Reverse osmosis membrane and module improvement roadmap for maximum impact

Advancements in Reverse osmosis (RO) membrane modules towards lowering specific cost of pure water can be quantified in terms of two non-dimensional numbers: specific cost of membrane conductance (viz., product of membrane area and permeability) relative to electricity price, and the channel mass-transfer coefficient relative to permeability. Reducing specific cost per unit conductance is more important than simply improving permeability, since high permeability at correspondingly large cost per unit area will not yield cheaper permeate production. The impact of technology improvements is assessed considering RO systems operated at an optimized average flux which minimizes the sum of capital and operating costs. Developing cheaper membrane modules is likely to be more impactful than increasing permeability or mass transfer coefficient. For example, for seawater RO, a 5× increase in permeability or mass transfer coefficient yields 6–8 % reduction in water cost, whereas, a 2× reduction in the amortized specific cost of membrane module yields more savings. Keeping membrane costs unchanged, both permeability and mass transfer coefficient have to be increased simultaneously by 2× to achieve similar savings. Multi-staged (and by extension batch) processes yield larger savings with improved membrane modules, and hence will likely become more prevalent in the future.