Drinking water from the Mediterranean Sea

The process ends with a taste test. After flowing down a long path of metre-thick pipes and through a filter only a tenth of a nanometre thick, the water drawn from the Mediterranean Sea is tapped and passed around. A sip, a nod. It tastes sweet.

50,000 cubic metres of fresh water are produced from seawater every day on the Tunisian island of Djerba. The quality is so good that “babies can drink it”, says Mosbah Helali, Managing Director of SONEDE (Société Nationale d’Exploitation et de Distribution des Eaux), Tunisia's national water distribution utility. The desalination plant is one of the first of its type in Tunisia and began operation last year. It cost 75 million euros, 60 million euros of which was financed by KfW on behalf of the German Federal Government. The system, which runs around the clock, can alleviate chronic water scarcity in Tunisia’s southern region.

Sea water becomes drinking water

The heart of the plant is a battery of 350 white filter modules containing nearly 7,000 membranes. Seawater with a salinity of around 40 grams per litre is pressed through these membranes at high pressures. Water molecules pass through the membranes. Everything else, including the salt components, is filtered out. The process is known as reverse osmosis and is a standard method of desalination.

In total, over 20,000 plants all over the world produce drinking water from seawater. The entire drinking water supply for the Gulf States is based on this technology. The only German plant is located on the island of Heligoland. It produces as much in one day as the plant in Djerba produces in a half hour.

The price of producing drinking water from seawater is high energy consumption. The high-pressure pumps require a great deal of power. According to the head of the national water distribution utility, they are the largest energy customer of the state’s electricity supplier, STEG. But Helali says, “our plant has the lowest energy consumption of comparable plants around the world with 2.5 kilowatt hours per 1,000 litres of drinking water.” This was one of the reasons that the island of Djerba, which also recovers energy with its plant, won second place from around 100 participants at the Global Water Summit 2019 in London as part of the “Desalination Plant of the Year” award.

The salt content is what counts

The Djerba plant’s concrete blocks are as high as a house and located at the edge of a hotel complex near the shore of the Mediterranean Sea. The seawater is taken from a depth of eleven metres two kilometres away from the coast and then fed to the plant through pipes under the beach. After desalination is complete, 35 litres of fresh water are made from 100 litres of seawater. The other two thirds are fed back into the sea. This is done using technical facilities that quickly dilute the salt water as this water contains high concentrations of all the components of seawater like salt and minerals.

The salt and mineral content of the fresh water produced through reverse osmosis tends to be around zero. Before it can be fed into the drinking water system, it first needs to be remineralised and mixed at a ratio of 5:1 with water from deep wells to raise the salinity to 1.5 grams per litre.

Loans from KfW

Without desalination, the population in the arid southern regions of Tunisia would not have sufficient drinking water. Many of these areas are desert environments. So KfW used 25 million euros in funds from the Federal Ministry for Economic Cooperation and Development (BMZ) to support a brackish water desalination programme that began in 2005. Ten plants that desalinate saline ground water are already in operation. To this end, KfW is providing 55 million euros for six additional plants in the form of a loan on behalf of the German Federal Government. These funds cover 70 per cent of the total investment.

With annual average rainfall of 150 millimetres, southern Tunisia is one of the most arid regions in the world. Up until construction of the desalination plant, Djerba’s population and one million tourists every year were supplied with a mix of brackish water from deep wells on the island that was desalinated in a plant and saline fresh water from the mainland. However, this method became insufficient to provide a continuous supply of hygienically safe water years ago. Particularly during the peak season in the summer, SONEDE needed to pump sulphurous and saline water from additional deep wells into the pipelines.

“We want to provide the southern population with drinking water, particularly those people living in the desert”, says Helali, the head of SONEDE. During tourist season, the island needs the entire supply of desalinated seawater. During other times, water from the new plant is also fed into the mainland supply network. This conserves the limited ground water resources in the south. Capacity for desalination of seawater on Djerba can be expanded by another 50 per cent within the existing building.

In 2021 SONEDE, which supplies the country’s three million households with drinking water, will put its second desalination plant into operation in Zarat. The site is on the mainland across from Djerba. Construction of the plant in Zarat will also be financed with a loan from KfW consisting of German federal government funds. Zarat will also supply 50,000 cubic metres of fresh water per day; the capacity of the plant can even be doubled.