desalination full report

8/16/2019 Desalination Full Report

1/80

D E T A I L E D R E P O R T

Economic and Technical Assessment of DesalinationTechnologies in Australia: WithParticular Reference to National

Action Plan Priority Regions

Prepared for

Agriculture, Fisheries & Forestry - AustraliaC/-Land & Water AustraliaLevel 2, UNISYS Building91 Northbourne AvenueTurner ACT 2612

2 September 2002


2/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions i

URS Australia

CONTENTS

1 The study 11.1 Background ............................................................................................. 1

1.2 Salinity units and quality guidelines......................................................... 21.3 Report structure....................................................................................... 3

2 Desalination today 42.1 Desalination as a source of fresh water .................................................. 4

2.1.1 Worldwide ....................................................................................... 42.1.2 Australia .......................................................................................... 5

2.2 Desalination as a tool to manage salinity ................................................ 62.2.1 Dryland salinity................................................................................ 62.2.2 Surface water salinity...................................................................... 7

2.3 Potential users of desalination................................................................. 8

3 Desalination technologies 93.1 Membrane processes .............................................................................. 9

3.1.1 Reverse Osmosis.......................................................................... 103.1.2 Electrodialysis ............................................................................... 13

3.2 Distillation processes............................................................................. 15

3.2.1 Multistage Flash Distillation........................................................... 163.2.2Multi Effect Distillation................................................................... 183.2.3 Vapour Compression Distillation ................................................... 20

3.3 Comparison of distillation and membrane processes............................ 213.4 Alternative processes ............................................................................ 22

3.4.1 Renewable energy powered conventional desalination ................ 223.4.2 Solar humidification....................................................................... 243.4.3 Freeze desalination....................................................................... 25

3.4.4 Membrane distillation .................................................................... 25

4 Cost comparisons 264.1 Summary ............................................................................................... 26

4.1.1 Methodology ................................................................................. 274.2 RO systems comparative cost analysis ................................................. 28

4.2.1 Capital cost analysis ..................................................................... 284.2.2 Operating cost analysis................................................................. 30

4.3 MED systems comparative cost analysis .............................................. 33

4.3.1 Capital cost analysis ..................................................................... 334.3.2 Operating cost analysis................................................................. 34


3/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions ii

URS Australia

4.4 EDR systems comparative cost analysis ............................................... 354.4.1 Capital cost analysis ..................................................................... 354.4.2 Operating cost analysis................................................................. 36

5 Factors affecting technology selection 38

5.1 Main factors affecting the cost of desalination....................................... 395.1.1 Energy source............................................................................... 395.1.2 Feedwater source ......................................................................... 405.1.3 Land availability ............................................................................ 435.1.4 Concentrate/brine disposal ........................................................... 435.1.5 Environmental management factors ............................................. 44

5.2 Opportunistic production and other offsetting benefits .......................... 445.2.1 Concentrate/brine disposal and value adding ............................... 44

5.2.2 Production on reclaimed land ....................................................... 485.2.3 Salt credits .................................................................................... 48

5.3 Cost competitiveness ............................................................................ 485.3.1 Improving the competitiveness of desalination ............................. 49

5.4 Cost sharing .......................................................................................... 50

6 Dryland salinity 526.1 Groundwater Flow Systems (GFS)........................................................ 52

6.1.1 Background and description ......................................................... 52

6.1.2 Management implications ............................................................. 546.1.3 Engineering options ...................................................................... 55

7 Desalination - Does it apply to you? 577.1 Decision tree.......................................................................................... 57

7.1.1 Does desalination stack up as a tool for treating salinity problems?...................................................................................................... 57

7.1.2 Does desalination stack up as an alternative to conventional formsof supplying fresh water? .............................................................. 58

7.1.3 Interpretation of the decision tree and examples .......................... 607.2 Further information ................................................................................ 60

8 Conclusions 618.1 Recommendations................................................................................. 61

TABLES

Table 1: Unit conversions for soil and water salinity............................................. 2

Table 2: Quality categories for water salinity........................................................ 3


4/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions iii

URS Australia

Table 3: Quality categories for soil salinity ........................................................... 3

Table 4: Status of renewable energy-assisted desalination options................... 22

Table 5: Summary of application of desalination technologies........................... 27

Table 6: RO capital cost by feedwater salinity and product flow rate (A$) ......... 28Table 7: RO operating costs by feedwater salinity and product flow rate (A$/kL)30

Table 8: MED capital cost by feedwater salinity and product flow rate (A$)....... 33

Table 9: MED operating costs by feedwater salinity and product flow rate (A$/kL)...................................................................................................... 34

Table 10: EDR capital cost by feedwater salinity and product flow rate (A$) ..... 35

Table 11: EDR operating costs by feedwater salinity and product flow rate(A$/kL) .......................................................................................... 36

FIGURES

Figure 1: National Action Plan priority regions...................................................... 1

Figure 2: Installed worldwide desalination capacity.............................................. 5

Figure 3: Installed Australian desalination capacity.............................................. 6

Figure 4: Basic illustration of membrane processes........................................... 10

Figure 5: Basic illustration of MSF process ........................................................ 16

Figure 6: Basic illustration of the MED process.................................................. 18

Figure 7: Basic illustration of the solar humidification process ........................... 24

Figure 8: Cost comparison of desalinated and piped fresh water ...................... 49

Figure 9: Basic Groundwater Flow Systems (GFS) of Australia ......................... 54

Figure 10: Desalination decision tree ................................................................. 59

ANNEXES

1 References2 Glossary of terms3 Consultancy terms of reference

S:\NRFD\PROJ\A10065 DESALINATION REVIEW\CLIENT REPORTS\FINAL\LONG REPORT\DETAILED REPORT (LONG) FINAL.DOC

2.09.02


5/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions iv

URS Australia

Executive summary

Introduction

Studies undertaken as part of the Murray-Darling Basin Salinity Audit and the NationalLand and Water Resources Audit have highlighted a likely decline in the quality of water

supplies over the next fifty years arising from the impacts of salinity on groundwater andsurface water resources. Some 2.5 million hectares of land in Australia have becomeaffected by dryland salinity processes over the past four decades and the trend is for this toworsen before it improves.

The National Action Plan (NAP) for Salinity and Water Quality to address water qualityand salinity related problems has identified 21 priority regions within which salinity andits management is a particular priority. This study assesses the technical and financialaspects of desalination in the NAP regions as a source of fresh water for human use and asa salinity management tool.

Status of desalinationSince WWII, desalination of saline water has become a reliable and cost effective meansof providing fresh water for human use, particularly in the arid and isolated regions of theworld. Middle East counties such as Saudi Arabia possess the greatest number and largestcapacity desalination plants in the world. Despite its seemingly suitable criteria of isolatedcommunities and aridity, Australia has comparatively limited operational expertise indesalination. Isolated mining towns and small communities as well as industrial processessuch as power stations that require exceedingly high qualities of water are the main usersof desalination technologies in Australia.

Alternative desalination processes

The techniques for desalination may be classified into three categories according to the process principle used:

process based on a physical change in state of the water – i.e. distillation or freezing; process using membranes – i.e. reverse osmosis or electrodialysis; and process acting on chemical bonds – i.e. ion exchange.

Variations in the design of each approach, such as the use of different energy sources, anda vast range of operational parameters, mean there are many ways in which saline water

can be desalinated. Each approach has its advantages and disadvantages – the choice of which approach to use is highly dependent on the requirements at hand and therestrictions faced at the site being considered. It is apparent that there is no one rightdesalination technology.

However, throughout the world, more Reverse Osmosis (RO) plants are being constructedthat the previously popular distillation techniques. Small scale and capacity renewableenergy powered desalination plants are also receiving interest for particular applicationswhere mains electricity is not available and solar insulation is high.

