Introduction: Desalination by reverse osmosis

Stewart Burn and Stephen Gray

Climate change in many nations is predicted to reduce the long-term yield of dams and groundwater systems, which when linked to increased urbanisation and population growth will see increasing water shortages in many cities. Higher temperatures and reduced precipitation could also increase urban water demand because many cities use about 30–40% of residential water for irrigating domestic gardens and public parks. More water tends to be used when the weather is warm and dry as water for irrigation and evaporative cooling. In many cities, new sources of water will be required to meet the increasing demand for water, giving an opportunity to undertake new solutions. Whilst options such as stormwater capture and reuse (potable or non potable) are possible for some cities, for many the combination of these with desalination or desalination alone is a reliable and attractive option, as exemplified by the desalination plant built at Kwinana, Western Australia and shown in Figure 1.1.

Water desalination has a long history. Initial impetus was the need for potable water from the sea, or from brackish groundwater in the case of arid and remote communities. Early applications of desalination were small-scale plants deploying a range of technologies. However with the technological developments in Reverse Osmosis, most new plants use this technology because it has a proven history of use and low energy and capital costs compared with other available desalination technologies. This has led to the recent trend for larger seawater desalination plants in an effort to further reduce costs, and 1,000 MLD seawater desalination plants are projected by 2020.

Reverse osmosis (Figure 1.2) uses a membrane to filter and remove salt ions, large molecules, bacteria, and disease-causing pathogens from sea water by applying pressure to the water on the feed side of a semi-permeable membrane. The salt is retained on the concentrated side of the membrane and pure water passes to the other.

Reverse osmosis has a number of shortcomings. Although the membrane is impervious to salt, it can let through small neutrally charged compounds such as N-nitrosodimethylamine (NDMA) and boron, requiring further or enhanced treatment before the desired water quality is met. Reverse osmosis removes all the naturally occurring salts to give un-buffered water that is deficient in calcium and other essential minerals, making it corrosive to distribution systems and inappropriate to drink. Minerals are, therefore, added back into the water to stabilise its corrosive nature and make it palatable for potable use. Brine streams are inevitable part of desalination, and management of concentrated brine requires careful attention, particularly in environmentally sensitive regions.

It is expected that the existing trend to use reverse osmosis for urban and industrial water desalination will continue and research is examining ways to make the process more efficient and reduce the amount of energy needed. For high salinity feed waters such as seawater, energy costs are a significant proportion of the operating cost and improvements in consumption stands to provide significant financial benefit. A range of emerging technologies increase efficiency by either pre-treating water, reducing membrane fouling, improving the throughput of water and rejection of pollutants, reducing the pressure at which the systems operate or recovering energy from the brine stream.

Pre-treatment is essential for all Reverse Osmosis plants and is needed to prevent fouling of reverse osmosis membranes. Inorganic salts, colloidal and particulate matter, organic compounds and microorganisms present in the feed water reduce membrane efficiency and lifespan. The main pre-treatment used is coagulation. However, coagulation only removes some pollutants and can produce small flocculants that fouling downstream membranes. New coagulants formulated for a number of water sources aim to greatly improve flocculent size, capture more pollutants, reduce membrane fouling, and can be easily washed from membranes. Technologies are also being developed to allow membrane surfaces to be treated with natural poly saccharides that have excellent anti-fouling properties.

Several emerging technologies have the potential to improve the efficiency of reverse osmosis membrane. For example, improved polymer membranes based on controlling the pore shape as shown in Figure 1.3 and nancomposite materials such as those based on carbon nanotechnology that could produce membranes composed of forests of microscopic tubes. However, it may be decades before some of these are mature enough for commercial application.

This book recognises that desalination by reverse osmosis has progressed significantly over the last decades, and provides an up to date review of the state of the art for the reverse osmosis process, issues that arise from desalination operations, environmental issues and ideas for research that will bring further improvements in this technology.

The process of reverse osmosis

Sergio G. Salinas-Rodriguez, Jan C. Schippers and Maria D. Kennedy

2.1 INTRODUCTION

Reverse osmosis (RO) systems are capable of separating dissolved ions from a feed stream (Figure 2.1). In RO systems, feed water is split into two streams: one has no (low) salinity and the other one has high salinity. The low salinity stream is known as 'permeate or product water' while the high salinity stream is known as 'concentrate, brine, or reject'.

The quantity of water (Qw) flowing through a membrane is proportional to the differential pressure feed-permeate (ΔP), membrane surface area (A) and permeability of the membrane (Kw). This relationship is expressed with the following equation:

[MATHEMATICAL EXPRESSION OMITTED] (2.1)

Qw = permeate flow (m3/h)

V = total filtered volume water (permeate) (L or m3)

t = time (h, min, s)

ΔP = differential pressure (pressure feed – pressure permeate) (bar)

Δπ...