If one were to select a single word describing the most important factor when applying membrane technology to a wastewater application, it would be chemistry. That is not to say that other factors are not important; certainly fluid dynamics and other physical science factors play a major role as well. But it is the chemistry that is so important to success.
And it is the chemistry that is so often overlooked.
All too often reverse osmosis, or any membrane system for that matter, is considered the quintessential 'black box', being viewed as merely an arrangement of pipes and membrane elements that will take a dirty wastewater and 'clean it up'.
This line of thinking frequently ends up as being a prescription for disaster. Since a membrane system is a dynamic operation, one can not view the system as being some sort of static instrument. And it is the chemistry that is the most dynamic of all.
The chemistry plays such an important role because the system is constantly concentrating contaminants. In fact, one shouldn't think of a membrane system for wastewater processing as a 'filtration system', or a 'purifier', but, rather as a 'waste concentrator', constantly increasing the contaminant concentrations, producing purified water as a byproduct.
This is important since it is the 'concentration' aspect that is so often taken lightly. And it can be the key to the successful design of a wastewater membrane application. This concentration 'factor'- that is the degree of concentration - relates to both soluble solids and insoluble solids alike. Clearly, if one concentrates insoluble solids to a point where they tend to accumulate within a system, problems will develop.
Concentration of dirt is fairly straightforward. What often does not appear to be all that clear are potential problems harbored within the make-up of the soluble solids. As they become concentrated, certain species can surpass their limit of solubility, creating a problematic insoluble solid.
If one does not understand the chemistry of the soluble solids (or the insoluble solids, for that matter) membrane fouling problems typically result.
Concentration factor is a simple physical phenomenon - as one removes purified water from a aqueous solution, the solute remaining becomes more concentrated. If, for example, one removes fifty percent of the water, the remaining solids are concentrated by a factor of two; if 75% of the water is removed, the factor is four, and so on.