Choice of technology, costs and cost influencing factors

In general, the costs for membrane plants tend to be lower than for distillation plants of asimilar capacity, but particularly for plants generating fresh water of less than 300 to 400kL/day where distillation is not financially feasible. Distillation is typically only viable for


6/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions v

URS Australia

plants of higher capacity than this, and particularly where a low cost, high quality wasteheat source (i.e. latent heat from an industrial process, such as an electricity or manufacturing plant) is readily available.

If the feedwater TDS is greater than 10,000 mg/L and a low cost, high quality waste heatsource is available, distillation processes are generally selected. Other than this scenario,distillation processes are only really considered where very high feedwater TDS valuesgreater than 50,000 mg/L occur, and for high capacity plants greater than 300 to 400kL/day.

The technical operational boundaries and some cost comparisons for the threedesalination technologies that were considered in detail in this report are summarised

below.

Parameter Seawater RO Brackish RO Multi EffectDistillation

ElectrodialysisReversal

Feed Water Salinity(mg/L TDS)

> 32,000 < 32,000 > 35,000 3,000 – 12,000

Product Water Salinity(mg/L TDS)

< 500


7/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions vi

URS Australia

Comparison with traditional forms of water supply

The cost of desalination, particularly for membrane technology, has fallen considerablyover the last few decades. However, for desalinated fresh water to be a cost effectivemeans of supply is dependent on the absence and/or high cost of traditional forms of mains water supply. This would occur mainly in the more remote rural areas, some of which are located in the NAP regions of Australia.

There are a number of ways to reduce the net cost of constructing and operating adesalination plant which will increase its competitiveness against traditional forms of water supply and its cost effectiveness as a salinity management tool. Chief among theseis value adding to the highly saline waste stream.

Likewise, as the scarcity and cost of high quality mains water increases, so to does theattractiveness of desalination. Regulatory, market and policy changes that enable the priceof water to reflect its true value will accelerate this process.

Recommendations

Recommendations arising through the course of this study include developing:

A compilation of visual guides and maps which overlay existing geo-referenceddatasets of criteria that influence the choice of desalination. For example, demand,supply and price of traditional forms of water supply, presence and type of energy(including renewable), high yielding groundwater aquifers, etc. These will highlightthose areas that are most suitable for desalination and prompt decision makers in thoseareas to consider using it.

Fully specified BCA of an existing desalination plant or a site specific desalination plant proposal. This would enable a more accurate assessment of the cost

effectiveness of desalination. Integrated biophysical and economic models which process user-entered data to

recommend appropriate desalination technologies for particular scenarios. These existfor other countries but have not been developed for Australia.

Further technical development of renewable energy augmented desalination plants andtechnologies that require little maintenance and technical know-how to operate. Thesetypes of plants are likely to be suitable for many of the NAP regions in Australia.


8/80

Economic and Technical Assessment of Desalination Technologies in Australia: With Particular Reference to National Action Plan Priority Regions 1

URS Australia

1 The study

1.1 BackgroundIn an arid continent like Australia, supplies of potable water are a very limited resource.Recent studies undertaken as part of the Murray-Darling Basin Salinity Audit and the

National Land and Water Resources Audit have highlighted the potential decline in thequality of water supplies over the next fifty years arising from the impacts of salinity ongroundwater and surface water resources. Rising saline groundwater also threatens toseverely damage or destroy infrastructure, urban environments and key environmentalassets as well as reducing the productive potential of the land.

The National Action Plan (NAP) for Salinity and Water Quality to address salinity related problems has identified 21 priority regions within which salinity and its management is a particular priority. Figure 1 illustrates these areas.

Figure 1: National Action Plan priority regions

A range of solutions aimed at using rainfall more effectively and/or interceptinggroundwater thereby reducing likelihood of dryland salinity in the NAP regions have been

proposed. Where the problem is so great that it is neither technically nor financially

feasible to halt salinisation, adapting to and ‘living with’ salinity may be the best option,as demonstrated in the PMSEIC Report; “Dryland Salinity and Its Impact on RuralIndustries and the Landscape”.


9/80


URS Australia

Engineering approaches to intercept saline surface and groundwater resources is another option for managing the salinity problem. Groundwater pumping to provide feedwater for desalination processes provides not only a source of fresh water for human use but alsohas the potential to provide environmental benefits via drawdowns in salty groundwater levels and the resultant protection this affords to infrastructure, urban environments andkey environmental assets. Advances in desalination technology are continuing to bringdown the cost of supplying water from such means.Desalination in this study is therefore assessed in its capacity both as a source of freshwater for a variety of uses and as a salinity management tool. Rather than examine each

NAP region for its desalination potential, the range of conditions faced in the NAPregions are used as the basis for assessing the desalination technologies.

1.2 Salinity units and quality guidelinesWhere possible, salinity units reporting in the following chapters are converted to mg/LTDS. Several of the most common unit conversions for soil and groundwater salinity aredescribed in Table 1.

Table 1: Unit conversions for soil and water salinity

Convert… to… by multiplying with…

uS/cm mS/cm 0.001

mS/cm dS/m 0.01

ppm (mg/L) S/cm 1.4*

m3 ML 0.001

m3

kL 1m3 L 1000

ppm (mg/L) g/L 0.001

Bar kPa 100

Atm kPa 101* Rule of thumb only

For the purposes of this study, quality categories of water salinity are provided in Table 2.The Australian drinking water guidelines (ARMCANZ, 1996) state that water of less than

100 mg/L TDS (total dissolved solids, ie, not just sodium) is considered an excellentquality source of everyday drinking water. For limited periods of time, water of up to1,200 mg/L TDS may also be acceptable depending on taste. Maximum advisableirrigation water salinities for healthy growth range from 0 to greater than 3,500 mg/L TDSdepending on the type of plant being irrigated, the soil type, and irrigation frequency.Water for consumption by stock also varies depending on the animal, ranging up toapproximately 7,000mg/L TDS in the case of sheep.

Most desalination techniques are able to reduce sea water level salinity down to 500 mg/LTDS levels and less.


10/80


URS Australia

Table 2: Quality categories for water salinity

Use Rating Approximate Salinity Range(mg/L TDS)

Human consumption Excellent < 100

Human consumption Good to fair 100 – 1,000

Human consumption Poor 1,000 – 1,200

Human consumption Unacceptable > 1,200

Irrigation Maximum for healthy growth 0 – 3,500 depending on plant

Stock watering – dairy cattle Maximum for healthy growth 3,500

Stock watering – beef cattle Maximum for healthy growth 4,800

Stock watering – sheep Maximum for healthy growth 7,150Source: ARMCANZ (1996)

Soil salinity categories used by the Victoria Department of Natural Resources andEnvironment were developed through plant productivity studies and are listed in Table 3.

Table 3: Quality categories for soil salinity

Soil class Soil salinityrange (dS/m)

Comments

A 0 - 3.8 Supports growth of all pasture speciesincluding salt-sensitive clovers

B 3.8 - 6.5

C 6.5 - 8.6

D > 8.6 Supports growth of salt-tolerant plant speciesonly

1.3 Report structureChapters 2, 3, 4 and 5 of this report introduce desalination technologies and their usethroughout Australia and the world, provide a description of each type of technology andsome indicative cost estimates for particular sized plants desalinating water over a rangeof salinity.

Chapters 6 and 7 apply the concepts introduced in the preceding four chapters to the issueof dryland salinity. A ‘decision tree’ schematic is presented to assist potential users decidewhether desalination is suited to their particular circumstance.


11/80


URS Australia

2 Desalination today

2.1 Desalination as a source of fresh water Of all the Earth’s water, 94 percent is salt water from the oceans and 6 percent is fresh. Of the latter, 27 percent is in glaciers and 72 percent is underground (Buros, 2000). While theEarth’s salt water resources support commercially important activities such as fishing andtransport, it is typically beyond the limits to support human life or farming. Desaltingtechniques have therefore captured attention as an option to increase the range of water resources available for use by a community.

