Pilot filtration study to reduce fouling on Marbella seawater RO plant

The seawater RO plant at Marbella had operated intermittently, and at low flowrates, since its construction 10 years ago. This was because the plant had been designed to operate at times of water shortage, to provide water when the conventional supplies were not available. During the early part of the 21st century, the mode of the plant changed, and it was expected to operate at design capacity for most of the time. This brought two facts to light — that during high abstraction rates the intake water quality deteriorated significantly, and that the filtration system that had been installed was not capable of adequately filtering the feed water to make it suitable for feeding to a reverse osmosis plant. The plant was originally built using DuPont B10 permeators. Due to the unavailabilty of these permeators, the plant is currently being converted, stream by stream, to spiral wound membranes supplied by Hydranautics. At design flowrates, the DuPont system fouled rapidly, and required cleaning every two weeks. The spiral trains fouled more slowly, but still required cleaning at a higher frequency than would be considered normal for this type of plant. A pilot filtration plant was installed on site to attempt to find the following; • An optimum coagulant for the water. • An approximate dose rate for that coagulant. • The effect of different media on the quality of filtered water. • The length of run between backwashes using different media. • The quality of water that can be achieved using this coagulant and media. By installing pressure tapping points along the length of the filter, the area of differential pressure could be measured. This was used to ensure depth filtration was taking place, and the foulants were being removed through the length of the bed rather than surface filtration. *Corresponding author. Presented at the conference on Desalination and the Environment. Sponsored by the European Desalination Society and Center for Research and Technology Hellas (CERTH), Sani Resort, Halkidiki, Greece, April 22–25, 2007. 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.0000.00.000

Operating a pilot filtration plant in close cooperation between plant operator, membrane supplier and chemical supplier can provide useful data for use in designing or upgrading media filtration plant and solving plant operational problems. Already first results from coagulation– flocculation tests optimization of pH for coagulation and tuning of the chemistry of the pre-treatment were implemented and brought immediate improvement in plant performance what is obvious in presented charts from differential pressure behaviour in two typical trains. Filtration pilot trials on the Marbella feed water, using a multimedia design that gave good separation resulted in better sludge loading of the media, and a lower filtered water silt density index than existing filtration. It is anticipated that this will result in longer operation between cleans on the reverse osmosis unit.