water treatment of contaminated groundwater; Arsenic

 

 

 

Major groundwater contaminant

 

 

2.1 Arsenic

Arsenic is a chemical element that comes in deeper levels of groundwater and naturally occurs in the rocks of earth’s crust and mountain. Arsenic is regarded as cancer causing material and a poison. Unfortunately it can be only found by conducting water tasting because it has no taste or taste.  

 

 

 

Fig. 1 Arsenic occurrence in groundwater

 

2.1.1 Bangladesh

  There are lots of locks containing high arsenic levels in Himalayas. As the locks were eroded by Ganges River and Brahmaputra River, the element arsenic of those locks has been dissolved into two rivers. The rivers flowed downstream and formed clay layer of flood plain and delta in which most people in Bangladesh lived. The people have used the river containing life-threatening arsenic for drinking. About 10 out of 1,000 people who drunk contaminated water have experienced waterborne disease, skin disease and cancer.

 

 

 

2.1.2 Vietnam

   There are few sources for drinking water in Vietnam. Groundwater is mainly used for drinking.

However, due to indiscriminate excavation of boreholes and well groundwater levels fell fast, which

leads to the depletion of groundwater water supply. Thus, they didn’t have enough water for daily life,

especially for drinking. So they had no choice but to drink groundwater that exceeds 0.05 mg / l arsenic

which is minimum acceptable concentration for drinking. Researcher who conducted water tasting

presumed that about 1200, half of whole wells, contain high arsenic concentration.

 

 

 

3.Removal of arsenic in groundwater

The general methods for removing arsenic from drinking water are divided into four process. The table below show technologies depending on each process [7].

 

 

Process	Technologies
Precipitation	coagulation/filtration, direct filtration, coagulation assisted microfiltration, enhanced coagulation, lime softening, enhanced lime softening
Absorption	adsorption onto activated alumina, activated carbon and iron/manganese oxide based or coated filter media
Ion-exchange	Anion exchange
Membrane	Nano-filtration, Reverse Osmosis, Electrodialysis.

 

 

 

3.1 Precipitation processes

Adsorption co-precipitation with hydrolysing metals such as Al3+ and Fe3+ is the most common treatment technique for removing arsenic from water. Sedimentation followed by rapid sand filtration or direct filtration or microfiltration is used to remove precipitate. Coagulation with iron and aluminium salts and lime softening is the most effective treatment process. To improve efficiency of this method, a priory oxidation of As(III) to As(V) is advisable. Hypochlorite and permanganate are commonly used for the oxidation. Atmospheric oxygen can also be used, but the reaction is very slow [7].

 

 

3.2 Adsorptive processes

Adsorptive processes involve the passage of water through a contact bed where arsenic is removed by surface chemical reactions. Activated alumina, activated carbon, iron oxide coated or based filter media are used for these processes [7].

 

 

3.3 Ion exchange processes

In these processes, ions held electrostatically on the surface of a solid phase are exchanged for ions of similar charge dissolved in water. Usually, a synthetic anion exchange resin is used as a solid. Ion exchange removes only negatively charged As(V) species. If As(III) is present, it is necessary oxidise it [7].

 

 

 3.4 Membrane processes

 Microfiltration (MF), utrafiltration (UF), nano-filtration (NF), reverse osmosis (RO) and electrodialysis reversal (EDR) can remove arsenic through filtration, electric repulsion, and adsorption of arsenic-bearing compounds. The use of MF and UF membranes is dependent on the size distribution of arsenic bearing particles in water. To increase removal efficiency with a low percentage of particulate arsenic content, MF can be combined with coagulation processes. Nano-filtration membranes are capable of removing significant portions of the dissolved arsenic compounds in natural waters. Reverse Osmosis (RO) is very effective in removing dissolved arsenic. Electrodialysis reversal (EDR) can also be used for removal of arsenic. A water recovery of 85% is achievable. Reported arsenic removal varies from 28% to 86% In general, membrane filtration is more effective for removal As(V) than for As(III) [7].

 

 

3.5 DPHE-Danida Fill and Draw Units

It is a community type treatment unit designed and installed under DPHE-Danida Arsenic Mitigation Pilot Project. It is 600L capacity (effective) tank with slightly tapered bottom for collection and withdraw of settled sludge. The tank is fitted with a manually operated mixer with flat-blade impellers. The tank is filled with arsenic contaminated water and required quantity of oxidant and coagulant are added to the water. The water is then mixed for 30 seconds by rotating the mixing device at the rate of 60 rpm and left overnight for sedimentation. The water takes some times to become completely still which helps flocculation. The floc formation is caused by the hydraulic gradient of the rotating water in the tank. The settled water is then drawn through a pipe fitted at a level few inches above the bottom of the tank and passed through a sand bed and finally collected through a tap for drinking purpose as shown in Fig. 7. The mixing and flocculation processes in this unit are better controlled to effect higher removal of arsenic. The experimental units installed by DPHE-Danida project are serving the clusters of families and educational institutions.

