A Fouling Index Reflecting Statistical Aspects of Fouling Process with Reverse Osmosis Desalination

Reverse osmosis membrane (RO) is becoming a most popular instrument to desalinate seawater as a result of new developments in membrane technology. The critical factor governing successful long-term reverse osmosis (RO) in desalination plants is fouling caused by the turbidity of seawater, such as by suspended solids, sparingly soluble salts, or biological growth. To reduce the turbidity, the seawater is first treated with sand, film, or other agents, resulting in what is called “pretreated seawater” for RO. The fouling potential of this pretreated seawater should be assessed quickly and accurately to maintain RO performance. Silt density index (SDI) is a widely used standard measure of the fouling potential of the pre-treated seawater on a membrane surface, and judging whether it should be supplied with the value of SDI is usually performed. However, some literatures indicate that SDI does not always correctly represent the fouling potential. Thus, in this study, we sampled seawater under various environmental conditions, such as a place, a season, turbidity, pH, and water temperature, and conducted filtering experiments. Statistically analyzing the relationships between the amount of filtrated water, elapsed time, and environmental factors, we developed a new fouling potential index, termed “permeation index β.“ Comparing performance between SDI and PI, we showed the predominance of the latter.