CHAPTER 1
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.
CHAPTER 2
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 = osmotic pressure difference (bar) (osmotic pressure feed – osmotic pressure permeate)
Kw = permeability constant for water (m3/m2/s/bar)
A = surface area of the membrane(m2)
(ΔP - Δπ) = net driving pressure (NDP) (bar)
In membrane technology, flux is defined as the ratio of the permeate flow and surface area of the membrane. It is expressed as:
[MATHEMATICAL EXPRESSION OMITTED] (2.2)
Flux (J) is the permeate flow through a membrane surface area (Qw/A) (m3/m2 · h or L/m2 · h). Then,
[MATHEMATICAL EXPRESSION OMITTED] (2.3)
2.2 OSMOTIC PRESSURE
For Reverse osmosis to work, a pressure greater than the osmotic pressure needs to be applied to the seawater side of the membrane. Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane (Voet et al. 2001). Osmotic pressure is also defined as the minimum pressure needed to cancel out osmosis. Figure 2.2 illustrate the process of reverse osmosis in which the applied pressure needs to overcome the osmotic pressure head to force water to pass through the semipermeable membrane.
The osmotic pressure reduces the effect of hydraulic pressure, as a consequence, the effective pressure or net driving pressure is equal to the hydraulic pressure minus the osmotic pressure.
NDP = ΔP - Δπ (2.4)
where:
ΔP = differential hydraulic pressure (pressure feed – pressure permeate) (bar)
Δπ = difference osmotic pressure (osmotic pressure feed – osmotic pressure permeate) (bar)
In membrane filtration, the osmotic pressure hinders the water flow as illustrated in Figure 2.3.
In reverse osmosis systems the recovery at which the RO system operates is governed by the osmotic pressure of the feed water.
2.2.1 calculation of osmotic pressure
In practice, feed water can be classified according to the amount of salts it contains as follows: brackish water with total dissolved solids-salts (TDS) between 1,000– 5,000 mg/L, highly brackish 5,000–15,000 mg/L, saline 15,000–30,000 mg/L and seawater with TDS content higher than 30,000 mg/L. In Table 2.1 the osmotic pressure is presented for low salinity brackish water and for seawater.
In practice, salinity about 1,300 to 1,400 mg/L equals to 1 bar of osmotic pressure. From this figure, the osmotic pressure (in bar) can be estimated with a rule of thumb, as follows:
π ≈ 0.7 x 10-3 x C (2.5)
Where the salt concentration (C) is in milligrams per liter (mg/L). Hydranautics suggests that an approximation for osmotic pressure may be made by assuming that 1,000 mg/L of Total Dissolved Solids (TDS) equals about 11 psi (0.76 bar) of osmotic pressure (Hydranautics, 2001).
On the other hand, the feed osmotic pressure can be calculated more accurately by using the equation provided by ASTM (2000) based on the van't Hoff equation:
[MATHEMATICAL EXPRESSION OMITTED] (2.6)
where:
πf = osmotic pressure (kPa)
φ = Osmotic coefficient.
= Osmotic coefficients estimates for brackish and seawater, 0.93 and 0.90, respectively.
Tf = temperature of feed stream (°C)
[summation]mi = summation of molalities of all ionic and non-ionic constituents in the water
Membrane manufacturers make use of similar formulas, for instance DOW in its Filmtec technical manual (DOW, 2005) recommends the following equation for the feed water osmotic pressure:
[MATHEMATICAL EXPRESSION OMITTED] (2.7)
where:
πf = osmotic pressure (psi). 1 psi = 6.8948 kPa
T= temperature of water (oC)
[summation]mi = summation of molalities of all ionic and non-ionic constituents in the water
2.3 WATER FLOW
Theory suggests that the chemical nature of the membrane is such that it will absorb and pass water preferentially to dissolved salts at the solid/liquid interface. This may occur by weak chemical bonding of the water to the membrane surface or by dissolution of the water within the membrane structure (Solution Diffusion Theory). The chemical and physical nature of the membrane (e.g., surface charge and pore size) determines its ability to allow the preferential transport of water over salt ions.