2.1.1 WorldwideThe application of desalting technologies over the last half of the 19 th century has changedthe way people live their lives and where they choose to live. Villages, cities and

industries have now developed in many of the arid and water short areas of the worldwhere sea or brackish waters (a salt level between fresh and sea water) are available andhave been treated with desalting techniques.

The change is most apparent in parts of the arid Middle East, North Africa, and some of the islands of the Caribbean, where the lack of fresh water previously limiteddevelopment (Buros, 2000).

The requirement to provide fresh water to people in areas with little or no infrastructurewas highlighted during WWII. The potential of desalination was realised during this timeand the technology underwent its first intensive period of development following the war.The American government, through the creation and funding of the Office of Saline

Water in the early 1960’s, and its successor organisations, led the worldwide researcheffort. The work of these organisations underpins much of the knowledge andunderstanding that exists today.

By the late 1960’s, commercial thermal approaches to desalting water were common place, with capacities up to 8,000 kL/day (Buros, 2000) being achievable. In the 1970’s,commercial scale membrane processes such as Reverse Osmosis (RO) and Electrodialysis(ED) were introduced and used more extensively. As the technology progressed andoperational experience increased through the 80’s and 90’s, the cost of construction andoperation reduced significantly. This was particularly the case for the membranetechnologies which are a considerably cheaper prospect now for certain applications than

the tried and trusted thermal/distillation approach (Buros, 2000).The International Desalination Association’s (IDA, 1998) most recent audit of worldwidedesalting capacity states a total installed figure of approximately 22,700 ML/day of whichabout 85 percent is still in operation. Water Corporation (2000) provides a more up todate, but unreferenced, figure of 25,600 ML/day from 13,885 desalting units as of 31 st

December 1999.

Almost half of the world’s capacity is used to desalt seawater in the Middle East and North Africa for municipal water supplies. Saudi Arabia ranks first in total capacityinstalled (approximately 24 percent of total world capacity), with the United States second(16 percent). The IDA inventory (IDA, 1998) indicates that the world’s installed capacity

consists mainly of multi-stage flash distillation and RO processes as indicated in Figure 2.


12/80


URS Australia

Figure 2: Installed worldwide desalination capacity

44%

42%

6% 4%4% Multi-Stage Flash

Reverse Osmosis

Electrodialysis

Multi-Effect Distillation

Vapour Compression

2.1.2 AustraliaAs of September 2000, the total installed capacity in Australia of desalination plantsgreater than 100kL/day was 90ML/day (Water Corporation, 2000). The largest of these isa 35ML/day RO plant at Bayswater, NSW supplying process water for a zero-discharge

power station (the largest of its kind in the world). A substantial number of mines and power stations in Australia use desalination for production of process and boiler feedwater, or to process effluent to comply with zero discharge commitments. Theaverage capacity of desalination plants in Australia is 2.6ML/day (Water Corporation,2000).

A limited number (less than 10) of small desalination plants are used for public water supplies in Australia. The low number is primarily due to the cost of providing water viadesalination being higher than the costs of conventional water supplies. Some examplesinclude:

an A$3.5 million RO unit on Kangaroo Island, SA to supplement the township of Penneshaw’s domestic water supply;

since 1995, Rottnest Island in WA has operated an RO plant for a variety of freshwater uses;

a desalination facility to supplement municipal water supplies for Port Lincoln andlower Eyre Peninsula, SA is currently being investigated (pers. comm., GlennWalker); and

the Western Australian Water Corporation is investigating the feasibility of developing several desalination facilities for industrial and urban applications (Water Corporation, 2000).

Most desalination plants in Australia were installed in the 1980’s and 1990’s. Themajority of these use the RO process as indicated by Figure 3.

As the scarcity and price of conventional sources of fresh water rise over time, and as fullcost recovery as a principle for water charging becomes more widespread, desalination asan option for supplying fresh water for human consumption and irrigation is expected to

become more popular in Australia.


13/80


URS Australia

Figure 3: Installed Australian desalination capacity

2.2 Desalination as a tool to manage salinityThe benefits of desalination plants that treat pumped groundwater, include not only thefresh water product, but also the benefits that may occur via the lowering of salinewatertables and the prevention of dryland salinity. Desalination of brackish surface water resources has also been undertaken to manage salinity threats.

2.2.1 Dryland salinityTo understand how desalination can help manage dryland salinity, the hydrogeologicalfactors which underpin the process of dryland salinity must also be understood. Thetechnical understanding of how dryland salinity occurs in Australia has improvedsignificantly in recent years. The concept of Groundwater Flow Systems (GFS) wasdeveloped out of the National Land and Water Resources Audit and recognises that

dryland salinity is highly correlated to a landscape’s underlying geological characteristicsand landform (Refer to Section 6 for more discussion of this topic). Coram et al (1999)identified in which GFS’s particular types of dryland salinity mitigation techniques areappropriate as a management tool. More recently, LWRRDC (2001) has focused on theefficacy of engineering options (including groundwater pumping for desalination) as asalinity management tool.

The efficacy of desalination as a dryland salinity management tool should be evaluatedagainst its technical ability to draw-down and manage the watertable (LWRRDC, 2001).This in turn is dependent on the conceptual mechanism of how the relevant groundwater flow system underlying the salinised landscape contributes to dryland salinity.

Generally, it is accepted that groundwater pumping for desalination can be an effectivemeasure against raising saline groundwaters, particularly in those regions where aquifersare highly permeable and significant enough in size to generate suitably large yields(LWRRDC, 2001; Coram et al , 1999; NDSP, 2001). However, engineering options will

be ineffectual against the highest permeability and largest regional GFS’s because anygroundwater pumped will be immediately replaced and no draw-down will occur (pers.comm., Glenn Walker).

Good examples of where desalination is being considered as part of a larger plan tomanage dryland salinity are Wellington Reservoir Catchment in southern WA and anumber of small towns in the Goldfields and Agricultural region of WA, such as Merredin

(Water Corporation, 2000).Chapter 6 discusses in more detail those conditions where desalination can be an effectivetool for managing dryland salinity.

12%

64%

18%

6% Multi-Stage Flash

Reverse Osmosis

Vapour Compression

Other


14/80


15/80


URS Australia

2.3 Potential users of desalinationThe users of desalination are many and varied. For fresh water, desalination plants havethe potential to supply drinking quality water and water for non-consumptive uses (eg,washing, cleaning) for populations ranging in size from individual households right up tosmall cities. In Australia, desalination plants designed for such uses are very small in

capacity and in number (Water Corporation, 2000). Most owners/operators of this type of desalination plant are individuals, households or private companies providing water for remote mine sites and base camps, however water supply, treatment and distributioncorporations are also potential users for the larger capacity units. Indeed, most research into desalination plants for drinking water purposes in Australia appears to be undertaken bysuch bodies (Water Corporation, 2000; Dames and Moore, 1993).

Irrigation schemes, power stations, industrial plants and other bodies overseeing thedischarge of effluent water may choose to invest in desalination plants to meet their discharge regulations criteria. This is especially the case where their effluent water can

potentially flow in to drinking water supplies either directly via surface water systems or indirectly via groundwater. These groups would also be interested in desalination at theother end of their operations, where particularly clean feedwater is required (eg, power stations, highly salt-sensitive and high value horticultural products).

There are a great number of organisations in Australia whose role it is to manage and protect the environment. These represent another type of user whose main interest lies indesalination’s ability to generate environmental benefits. Governments of all levels makeup the majority of this user group, as well as quasi-government bodies and governmentfunded organisations such as catchment management authorities, research anddevelopment organisations, park managers. Private firms looking to generate ‘kudos’ for their environmental stewardship may also see merit in owning/operating a desalination

plant designed primarily for environmental outcomes.

Currently the financial costs of desalination may be hard to justify but as the technologydevelops further it is likely to become more attractive.


