Desalination

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Globally, desalination now competes favourably with traditional sources of water that are becoming expensive due to the costs of containment (dams), inter-basin transfers, contamination, accessibility and environmental impingement.

Desalination is a process that removes dissolved organics, salts and other impurities from sea water, brackish water, or treated waste water. Currently there are over 12 450 large scale desalting plants worldwide producing more than 27.28 million cubic metres (kilolitres) of pure water per day for potable, industrial, agricultural and landscape irrigation purposes.

Originally, desalination was the preserve of Middle Eastern oil producing countries and was achieved through high energy consuming thermal vaporisation processes, with the first major desalination plant installed in Kuwait in 1957. In recent years, membrane based Reverse Osmosis (RO) systems that became commercially available from the mid-1960s have taken over from these evaporation processes.The RO process involves forcing water, under pressure, through a membrane to remove unwanted ionic constituents.

Sea water reverse osmosis (SWRO) has been extremely successful in the popular tourist regions of the Mediterranean where plants have delivered reliable water supplies since the 1970s. Spain has been using RO technology for 30 years and has 700 desalination plants, having installed Europe’s biggest reverse osmosis desalination facility (120 000m3/day) at Carboneras.

Madrid recently decided to build an additional 20 desalination plants on its east coast to provide water for agriculture and tourism along its parched south east, shelving an earlier controversial plan that would have piped water from the Ebro River in the wetter north.
South Africa installed its first bulk water SWRO plant (500m3/day) at the Albany Coast Water Board in the Eastern Cape in 1997, supplying the communities of Bushmansriver and Kenton-on-Sea.

Just ten years ago, desalinated water would have cost much more than treated surface water. However, desalination technology has significantly improved which has lowered operational and maintenance costs (approximately US$0.19/m3) whilst market developments in membrane-based technologies have resulted in reductions of up to 50% in the overall cost of equipment and consumables.
Conversely, limitations on availability and the growing complexities of treating conventional surface water has increased – a trend that is continuing, while the costs to turn salt water fresh is falling some 4% annually. A recent comparative cost evaluation undertaken for a coastal municipality in South Africa determined that a desalinated sea water supply option would provide cheaper water (R3.11/m3) than a conventionally treated surface water option (R3.47/m3). Moreover, these costs did not reflect desalination’s compelling triple bottom line benefits.

In the past few years a number of other regions, not traditionally associated with desalting, have selected desalination as the preferred process over conventional supply due to its lower cost, higher product water quality, reduced environmental impact and proximity to limitless and guaranteed supplies of water (albeit salty).
Thames Water’s 150ml/day Essex plant will supply desalinated water to 900 000 residents of the City of London and Perth’s 130ml/day Kwinana seawater desalination installation was selected over a pipeline importing water from the Kimberley river because capital costs as well as operating and maintenance costs were substantially cheaper.
Also, total energy consumption for the desalination plant would be a mere 5kWh/m cubed compared to the pipeline’s 15kW/m cubed energy requirement.

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In South Africa

Here in South Africa, with the lower cost and security benefits that desalination now provides, a number of coastal municipalities are seriously investigating desalinating seawater, underground brackish aquifers and secondary sewage effluent streams, to augment dwindling supplies that are now under pressure from increased populations, tourism and the wild card in any water resource management planning, drought.

Private developers of coastal estates are also busy tapping into on-site saline or brackish aquifers and adjacent limitless seawater reserves to provide both potable water for resident communities and landscape irrigation waters,as well as desalinating secondary waste water effluents to ‘make greens greener’ whilst ensuring the sustainability of their sizeable investments. Desalination is able to provide a range of eco-friendly solutions for coastal developments encountering localised water shortages or future developments where available resources are limited or too saline.

Water security and use is one of the most important issues facing the future of both new and existing golf and polo developments, particularly in the Western Cape at the present time. Since such facilities are highly visible features, they become targets for criticism during periods of drought when homeowners and others are restricted in their use of potable water. Developers and golf course owners are overcoming public resistance by conserving water with more efficient irrigation systems as well as using alternative water resources where possible. Many golf courses and developments requiring landscape irrigation use waste water streams reclaimed from secondary sewage works.

However, during low rainfall periods the dilution factor of irrigated waste water is lower. During the rainy season, natural rainfall dilutes the concentrated nutrients found in such irrigated secondary waste waters. However during low rainfall periods, natural rainfall dilution does not occur. This is further compounded by the increased concentration of the secondary effluent due to reduced storm water catchment. In landscaped areas where there isn’t sufficient drainage, a toxic build-up tends to form around the root zone.

