SELECTIVE TREATMENT OF SELENIUM AND MERCURY
Tecnoimpianti has faced in recent years the environmental problems of critical contaminants treatment, like selenium, arsenic, boron, mercury, vanadium and fluorine.
For selenium, which cannot be eliminated by traditional chemical-physical methods, TWT has developed and patented an innovative process on an industrial scale that is also effective for the removal of mercury and other heavy metals, creating the first selective removal system for selenium from wastewater in the Power sector.
The Selenium is present in many process and waste waters and in particular can be a problem for:
The removal of selenium solves a challenging environmental problem and allows to:
These targets can be easily achieved with the TWT treatment process, which can “block” both Se6+ and Se4+ in sludge, with abatement rate up to 99.9% in compliance with legal limits.
THE DeMS PROCESS
The TWT treatment method originates from the study of a natural oxide-reduction process of metals and metalloids.
By using simple installation solutions and by maintaining optimal redox, pH and reactor temperature parameters, a reliable solution can be provided on industrial scale both for selenium and other elements which are difficult to be removed using conventional processes (mercury, vanadium, chromium, etc.).
Today, is operating in Italy the first selenium selective treatment plant with capacity of 120m3/h on TSD effluent of a thermoelectric power plant.
COAL POWER PLANTS: TREATMENT OF DESOX DISCHARGE
In the power plants sector, selenium can be found in fossil fuels, especially coal; it is dragged in the combustion fumes with one part ending up solubilised in the fumes washing process of the DeSOx plant (pre-scrubber/scrubber/washers).
The oxidizing environment of the DeSOx process contributes to the increase in the Se6+ fraction, which is restive to removal and cannot be eliminated in the existing DeSOX discharge treatment systems (TSD).
TWT has focused precisely on the application of the selenium reduction process with saline waste and on the optimal integration in existing TSD systems. In the specific application, the treatment system, developed on three lines in parallel, was designed to treat the output of the primary TSD and before the secondary clarification. In picture 1 is shown the measured treatment performance rate.
Developing the system represented an environmental priority for the power plant, and it also allowed the elimination of pre-existing constraints in the procurement of coal also with high selenium content, for a reduction in supply costs of about 6-10 $/ton compared to selenium-free coal.
This economic advantage, considering the capacities of a 600MW coal power plant, allows a savings of 1 €/MWh and an investment payback time of less than 1-1.5 years. The plant is configured as a chemical-physical treatment with sludge separation by means of lamellar clarifiers.
The filter-pressed sludge containing selenium is disposed of in the same way as the other sludge generated by treatment systems present at the power plant. The project took 8 months to develop, from design to construction, and the plant started to operate in January of 2012.