16/80


URS Australia

3 Desalination technologies

Desalination is a process that removes dissolved minerals (including but not limited tosalt) from feedwater sources such as seawater, brackish water or treated wastewater.

The techniques for desalination may be classified into three categories according to the process principle used:

Process based on a physical change in state of the water – i.e. distillation or freezing; process using membranes – i.e. reverse osmosis or electrodialysis; and process acting on chemical bonds – i.e. ion exchange.

Of the above processes, those based on chemical bonds such as ion exchange are mainlyused to produce extremely high quality water for industrial purposes and are not suited totreating seawater or brackish water. Consequently, this process is not discussed further inthis study.

The other two processes, based on physical change of the water and filtering viamembranes, are regularly used to treat seawater and brackish water and have beendeveloped over many years in large scale commercial applications. There are also somevariations on the design and application of these processes that have not yet reachedcommercial or widespread acceptance but which in certain circumstances are considered

potentially useful. The desalination processes investigated in this study are as detailed below.

Major Processes:

Membrane

Reverse Osmosis Electrodialysis

Distillation

Multi-Stage Flash Distillation Multiple Effect Distillation Vapour Compression Distillation

Alternative Processes:

Renewable Energy Powered Conventional Desalination Solar Humidification Freezing Membrane Distillation

3.1 Membrane processesMembranes are used in two commercially important desalination processes:

Reverse Osmosis (RO); and Electrodialysis (ED).


17/80


URS Australia

Each process uses the ability of the membranes to differentiate and selectively separatesalts and water. However, the membranes are used differently for each of these two

processes.

Reverse Osmosis is a pressure driven process, with the pressure applied used for separation, allowing the water to pass through the membrane while the salts remain.Electrodialysis is a voltage driven process, and uses the electrical potential to selectivelymove salts through the membrane, leaving the product water behind. Figure 4 provides a

basic illustration of the process and is discussed in more detail in the following sections.

Figure 4: Basic illustration of membrane processes

3.1.1 Reverse Osmosis3.1.1.1 Technical descriptionThe principle of osmosis is the transfer of a solvent, in this instance water, through asemi-permeable membrane under the influence of a concentration gradient. In a system of two compartments, one containing pure water and the other saline water, osmosis occurswhen the flow of water moves toward the saline solution through the semi-permeablemembrane wall.

By applying a pressure on the saline solution, the quantity of water transferred by osmosiswill decrease. A point is reached at which the applied pressure is such that there is no flowof water across the membrane. This equilibrium is called the osmotic pressure of the

saline solution. An increase in the pressure applied to the saline solution beyond theosmotic pressure will drive a flow of water in the opposite direction to the normal osmoticflow – this is the process of Reverse Osmosis (RO).

The quantity of pure water that passes through the membrane during reverse osmosis is afunction of the difference between the applied pressure and the osmotic pressure of thesaline solution.

In practice, the saline feedwater is pumped into a closed vessel where it is pressurisedagainst the membrane. As water passes through the membrane, the remaining feedwater increases in salt concentration. This water is discharged from the vessel in a controlledmanner in order to ensure problems such as precipitation of supersaturated salts and

increased osmotic pressure across the membranes does not occur in the system.


18/80


URS Australia

The amount of water discharged to waste in the brine stream varies from approximately20% to 70% of the feed flow, depending on the salt content of the feedwater, the pressure,and type of membrane.Product water with a salinity of less than 500 mg/L TDS can usually be obtained using asingle stage RO operation.

Pretreatment of the feedwater is an essential component of the RO plant in order to prevent scaling of the membranes by scale-forming foulants such as salt precipitation andmicrobial growth. Usually the pretreatment consists of fine filtration and the addition of antiscalants and/or dispersants to inhibit precipitation and the growth of micro-organisms.The use of micro, ultra and nanofiltration is becoming increasingly important as a

potential pretreatment alternative to the conventional pretreatment processes. This aids ineffectively ‘softening’ the feedwater, aiding in the removal of the calcium and magnesiumwithin the feed prior to RO.The high pressure pump within the RO system supplies the pressure needed to enable thewater to pass through the membrane, while rejecting the salts. This pressure ranges from

approx 17 to 27 bar for brackish water RO systems, and from 54 to 80 bar for seawater RO systems.The processes of brackish water and seawater reverse osmosis are essentially identical.There are however, substantial differences in the two processes pressure requirements, asstated above, with brackish water systems requiring substantially lower operating

pressures, and the rate of conversion of feedwater to desalinated product water, with brackish water systems able to achieve higher system recoveries. In addition to thediffering energy requirements of the two processes, the types of pretreatment required canalso vary considerably.

3.1.1.2 Application of technologyMost operational problems occur in RO plants because materials have deposited on themembrane surfaces or in the membrane elements, preventing the membranes fromfunctioning efficiently. Other problems occur due to mechanical failures, and poor operation.Hence the main problems associated with reverse osmosis plants are associated withmembrane fouling issues and the working life of the semi-permeable membranes. Correct

pretreatment of the raw feedwater is essential to avoid fouling and maintain desaltedwater output over the membrane lifetime, significant fouling can reduce the product water flux considerably. Furthermore, membranes are also subject to a degree of compaction

under the applied pressure such that their performance characteristics deteriorate withtime.Mechanical failures can occur due to the high pressures needed for the transport of water across the membranes, the piping, supports, machinery etc., which can therefore besubjected to water mechanical stresses such as high pressures and vibration.The rate of recovery of drinking water quality water from an RO plant is largely limited

by the concentration in the feedwater of membrane scale producing compounds, mainlyCaCO 3, CaSO 4, BaSO 4, SrSO 4 and SiO 2. If any of these components are present inconcentrations of less than 20% of their solubility limit, then they do not usually presentlimitations, and for a brackish water RO system, at least 80% recovery of the feedwater as

drinking quality product water is achievable. A seawater RO system is able to achieve atmost 30% recovery of the feedwater. However, there are many instances in Australiawhere scale producing compounds are present in groundwater aquifers and for


19/80


URS Australia

desalination plants processing this water, their efficiencies are normally lower (pers.comm., N. Wende).As the feedwater TDS level decreases higher recoveries and higher salt rejections can beachieved with RO membrane plants, provided scaling constituents are in acceptably lowconcentrations. For example, for brackish feedwater TDS levels in the range of 2500 to3000 mg/L TDS, typical brackish water RO plants can achieve up to 98% salt separationfrom the feedwater, at typical operating pressures in the range of 1400 to 1700 Kpa.

The salinity of the product water is therefore dependent on the salinity and chemicalcharacteristics of the feedwater, and would usually be of the order of 2 to 10 percent of thefeed salinity (as mentioned above for low feedwater TDS systems). Hence less than 500mg/L product water TDS for seawater feed systems is typically achievable, and less than200mg/L product water TDS is achievable for brackish water feed systems.

RO systems are found to be most suitable for use in regions where seawater or brackishgroundwater is readily available, such as throughout the NAP regions of Australia. RO is

by far the most widely used process for desalination in Australia. An example of adesalination plant using RO technology is at Penneshaw on Kangaroo Island, SouthAustralia (see case study below). Other examples include Ravensthorpe in southern WA,Denham north of Perth, Rottnest Island off the coast of Fremantle WA, and some of theremote roadhouses along the highway between Adelaide and Perth in the Great AustralianBight. The RO plant at Bayswater, NSW, is the largest zero-emissions plant in the world(35ML/day) and provides highly pure water for boiler processes in an adjacent power station.

Penneshaw Case Study

The reverse osmosis desalination plant at Penneshaw is run by the South Australian water

authority, SA Water . It provides potable water to the township of Penneshaw (population395) on Kangaroo Island a few hours south of Adelaide. Historically, farm dams wereused for water supply in the area. Today these would be too unreliable to sustain presentday populations as well as too polluted (due to agricultural activities in the surroundingareas).

