Chemical Addition prior to Membrane Processes for Natural Organic Matter (NOM) Removal

Abstract

Membrane processes for surface water treatment include microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF), depending on the target material to be removed and the limiting process economics. MF will remove turbidity, but no dissolved compounds, unless associated with colloids, UF will, depending on the molecular weight cut off (MWCO), partially remove NOM, and NF will remove NOM almost completely, but for a price often considered as uneconomic due to energy costs. Chemical addition prior to MF or UF may enhance the NOM removal capacity of these processes to a comparable range as achieved with NF. In this work the improvement of NOM removal by MF with chemical pretreatment was investigated using FeCl3 and hematite (a-Fe2O3) addition. The results achieved with the addition of ferric chloride as a coagulant prior to MF showed that 95% removal of NOM can be achieved at a dosage of 25 mgL-1. The flocs form a gelatinous deposit on the membranes and cause flux decline, however the resulting flux is still high compared to UF and NF. Higher dosage of 100 mgL-1 resulted in a very high flux decline. The addition of hematite synthesised as monodispersed, spherical colloids in the sizes 75, 250 and 500 nm showed the importance of colloid size on MF flux. Small colloids (75 nm) are not retained by the membrane when stabilised due to the adsorption of organics, but also adsorb larger amounts of NOM than do larger hematite particles. Aggregation of these colloids increased colloid rejection with a concomitant increase (to about 20% at a low dosage of 10 mgL- 1 Hematite) in removal of adsorbed organic matter. Aggregation of small colloids increases the adsorbant surface area significantly versus larger primary colloids. The structure of the aggregates was found to be important for membrane flux. Alternatively, tighter membranes can be used. UF membranes showed a NOM removal of 10 to 90% for a MWCO of 30 to 1 kDa (five membranes were investigated), respectively. NF removed > 95% of organics, independent of solution chemistry and could remove a large fraction of multivalent ions. The study shows that if no salt rejection (softening) but very high NOM removal (> 90%) are required in a water treatment application, hybrid processes of MF with chemical pretreatment may be a very attractive alternative to UF or NF.