In constant flux systems, the flux in a RO system can be expressed as:
J = (Net Driving Pressure) x Kw = (ΔP - Δπ) X Kw (2.8)
with:
ΔP = Pf - Pp (2.9)
and
Δπ = πf - πp (2.10)
where:
Jw = water flux (L/m2/h)
ΔP = hydraulic differential pressure (pressure feed – pressure permeate) (bar)
Δπ = osmotic pressure difference between the feed water and product water (permeate) (bar)
Kw = permeability constant for water (m3/m2/s/bar) (L/m2/h/bar)
2.3.1 Salt rejection
The salt rejection (SR) is by definition the ratio of the salt concentration in the feed water minus the salt concentration in the product water over the salt concentration in the feed water and it is expressed as percentage, as follows:
SR = Cf - Cp/Cf x 100% (2.11)
SR = (1 - Cp/Cf) x 100% (2.12)
Where Cf is the salt concentration in the feed water and Cp is the salt concentration in the product water.
2.3.2 Salt passage
The salt passage (SP) is by definition the ratio of the salt concentration in the product water over the salt concentration in the feed water expressed as percentage, as follows:
SR = Cp/Cf x 100% (2.13)
Salt passage is the opposite of salt rejection.
SP = 100% - SR (2.14)
2.4 SALT FLOW
Water can pass a reverse osmosis membrane; salts as well, however, at a much lower rate. The transport of salts through RO membranes is due to diffusion. Diffusion is a result of the motion of ions in water and the tendency of salts to move from high concentration to low concentration. Diffusion is a slow process, but cannot be neglected.
The salinity of the product water (Cp) depends on the relative rates of water and salt transport through a membrane. This relationship is expressed by the following equation:
Cp = Qs/Qw (2.15)
Where Qs is defined by the following equation:
Qs = ΔC x Ks x A (2.16)
With
ΔC = Cf - Cp (2.17)
where:
Qs = Flow rate of salt through membrane (kg/s)
ΔC = Salt concentration differential across membrane (kg/m3) = Cf – Cp
Ks = Membrane permeability coefficient for salt (m3/m2 x s)
A = Membrane area (m2)
Replacing the formula of Qs in the formula of Cp, we have:
[MATHEMATICAL EXPRESSION OMITTED] (2.18)
Replacing terms,
[MATHEMATICAL EXPRESSION OMITTED] (2.19)
Dividing the whole equation by Cf, and rearranging the equation, we have the salt passage:
[MATHEMATICAL EXPRESSION OMITTED] (2.20)
Then,
Salt passage (SP) = Cp/Cf x 100% (2.21)
Looking at the right side of the equation, since Cp is small compared to Cf, the ratio Cp/Cf is much smaller than 1, therefore, the salt transport (Qs) is constant at a certain Cp and is independent of the pressure. As a consequence, the salt passage (SP = Cp/Cf) is lower at high pressure (Pf) and vice versa. This is because the same quantity of salt (Qs) will be diluted by a larger volume of (product) water and vice versa.
The salt flux (Js) is defined as the salt transport per membrane area per hour (mg/m2/h) (mg per m2 per hour). The salt flux is proportional with the concentration difference between membrane surface at: feed water side (Cf) and product water side (Cp), and the permeability coefficient of membrane for salts (ions) Ks.
Js = (Cf - Cp) x Ks (2.22)
Remark: The larger the pores, the larger the permeability for salt and water. In general Cp is low in comparison with Cf. So, Cp can be neglected in this formula.
Js ≈ Cf x Ks (2.23)
where:
Js = salt flux (mg/m2/h)
Cf = concentration at feed side membrane (mg/L)
Cp = concentration at product side membrane (mg/L)
Ks = Permeability for salt [(mg/m2/h) / (mg/L)]