The desalination plant is of South African design and uses RO technology to supplementthe town’s existing dam water supplies. Given that most tourism in the region is basedaround the local environment, the plant was designed to minimise environmental impactsand avoid the use of chemical cleaners.

As the plant is located along the coastline, brine concentrate is disposed of into the ocean

via pipes. It is believed that alternative uses and value-adding to the brine water would betoo expensive to establish.

Seawater is used as the feedwater source for the plant with a salinity level of approximately 38,000-39,000 mg/L TDS. Between 35% and 40% of the 250KL of seawater treated daily is recovered (ie, converted to potable water) and desalinated to a levelof no more than 1,000 mg/L.

The plant operates continuously and is powered from mains electricity which is availableto all residents of the island via an underwater power cable from the mainland.

The plant cost AUS$3.5 million to construct (including associated civil works) – this issignificantly less than the cost of building infrastructure to link the township of Penneshaw to the mains water supply 60km away. However, the operating costs of the

plant are such that it is more expensive than the unit costs of water provided via mains.


20/80


URS Australia

3.1.1.3 Advantages and disadvantagesThe advantages of using the RO system for desalination are:

They are quick and cheap to build and simple to operate. There are few components,durable plastics and non-metal materials are mainly used - pre-treatment of thefeedwater to prevent fouling of the membrane is the only potential problem.

It can handle a large range of flow rates, from a few litres per day to 750,000 L/day for brackish water and 400,000 L/day for seawater. The capacity of the system can beincreased at a later date if required by adding on extra modules.

It has a high space/production capacity ratio, ranging from 25,000 to 60,000 L/day/m 2. Energy consumption is low. It can remove other contaminants in the water as well as the salt. The use of chemicals for cleaning purposes is low. There is no need to shut down the entire plant for scheduled maintenance due to the

modular design of the plant. The startup and shutdown of the plant does not take long.

The disadvantages of using the RO system for desalinisation are:

RO membranes are expensive and have a life expectancy of 2-5 years. If the plant uses seawater there can be interruptions to the service during stormy

weather. This can cause resuspension of particles, which increases the amount of suspended solids in the water.

There is a requirement for a high quality standard of materials and equipment for theoperation of the plant.

It is necessary to maintain an extensive spare parts inventory.

There is a possibility of bacterial contamination. This would be retained in the brinestream, but bacterial growth on the membrane itself can cause the introduction of tastes and odours into the product water.

Most failures in RO systems are caused by the feedwater not being pre-treatedsatisfactorily. Pre-treatment of the feedwater is required in order to remove

particulates so that the membranes last longer. Careful pre-treatment of feedwater isnecessary, especially if feedwater quality changes.

The plant operates at high pressures and sometimes there are problems withmechanical failure of equipment due to the high pressures used.

3.1.2 Electrodialysis3.1.2.1 Technical descriptionUsing a similar approach to that of RO, Electrodialysis (ED) involves the movement of water through a filtering membrane. However, instead of using pressure to overcome themembrane’s resistance, pretreated water is pumped between electrodialysis cells under theinfluence of a low voltage direct current (DC) electrical field.

An electrodialysis cell consists of a large number of narrow compartments through whichthe feedwater for desalination is pumped. These compartments are separated bymembranes that are permeable to either positive ions (cations) or negative ions (anions).Under the influence of the DC electrical field, cations and anions migrate through theappropriate membranes, forming compartments of electrolyte-enriched wastewater andelectrolyte depleted product water.


21/80


URS Australia

The partially deionised water is removed from the ED cell with the electrolyteconcentrations reduced by a factor of at least two. If further desalination is required thentreatment via one or more additional stages of cells may be necessary. Non-ionic

particulates, bacteria and residual turbidity may also pass through the cells with the product water, and therefore this may require further treatment to achieve the desired product water standards.

The basic electrodialysis unit consists of several hundred cell-pairs bound together withelectrodes on the outside and is referred to as a membrane stack. Feedwater passessimultaneously in parallel paths through all of the cells to provide a continuous flow of desalinated water and brine to emerge from the stack. Depending on the design of thesystem, chemicals may be added to the streams in the stack to reduce the potential for scaling.

The raw feedwater must be pre-treated to prevent materials that could harm themembranes or clog the narrow channels in the cells from entering the stack. Thefeedwater is circulated through the stack with a low pressure pump with enough power toovercome the resistance of the water as it passes through the narrow passages.

Recently, advancements to the ED technology in the form of Electrodialysis Reversal(EDR) have occurred. The EDR process involves a reversal of the water flow in order to

break up and flush out scales, slimes and other foulants deposited in the cells before theycan build up and create major fouling problems. This flushing also allows theelectrodialysis unit to operate with fewer pretreatment chemicals, hence minimising costs.

An EDR unit operates on the same general principle as a standard electrodialysis unit,except that both the product and the brine channels are identical in construction. Severaltimes per hour, the polarities of the electrodes are reversed and the flows simultaneouslyswitched so that the brine channel becomes the product water channel, and vice versa. Theresult of this is that the ions are attracted in the opposite direction across the membranestack. During this interval the product water is dumped until the stack and lines areflushed out and the desired water quality is restored. This flush normally takes 1 to 2minutes, with the unit returning to normal operation on completion of the flushing

process.

3.1.2.2 Application of technologyThe ED process is usually only suitable for brackish feedwaters with a salinity of up to12,000 mg/L TDS. With higher salinities the process rapidly becomes more costly thanother desalination processes. This is because the consumption of power is directly

proportional to the salinity of the water to be treated. As a rule of thumb, approximately 1kWh is required to extract 1kg additional salt using ED. The major energy requirement of the process is the direct current used to separate the ionic substances in the membranestack.

A variety of operational problems can be experienced with electrodialysis facilities. Themajor ones being scaling and leaks.

Scaling – scale formation will foul the membrane surfaces and block the passages in thestack. The result of this is that the slowly moving water then becomes highly desalted dueto the longer period of exposure to the electromotive force. This highly desalted water hasa low conductivity and offers a high resistance to current flow, thus decreasing the

efficiency of the process. Some scale can be removed by introducing chemicals into thestacks in an attempt to dissolve or loosen the scaling so that it can be washed out,however in more severe cases the stack will need to be disassembled.


22/80


URS Australia

Leaks – operating and/or maintenance problems can result from leaks in two parts of theelectrodialysis stacks, either between the stacked membranes and spacers, or through themembranes.

3.1.2.3 Advantages and disadvantages

The advantages of using electrodialysis plants for desalination are: They can produce a high recovery ratio (85-94% for one stage). Can treat feedwater with a higher level of suspended solids. Pre-treatment has a low chemical usage and does not need to be as precise. The energy usage is proportional to the salts removed, instead of the volume of water

being treated.

The membranes for EDR have a life expectancy of 7-10 years, which is longer thanfor RO.

EDR membranes are not susceptible to bacterial attack or silica scaling. Scaling can be controlled whilst the process is on-line, the membranes can also be

manually cleaned.

Can be operated at low to moderate pressure.The disadvantages of using electrodialysis for desalination are:

Periodic cleaning of the membranes with chemicals is required. Leaks sometimes occur in the membrane stacks. Bacteria, non-ionic substances and residual turbidity are not affected by the system

and can therefore remain in the product water and require further treatment before

certain water quality standards are met.

3.2 Distillation processesThe distillation processes are primarily:

Flash-type distillation; Multi-effect distillation; and Vapour compression distillation.

Distillation processes mimic the natural water cycle in that saline water is heated, producing water vapour, which is in turn condensed to form fresh water. Approximatelyhalf of the world’s desalination capacity is based on the Multistage Flash distillation

principle (MSF) (Buros, 2000). However, this is reflecting a continuing decline in themarket, with other water distillation technologies such as Multi-Effect (MED) and Vapour Compression (VC) distillation, rapidly expanding and anticipated to have a moreimportant role in the future as they become more accepted and understood.