 

 

 

Fig. 2 DPHE-Danida Fill and Draw Arsenic Removal Unit Attached to Tubewell

 

 

3.6 Arsenic removal methods used for household

Household level arsenic removal systems use adsorptive filtration or coagulation, ion exchange treatment or combination of coagulation and adsorption. Oxidation is sometimes used to improve As(III) removal efficiency. A comprehensive survey of POU arsenic removal systems based on a short-term performance test in terms of flow rate, storage capacity, breakthrough time, bacteriological performance, chemical use, costs, and user acceptability has been made by WaterAid. The results of this survey are presented in two reports (WaterAid, 2001a,b). UNESCO-IHE has developed a POU filter for arsenic removal with iron oxide coated sand (IOCS) as an adsorbent. The filter is simple, easy-to-use and does not require any chemicals. Alcan, Sidko (a granular ferric hydroxide filer system), READ-F and Sono are four commercial methods recently approved by the Government of Bangladesh for sale.. Good back-up and accepted methods for sludge disposal are essential for the operation of the POU systems (Arsenic project, 2007). Alcan and Sono filters are shown in figure below [7].

 

 

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Fig. 3 household

 

 

3.7 Emerging methods

 

3.7.1 Water pyramid

The water pyramid, developed for rural tropical areas, employs solar energy to produce potable water from saline, brackish or polluted water. The technology also removes fluoride. A water pyramid with a total area of 600 m2, placed under favourable tropical 82 Perspectives in Water Pollution conditions, can produce about 1250 litres of fresh water a day. The rate of production is however dependent on local atmospheric conditions such as climate, temperature, cloud-cover and wind activity. Solar energy drives the desalination while energy required for pressuring the WaterPyramid® is obtained using solar cells combined with a battery backup system. A small generator may be required to cater for intermittent peak demands in electricity [2].

 

 

 

Fig. 4 The WaterPyramid®

 

3.7.2 The Solar Dew Collector system

 The Solar Dew Collector system developed by Solar Dew is same as the WaterPyramid . This is a porous membrane that purifies water using solar energy. In this techniques water sweats through a membrane and evaporates on the membrane surface. This increases humidity in the evaporation chamber. As a result of temperature difference pure water condenses on the cooler surface of the system.

Larsen and Pearce, 2002, proposed a defluoridation method in which fluoride containing water is boiled with brushite ( CaHPO4.2H2O) and calcite ( CaCO3 ). Good results were obtained on laboratory scale. Larsen and Pearce concluded that boiling brushite and calcite in fluoritic water yields fluoroapatite which results in defluoridation [2].

 

                                              

                                                                                 Fig. 5 The Solar Dew Collector system

 

3.7.3 Memstill® technology

This technology advances ecology and economy of the existing technologies in brackish and sea water desalination. The technology also removes other anions such as fluoride and arsenic. In the Memstill® technology cold feed water takes up heat in the condenser channel through condensation of water vapour, then a small amount of (waste) heat is added, and flows counter currently back via the membrane channel. This small added heat evaporate water through the membrane. The water is discharged as cold condensate. The cooled brine is disposed, or extra concentrated in a next module. The Memstill® technology can produce potable water at a cost well below that of existing technologies like reverse osmosis and distillation. It is expected that the Memstill® technology will also be developed for small scale applications using solar heat [2].

 

 

 

Fig. 6 Memstill® technology

 

 

 

References

1. Groundwater Quality: Tanzania, WaterAid, 2001

2. Perspectives in Water Pollution; Chapter 4 Ground Water Contamination with Fluoride and Potential Fluoride Removal Technologies for East and

Southern Africa, InTech, 2013

3. US EPA. Water Treatment Technology Feasibility Support Document for Chemical contaminants. EPA-815-R-03-004, EPA 2003.

 

4. Dysart A. Investigation of Defluoridation Options for Rural and Remote Communities. Research Report No 41, The Cooperative Research Centre for Water Quality and Treatment, Salisbury SA 5108, AUSTRALIA 2008.

 

5. Zakia A, Bernard B, Nabil M, Mohamed T, Stephan N, Azzedine E. Fluoride removal from brackish water by electrodialysis. Desalination 2001; 133, 215 - 233.

 

6. http://www.bibliotecapleyades.net/salud/salud_fluor23.htm

 

7. http://www.un-igrac.org/dynamics/modules/SFIL0100/view.php?fil_Id=130

 

 

 

 

 

 

 

 

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