Radioactive water in Sicily purified by 99.9 percent

Overview_TEXT

Overview

One look at the chemical analysis of the water at the phosphorous gypsum stack in Gela, Sicily, and most water technologists would have cringed in horror: This polluted water showed high salinity combined with elevated concentrations of different heavy metals, phosphoric acid, fluorides and radioactive elements. 

Ulrich Bäuerle did not cringe. He saw a challenge and an opportunity. His company, Clear Water Technologies (CWT) of Grafenrheinfeld near Frankfurt, Germany, specialises in difficult water cleaning projects.

“You have to understand the chemistry,” says Ulrich Bäuerle. “Most people would look at the water and say it was impossible to purify, but we and our partner company Servizi Technologici per l'Ecologia Srl Uninominale (STE) used our experience and tried one thing and then another until it worked.” 

Used in the production of phosphoric acid for fertilizer (see story tab “A need to feed”), phosphate often contains naturally radioactive elements, like many other natural rocks. Making phosphoric acid leaves phosphorous gypsum, a by-product. Its disposal in a landfill over time contaminates the accumulated rainwater, says Ulrich Bäuerle.

The phosphoric acid production plant in question operated during the second half of the twentieth century and was closed down in the 1990s for industrial and market reasons. The problems related to the phosphorous gypsum landfill in Sicily, however, remained. 

This problem is not unique to Gela. Sites that process phosphate rock to produce fertiliser are present on all continents. According to a 2012 Greenpeace study, the problem is growing fast, since increased mining of phosphates to meet increased demand for fertiliser has meant that the quality of the rock is getting worse. Around 110 million tonnes of phosphorous gypsum is produced every year.

 

The situation

The pollution at Gela is a big enough problem to interest the International Atomic Energy Agency (IAEA), which says there are 15 million tonnes of material in the 55-hectare, 20-metre-deep phosphorous gypsum stack. In spite of retaining walls that run deep into the clay below, there is still a risk of radiation getting into the groundwater, so surface water is pumped into two huge artificial lakes alongside the site. 

In 2010, the IAEA dedicated significant effort into determining how to measure the risk posed by the site and how to deal with the radioactivity in the stack itself. The leachate water generally contains heavy metals and radioactive isotopes of uranium, lead, polonium, radium and potassium, emitting alpha, beta and gamma radiation. 

That is where most water engineers gave up, but not Ulrich Bäuerle. He had to build a pilot plant and demonstrated his water treatment solution to a committee of experts, with each of the stages of the process being examined by two independent laboratories.

At the plant operated by CWT’s Italian partners STE, the first stage – designed by Ulrich Bäuerle and built by STE – is chemical pre-treatment. The second stage, built by CWT for STE, treats the remaining water physically.

“The pre-treatment removes the bulk of salts, heavy metals, phosphorous and fluoride,” explains Ulrich Bäuerle, “while the second stage removes the rest of the dissolved salts and other solubles.”

 

The solution

The pre-treatment involves chemical processes to transform dissolved pollutants into solid particles. These can be separated from the water using some filter clothes in a big chamber filter press. Because biological contamination can create problems in the subsequent process steps, the chemical treatment has to be adapted according to the quantity of algae and bacteria present in the holding ponds.

“What emerges from the pre-treatment is white sludge in solid panels, which can turn a bit green if there are a lot of algae,” says Ulrich Bäuerle. “The pre-treatment removes most of all problematic ingredients that would prevent the membrane treatment in the second stage from working well. What emerges is clear water, but with all the soluble and partially radioactive salts still in it.”

The solid sludge is returned back to the stack. 

Meanwhile, the water continues on to the treatment plant, which is neatly packed inside a pair of sea containers. That is a speciality of CWT: its plants are assembled in Grafenrheinfeld in standard shipping containers and can be transported and installed quickly wherever they are needed. In the case of Gela, installation of the insulated and air-conditioned containers took just six weeks. 

While much of what happens in the treatment plant is secret, Ulrich Bäuerle explains the basic flow (see illustration).

The first filter in the container is a porous membrane –a physical filter that is so fine, it removes suspended solids as well as bacteria and viruses. The water then undergoes reverse osmosis (RO), in which it is forced through three membrane stages. At each stage, the concentrate of pollutants and chemicals will be returned to an earlier stage so that it can be retreated. Ulrich Bäuerle monitors plant operation from his office in Germany using a web-based control interface. He can thus recommend adjustments to the on-site engineer.

 

The outcome

At the end of the process, all suspended solids and 99.9 percent of the salts will have been removed. The 13,000 kilogrammes/hour of surface water will have become 10,000 kilogrammes/hour of pure water, 2,500 kilogrammes/hour of RO-concentrate and approximately 710 kilogrammes/hour of camber filter-press sludge. The concentrate goes back to the stack, with new and patented re-infiltration technology supplied by German IEG Technologie GmbH that avoids a water short circuit, ensuring even distribution of the concentrate across the gypsum volume.

The water is now clean enough to meet the discharge limits set by the environmental authorities and is fed into a nearby stream. 

One treatment plant turned out not to be enough, however. Now two identical plants work 24 hours a day, seven days a week, with a total capacity of 600 cubic metres of water per day.

This was a challenge for CWT. Although it always deals with difficult water, this was a step further than anyone had gone before.

“There are around 20 other locations around the world that face the same problems,” says Ulrich Bäuerle, “and until now, nobody has known how to deal with them. Now we want to take [our solution] elsewhere, and we can adapt this experience for use on other equally difficult but different problem sites.”

Topic:

Industrial water treatment

Location:

Google Map

Gela, Sicily

Company:

Clear Water Technologies

One of the Grundfos Digital Dosing pumps inside the CWT water treatment container in Gela, Sicily.

 

Carmelo Curvà walks through the plant in Gela, Sicily, which treats highly polluted, radioactive water contaminated from the production of phosphoric acid for fertilizer.

 

Radioactive water in – clean water out. The CWT water treatment system is illustrated here.

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