Technologies for Arsenic, Fluoride and Iron Removal
2.2 HOUSEHOLD LEVEL TECHNOLOGIES
2.2.1 Household Level Technologies for Arsenic Removal
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However, spontaneity of the biosorption decreased with increase in the metal concentration from 5 to 50 ppm. For metal concentrations higher than 50 ppm, the adsorption became non-spontaneous. Scanning electron microscopic (SEM) images of wheat straw were also taken to examine the surface structure of the wheat straw along with the energy dispersive spectrum (EDS) analysis. The results obtained confirmed the adsorption of Ni2+ and Zn2+ on wheat straw, and showed that the inner surface of the wheat straw appeared to provide more adsorption sites for metal binding.
Technologies for Arsenic, Fluoride and Iron Removal
household level removal technologies are still under development in order to reduce the concentration of arsenic from groundwater. Some of the promising household level technologies are discussed below.
2.2.1.1 Star Filter
The star filter was developed by the Stevens Institute of Technology (Ahmed, 2001). The patented technology consisted of two buckets, first one to mix chemicals and the other one to separate flocs. Mixing of chemicals (iron sulphate and calcium hypochloride) was carried out in the first bucket (Ahmed, 2001, Hemda and Huw, 2009). The water was then transferred to the second bucket where coagulated flocs were separated by the process of sedimentation and filtration. Filtration was carried out by a sand filter placed at the bottom of the lower bucket. The treated water was then stored in a container via a tube. The technology was able to reduce the level of arsenic to less than the guideline value (WHO, 1993). The sand bed used for filtration was getting clogged quickly by flocs and required washing at least twice a week.
2.2.1.2 Two-Bucket Treatment Unit
The bucket treatment unit developed by the Department of Public Health Engineering (DPHE) and Danish International Development Agency (DANIDA) Project and further improved by the Bangladesh University of Engineering and Technology (BUET) was based on coagulation, co-precipitation, and adsorption processes (Sarkar et al., 2000; Tahura et al., 2001). It consisted of two buckets each with a capacity of 20 L and placed one above the other. Chemicals were mixed manually with arsenic-contaminated water in the upper bucket by vigorous stirring with a wooden stick and then flocculated by gentle stirring for about 1-2 min. Chemicals added were mixture of coagulant (e.g. alum, ferric chloride or sulphate) and oxidant (potassium permanganate, calcium hypochloride) in crushed powder form. The mixed water was allowed to settle, passed into the lower bucket and then water was collected through a sand filter installed in the lower bucket (Sarkar et al., 2000; Tahura et al., 2001). Although the unit was very efficient in arsenic removal but residual
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concentrations of aluminum and manganese in the water were of concern. Aluminum was suspected as a neuro-toxic and manganese might have synergistic effect on aluminum toxicity (Tahura et al., 2001). Sand filter in this system needed regular washing, along with other different parts of the unit because carelessness might promote bacterial contamination of the unit posing another health risk to the users.
2.2.1.3 Sono 3–Kolshi
The Sono 3-Kolshi (or three-pitcher) filter used zero-valent iron filings (cast- iron turnings), sand, brick chips and wood charcoal to remove arsenic and other trace metals from groundwater (Munir et al., 2001; Khan et al., 2000). The filtration system consisted of three kolshi (burnt clay pitchers) widely used in Bangladesh for storage of drinking and cooking water. The top kolshi contained 3 kg cast-iron turnings and 2 kg sand on top of the iron turnings. The middle kolshi contained 2 kg sand, 1 kg wood charcoal, and 2 kg brick chips. Brick chips were also placed around the holes to prevent leakage of finer materials. Tubewell water was poured in to top kolshi and filtered water was collected from the bottom kolshi. The three-pitcher filter was found to be very effective in arsenic removal for the first four to six weeks. Normally the three- pitcher filter needed to be replaced after 3-4 months of operation due to decrease in efficiency as well as clogging and hardening of iron filings used in the first kolshi which could not be removed. Although this method was effective in arsenic removal, the bacteriological contamination in effluent water was sometimes high as the open filter media encouraged growth of micro-organisms.
