Numerous tests around the ...
Published by Prem Baboo, Researcher at www.researchGate.net
Numerous tests around the world have proven that ultrafiltration (UF) provides optimum pre-treatment for seawater desalination based on reverse osmosis membranes (SWRO). The use of pressurized ultrafiltration as a pre-treatment for the reverse osmosis membranes in seawater desalination has experimented an impressive increase as a result of the continuous search for cost-effective technologies which enable a sustainable production of water Ultrafiltration will remove all suspended solids and will provide a substantial reduction in micro biological activities. . Even more importantly, the ability to reliably provide good quality filtrate water to the downstream reverse osmosis are the most remarkable benefits associated with this technology. Plugging of RO spacers is completely eliminated and the RO cleaning frequency can be substantially reduced. Key benefits associated to the ultrafiltration technology versus conventional pre-treatment are a low footprint, the ability to remove virus and bacteria and to significantly reduce colloids, suspended particles, turbidity and some total organic carbon. The main obstacle against the use of ultrafiltration membranes for SWRO pre-treatment has always been the higher operating cost of ultrafiltration when being compared with conventional pre-treatment. Up until now this higher cost has prevented implementation of UF in all but a few SWRO plants. A new membrane has been designed with the aim of tailoring it towards lowest total cost of ownership. Typical operating conditions have been used to quantify the parameters for a potential large scale desalination system (UF + SWRO): amortization of investment in UF membranes and equipment; operating costs of the UF system; reduction in operating costs of the SWRO desalination plant, when being compared against a conventional pre-treatment + SWRO system; the increased output of the SWRO desalination plant due to higher availability and shorter construction time. The total cost of ownership of a UF based SWRO desalination plant has been determined (expressed in US$/m 3 of water produced). Taking all factors into account the total cost of ownership of a dual membrane desalination plant (Seaguard UF + SWRO) will be 2–7% lower than the total cost of ownership of a SWRO plant based on conventional pre-treatment.
Backwash is identified as the most important cleaning process to be improved in order to increase the efficiency of the ultrafiltration process. Compared to the Chemical Enhanced Backwash (CEB) and the Clean in Place (CIP), the backwash consumes a huge amount of time, because it takes place more often. This thesis researches how to improve the effectiveness of the cleanings done in the ultrafiltration in order to improve the process efficiency. Thanks to this research, ultrafiltration efficiency is increased from its original value of 88% to 98% in desalination plants. This represents filtrating 96 minutes extra per day and a reduction of 100% in the filtrated water used during backwashes. This represents a cost decrease in the ultrafiltration process of 7.1%, and a cost decrease of 1.2% in the whole desalination process. Moreover, sodium hypochlorite chemical equivalent concentration is reduced from 0.28 mg/l to 0.06 mg/l. Backwash sequence is also simplified from five cleaning steps to only two cleaning steps. These are the backwash top with air scour and the forward flush. The steps eliminated are the air scour, the draining and the backwash bottom. Backwash frequency is optimized from 30 minutes to 90 minutes. Backwashing with reverse osmosis brine is also proven feasible in an ultrafiltration and reverse osmosis integrated process. Chemical Enhanced Backwashes frequency is decreased from one CEB per day to one CEB every five days. Polyvinylidene difluoride (PVDF) fibers are assessed against Polyethersulfone (PES) fibers. PES fibers show initially higher permeability, but are less fouling resistant. Therefore, they need 2.5 more CEBs to sustain the same operating flux. Moreover, if these CEBs are not done, PVDF membranes show 55% higher permeability. If the smaller active filtration area of PES modules is taken into account, savings of 18% in the ultrafiltration step and of 2% in the desalination plant are achieved.