Systems used for the purification of water contaminated by arsenic
The contamination of water by arsenic is just an Italian problem?
Unfortunately, the contamination of water by arsenic affects the entire planet: the continent of Asia is the most affected, particularly in Bangladesh where the concentrations are also coming to some g / L. Australia also there are problems of arsenic as well as South America, particularly Argentina, but is also an issue in North America, Northern Europe, etc. ...
In Italy, which are the areas most heavily penalized by arsenic?
In Italy, the presence of arsenic in water intended for drinking has taken on aspects of a social problem when you consider that affects over 100,000 people in regions such as Lazio, Tuscany, Umbria, Trentino Alto Adige and Lombardy: in fact amounted to 128 municipalities whose drinking water showed an arsenic content exceeding the limits of concentration, 10 g / L (ie 10 ppb), provided by the European standards. In some isolated cases, these values have reached a concentration of 100 ppb.
Why do we find arsenic in the water?
Because groundwater basins or ground, encountering volcanic rocks, solubilize arsenic, which then contaminates the water. Arsenic is then a natural contaminant, present in the chemical form of arsenate (As +5) species oxidized, and arsenites (As +3) in the reduced form.
Because arsenic is harmful to health?
Both chemical forms of arsenic are toxic to humans, arsenite is more dangerous dell'arseniato, with effects that may prove carcinogenic: long periods of exposure can also cause cancers of the skin or internal organs such as liver, colon, brain. For this reason, the WHO, World Health Organization, has established an upper limit of tolerance at 10 g / L.
How did the ENEA Research on the process of de-arsenificazione of drinking water?
As we mentioned above, the research comes from a real need for some 100,000 Italian citizens.
In line with the mandate of the Agency and with our mission we took on board to answer technical-scientific public administrations: Regions, Provinces, Municipalities, etc. ...
What is the technical know-how on the topic of ENEA arsenic?
Some of our Technical Units dealing with the arsenic problem for many years, especially in terms of water monitoring, especially in Lazio.
My group is involved in the processes of cross-flow filtration for 20 years, then joining other skills ENEA (engineers, chemists, geologists, biologists and process experts) we came to have the results that we consider useful for the country.
How did you come to the redesign of a process of de-arsenificazione?
First of all we studied the techniques used by conventional systems of de-arsenificazione starting from that of the Centre of Casaccia, we have highlighted the critical points of operation, so we studied technical solutions to ensure the quality of the water produced.
Moreover, almost all companies, including international ones, which use or market for arsenic adsorbent materials (iron and manganese, iron hydroxide, etc..) When quoting the membrane technologies, as an alternative to their processes, they claim that our technology is ineffective for the removal of such a compound. In fact, these considerations are incorrect, as shown by the results of our experimental campaigns.
What is the main innovation of your process?
Today we are able to indicate a process of de-arsenificazione water, safe and reliable for different types of primary water, using all available technologies in optimal condition for each of them.
One thing is proven: the technique of reverse osmosis completely removes arsenic and greatly simplifies the process, but not enough to say we must consider that reverse osmosis to close the topic. On the world market there are at least 400 different types of osmosis membranes produced by countries like USA, Japan, etc ... should also be considered a set of parameters which affect the plant by osmosis, the process conditions, maintenance and assistance .
The membrane systems are highly selective and effective in the removal of arsenic and minerals in general, but are very critical and require a controlled management and skilled labor, not always readily available at the level of public administrations or operators of water supply.
Because the adsorption systems not convinced?
The adsorbents are generally of the products obtained from industrial processes, such as that of the industrial production of aluminum (Bauxite), and can therefore contain traces of pollutants in water releasable then purified.
Moreover, they are applied in practice with two process methodologies that are flotation and fixed bed adsorber.
Both processes show significant functional limitations and, most important, do not guarantee the purifying effect of water in long cycles of operation, with the result that the consumer is convinced to have purified water instead it is not.
These materials have run out of charge, to work better in an acidic environment, often require a chemical pre-treatment of water, release alkalis, in short, should be used very carefully.
Should not be neglected also technological problems related to the adsorption process in a fixed bed: often the water follows the preferential sliding ways within the adsorbent bed, in fact reducing the operating time of "effective" of the same that is not used for its entire volume as designed. This translates into a lack of water purification in which the values of arsenic may exceed those that we inadvertently had prefixes. Add that adsorption systems have a negligible effect in the removal of arsenites ..
On the contrary the treatment with membrane technology is effective both on arsenites that on arsenates, although with slight differences, for another negligible for practical purposes.
What are the technical limitations of membrane processes?
On the issue of removal of arsenic in drinking water do not see anyone. Some argue that the membranes waste too much water, and are conditioned by the presence of calcium in primary water.
Our experiments show that the water of the Casaccia "wastes" as effluent concentrated around 5% by volume of the inlet water, for water saltier than this percentage may be increased to a maximum of 15%, but the water produced is always free of arsenic.
The presence of salts in the form of bicarbonates, carbonates, silicates, etc. ... in the water can be a primary problem of clogging of the membranes, but the righteous have largely technical measures to eliminate such hazards.
There are no industrial facilities osmosis to purify the waters also of contaminants?
The reverse osmosis technique is widely mature if you think that all over the world produce everyday million m3 of drinking water from the sea.
On the issue of potable fresh water there is a reverse osmosis system that produces drinking water for the entire city of Paris from the Seine. The plant is located in Mery-sur Oise and produces 6,000 m3 / h of drinking water. Even if there is arsenic in the Seine osmosis removes still 99% of mineral salts.
The membrane systems are expensive? They consume in terms of energy?
The costs of membrane systems are of the same order of magnitude than absorption, and when the sizes production increased over 50 m3/hi costs of the membranes is reduced. The energy consumption of the processes of reverse osmosis depends on the salinity of the primary water. Our data obtained show that the water of the Casaccia operand with the osmosis purified water is the energy cost of € 0.067 / m3, for that at low pressure of 0.31 € / m3 for that high pressure.
You can reconcile the adsorption technologies with those of membrane?
Certainly yes, and it is in this direction that we are creating new experimental campaigns.
Can you give me an example?
If we use the techniques of membrane technology as the primary de-arsenificazione adsorbers can be used to treat the retentate (concentrate) of the membrane.
The other example concerns the application of the technique of micro-filtration with adsorbent used in the flocculation (physical-chemical process that does increase the weight of the particles to settle better) adsorbents. Our experiments performed with fine dust using microfiltration membranes have shown the following advantages:
1. 50% increase of the efficiency of adsorption of arsenic
2. lower consumption of adsorbent 30%
3. reduction of production times purified water for 48 hours with the flocculation 7 minutes with the microfiltration
4. better quality of the produced membranes that is purified by arsenic from airborne particles and bacteria.
How to translate these improvements on the cost of the process?
I can not give precise because these data depend on a number of factors, among which the most important is the composition of the outgoing water, which also varies in the same geographical area. Our studies also show concern for the environmental sustainability of the process, eg. if arsenic is chemically bonded with the adsorbent (strong bond) reduces the risk of it leaching at the level of landfill, so our goal is to work in a logic of LCA (Life Cycle Assessment).
20/07/2011
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Source:
Italian version of ReteArchitetti.it