Insitu vs Other filtration techniques

In-situ Treatment vs Filtration Techniques:

Comparison of Main Arsenic Removal Technologies
Technology	Advantage	Disadvantage
Oxidation and sedimentation: air oxidation, chemical oxidation	Relatively simple, low cost, but slow process (air) Relatively simple and rapid process (chemical) Oxidizes other impurities and kills microbes	Processes remove only some of the arsenic Used as pretreatment for other processes
Coagulation and filtration: alum coagulation, iron coagulation	Relatively low capital cost Relatively simple in operation Common chemicals available	Not ideal for anion-rich water treatment (e.g. containing phosphates) Produces toxic sludge Low removal of As(III) Preoxidation is required Efficiencies may be inadequate to meet strict standards
Sorption techniques: activated alumina, iron-coated sand, ion exchange resin, other sorbents	Relatively well known and commercially available Well-defined technique Many possibilities and scope for development	Not ideal for anion-rich water treatment (e.g. containing phosphates) Produces arsenic-rich liquid and solid wastes Replacement/regeneration is required High-tech operation and maintenance Relatively high cost
Membrane techniques: nanofiltration, reverse osmosis	Well-defined and high removal efficiency No toxic solid wastes produced Capable of removal of other contaminants	High capital and running costs High-tech operation and maintenance Arsenic-rich rejected water is produced
Source: World Bank Technical Report, Volumn II: Arsenic Contamination of ground water in South and East Asian countries

In-situ treatment Vs Filtration Techniques
In-situ Technique	Point of Difference	Filter techniques
After treating water for 45 days, As & Fe content went well below WHO guidelines. Fe content in the water is a boon to this technique.	Efficiency	The expensive filters get clogged up with iron and needs recharging/re-activation/change within a few months of installation.
With a little maintenance, it can run for 10 years. But within that time, the aquifer will be treated. So, a normal shallow tube-well beside the plant will deliver arsenic free water.	Longivity	Not as long as 10 years. They require frequent sludge removal & recharge every 4-5 month.
1 to 2 months after starting of the plant.	Start-up time	No start up time required.
$ 3 per month	Maintenance Cost	Cost of recharging or replacing the filters every few months. This varies with the filtration techniques, the chemicals required and the Iron & Arsenic concentration in water.
Village plumbers & electricians can maintain it.	Expertise to maintain	Specially trained technicians & particular chemicals are required for maintenance.
Operators with basic training working 2 hrs per day will be required with minimum salary of $ 0.75 per hr.	Operation	A caretaker may be needed.
Highly sustainable process. Not only the pumped up water, but also the aquifer is treated for arsenic & iron.	Sustainability	No sustainability issue. The water coming out from the filter is treated only.
The ground water is being restored to its previous arsenic free condition.	Natural resource restoration	Only the pumped up ground water is treated.
Minimum iron precipitate generation for 3 months. After that, waste generation is nil.	Waste generation	A cosiderable quantity of iron & arsenic sludge is produced every day. The waste management becomes a problem.

The Solutions:

There are broadly three ways to access clean water in places where arsenic is found in the ground. The first is to tap from a clean source, the second is to clean the source (in-situ treatment) and the third is to clean the polluted water (post-treatment of polluted water). There is a wide range of solutions that fit in one of the three.

1. Tapping clean source

a. Rainwater harvesting

The harvesting of rainwater seems to be the most sustainable way to access clean water. The source may not last the whole dry season, however, and therefore promotion of rainwater harvesting will need to be combined with other solutions. Good designs of rainwater tanks are available and at relatively low cost (Howard, 2003).

The main risks are:

i. The feaces that gets in the tank, especially from birds, but this is relatively easy to deal with (ibid.).

ii. Besides, close to urban areas, and when metal roofs are used, collected rainwater can contain unsafe levels of lead and zinc, and possibly other metals (Johnston et al., 2001).

iii. Some users don’t like the taste of the water, (The World Bank, 2005)

iv. That it has been reported from Bangladesh that the return to rainwater harvesting may be viewed as a step backwards to several decades ago when it was quite widely used. (The World Bank, 2005)

b. Surface water

The per capita available surface water in arsenic affected areas of West Bengal is about 7000 cubic meters (Hossain et al., 2005). During the monsoons, the average annual rainfall in this region is about 1600 mm (ibid.). In addition, West Bengal is richly endowed with other available surface water resources such as wetlands, flooded river basins, lagoons, ponds, and ox-bow lakes (ibid.). This available surface water can be tapped as an important source of drinking water.