MSF and MED are generally used in most of the larger scale seawater desalination plants.These processes generally require high amounts of energy to desalinate water regardlessof the level of salt concentration, hence brackish water desalination (which requires lessenergy) is usually not a viable option for this technology.

The evaporative processes require thermal or mechanical energy to cause evaporationfrom a brackish or saline feedwater, and as a result tend to have operating cost advantageswhen low cost thermal energy is available.


23/80


URS Australia

3.2.1 Multistage Flash Distillation3.2.1.1 Technical descriptionMultistage Flash distillation (MSF) accounts for the major portion of desalinated freshwater currently produced and is used primarily for desalting seawater. This process has

been in large scale commercial use for over thirty years and is illustrated in Figure 5.

Figure 5: Basic illustration of MSF process

The principles of MSF involve seawater feed being pressurised and heated to the plant’smaximum allowable temperature. When the heated liquid is discharged into a chamber maintained slightly below the saturation vapour pressure of the water, a fraction of itswater content ‘flashes’ into steam. The flashed steam is stripped of suspended brinedroplets as it passes through a mist eliminator and condenses on the exterior surface of theheat transfer tubing. The condensed liquid drips into trays as hot product (fresh) water.

The recirculating stream, flowing through the interior of the tubes that condense thevapour in each stage, serves to remove the latent heat of condensation. In doing so, thecirculating brine is preheated to almost the maximum operating temperature of the

process, simultaneously recovering the energy of the condensing vapour. This portion of the MSF plant is called the ‘heat recovery’ section. The preheated brine is finally broughtup to maximum operating temperature in a brine heater supplied with steam from anexternal source.

A once through MSF plant will generally recover no more than 10% of the feed as productwater, however higher recoveries of between 25 to 50% of the feed flow can be achievedas product water in a modern well-designed and high temperature recyclable MSF plant.

The salinity of water desalinated by the MSF process is typically less than 50 mg/L TDS,and as a result may require blending with a small amount of brine to increase the salinityand buffering salts to acceptable levels. Blending is often used with product waters of lessthan 50 mg/L TDS for the following reasons:

Water with TDS


24/80


URS Australia

There is a general consensus that water for drinking purposes should contain certainquantities of minerals. Blending will ensure the addition of these minerals back intothe water. It should be noted however that this point is of debatable merit.

3.2.1.2 Application of technology

The MSF process is energy intensive due to the requirement to boil the feedwater,although energy efficiency is substantially enhanced via the heat recovery process.

The advantages of MSF plants lie in their ability to be constructed in large capacities,their reliability over a potentially long operating life, and the design and operationalexperience in operating these units that has been accumulated over many years. A further advantage lies in the fact that boiling does not occur on a hot tube surface, as it flashesinstead, thereby reducing the incidence of scaling.

Their disadvantages come in the form of large capital investment due to the extensive useof high quality stainless steel and alloys required to prevent corrosion. Following on fromthis, due to the high temperatures required for the flash process, severe corrosion

problems can occur, particularly if the feedwater requires acid dosing or if carbon dioxideconcentrations are high within the cells due to inadequate degassing of the feedwater.

Further disadvantages include substantial logistical difficulties involved with plantconstruction and start-up, the inflexibility to operate the plant below 70-80% of designcapacity, the need for and difficulty in designing and constructing a highly efficient plant,and the necessity for pumping, treating and heating large quantities of feedwater relativeto product due to low process recoveries. In areas where brine disposal is a potential

problem, processes such as MSF that create large quantities of brine, are often consideredinappropriate.

3.2.1.3 Advantages and disadvantagesIn summary, the advantages of using multi-stage flash distillation for desalination are:

MSF plants can be constructed to handle large capacities. The salinity of the feedwater does not have much impact on the process or costs. It produces very high quality product water (less than 10 mg/L TDS). There is only a minimal requirement for pre-treatment of the feedwater. The strict operational and maintenance procedures for other processes are not as

rigorous for MSF.

There is a long history of commercial use and reliability. It can be combined with other processes, eg using the heat energy from an electricity

generation plant.

The disadvantages of using multi-stage flash distillation for desalination are:

They are expensive to build and operate and require a high level of technicalknowledge.

Highly energy intensive due to the requirement to boil the feedwater, although energyefficiency is substantially enhanced via the heat recovery process.

The recovery ratio is low, therefore more feed water is required to produce the sameamount of product water.


25/80


URS Australia

The plant can not be operated below 70-80% of the design capacity. Blending is often required when there is less than 50mg/l TDS in the product water.

3.2.2 Multi Effect Distillation

3.2.2.1 Technical descriptionMulti Effect Distillation (MED) is stated as being the most important large-scaleevaporative process, and offers significant potential for water cost reduction over other large-scale desalination processes (pers. comm., N. Wende). It is predicted that the use of this distillation technology will expand in the future, over and above the usage of theMultistage-Flash distillation process ( ibid. ). MED plants are typically no smaller than300kL/day capacity, as anything less than this is not financially viable given thesignificant advantages of economies of scale that are available to this technology.

Multiple effect distillation units operate on the principle of reducing the ambient pressureat each successive stage, allowing the feedwater to undergo multiple boilings without

having to supply additional heat after the first stage. This process is illustrated in Figure 6.

Figure 6: Basic illustration of the MED process

In MED units, steam and/or vapour from a boiler or some other available heat source isfed in to a series of tubes where it condenses and heats the surface of the tube and acts asa heat transfer surface to evaporate saline water on the other side. The energy used for evaporation of the saline water is the heat of condensation of the steam in the tube.The evaporated saline water, now free of a percentage its salinity and slightly cooler, isfed in to the next, lower-pressure stage where it condenses to fresh water product, whilegiving up its heat to evaporate a portion of the remaining seawater feed.There is typically a series of these condensation-evaporation stages taking place, each one

being termed an “effect”. The process of evaporation-plus-condensation is repeated fromeffect to effect, each at successively lower pressures and temperatures. The combinedcondensed vapour constitutes the final product water.


26/80


URS Australia

A well designed multi-effect distillation plant will recover approximately 40 to 65% of the feed as product water. Product water quality is highly pure with TDS values typicallyless than 10 mg/L TDS.

MED plants typically derive their energy from low pressure steam generators or industrial process steam. MED units are also unique in their ability to recycle waste heat fromthermal power plants, diesel generators, incinerators or industrial processes and as aconsequence, are often sited adjacent to such plants or incorporated with them at thedesign stage.

3.2.2.2 Application of technologyThe essential difference between MED and MSF is that flashing of the steam plays only aminor role in the process, and that the condensing steam evaporates seawater via the heattransfer surface in each cell, or ‘effect’. Therefore, in a MED system, steam produced then

passes to the next, lower temperature, effect where it condenses, evaporating moreseawater and the process is repeated in each subsequent effect. Thus, due to the lower

temperature operation of these units and the pressure reduction technique, the MEDspecific power consumption is approximately half that required for the MSF process.

Another benefit of the MED process is in the event of a leaky tube wall occurring, thevapour would tend to leak into the brine chamber, thereby avoiding contamination of the

product water.

Also, the number of effects required for an MED plant is generally not more than 10,compared to the larger MSF plants where typically 20 to 40 stages are required before it isconsidered a cost-effective option (Buros, 2000). As a result, MED plants areconsiderably smaller in physical size than MSF. Following on from this, the major advantage of the MED process is its ability to produce significantly higher performance

ratios than the MSF process. This is a significant factor to consider in environmentallysensitive areas and/or where brine disposal is an issue.

3.2.2.3 Advantages and disadvantagesThe advantages of using multi-effect distillation for desalination are:

The pre-treatment requirements of the feedwater are minimal. Product water is of a high quality. MED plants are very reliable even without a strict adherence to maintenance.

The plant can be combined with other processes, eg, using the heat energy from a power plant. The plant can handle normal levels of biological or suspended matter. The requirements for operating staff are minimal.