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500m3 per day sea water reverse osmosis skid – front inlet perspective
500m3 per day sea water reverse osmosis skid – back outlet perspective illustrating the brine reject piping manifold and the product water manifold

Furthermore, the use of reclaimed secondary sewage water for potable and industrial re-use raises concerns for the security of this resource’s future use by the Green Industry.Water and Forestry Affairs Minister Buyelwa Sonjica has publicly stated that the department’s first priority is ‘stopping waste…then there is the option of recycling waste water …then we are looking into desalination.’ The RO desalination process used to treat secondary sewage is similar to that used to desalt seawater. The water produced is ‘pure’, free of all pathogens, viruses and bacteria. Singapore’s recycled sewer or waste water called “NEWater” is reclaimed via an RO desalination process and currently contributes 1% to Singapore’s bulk water supply. Golf courses and polo fields cannot rely on future supplies of treated secondary sewage to quench their thirsty greens when there is a shortage of local water supplies.

A few forward thinking developers are looking at entering into Public Private Participation (PPP) initiatives with local water strapped municipalities, effectively ‘trading’ desalinated potable water for access to low cost secondary sewage streams whilst also reducing their own per cubic metre desalination costs through the economies-of-scale achieved by producing greater quantities of desalinated water per day than their own immediate requirements. Profits from the sale of this additional water can be used to reduce the capital expenditure loan repayment period, further reducing the overall per cubic metre production costs for desalinating water.

Desalinating brackish water costs about half that of desalting sea water or secondary sewage effluents due to lower operating pressures and reduced initial capital outlay for equipment. Sea water systems require 60 bar pressures to drive the process, whereas brackish water RO systems or nanofiltration systems in some instances, range between 5 and 20 bar, depending on the total dissolved solids (TDS) count. Sea water RO has an average conversion rate of between 35% – 50%, which means 35% – 55% of the feed-water is recovered or permeated whilst the remainder is concentrated and returned to source in most instances.The conversion rate of brackish RO ranges from 55% – 90%.

A significant advantage of RO systems over previous distillation processes is their modular construction. Skids can be added incrementally, as and when community water demands increase. The Albany Coast Water Board plant provides a classic case in point. The initial 500m3/day skid installed in 1997 has been up-scaled every second year. The facility now supplies six local municipalities and has an overall capacity of 1 500m3/day. The initial 500m3/day installation abstracted sea water from a beach-well located alongside the lagoon. Beach-wells are preferred over direct sea water intake systems as the sand provides a natural, dynamic media filtration, removing all suspended matter prior to the RO process. However, the beach-well abstraction capacities were insufficient for the up-scaled RO desalination works and due to environmental limitations, a second beach-well was not possible, and an additional direct sea water intake works was therefore required. Ocean Horizon is currently installing the mechanical and electrical works for the new intake works together with a new 2-stage filtration works prior to the up-scaled RO system.

Configuration

A typical RO desalination plant may be configured in four distinct stages: intake works, filtration works, RO desalination works and post-treatment stabilisation and product water transfer works. The filtration works usually incorporates a 2-stage filtration process for removal of suspended solids prior to the RO desalination process as the more effective the filtration, the longer the RO membrane life expectancy. The primary filtration process is usually comprised of media filters for the removal of suspended solids including iron. The secondary filtration process may incorporate bag or cartridge systems, or membrane-based micro-filtration (MF) or ultra-filtration (UF) processes, and produces a ‘fine’ filtrate for the RO process, wherein all unwanted dissolved solids are removed.

The RO process is comprised of a pump, membrane pressure housing, membrane element(s), plumbing connections and electrical and mechanical control systems. The number of membrane elements used in each desalination system is related to the quantity of water produced by the system. There are many types of membranes, each characterised by their particular salt rejection. The pump is used to pressurise the feed water to create the RO effect and distribute the product water to the storage reservoir.

Cost effective

In recent years, several factors have emerged to make desalination more cost effective than in the past. These include a dramatic reduction in the cost of RO membranes combined with improvements in membrane technology and significant reductions in energy consumption. RO desalination technology is now able to provide guaranteed, limitless supplies of consistently high quality water at an increasing number of new facilities, a lower cost per cubic metre than conventional supplies and with the added benefit of being independent of climate and rainfall. Furthermore, sea water desalination is able to provide other triple bottom line benefits such as reducing environmental impacts by locally sourcing and controlling readily available and easily accessible sea, brackish and waste water resources and alleviating pressure on surrounding resources, outlying communities and agricultural regions.

In South Africa, private developers busy installing desalination plants of smaller capacities (500m3/day – 1000m3/day), appear to be driving our trend of tapping into limitless saline reserves. It is as much about money as it is about water.

About the author

Elza-Lynne Kruger is managing director of Ocean Horizon (Pty) Ltd, a new South African company providing eco-friendly water treatment solutions that specialise in membrane-based technologies. Senior personnel of Ocean Horizon have been involved with the supply, installation and commissioning of sea water desalination plants worldwide for over a decade.