New granular materials for dual-media filtration of seawater: Pilot testing

摘要

The scope of this study was to test new granular materials for dual-media bed filtration of seawater and to assess the quality of the filtrate with regard to criteria for feeding reverse osmosis desalination installations. Two different filter columns, one with the new materials (expanded clay), i.e. Filtralite NC 1.5-2.5 mm on top of Filtralite HC 0.8-1.6 mm (called Filtralite MonoMulti filter), and another with anthracite coal 1.2-2.5 mm on top of a sand layer 0.8-1.25 mm (called sand/anthracite filter), were tested in parallel operation under various conditions in the summer of 2007. Both filters exhibited similar performance, removing particulates from feed water and producing filtrates of acceptable quality for feeding RO systems (SDI15,15-min Silt Density Index, values lower than 5). The relatively high temperatures prevailing during the summer apparently favoured the formation of larger aggregates and their subsequent effective capture within the filter media. Regarding the hydrostatic head development, it was observed that the Filtralite MonoMulti column had slower head build up than the sand/anthracite column for the same duration of filtration, especially at the higher filtration velocities (10 and 15 m/h). For the lower filtration velocity tested (5 m/h) both filters demonstrated almost the same rate of head build up. Regarding the duration of filtration cycles, Filtralite MonoMulti exhibited either the same (in the cases of filtration velocities 5 and 10 m/h) or longer duration (at the highest filtration velocity tested of 15 m/h) compared to the conventional sand/anthracite filter. A comparative assessment of data from dual-media filters tested in this as well as in previous campaigns with similar materials shows that the combination involving Filtralite NC 1.5-2.5 mm on top of Filtralite HC 0.8-1.6 mm exhibits, overall, the best performance. The latter is attributed to the rough surface texture of Filtralite grains, which are characterized by extensive crevices, with an increased capacity of retaining micro-particles and aggregates.