The disadvantages of using multi-effect distillation for desalination are:

They are expensive to build and operate - energy consumption is particularly high. The plant can be susceptible to corrosion. This can usually be controlled by the choice

of material.

The product water is at an elevated temperature and can require cooling before it can be used as potable water.

The recovery ratio is low, although not as low as for MSF.


27/80


URS Australia

3.2.3 Vapour Compression Distillation3.2.3.1 Technical descriptionThe low temperature Vapour Compression Distillation (VCD) method is a simple, reliableand highly efficient process. Its efficiency comes largely from a low energy requirementand its design that is based on the ‘heat pump’ principle of continuously recycling the

latent heat exchanged in the evaporation-condensation process.VCD is similar in process operation to multi-effect distillation. The main difference is thatthe vapour produced by the evaporation of the brine is not condensed in a separatecondenser. Instead a compressor returns it to the steam side of the same evaporator, inwhich it originated, where it condenses on the heat transfer surfaces, giving up its latentheat to evaporate an additional portion of the brine.

The energy for the evaporation is not derived from a prime steam source as in the preceding two distillation processes, but from the vapour compressor. In addition, thelatter raises the temperature of the vapour by its compressive action, thereby furthering thedriving force for the transfer of heat from vapour to brine.

Typically these units are no smaller than 300 to 400 kL/day, and are most economic withfeedwater of high TDS levels, typically greater than 50,000 mg/L TDS (higher thanseawater). High quality product water can also be achieved with VC units, generally lessthan 10 mg/L TDS, and in some cases even as low as 2 mg/L TDS. Recoveries of approximately 50% can be achieved with these units.

3.2.3.2 Application of technologyVCD units are usually built in the 20 to 8,000 kL/day range, and are often used for resorts,industries, and drilling sites, where fresh water is not readily available. The VC process

benefits from low energy demands mainly in the form of mechanical energy to drive acompressor rather than the large amounts of high grade thermal energy required with theMSF and MED processes.

Furthermore, with the low temperature VC distillation process, using a high capacitycompressor, operating temperatures of below 70ºC are possible, thus reducing the

potential of scale and corrosion. In comparison with thermal desalination plants, nocooling water is required, resulting in smaller intake and pumping systems, and lower energy requirements, with no need for a heat rejection section.

The main disadvantage with these units is that starting the plants can be a problem -usually an auxiliary heater must be fitted to raise the feed temperature so that some vapour is available before the compressor can take over.

3.2.3.3 Advantages and disadvantagesIn summary, the advantages of using vapour compression distillation for desalination are:

The plants are very compact and can be designed to be portable. Minimal pre-treatment is required. The capital cost of the plant is reasonable and operation is simple and reliable. The recovery ratio is good.

The product water is of a high quality. The energy requirements are relatively low, although not as low as RO.


28/80


URS Australia

The disadvantages of using vapour compression distillation for desalination are:

Starting up the plant is difficult. An auxiliary heater is normally required to get thetemperature of the feed water up to a point where some vapour is formed. After thisthe compressor can take over.

It requires large, expensive steam compressors, which are not readily available.

3.3 Comparison of distillation and membrane processesTo summarise the above descriptions of the major desalination processes, a comparison of each approach is provided below.

The advantages of using membrane processes over distillation processes are:

Membrane plants normally have lower energy requirements. The capital cost for membrane plants is lower than distillation plants. Membrane plants have a high space/production capacity ratio.

Membrane plants generally have higher recovery ratios than distillation plants. Membrane plants operate at ambient temperature. This minimises the scaling and

corrosion potential, which increases with higher temperatures.

Membrane plants can easily be downgraded simply by taking sections out of the plant.

The disadvantages of membrane processes when compared to distillation process are:

Membrane processes do not destroy biological substances, unlike distillation processes. Therefore they must be removed in either pre-treatment or post-treatment if the water is to be used for potable water or process water.

Membranes that are of the polyamide type can not be used if there is chlorine in thewater. The chlorine must be chemically removed. The performance of membrane plants tends to decline progressively with time due to

fouling of the membrane.

Membrane plants need to be cleaned more regularly than distillation plants. Membrane plants need more rigid monitoring than distillation plants.

The advantages of distillation plants over membrane plants are:

Distillation plants have been established for a long time and have proven to be areliable means of desalination.

Distillation plants produce higher quality product water than membrane plants. Distillation plants do not need to be cleaned as often as membrane plants. Distillation plants do not need to be monitored as strictly as membrane plants. Distillation plants only require a minimal amount of operating staff.

The disadvantages of distillation plants when compared to membrane plants are:

Distillation plants require more feedwater for the same amount of product water dueto their lower recovery ratio.

Distillation plants are more vulnerable to corrosion than membrane plants. This iscontrolled by the selection of materials.

Distillation plants require more room for a given capacity than membrane plants.


29/80


URS Australia

Distillation plants have a higher capital cost than membrane plants. Distillation plants consume more energy than membrane plants. The temperature of the product water is higher for distillation plants, than for

membrane plants. This means that the product water needs to be cooled to be used as potable water.

3.4 Alternative processesVariations in the application of the two major desalination processes have led to thedevelopment of a number of alternative ways to desalinate saline water. These processeshave not as yet achieved the level of commercial success and viability that the abovementioned conventional processes have, however, under certain circumstances they have

proved to be viable. The conditions in which these processes are viable are likely to become more common place in the future.

3.4.1 Renewable energy powered conventional desalinationMany remote towns and communities rely on costly and often limited supplies of dieselfuel for their energy needs. These and other forms of fossil fuels are sometimes heavilysubsidised by government to meet community service obligations (Water Corporation,2000). Most desalination techniques consume a large amount of energy, therefore findingmethods of using renewable energy to power the desalination process is desirable.

Solar collectors or wind energy devices can be used to provide the heat or electricalenergy requirements to operate a standard desalination plant using membrane or distillation. The National Renewable Energy Laboratories in the USA conducted a surveyto identify actual examples and potential ways in which renewable energy could be usedwith desalination (NREL, 1998). Table 4 illustrates pairings of renewable energy anddesalination processes. In many cases actual working case studies or pilot projects exist,

but for some pairings where the process seems quite viable, the concept remains untested(represented by a blank cell).

Table 4: Status of renewable energy-assisted desalination options

Desalination technologyRenewableenergy source Multiple

EffectDistillation

MultistageFlash

Distillation

VapourCompression


Solar thermal Pilot plants(Spain, 1988;UAE, 1984)

Pilot plants(Kuwait, 1984;Mexico, 1978)

N/a N/a N/a

Solar thermalelectric ormechanical

Pilot plant(thermal)(USA, 1987)

Pilot plants(mechanical directdrive, France, 1978)

PV-batteryinverter

N/a N/a Commercial Pilot plant (Japan,1988)

PV, noinverter

N/a N/a Commercial (directdrive, Australia,1996)

Commercial(battery/all DC)(NewMexico, 1995)

Wind-battery N/a N/a Pilot Plant(Spain, 2001)

Pilot plants (France,1990; Spain, 2001)

Pilot plant (Spain,2001)

Wind-diesel Pilot plants (Spain,Greece, 2001)


30/80


URS Australia

Desalination technologyRenewableenergy source Multiple

EffectDistillation

MultistageFlash

Distillation

VapourCompression


Wind-mechanical

N/a N/a Pressurised water storage pilot plant(Australia, 1990)

N/a

Wind-electricdirect drive

N/a N/a Cut in/cut outcontrol pilot plants(Germany, 1979;France, 1987)

Source: National Renewable Energies Laboratory (1998)

Today, in remote locations and/or where energy costs are high, desalination plants existthat either fully or partly rely on renewable sources for their energy. Most have capacitiesof less than 20 kL/d (Buros, 2000). The economic viability of operating these plants ishighly correlated to the cost of producing the solar or wind energy, hence they are best

located where average yearly solar insolation is high or where prevailing winds are strong.A large example of solar power desalination is the Abhu Dhabi solar distillation plant inthe United Arab Emirates. This plant was commissioned in 1984 and has an output of 85kL/day of fresh water. The solar collectors take up an area of 1,862 m 2. The recovery ratioof such plants range from 43%-55% (unknown, 2001).