2.2.1.4 BUET Activated Alumina Unit
The BUET activated alumina arsenic removal unit (ARU) consisted of subunits for oxidation-sedimentation, filtration and activated alumina adsorption (Feenstra et al., 2007; David et al., 2008). Oxidation and sedimentation was performed in a 25 L plastic bucket. Approximately 1 mg/L potassium permanganate was added to the water in the bucket to oxidize As(III) to As(V); the mixture was stirred vigorously with a wooden stick and then allowed to settle for about 1 h. The settled water was filtered through a
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sand bed and was then passed through the activated alumina column. Lack of personal hygiene of users might result in bacterial contamination of the treated water needing weekly chlorination or hot water treatment of the ARU. The height of the unit, difficulty in flow control and inadequate flow rate were some of the limitations needed to be corrected to increase its acceptability by users.
2.2.1.5 Alcan Enhanced Activated Alumina Unit
This technology was widely used in field conditions in Bangladesh, where it gave good results as far as arsenic removal was concerned (Ahmed, 2001). The sorbent used was enhanced activated alumina – the ActiGuard media, which differed from activated alumina by its greater surface area and larger internal porosity. Therefore, more sites were available for contaminants adsorption, which was consequently more efficiently removed from the water. The filtration units were designed for a single family use or a larger scale application. It was made of two plastic buckets or tanks placed side by side on a stand, the first being gently elevated compared to the second one. The first tank contained an enhanced activated alumina filter media which treated the water previously poured in it. The bottom of the tank was provided with a pipe that connected both the tanks. The water was allowed to flow upwards through other ActiGuard media placed in the second tank before being collected by the means of a tap placed at the top of the second tank. Alcan’s enchanced activated alumina unit was designed for single use and therefore saturated media needed to be disposed off in an environmentally safe manner.
A number of household treatment technologies for the removal of arsenic (Alcan, BUET, DPHE/DANIDA, Sono, Stevens) were each evaluated using water from 63 different tube wells taken from 3 different regions of Bangladesh (Sutherland et al., 2002) having influent arsenic concentrations up to 600 µg/L. The more advanced treatment methods using: activated alumina (Alcan, BUET); metallic iron (Sono) and iron coagulation (Stevens) were found to be most easily used and efficiently reduced arsenic concentrations to below the Bangladesh drinking water standard (0.05 mg As/L). The use of aluminium sulphate coagulants and permanganate oxidants in the
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DPHE/DANIDA technology introduced unacceptably high concentrations of aluminium and manganese into the treated waters and were not recommended in household water treatment applications. While arsenic concentrations were initially considered to be of paramount importance, it became clear that such technologies could increase the risk of bacterial contamination in the treated water and this needed serious consideration as this could create a hazard much greater than the arsenic contained in the water.
2.2.1.6 SAFI Filter
The SAFI filter – a type of household candle filter – was made of composite porous materials such as kaolinite and iron oxide on which hydrated ferric oxide was deposited by sequential chemical and heat treatment (David et al., 2008). The candle filter worked on the principles of adsorption and filtration on chemically treated active porous composite materials. The oxyhydroxides of Fe, Al and Mn assisted in removal of arsenic, iron and bacteria. The SAFI filter was reported to have good arsenic removal capacity initially, but efficiency declined with time. Moreover, the filter media used to get clogged and the unit suffered rapid erosion from mechanical cleaning indicating poor workmanship (David et al., 2008). The filter candle, in many cases, was found to leak at joints and used to disintegrate because of inadequate strength.
