(177d) Improving Membrane System Productivity: From the Local to the Module to the System Level

The birth of industrial membrane processing can be traced back to the discovery of the Loeb-Sourirajan membrane in the 1960s. That discovery spawned not only a major international research effort but also a major international industry. Membrane technology have now developed to the point where there is virtually no separation process for which it would not be considered as a serious alternative when designing and installing equipment. Membrane technology is now routinely applied to significant global issues that include water and wastewater treatment and recycling, product recovery and concentration, gas separation, to name but a few. The development of the technology has been driven by continuous reductions in equipment and operating costs alongside enhancements in membrane separation capabilities and design innovation (Fane, 2005).

This paper focuses on the role of membrane system design and operation in increasing the throughput and productivity of a membrane plant in such a way that leads to decreases in capital and operating costs and, hence, increases in profitability. It considers the research and process innovations starting at the local membrane-solution level, through to the module level and then to the system level. To optimize overall membrane system productivity, it is vital to consider these three levels because a membrane plant is a complex system.

The smallest component in the membrane system is the module. Up to hundreds of modules can be operating in parallel in what is called a stage. Up to a dozen or more stages can be connected in a cascade to form a network. Fluid can be recycled, repressurized, added and/or removed within and between stages. Behavior varies inside modules and between different modules and different stages. Factors affecting the behavior of an individual module include not only the design and operation and feed conditions of each module (i.e. local effects) but also the design and operation and feed conditions of each stage and of the entire system (i.e. global effects). Other factors can also affect the module and stage behavior, such as cleaning and replacement schedules, the age of the membranes, and, local and global changes in operating and feed conditions. The overall system behavior, which is what matters from the point of view of the end-user, is not only affected by the behavior of each module and stage but also by the network structure and the operating configuration (e.g. with or without fluid recycling).

Due to the complexity of interactions between the module, stage and system level behavior, optimizing only the local or module and stage level performance independently of the network performance and vice versa is unlikely to achieve overall optimum system performance. This is because even though the design and operation of each module at the local level does have a profound effect on the overall system performance, nevertheless the overall system dynamics is not solely determined by the behavior of the individual modules and indeed differs significantly for systems with different network structures and operating configurations, particularly those involving fluid recycling. Thus, ideally, module and network design for fluid management need to be integrated with operation and control strategies for fouling management in order to achieve optimal system performance and economic viability.

Strategies that improve productivity rely on one of three mechanisms: (1) reducing the attraction between the membrane and the solution components such as solutes, particulates or biological species by pre-treating the feed or altering the chemical interactions with the membrane surface; (2) reducing the amount of solute, particulates and/or biological species that get close to the membrane by appropriate choice of operating conditions and improving the mixing; and, (3) restoring productivity by cleaning the membrane after fouling has occurred.