Nothing, Everything _ Soli Deo Gloria

water treatment of contaminated groundwater; Arsenic

2013. 7. 1. 22:40 자료공유/물, 인간의 최소한의 권리

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].


내용을 입력하세요		내용을 입력하세요

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

If this article is helpful to your research, click the button 'like'.

By doing this, I can share the article with more people.

저작자표시 비영리 동일조건

'자료공유 > 물, 인간의 최소한의 권리' 카테고리의 다른 글

알베도(albedo) - 빛을 반사하는 정도(반사율) (0)	2013.11.12
4대강 사업(낙동강)으로 인한 피해사례 및 원인과 대책 (1)	2013.11.10
Water treatment of contaminated groundwater(오염된 지하수처리) - Fluoride(불소) (0)	2013.06.18
빗물과 당신 - 4장 지하수에 섞여 있는 것들; 비소, 방사능, 불소 (0)	2013.06.01
아프리카 지하수 오염 실태(불소, fluoride) - 동, 서 아프리카 (2)	2013.06.01

Trackbacks / Comments

Water treatment of contaminated groundwater(오염된 지하수처리) - Fluoride(불소)

2013. 6. 18. 22:41 자료공유/물, 인간의 최소한의 권리

Water treatment of contaminated groundwater

1. Introduction

Groundwater is the main source of water supply worldwide. Especially local people in developing countries have increasingly used groundwater for daily life as well as for agriculture. Unfortunately, however, groundwater is not considered desirable for drinking because groundwater is sometimes contaminated with naturally occurring contaminates through several ways. Many who drunk the contaminated groundwater have experienced waterborne diseases, skin problem and even cancer. Therefore, this report focused on searching for status of groundwater contaminated by fluoride and arsenic and studied water treatment of contaminated groundwater to lower contamination levels of groundwater.

2. Major groundwater contaminants

2.1 Fluoride

Fluoride is a chemical that naturally occurs in a variety of locks and soils or deposition of atmospheric volcanic particles. Also fluoride can come from infiltration and runoff of chemical fertilizers in agricultural areas and liquids waste from industrial sources.

2.1.1 India

In India, ground mostly consisted of granites containing fluoride. When it rains, the element of fluoride with rainwater flows down to aquifer and then is dissolved into groundwater. According to report, about 6,000 people who used it for living life have experienced skin diseases and another symptom that could scarcely breath. Medical officer considered groundwater containing high fluoride levels as a reason of those diseases.

2.1.2 Africa

Groundwater is main resource for drinking in Africa. As we mentioned earlier, unfortunately, groundwater has been contaminated by fluoride, one of naturally occurring materials, which is contained in many kind of locks. In some areas of Africa amount of fluoride go beyond upper limit of 1.5 mg/l recommended by World Health Organization(WHO). The Fig. 2 shows fluoride levels of groundwater in Africa.

In Tanzania, fluoride is the most severe of the known water quality problems. The problem occurs in both the Rift zones in northen and south-western Tanzania, and in the crystalline basement complex of the central plateau. High concentration of fluoride have been found in soda lakes and some rivers, concentration of which is 60-690 mg /l and 12-26mg / l respectively.[1] According to the report, more than 30% of groundwater for drinking exceeds 1.5 mg/l fluoride. [2]

3. Water treatment of contaminated groundwater.

3.1 Removal of fluoride in groundwater

The general methods used for removal of fluoride from drinking water are mainly divided into three basic types depending upon process. The table below shows methods applied to removal of fluoride [6].

Process	Technologies
Chemical reaction with fluoride	Nalgonda technique, Lime
Absorption	Bone charcoal, processed bone, tricalcium phosphate, activated carbons, activated magnesia, tamarind gel, serpentine, activated alumina, plant materials, burnt clay
Ion-exchange	Anion/Cation exchange resins
Membrane	Reverse Osmosis, Electrodialysis

3.1.2 Nalgonda Technique

The Nalogonda technique employs flocculation principle 1. Nalgonda technique is a combination of several unit operations and the process invloves rapid mixing, chemical interaction, flocculation, sedimentation, filtration, disinfection and sludge concentration to recover waters and aluminium salts. Alum (hydrated aluminium salts) - a coagulant commonly used for water treatment is used to flocculate fluoride ions in the water. Since the process is best carried out under alkaline conditions, lime is added. For the disinfection purpose bleaching powder is added. After thorough stirring, the chemical elements coagulate into flocs and settle down in the bottom [6].