2.2.1.7 Well-head Arsenic Removal Units
Activated alumina was also used in more than 135 well-head arsenic removal units which were installed in remote villages of West Bengal (India) bordering Bangladesh (Sudipta et al., 2005). Each well-head arsenic removal unit containing about 100 L of activated alumina was designed to serve approximately 200–300 households without any chemical addition, pH adjustment or electricity for operating these units. The arsenic concentration in the influent varied from around 100 µg/L to more than 500 µg/L while the treated water contained arsenic concentration well below 50 µg/L. The units were capable of removing both As(III) and As(V) from the contaminated groundwater for several months, often exceeding 10,000 bed volumes. In
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the top portion of the unit, the dissolved iron present in groundwater was oxidized by atmospheric oxygen which in turn selectively bonded both As(III) and As(V). Upon exhaustion, these units were regenerated by caustic soda solution followed by acid wash. Many units had operated for several years without any significant operational difficulties. The treated water was used for drinking and cooking purposes.
2.2.1.8 IHE Arsenic Removal Family Filter
UNESCO-IHE had been developing an arsenic removal family filter with a capacity of 100 L/day based on arsenic adsorption onto iron oxide coated sand – a by- product of iron removal plants (Petrusevski et al., 2008). The longer term and field conditions performance of the third generation of eleven family filters prototypes were tested in rural Bangladesh for 30 months. All filters achieved initially highly effective arsenic removal irrespective of arsenic concentration and groundwater composition.
Arsenic level in filtrate reached 10 µg/L after 50 days of operation at one testing site and after 18 months of continuous operation at other 3 testing sites. Arsenic level at other 7 sites remained below the WHO guideline value (WHO, 1993). Positive correlation was observed between arsenic removal capacity of the filter and iron concentration in groundwater. In addition to arsenic, iron present in groundwater at all testing sites was also removed effectively. Manganese removal with IHE family filter was effective only for groundwater with low ammonia concentration. A simple polishing sand filter after IHE family filter resulted in consistent and effective removal of manganese. IHE family filters were easy to operate and were well accepted by the local population.
2.2.1.9 Kanchan Arsenic Filter
In order to combat crisis of arsenic in drinking waters of Nepal, Massachusetts Institute of Technology (MIT), Environment and Public Health Organization (ENPHO), Kathmandu, Nepal and Centre for Affordable Water and Sanitation Technology (CAWST) together researched, developed and implemented a household water treatment technology known as Kanchan Arsenic Filter (KAF) (Ngai et al.,
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2007). A two-year technical and social evaluation of over 1000 KAFs deployed in rural villages of Nepal revealed that the KAF typically removed 85-90% arsenic, 90-95%
iron, 80-95% turbidity, and 85-99% total coliforms. The 83% of the households continued to use the filter after 1 year mainly motivated by the clean appearance, improved taste, and reduced odour of the filtered water as compared to the original water source.
2.2.1.10 Household Sand Filter
Arsenic removal efficiencies of 43 household sand filters were studied in rural areas of the Red River Delta in Vietnam by Berg et al. (2006). In addition, raw groundwater from the same households and other 31 tube wells were also investigated simultaneously to find out the arsenic co-precipitation with hydrous ferric iron from solution without contacting sand surfaces. From the groundwater containing 10-382 µg/L As, < 0.1-48 mg/L Fe, < 0.01-3.7 mg/L P, and 0.05-3.3 mg/L Mn, average As removal rates of 80% and 76% were observed for the sand filter and co-precipitation experiments respectively. The concentration of dissolved iron in groundwater was the decisive factor for removal of arsenic. Residual arsenic levels below 50 µg /L were achieved by 90% of the studied sand filters, and 40% were even below 10 µg /L. Fe/As ratios of greater than or equal to 50 or 250 were required to ensure arsenic removal to levels below 50 or 10 µg/L respectively. Phosphate concentrations greater than 2.5 mg/L slightly hampered the sand filter and co-precipitation efficiencies. Interestingly, the overall arsenic elimination was higher than predicted from model calculations based on sorption constants determined from co-precipitation experiments with artificial groundwater. The investigated sand filters were fast to operate and robust for a broad range of groundwater composition and were a viable option for mitigation in other arsenic affected regions.