Optimizing low-pressure ultrafiltration pretreatment systems for the removal of seawater organic matter and the effects of marine algae

Abstract

Membrane fouling due to organic matter in seawater is a key factor that contributes to the overall performance of desalination plants. With the increasing use of ultrafiltration (UF) membranes in desalination pretreatment systems, the challenge of controlling fouling becomes even more important. During periodical marine algal blooms, the organic content in seawater increases significantly, presenting greater problems. Various fouling control techniques can be applied in water treatment plants to improve the performance of UF membranes, such as dissolved air flotation (DAF), in-line coagulation, and disinfection by chlorination. In this thesis, the impact of these techniques on membrane fouling layer mechanisms was assessed, and the potential limitations of their applications were identified. During the initial study with model organic solutions, it was found that substituting permeate water backwash with deionized water resulted in lower membrane fouling. However, it has the potential negative impact of long term irrecoverable fouling. While this method of fouling mitigation could be applied under normal seawater conditions, it was not suitable during marine algal bloom periods. The use of chlorine disinfection was found to improve membrane fouling by eliminating live algae cells in solution preventing the generation of algal organic matter. However, it was found that high dosages of chlorine would result in significant cell lysis, releasing organics from inside the cell walls. This was found to have a negative impact on permeate quality. Similar reactions were observed when high levels of physical shear were applied. In-line coagulation and DAF were both very effective at mitigating UF membrane fouling, and could be applied during normal seawater conditions and marine algal bloom periods. DAF was found to improve membrane longevity significantly and was still effective when coupled with industrially aged pretreatment membranes. However, it has higher operating costs compared with other fouling mitigation strategies.