3.1.3 Precipitation

Method involving the addition in sequence, of an alkali, chlorine and aluminium sulphate or aluminium chloride or both was developed. It is cheap and is used extensively in India. Though lime softening accomplishes fluoride removal, its high initial cost, large dosage and alkaline pH of the treated water renders it unsuitable for field application. Large dosage and alkaline pH of the treated water renders it unsuitable for field application [6].

3.1.4 Bone Char

The uptake of fluoride onto the surface of bone was one of the early methods suggested for defluoridation of water supplies. The process was reportedly one of the ion exchange in which carbonate radical of the apatite comprising bone, Ca(PO4)6.CaCO3, was replaced by fluoride to form an insoluble fluorapatite. Bone char produced by carbonizing bone at temperature of 1100-1600ºC had superior qualities than those of unprocessed bone and hence replaced bone as defluoridating agent [6].

3.1.5. Contact Precipitation

It is a technique by which fluoride is removed from the water through the addition of calcium and phosphate compounds and then bringing the water in contact with an already saturated bone charcoal medium [6].


내용을 입력하세요		내용을 입력하세요

3.1.6. Degreased and alkali treated bones

Degreased and alkali treated bones are effective in the removal of fluoride from initial fluoride concentration ranging from 3.5 mg fluoride/L to 10 mg fluoride/L to less than 0.2 mg fluoride/L Bone contain calcium phosphate and has a great affinity for fluoride. The bone is degreased, dried and powdered. The powder can be used as a contact bed for removal of fluoride in water. The exhausted bed is regenerated with sodium hydroxide solution [6].

3.1.7 Serpentine

Serpentine is a mineral name, which applies to the material containing one or both of the minerals, chrysotile and antigorite1. The composition of the mineral closely corresponds to the formula Mg6Si4O10 (OH). The material is green or yellow and is available in Andhra Pradesh. To test the capacity of serpentine to remove fluorides from waters, the green and yellow varieties were studied for their defluoridation capacity. Extensive laboratory investigations were conducted with a view to popularize the mineral, if found suitable as a defluoridating medium. [6].

3.1.8 Activated Carbon

Most of the carbons prepared from different carbonaceous sources showed fluoride removal capacity after alum impregnation. High Fluoride removal capacities of various types of activated carbons had been reported. Alkali digested alum impregnated paddy husk carbon was an efficient defluoridating agent. Investigations have shown that carbonized saw dust when quenched in 2% alum solution forms an excellent defluoridating carbon. The defluoridating process is stoichiometric and equilibrium is established between carbon & fluoride. On exhaustion (after continued use) the carbon can be regenerated by passing 0.2 to 0.5% alum solutions. Activated carbon prepared by other workers from cotton waste, coffee waste, coconut waste etc., was tried for defluoridation but all these materials proved to be of academic interest only [6].

3.1.9 Membrane filtration

Of advanced water treatment technologies, Membrane filtration processes are the most advanced of the reported technologies that have been applied to in treatment of pure and ultra pure water. Reverse osmosis(RO), one of the best defluoridation technologies, is recommended by US EPA in 2003[3]. RO and nano-filtration (NF) are the well known membrane technologies that can remove a large spectrum of pollutants from water such as turbidity, salinity, pathogens, heavy metals, natural and synthetic organics, and hardness[4]. These processes are reported to be effective in in water defluoridation and produce high quality water during water treatment. NF membranes runs at lower pressure and have lower capacity as compared to RO membranes. electrodialysis is another membrane technology that is applied to big scale plant water treatment of high fluoride brackish water for drinking water supply[5]. Electrodialysis is same as RO, except that it uses an applied direct current potential instead of pressure, to separate ionic contaminants from water. Water does not physically go through the membrane in the electrodialysis process as such a particulate matter is not eliminated. Thus, the ED membranes are not technically regarded as filters. Comparing to RO, the water quality from electrodialysis treatment may require post-treatment stabilization. The process tends to be most economical for source water with TDS levels in excess of 4,000 mg/L. It is established that RO and electrodialysis have very high defluoridation capacities (85 – 95 %) and function effectively in any pH range. However the water loss is high (20 - 30 % for electrodialysis, 40 – 60 % for RO), have high capital cost and are energy intensive [5]. For membrane technologies, It often require special instrument, electrical energy and specialized training for operators as such the capital and operation costs are high. Low applicability is therefore envisaged for rural sectors of the developing countries where energy and trained human resource are often deficient [6].