Nanofiltration concentrate treatment for increasing the water recovery in drinking water production

Nanofiltration is appropriate for the production of drinking water out of surface water. Due to scaling and fouling of the membranes, the water recovery is generally limited to about 80%. As problems with water scarcity are expected to grow worse in the coming decades, it cannot be tolerated that 20% of the feed water is wasted. The purpose of the study is to increase the water recovery to 96% by devising a closed cylce where the concentrate stream is treated so it can return to the feed side of the membrane without increased membrane fouling. Natural organic matter (NOM) and sparingly soluble salts foul the membrane. NOM is decomposed by ozone oxidation and the salts are removed by electrodialysis (ED). Experiments with solutions of natural humic acids show that small ozone doses of 1 mg ozone per mg COD can accomplish 40% COD reduction. It is not recommended to use higher ozone doses, because COD removal cannot be improved appreciably. Although it is not possible to achieve a high mineralization degree with ozone oxidation, ozone greatly alters the structure of the humic acids. The hydrophobic fraction of the COD is selectively removed and transformed into hydrophilic carboxylic acids, which adsorb less on the membrane surface. The high molecular mass fraction of the humic acids is decomposed into smaller fragments, although the COD rejection of the membranes stays stable. The removal of hydrophobic organics and high-molecular weight chains have both a positive effect on the membrane flux, which is proved by experiments. The only problem is to obtain a sufficient high mineralization degree in order to prevent accumulation of organic matter in the closed cycle. The addition of hydrogen peroxide to the ozonated solution could slightly increase the mineralization degree, but not to a sufficient level yet. For this application, ED membranes with a high permselectivity for divalent ions are desired. FTAM membranes of Fumatech have a high affinity for sulphate ions. The transport number of sulphate ions relative to chloride ions was higher than two, although steric hindrance typically obstructs the transport of sulphate ions through anion selective membranes. However, permselectivity of specific ions through anion exchange membranes is governed mainly by the balance of hydration energy of anions with hydrophilicity of the membranes (affinity), instead of the difference of hydrated ionic size of anions.status: publishe