Water treatment options for safe drinking water at the household level

EAWAG-Group, Principal Investigators: Dr. Stephan HUG, Dr. Urs von GUNTEN, Martin WEGELIN. Students: Olivier Leupin Water treatment

Until alternative water supplies in Bangladesh are available, water treatment is one of the options that need to be implemented most urgently. In industrialized regions, a number of more or less costly technologies are able to remove arsenic to levels below the current limit of 50µg/L. However, adapted technologies that can be broadly implemented in rural Bangladesh are still in development or in evaluation.

Arsenic removal methods

Most arsenic removal methods work by adsorption of the dissolved arsenic to surfaces of solid particles. Adsorbing materials can be sand, clay, charcoal, etc., which can be used as filters. Particularly effective are small particles which form directly in the water after adding dissolved iron or aluminum. Within minutes after iron- or aluminum salts are added to water, extremely fine iron- or aluminum(hydr)oxide particles form and adsorb arsenic. With time, the particles grow larger and can be removed, together with the arsenic, by filtration or by settling to the bottom of containers. In groundwater, arsenic occurs in two forms (oxidation states): as trivalent arsenic, As(III), and as pentavalent arsenic, As(V). Since As(V) generally adsorbs much better to solids under typical treatment conditions, all the arsenic needs to be converted to As(V). Normally, this is done by addition of oxidizing chemicals such as chlorine, bleach, ozon or permanganate. However, chemicals are often not available in villages in Bangladesh and people sometimes object to adding chemicals.

Naturally occurring chemical processes in Bangladesh groundwater Among other components, groundwater in Bangladesh typically contains 5-20mg/L of dissolved divalent iron, (Fe(II)). Immediately after pumping, the water still lacks oxygen and is clear and colorless. As the oxygen of the air enters the water, it oxidizes the dissolved and colorless Fe(II) to Fe(III). The Fe(III) is no longer soluble and forms small iron(hydr)oxide particles, which turn the water turbid and brown.

As described above, these particles are able to adsorb and remove arsenic from the water, but the removal is only efficient for As(V). Unfortunately, the conversion of As(III) to As(V) with oxygen is a very slow process. If ways can be found to speed up the conversion As(III) to As(V), removal of arsenic might be possible without adding chemicals. Other constituents in the groundwater are partly helpful and partly detrimental for treatment. Generally, Bangladesh groundwaters are very hard and silicate-rich, with up to 500mg CaCO3/L and 70mg SiO2/L. Dissolved iron and manganese impart an unpleasant metallic taste. After the pumped water comes in contact with air, silicate and iron(III)(hydr)oxide particles start to precipitate. These precipitates stain food and clothes brown and they pose a problem to many water treatment methods by clogging the pores of filter materials.

Our objectives and goals

The goal of our work is twofold:

1) to investigate the properties of the pumped groundwater and to understand the chemical reactions that occur in these waters, in order to develop different pumping strategies and new water treatment methods.

2) to develop adapted arsenic removal methods which do not require chemicals or special equipment.

Can sunlight be used to remove arsenic?

A possible method to achieve rapid oxidation of As(III) to As(V) without the use of chemicals is to use sunlight for the production of reactive oxidants. The light induced production of reactive oxidants can be promoted by adding a few drops of lemon juice per liter of water. We tentatively call this method Solar Oxidation and Removal of Arsenic (SORAS).

For treatment by SORAS, water is placed in PET or other transparent bottles and 4-5 drops of lemon juice is added per liter of water. The bottles are then shaken vigorously to saturate the water with air. After this, the bottles are exposed to sunlight for several hours. The bottles are then left in vertical position overnight, to allow the iron(hydr)oxide particles with the adsorbed As(V) to settle.

The clear water above the settled brownish sludge is carefully decanted or filtrated through fine cloth.

Sunlight produces oxidants in photochemical reactions with Fe(III) and citrate. The oxidants oxidize As(III) to As(V), which adsorbs almost completely to the precipitating Fe(III)(hydr)oxides formed from oxidation of the naturally present Fe(II). (sketch)

Arsenic removal efficiencies in the laboratory with synthetic groundwater were around 90%. Removal efficiencies in field tests in Bangladesh were dependent on the water composition of individual wells and varied between 45% and 80%. In waters with more than 10mg Fe/L, removal efficiencies were high, while they were low with less than 5mg Fe/L. Villagers can estimate the iron content by the brown color that develops after aeration and by the amount and the color of the precipitates (picture).

With the iron removed, the treated water is clear and colorless. Bangladeshis described the taste of treated water as light and refreshing, compared to the metallic taste of untreated water. SORAS appears to be a working treatment option for water with high iron contents. However, it is currently not able to achieve the current limit of 50ppb in all wells. Nevertheless, even with an average arsenic reduction of 60%, consistent water treatment could lead to an estimated reduction of arsenic related diseases by a factor two to three.

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