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regarded as medium sized in global terms. The stainless steel would be used to construct the desalination unit, connecting pipework, other equipment such as pumps and valves for the unit, and its auxiliary systems and general infrastructure. Stainless steel in reverse osmosis systems The temporary plants used around Cape Town were of the reverse osmosis types. In reverse osmosis plants, there are much lower pressures and thus a high use of PVC and other plastics. The use of stainless steel is limited to the pumps, valves and tubes working at higher pressures. These plants require far less stainless steel than thermal desalination plants; for example, a plant producing 1500m3/h of permeate requires about 30-40 tons of the aforementioned materials. Conclusion Desalination is already widespread in some water-stressed areas, like the Gulf region, where Kuwait gets up to 90% of its drinking water from desalination, where Kuwait gets up to 90% of its drinking water from desalination 42% of the United Arab Emirates’ water needs. While desalination is a reliable process used by over three hundred million people worldwide, it has some negative environmental impacts: • Fossil fuels - Desalination is energy-intensive and uses fossil fuels contributing to global warming. • Toxic brine - The brine produced by desalination plants can pollute coastal ecosystems. • Waste and chemicals - Desalination plants produce waste and toxic chemicals that can harm wildlife. • Greenhouse gas emissions - Desalination plants that use diesel produce greenhouse gas emissions. • Temperature difference - The temperature of rejected brines is often 10 to 15 degrees Celsius higher than that of ambient saltwater, which can be harmful to marine ecosystems. • Marine life - Desalination surface water intakes are a huge threat to marine life. For example, the sodium metabisulphite discharged from desalination plants can negatively impact seagrass habitats. It is clear that desalination is expensive and has significant environmental sideeffects but in an arid country such as South Africa what would be the alternatives? Additional reading 1. https://sassda.co.za/wp-content/uploads/2024/03/ ISSF_Desalination_in_Stainless_Steel.pdf 2. www.dailymaverick.co.za/article/2023-12-03-cape- town-set-to-purify-treated-sewage-water-for-drinking- purposes
salt and other minerals can be removed. The water is pushed at high pressure (60bar) through the semi-permeable membranes, which function as a filter for the salt and minerals and only allow fresh water to pass through. The freshwater produced by the reverse osmosis process requires minerals to be added before it is fluoridated and chlorinated, in line with Australian Drinking Water Guidelines and NSW Health requirements. Two large pumps transfer the water from the drinking water storage tank into a pipeline to the end-users. Approximately 58% of the water used during the desalination process is returned to the ocean. This water is known as seawater concentrate (effectively saltier seawater) and is transported back to the ocean via an outlet tunnel and specially designed outlet dispersion nozzles. These nozzles make sure the water mixes rapidly and returns to normal seawater salinity and temperature within a short distance of its discharge point so as not to harm the local marine environment. How much stainless steel is used in desalination? One should distinguish between the desalination processes when trying to determine the quantities of stainless steel used in such applications. The tonnage would also depend on the plant’s capacity. The types or grades of stainless steel used in desalination would also be noble or with higher alloying content to protect against harsh conditions in the process. Since desalination is not common in South Africa, we have no real data on local projects. South Africa is only beginning its feasibility studies on the suitability for such a facility and currently, a study is being finalised for a proposed plant near Cape Town. If this report shows that such a project is feasible, this will be the first permanent installation with this level of capacity in Southern Africa. Stainless steel in thermal desalination It would seem that empirical data from the proposed Cape Town project shows that a thermal desalination plant, which produces about 1500 m3/h of distillate, requires 900- 1000 tons of stainless steel . A plant of this capacity will be
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Issue 1 – 2024
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