Surface water is microbiological unsafe, contains too much salts and is polluted with anthropogenic substances like pesticides, medicines, endocrine disruptors and industrial chemicals. The production of safe drinking water out of this water is only possible by applying a robust treatment. A process that can remove virtually all unwanted substances from the water in one step is reverse osmosis (RO).
Reverse Osmosis
Osmosis is a naturally occurring process where water diffuses through a semipermeable membrane going from a solution with a low concentration of dissolved solids to a solution with a high concentration of dissolved solids. Flow will continue until concentrations are equalized reaching equilibrium, in this moment, the water level will be higher for the solution that originally had the higher concentration. This height difference is referred to as osmotic pressure. Water flow can be reduced or completely stopped if certain pressure is added over the highest concentration liquid and, if this pressure exceeds the osmotic pressure, the flow will be reverted and pure water will be produced. This principle is referred to as Reverse Osmosis.
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Osmosis and Reverse Osmosis Source | Source: (Wantha, 2015)
​
Water Source
As it can be observed in the table, water can be classified in different ranges according its Total Dissolved Solids content. However, in the case of membrane usage, this classification is reduced to only two categories; Brackish Water (from 1,000 to 10,000 ppm) and Sea Water (above 10,000ppm).
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Table | Water classification according to TDS content Source: (WQA, 2016)
​
​
​
​
​
​
​
​
​
​
​
​
Total water distribution in the world | Source: UNESCO
Sea Water
Oceans have a fairly predictable ions content distribution; more than 99% of their components are spread between 4 Cations (Sodium, Magnesium, Calcium, Potassium) and 4 Anions (Chloride, Sulfate, Bicarbonate, Bromide)(Prankratz et al., 2004). Boron exists in sea water as Boric acid in a concentration range from 4.5-6 mg/l. Although Sea Water Reverse Osmosis membranes are designed to achieve high rejection of salts, usually around 99.7% at a neutral PH.
Membrane filtration
Membrane filtration is a process in which a membrane is used as a selective physical barrier to separate compounds by applying a driving force across the membrane. In a membrane system, the feed water stream will be separated into two streams; the product or permeate streams, which contains the solutes that passed through the membrane (part of small yellow bullets) and the concentrate stream that contains solutes and particles rejected by the membrane (large red squares and part of small yellow bullets).
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Scheme of pressure driven membrane unit (A) and membrane performance indicators (B): feed channel pressure drop, normalized flux and salt (solute) passage.
​
​
​
​
​
​
​
​
​
​
Consequences of changes in operation conditions | Source: Dow Chemical Company.
Reverse Osmosis Membrane
A membrane element is the operational unit containing membranes, hydraulic parts and transporting parts. Several element configurations have been developed. Nowadays the most widely used RO elements in practice have a spiral wound configuration. Spiral wound membrane elements are produced from membrane sheets, which are wound along a central perforated permeate collection tube. Two flat-sheet membranes are glued together on the inside of two of its edges, making an envelope. The remaining open edge is glued to the central collection tube. In the envelope, the membranes are separated by a porous mesh named product spacer, facilitating the transport of product water to the central product collection tube. A membrane element contains a number of these envelopes, which are separated from each other on the feed side of the membrane envelopes by a feed spacer. The feed spacer separates the membranes and generates turbulence and mixing, improving mass transport near the membrane surface. The figure shows a feed spacer and a membrane element feed side with narrow flow channels containing feed spacers. The wound membranes and spacers with an end cap at each end of the element are cast in a fibre glass casing.
​
​
​
​
​
​
​
​
​
​
​
​
​
Spiral wound membrane element configuration
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Front view of a spiral wound RO element (A), detail of feed channel with restricted thickness (B) and the feed spacer (C) |Source: Vrouwenvelder, 2009
