Diagnostic and membrane autopsy of Djerba Island desalination station

Tunisia is a semi arid country; the shortage of good water quality in particular at the south regions forced the government to desalt brackish water via membrane technology such as RO to supply these regions with potable water. Station of Djerba Isle established in 2000 by SONEDE has a nominal capacity of 15 ⋅ 103 m3 per day and recovery rate of 75%; it is fed from an artesian well with the water salinity of 5500 ppm. Three RO units compose station; each one has configured in two stages a 2:1 array with six RO polyamide elements per pressure vessel. Membranes have been cleaned or replaced, during four operating years, as much time. In the aim to determine the reason of membrane fouling, this work occurs to analyse fouling layers. The membrane autopsy has achieved by different analytical methods, such as COT measurement, SEM (scanning electron microscopy), IR analysis, and diffraction by X-ray. Obtained results show that the deposit composed mostly of organic matter, silica, iron and CaCO3. The debris accumulate on upstream of pressure vessel is composed mainly of silica, metallic matter (iron and chromium) and aluminium phosphate. The presence of TOC indicates the presence of microorganism. The metallic products come from stainless steel fittings and internal coating film of cartridges filter that are damaged by the silica particle probably rejected by sand filters. The scale of CaCO3 deposition is owing to the bad anti-scaling organophosphonate-based, which can form aluminium phosphate by reaction with aluminium composing cartridges filter internal coating.

In the present work, we adopted the approach based upon the interpretation of the declines performances, of Djerba desalination plant, on basis of destructive element autopsy. Fouling layer of element removed from 1st stage of 2nd unit was analysed by different techniques. Results of analysis by scanning electron microscopy showed the presence of silica (7.12%), iron (13.81%), chromium (2.42%) aluminium (2.34%) and calcium (2.23%). The debris analysis showed the presence of silica (14%), iron (18.33%), chromium (4.17%) and aluminium (1.15%). The proportions of total organic carbon are respectively 14.27 and 11.22% in the fouling layer and in the debris. The deposit and debris composition are due to the hydrodynamic conditions, of water flow through the bed filter, causing the escapes of the sand particles. The introduction of the mixture sand/water in the pre-treatment compartment generates the stainless steels corrosion (pipes, pumps, etc) and the detachment of coatings film of cartridge filter and head pumps. Generated by-products are accumulated on membrane surface as foulant and debris forms. The deposition of calcium carbonate (and other scaling) is due to the inefficient anti-scalant. According to these results, it is necessary to take the following prevents: • Verify different parameters of sand filters (composition, particle size, density, depth bed, input and output flows), and regular maintenance with a view to avoid the flight of sand particles.