UF/MF as RO pre-treatment: the real benefit

Abstract

Ultrafiltration (UF) and microfiltration (MF) are theoretically the best pre-treatment upstream reverse osmosis, removing from the feed water most of the potential elements responsible of desalinating membranes fouling such as particles, turbidity, bacteria and large molecular weight organic maters. Those clarification membranes reduce also significantly the silt density index below 3 (100%) [1–6] and generally below 2. Many papers present results about the efficiency of UF/MF regarding clarification efficiency. UF/MF can be an economical option for RO pre-treatment. Several evaluations of the cost of UF/MF upstream RO demonstrate the interest of this pre-treatment technology on average to bad water qualities, compared to a double filtration steps conventional pre-treatment. In that case, and on a long-term operation basis, the treatment line is economically viable [7]. However, UF/MF remains more costly than a single filtration on mono or multimedia filter: UF/MF is not economically applicable on the best water quality to pre-treat. Nevertheless, UF/MF technology has some limits. If this technology is economically viable on average to bad water qualities, UF/MF direct filtration can overpass the technical limits of the process, generating the need of frequent cleanings. In that case, the RO fouling is moved upstream to the pre-treatment stage. Moreover, the clarification membranes, with cut off generally higher than 100,000 kd, do not remove the smaller organic matter molecules responsible of the biofouling development on RO membranes. In this situation, it is very difficult to demonstrate the real technical advantage of UF/MF on the RO operation, since this demonstration needs a long-term study (more than one year depending on the application) with side-by-side technologies to compare, and the results appear to be site specific. Solution of this double problem, UF/MF fouling on average to bad surface seawater and biodegradable organic matters removal, can be found in the treatment line, enhancing the UF/MF treatment by adding a pre-coagulation/settling of flotation. The benefits of this association of technologies are multiples, as it was developed for the treatment on surface water one decade ago for drinking water production. Moreover, the benefit of the coagulation/separation on the design (filtration flux) and operation (frequency of cleaning, membrane life duration) can significantly improve the economical balance of the process. It also allows a definitive advantage concerning RO membrane fouling and its related impact on cleaning frequency, RO membrane replacement and plant availability.