Membrane fouling by natural organic matter is one of the main problems that slow down the application of membrane technology in water treatment. O(3) is able to efficiently change the physico-chemical characteristics of natural organic matter in order to reduce membrane fouling. This paper presents the state-of-the-art knowledge of the reaction mechanisms between natural organic matter and molecular O(3) or *OH radicals, together with an in-depth discussion of the interactions between natural organic matter and membranes that govern membrane fouling, inclusive the effect of O(3) oxidation on it.

Nanofiltration is an attractive technology for the production of drinking water from surface water. However, membrane fouling, mainly caused by natural organic matter, increases the costs and limits the water recovery. In this study, it was investigated whether the decomposition of natural organic matter in the concentrate stream by O 3 , has a positive effect on the membrane flux of four nanofiltration membranes (NF 90, NF 270, Desal 51, NF-PES 10). The results show that O 3 oxidation causes a significant alleviation of membrane fouling for all investigated membranes. This is caused by the selective removal of unsaturated bonds and hydrophobic components in the dissolved organic matter, and by the decomposition of molecular chains into smaller fragments by O 3 . However, the chemical oxygen demand could not be reduced by more than 20%. The performance of O 3 + H 2 O 2 was only slightly superior to O 3 alone, using an identical O 3 dose.

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Nanofiltration is an attractive technology to treat surface water for the production of drinking water. Due to scaling and fouling of the membranes, the water recovery in nanofiltration is generally limited to about 80%. This paper is part of a project which elaborates the concept where the concentrates are treated, so they can return to the feed side of the membrane, without increased membrane fouling, in order to reduce the water loss. The efficacy of ozonation and perozonation to remove humic acids in the concentrate is evaluated in this paper. In particular, the degree of mineralization, the amount of hydrophobic components and the destruction of high molecular mass fractions are considered. Chemical oxygen demand (COD) decreases fast with increasing ozone concentrations but reaches an asymptotic value of 40% of the initial COD, which is too high for this case. There is a selective removal of hydrophobic COD and high molecular mass chains are decomposed efficiently. The process could not be improved by changing the pH or by simultaneous adding hydrogen peroxide to the solution.