RO membrane autopsy of Zarzis brackish water desalination plant

Tunisia, located in a semi-arid zone, has limited water resources. The shortage of good water quality in particular at the south regions required a brackish water desalination to supply these regions with potable water. Zarzis desalination plant, working since 1999, has revealed difficulties related to the loss of membrane performances. In the aim to determine the reason of membrane fouling, foulant layer were analysed. The membrane autopsy has achieved by different methods, such as TOC (total organic carbon) measurement, SEM (scanning electron microscopy), AFM (atomic force microscopy), FTIR (fourier transform infrared spectroscopy) analysis, diffraction by X-ray. Results show that the foulant layer composed mostly of SiO2, clay, organic matter (polysaccharide, protein), CaSiO3, Fe3O4, AlPO4, and CaSO4. The characteristics bonds obtained by IR indicate the presence of polysaccharides and proteins that constitute a source for further microbiological growth; this is the problem of biofouling. The metallic elements results of corroded stainless steel and internal coating of cartridges filter that are damaged by the silica particle rejected by sand filters. The scale of CaSO4 and CaSiO3 deposition is owing to the bad antiscaling efficiency. The results are presented and discussed in the light of new trends in material.

Fig. 8. Cleaned membrane surface. show the remarkable differences between the surface morphologies of the two membranes. While the fouled membrane exhibits large-scale surface roughness of ridge and valley structure of which roughness mean is 141.19 nm, the cleaned membrane surface is relatively smooth 92.32 nm. This remarkable surface roughness of the tow membranes has a dramatic effect on the deposition rate of colloids to the membrane surface. It is worthwhile to mention that the marked difference in the surface roughness of the two membranes was also confirmed by SEM micrographs (Figs. 7 and 8). The permeability of membrane studied, before and after cleaning, was determined and results were: Permeability of fouled membrane 1.1637 L h−1 m−2 bar−1 Permeability of cleaned membrane 1.4461 L h−1 m−2 bar−1 The permeability is ameliorated with cleaning cycle, (EDTA, NaOH and citric acid) which in concordance with decreasing of roughness In the present work we adopted the approach based upon the interpretation of the results of performance decline, of Zarzis desalination plant, on basis of destructive element autopsy. Fouling layer of element removed from stages 1 of unit 2 was analysed by different techniques. Results of analysis by energy dispersive X-ray showed the presence of silica (56%), iron oxide (3%), calcium silicate (6%), calcium sulphate hydrate (3%), aluminium phosphate (3%) and clay (16%). The proportion of organic matter in the fouling layer (13%) has been identified by FTIR which showed the presence of polysaccharide and protein. The results were discussed on the basis of recent works related to the new trends in conception. The following recommendations can be proposed: • Regular maintenance of sand filters, to avoid the flight of sand particles. • Use an antiscalants more efficient, able to inhibit precipitations of salts and a specific antiscalants for silica, • Replace the external lid of cartridges filters corroded, • Replace the installations in iron or in corroded stainless steel, • Specific treatment is necessary to eliminate the clay, which forms a hard layer on the membrane surface difficult to eliminate. • Clean membranes when performances of plant are declined. The cleaning solution proposed according to the foulant: EDTA and soda to eliminate salts and the organic material, as it is recommended to use with the previous solution a detergent as agent disinfecting. Cleaning with the citric acid is necessary in order to eliminate the metallic oxides.