Evaluation of Rock Sand as a Filter Medium in the Slow Filtration Unit of Water Treatment in Mauritius

Abstract

Coral sand has been used as a filter medium in both the slow and the rapid filtration units of the water  treatment plants in Mauritius for a long time. Coral sand is obtained by either the dredging of lagoons or  from inland sand quarries. With time, extraction of coral sand was becoming detrimental to aquatic life in  the lagoons. A number of legal measures were adopted to control the extraction, but these proved to be ineffective. Eventually, in October 2001, Government officially banned the dredging of lagoons for coral  sand removal. The present study evaluated the use of rock sand as an economic substitute to coral sand  in the slow filtration process. Test results on the filtrate from laboratory and full-scaled models indicated  that rock sand  produces good quality filtrate, as far as physical and chemical characteristics of water quality is concerned.  However, the results from the fullscaled model also indicated that the major drawback is related to the  cleaning of the filter bed. Modification of both grading of the rock sand and the cleaning mechanisms may  improve the lifespan of rock sand filters, but these processes would add to the cost of production of potable water. Hence, in this study, results obtained from the full-scaled rock sand filter confirmed that rock sand is not an economic substitute to coral sand in the water filtration units of slow sand treatment plants.

Keywords: water treatment, filtration, rock sand, coral sand, filter medium