Risks are:

i. Surface water is often heavily polluted with faeces as a result of poor sanitation and hygiene

ii. It may also be contaminated with chemicals from industrial or agricultural runoff, such as heavy metals, pesticides, phosphate or nitrate.

iii. Surface water is usually free from arsenic contamination. However, there are cases where surface water was contaminated because the source of the water originates from arsenic rich rocks (Johnston et al., 2001) or waters affected by mining activities (World Bank, 2005).

iv. Surface water always needs to be purified. Usually, it is best to include multiple barriers to purify surface water (Johnston et al., 2003).

v. The water might still be needed to be disinfected to kill pathogens by boiling, ultraviolet (solar or artificial) radiation (e.g. Acra et al., 1989; EAWAG, 1999), or chlorination (see Singer, 2000; WRC, 1989; WHO, 1997b). All these processes are complex enough to apply in the rural areas.

c. Dug wells or ring wells:

Dug wells are traditionally the most wellknown method of groundwater use. The water from dug wells has been found to be relatively free from dissolved arsenic and iron, also in locations where neighbouring tube wells are severely contaminated (World Bank, 2005). The reasons for the relatively low concentrations of arsenic in dug wells are not fully known, but possible explanations include (ibid.):

• The water in the dug well slowly oxidizes due to its exposure to open air, large diameter and agitation during water withdrawal which can cause precipitation of dissolved arsenic and iron (ibid.).

• Dug wells accumulate groundwater from the top layer of a water table, which is replenished each year by arsenic-safe rain and percolation of surface waters through the aerated zone of the soil (ibid.).

Risks are:

i. Construction of such wells with cement ring walls provide bacteria free water, if the place is sunny and without trees. Caution should be taken however; the water should be well prevented from bacterial contamination etc. Recommended is to completely seal the well and withdraw the water by a hand pump. However, the lack of oxygen then might put the oxidation process at risk.

ii. The water can be treated further with simple sand filters, or chlorination for disinfection.

iii. Care has to be taken however, despite the tendency for low arsenic concentrations in dug well waters, not all are found to be below acceptable limits (World Bank, 2005).

iv. They may run out of water supply during the dry season.

d. Deep tube wells

Deep tube wells are an attractive option. The middle-level aquifer contaminated with arsenic is passed over and the risks on microbial hazards are low because of the natural filtering of aquifer materials, and long underground retention times (Johnston et al., 2001).

Risks are:

i. Questions arise on the sustainability in terms of arsenic leaching into the deep layer

ii.The sinking of water table due to exploitation of water.

iii. There is still uncertainty on the arsenic movement in the sub-surface and the scale and degree of arsenic contamination in the deep aquifer (ibid.).

iv. Rapid depletion of deep aquifers results in a deleterious influx from the Ascontaminated aquifer above. Intensive efforts to provide deeper tube-wells for supplying drinking water may be counterproductive if the aquifer is simultaneously depleted by irrigation demands.

2. Pre-treatment (in situ treatment): Clean the Source

The technology that is tested in the TIPOT project / DM06-880 is an in-situ treatment. Quoting World Bank (2005) on this technology: “In situ oxidation of arsenic and iron in the aquifer has been tried in Bangladesh under the Arsenic Mitigation Pilot Project of the Department of Public Health Engineering (DPHE) and the Danish Agency for International Development (Danida). The aerated tube well water is stored in feed water tanks and released back into the aquifers through the tube well by opening a valve in a pipe connecting the water tank to the tube well pipe under the pump head. The dissolved oxygen in water oxidizes arsenite to less-mobile arsenate and the ferrous iron in the aquifer to ferric iron, resulting in a reduction of the arsenic content in tube well water. Experimental results show that arsenic in the tube well water following in situ oxidation is reduced to about half due to underground precipitation and adsorption on ferric iron. The method is chemical free and simple and is likely to be accepted by the people but the method is unable to reduce arsenic content to an acceptable level when arsenic content in groundwater is high.” Johnston et al. (2001) state that the technique should be considered with caution. First they state that oxidants are by definition reactive compounds, and may have unforeseen effects on subsurface ecological systems, as well as on the water chemistry. Secondly, they mention that care must be taken to avoid contaminating the subsurface by introducing microbes from the surface. Finally, at some point pore spaces can become clogged with precipitates, particularly if dissolved iron and manganese levels are high in the untreated water.

DM06-880 delved deeper into this technology and applied this to field with slow but steady success. The main fault of the former work that were undertaken for in-situ treatment were:

i. High & extremely quick recharge rate

ii. Uncalculated & uncontrolled recharge volume

iii. Unscientific Fe:As ratio

The iron-arsenic flocs produced from this SAR Technology are so small in structure that they are not capable of clogging the aquifers.

Once all of these issues were taken care of, the technology worked better.

3. Post-treatment of arsenic rich water

Many solutions are found to remove arsenic from the water. There are many sources that describe and compare the various technologies, for instance Parga et al. (2005), Johnston et al. (2001). Parga et al. (2005) describe that the removal efficiency for arsenic is often much lower for As(III) than for As(V) by using anyone of the conventional technologies for elimination of arsenic from water, so either elevation of pH or oxidation of arsenite to arsenate is considered a prerequisite for any treatment method to be efficient. The most common arsenic removal technologies are grouped into the following four categories:

• Oxidation

• Coagulation

• Sorptive filtration

• Membrane filtration

These filters pump up the arsenic contaminated water on the earth surface; pass it through chemical filter beds and trap the arsenic. Once the chemicals get exhausted, the filters stop working. They need to be recharged or replaced. Also, due to high iron content of ground water, they get clogged very easily & the toxic wastes they are filtering off may enter the food chain after its disposal. None of the above techniques provide a long term sustainable solution to this problem.

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