log. In addition to satisfactory performance, it has been found that Cryptosporidium oocysts were retained in the upper layers of the bed and that the slow sand filter biofilm is the main removal mechanism of the technology. The slow sand filtration also effectively removes turbidity, suspended solids and heavy metals in the treated water. Due to its size and difficulty of removal, besides persistence in the environment and resistance to disinfection processes, removing Cryptosporidium can also mean the removal of all other protozoa present in the water. To detect the presence of Cryptosporidium oocysts in the treated water a particle counter may be used. This paper intends to verify the performance of a slow sand filter; and the coagulation insertion preceding the slow sand filter; for the treatment of the water of a coastal lake located in an urbanized basin.