Ultrafiltration (UF) is a proven technology in water treatment nowadays. However, fouling remains a major challenge in the operation of UF, especially in regard to colloidal NOM fouling. In general, a number of colloidal NOM fouling mechanisms may occur, such as adsorption, gel formation. Colloidal NOM fouling is influenced by multivalent cations, ionic strength and pH. In order to control membrane fouling, different pretreatments such as powder activated carbon adsorption, lime softening, ion exchange, conventional media filtration and coagulation with inorganic and polymeric coagulant have been investigated. In-line coagulation is the most commonly used pretreatment for UF of surface water. However, the problem with in-line coagulation is that a large amount of backwash-derived waste sludge containing dosed coagulants is produced. Since the backwash waste sludge with coagulant has to be treated before its discharge (in especially Western Europe), this option creates additional cost for the membrane treatment plant (up to 20% of the total cost of the whole plant). This dissertation investigates the technical possibility of controlling the UF fouling by backwashing with low ionic strength water (demineralized water in the Netherlands), in order to reduce the ionic strength and the amount of multivalent cations and thus reduce NOM fouling. Chapter 1 briefly introduces this dissertation. The effectiveness of deminerlaized water backwashing is generally investigated on a pilot scale (with a 2.4m2 membrane) in Chapter 2. Results show that regarding the removal of NOM foulants via hydraulic backwashes, demineralized water is better than UF and NF permeate. That is probably due to the absence of cations, reducing the charge screening effect and/or Ca-bridging effect between the negatively charged membrane and NOM, leading to a restoration of repulsion force and consequently an easy removal of fouling layer. However, it is not clear which components in backwash water lead to the low foulant removal in this chapter. Therefore, Chapter 3 investigates the influence of backwash water composition on fouling control. Different amount of CaCl2 and NaCl was dosed in demineralized water to test their effect on fouling control. It became clear that the presence of monovalent and divalent cations in backwash water reduces the fouling control efficiency. Moreover, by isolating the organic matter in UF permeate for backwashing, it is found that the organic matter in UF permeate itself does not cause fouling problems when they are in backwash water. In terms of the influence of monovalent and divalent cations, both the elimination of the charge screening effect and the breakdown of the calcium bridging effect are possible mechanisms to explain this improvement. Therefore, these two effects are presumed to be the mechanisms of demineralized water backwashing. The investigation of the hypotheses of demineralized water backwashing is reported in Chapter 4, including the charge screening and the calcium bridging effects. By determining the zeta potential of the membranes and the colloidal NOM compounds at different conditions, the impact of pH and electrolyte valence and concentration on their charge was assessed. Furthermore, the adsorption of calcium on the membranes and the NOM compounds was also illustrated. Results showed that a membrane became less negatively charged when the pH decreased and the concentration of electrolyte increased, proving the presence of the charge screening effect. Furthermore, divalent cation has a much stronger effect on the increase of membrane zeta potential than monovalent cations which is generally in consistent with the DLVO theory. Calcium ions indeed adsorbed on either new or fouled membranes, and bridged NOM and membranes afterwards. However, the interaction of calcium with fouled membranes is more substantial than with new membranes. However, the charge screening effect played a dominant role in the membrane fouling and fouling control by demineralized water backwashing. Most of the fouling caused by calcium bridging is difficult to remove even with a demineralized water backwash. Chapter 5 illustrates the effectiveness of demineralized water backwashing on ultrafiltration fouling of different fractions of NOM. Results of natural waters show the same fouling removal via demineralized water backwashing as the previous chapters. Furthermore, LC-OCD analysis of Schie Canal water showed that biopolymers can be flushed away by hydraulic backwashes of either demineralized water or UF permeate. Compared to almost zero removal of humics and LMW substances by UF permeate backwashes, demineralized water backwashing was able to remove a substantial amount of humics, and a small amount of LMW substances. Fouling of sodium alginate model compound showed a high reversibility no matter what kind of backwash water was used. This is also consistent with the LC-OCD analysis of Schie Canal water. However, not all biopolymers were removed by hydraulic backwashes. A low fouling reversibility was observed for BSA fouling, but BSA may be in the part of unremoved biopolymers with demineralized water. No improvement in fouling control for fouling of Suwannee River humic acid (SRHA) was observed as well when demineralized water was used for the backwash. This is probably because the calcium bridging via carboxyl functional groups is the main mechanism for SRHA fouling, which is difficult to break down. Since the charge screening effect is the main mechanism of demineralized water backwashing, theoretically speaking, its application on seawater treatment is also possible. Chapter 6 demonstrates that demineralized water backwashing can substantially improve seawater UF fouling control, similar to the previous findings in surface canal water. However, the duration of a successful demineralized water backwash should be extended from one to two minutes. This is due to the high salinity of seawater and thus more demineralized water was required to dilute the seawater and limit a higher dispersion effect of seawater than surface water. Monovalent cations in backwash water showed their impact on the fouling control efficiency, indicating the existence of a charge screening effect. Furthermore, the different UF membrane fouling behaviors in winter and spring indicated the impact of a seasonal influence on UF membrane fouling. In spring, the membrane showed more fouling probably due to the algae bloom which is widely considered an important fouling factor. The results of the long-term experiment reconfirmed the effectiveness of backwashing with SWRO permeate (similar quality as demineralized water) on the fouling control of seawater UF. Since it is very easy to access SWRO permeate in a UF-RO desalination plant, this approach can be implemented easily. In order to apply this technique in industry, optimization work was conducted on a pilot scale with a standard membrane element (40 m2) and reported in Chapter 7. Results show that SWRO permeate (having similar qualities as demineralized water) backwashing substantially improved the seawater UF fouling control, consistent with the previous studies with small-scale membrane modules. The effectiveness of SWRO permeate backwashing on UF fouling control was observed at a recovery rate up to 95.8% during a low fouling period. Furthermore, the results of the DEMIFLUSH pilot were similar to the Evides desalination plant having the same operational settings, suggesting that the results obtained from the DEMIFLUSH pilot are applicable to full-scale plants. However, the results of high fouling period are missing. If the optimization results can be repeated in high fouling period as well, the application of SWRO permeate is also economically feasible due to the low consumption of SWRO permeate. Optimization work should be continued to reduce the consumption of SWRO permeate or demineralized water, since the usage of these water is expensive.