Solar powered RO desalination units have been developed and are in operation in ruralareas of Australia. An example of this is the solar powered reverse osmosis unit‘Solarflow’ developed by The Remote Area Developments Group at Murdoch University.This unit has a capacity to desalinate 400L/day from brackish salinity water of up to 5,000mg/L TDS using a 120 watt photovoltaic array. This is enough to provide 2 people withtheir complete water requirements (washing, cooking, bathing, and drinking) for a day.Alternatively, it can be used to augment existing water supplies or used for one or two

purposes only in which case it can service many more people.

Costs for a fully operational 400L/day unit amount to approximately $22,000 for purchaseand installation (Mathews, 2001). Annual operating costs are comprised mainly of capitaldepreciation and also minor costs for repairs and maintenance as needed.

‘Solarflow’ has recently been commercialised by the manufacturer in regions where mainselectricity is not available and currently more than 20 are in operation throughoutAustralia and south-east Asia. A 1,500 L/day model is also currently undergoing testing.

Another Australian example is ‘Solar-Sustain’ which uses solar power as part of a solar humidification process and is described below in section 3.4.2.

Currently, the use of conventional energy such as mains electricity, to drive desalinationdevices is still generally more cost effective than using wind and solar power (Buros,2000). However, as technology improves and the cost of traditional sources of fresh water and energy rise, then renewable energy powered desalination units are likely to becomemore widespread. This is particularly the case in remote areas without access to reliableand affordable sources of energy where solar powered desalination plants such as

‘Solarflow’ have already been shown to be the optimal choice (Winter, 2001).


31/80


URS Australia

3.4.2 Solar humidificationSolar humidification has been a legitimate option for desalinating saline water since the19 th century (Kunze, 2001; Buros, 2000). In WWII the use of small solar stills on life raftsto provide fresh water was investigated in detail. Solar humidification involves the directuse of solar energy for heating saline water to increase the production of water vapour.The water vapour is then condensed on a cool surface, and the condensate collected asfresh product water.A green house solar still is a good example of this process. Here the saline water is heatedin a basin on the floor and the water vapour condenses on the sloping glass roof thatcovers the basin, and is collected as illustrated in Figure 7.

Figure 7: Basic illustration of the solar humidification process

As a general rule of thumb, well-managed and maintained solar stills require a solar collection area of about one square metre to produce up to six litres of fresh water per day,

but on average usually return nearer 3L/m 2/day (Kunze, 2001). Thus, for an 800L/dfacility (representing total daily water requirements for four people), a land area rangingfrom 130-260m 2 would be required depending on efficiency. New breakthroughs in solar still technology such as heat recovery and air mass circulation can reputedly improve the

production ratio up to 20L/m2

/day and thus reduce the area required to provide a givenamount of water ( ibid. ).

In Australia, a new adaptation of the humidification process has been developed by‘Solar-Sustain’ . Instead of large basins and overhead collectors, the Solar-Sustaintechnique uses pipes. Solar power is used to heat saline water which is pumped through

pipes at 1/3 of its capacity. The trapped air in the pipes rises to 100% relative humidity, isducted out of the pipes where it is cooled and condenses in to fresh water. The technologyis effective and reliable but in its current form is prohibitively expensive and isundergoing further development (pers. comm., A. Huffer). In its current (and now

outdated) level of development, unit costs for provision of water range from $3.50-$4.50/kL (this is mainly depreciation of capital costs of at least $50,000 over 5 years).


32/80


URS Australia

The advantages of the solar humidification process is its relative simplicity to operate andservice and obviously its ability to use solar or other renewable power as its source of energy, hence operating costs are very low. However there are restrictions in the use of this technique for large scale production such as:

large solar collection area requirements;

high capital cost; and vulnerability to weather related damage.

3.4.3 Freeze desalinationThe process of freeze desalination is based on the fact that dissolved salts are naturallyexcluded during the formation of ice crystals. In order to desalinate saline water using thismethod, the non-frozen saline component is removed at the appropriate time in thefreezing process, and the frozen (fresh) water washed and rinsed to remove any remainingsalts adhering to the ice crystals. The ice is then melted to produce fresh product water.

There have been a small number of plants developed and constructed over the past 40years (Water Corporation, 2000), however the process has not been commerciallydeveloped in the production of potable water for municipal purposes. At this stage,freezing desalination technology still has a better application in the treatment of industrialwastes rather than in the production of municipal water (pers. comm., N. Wende).Freeze desalination theoretically has some advantages over distillation methods, whichinclude a lower theoretical energy requirement, minimal potential for corrosion, and littlescaling or precipitation.The main limitation of this process is that it involves handling ice and water mixtures thatare mechanically complex to move and process. The freeze desalination process also has

high energy requirements and therefore cost, however it is capable of removing allharmful constituents that may be present, thus making it more suitable for the industrialwastes industry rather than purely for the production of municipal water.

3.4.4 Membrane distillationThis process combines the use of both distillation and membrane technology. Saline water is warmed to enhance vapour production, and this vapour is exposed to a membrane thatcan pass vapour but not water. After the vapour is passed through the membrane, it iscondensed on a cooler surface to produce fresh water. In liquid form, fresh water cannot

pass back through the membrane, thus trapping it to be collected as the product water.To date, this process has only been used in a few instances across the world (Water Corporation, 2000), and has not as of yet demonstrated any commercial success as thesource energy costs involved to produce the water are still too high to make the processcommercially viable.Being a combined process using both distillation and membranes, this process is subjectto the same performance limitations that are experienced with those technologies.

Namely, the larger space requirements, and considerable pumping energy requirements per unit of production.The main advantages of membrane distillation are in its simplicity, and the requirementfor only small temperature differentials to operate the process.Membrane distillation could be used most cost effectively in the desalination of salinewater where inexpensive low grade thermal energy is available from industry or fromsolar collectors.


33/80


URS Australia

4 Cost comparisons

4.1 SummaryConventional desalination processes have been developed to commercial capacity for approximately thirty years. MSF is commercially the oldest and our knowledge of it issuch that its best applications are well understood. The other distillation techniques suchas MED and VC are also well understood. The commercial development of membrane

processes such as RO and ED is relatively recent and their best applications are not aswell understood, but are nowadays also considered to be mainstream desalination

processes.

Three desalination technologies were selected for cost comparison purposes in this report,as these technologies are most likely to be financially viable for the low-capacity

production regimes outlined for this study. These technologies are:

Reverse Osmosis membrane systems (RO); Electrodialysis Reversal membrane systems (EDR); and Multi-Effect Distillation systems (MED).

In general, the costs for RO plants tend to be lower than for distillation plants of a similar capacity, but particularly for plants smaller than 300 to 400 kL/day where distillation isnot financially feasible. Distillation is typically only viable for plants of higher capacitythan this, and particularly where a low cost, high quality waste heat source is readilyavailable.

If the feedwater TDS is greater than 10,000 mg/L TDS and a low cost, high quality wasteheat source is available, than the MED process is generally selected. Other than for thewaste heat scenario, distillation processes such as MED are only really considered wherevery high feedwater TDS values greater than 50,000 mg/L TDS occur, and for highcapacity plants greater than 300 to 400 kL/day.

EDR systems tend to always be more costly than RO systems, however this becomes lessof an issue as the plant capacity increases - EDR systems are typically only 10% higher incosts than RO systems for plants greater than 100 kL/day. EDR systems have a feedwater TDS limit of 12,000 mg/L TDS, and are generally only considered when high scalingfeedwaters are present. EDR systems are therefore only economically viable over an ROsystem when the feedwate

desalination full report

Documents