REMOVAL OF ARSENIC THROUGH MEMBERANE FILTERTION TECHNIQUE
Nowadays arsenic has become one of the major cause for groundwater and environmental pollution. Arsenic is a heavy metal and it is denoted as As. Arsenic is found abundantly on earth’s crust (20th in earth crust, 12th in human body, 14th in sea water). It is lethargic well-known toxic compound. Moreover its impact on people and other living organisms is fatal or my cause severe diseases. Arsenic is carcinogenic when taken in small amounts. Arsenic poisoning was one of the major problem due to the use of tube wells in ganga-delta regions of India. Inorganic form of arsenic is highly toxic to humans (especially in drinking water), while organic can be consumed in suitable quantities.
Some of the major source of arsenic is from pharmaceutical companies via medicines (disposal), feed additives, from dye materials, preservatives, smelting of metals, pesticides and insecticides manufacturing and their waste disposal, petroleum, coal and due to waste incineration.
In humans arsenic is found in nails (0.89 ppb), hair (0.18), bone (0.07-0.12), heart kidney, liver, lung (0.03-0.05) increase in these amounts may even cause death.
It may cause acute effects like anaemia, peripheral neuropathy, renal and liver dysfunction, skin pigmentation and chronic effects like Blackfoot disease, Raynaud’s disease, peripheral neuropathy, loss of hearing .etc. Adverse health effects like cancer, tumour may happen if arsenic is consumed in the form of drinking water.
IMPACT ON ENVIRONMENT:
Plants absorb arsenic from soil or ground water. Plants contain these arsenic in small trace amounts gradually which are then consumed by humans and animals. Similarly, groundwater is contaminated and consumed as drinking water or for cooking. Moreover, it is also used for general purposes like washing, cleaning and bathing this causes skin infection or if severe cancer also. This in turn effects the lakes, ponds and then the aquatic plants and animals they consume this and die or have them accumulated in small to large quantities which we may consume and have adverse health issues (eg fish). Nearly 20 major countries have been reported of arsenic poisoning from the use of ground water. In Bangladesh, arsenic poisoning became a huge problem due to mass poisoning and death of people. According to 2007 reports 137 million people from more than 70 countries are effected due to arsenic.
REMOVAL OF ARSENIC THROUGH MEMBRANE FILTERATION TECHNIQUE:
Membrane technology is one of the improving and convenient methods of filtration made by us with the use of technology. Membranes are synthetic materials with many pores that act as selective barrier. This membrane only allows selected particles to pass through while others are prevented from entering the barrier. For the particles to move or pass through there needs to be a force that is pressure difference on both the sides. There are different types for removal of arsenic like – nanofiltration (NF), reverse osmosis (RO), ultrafilteration (UF), microfiltration .etc
The pore size range of membrane working are High-pressure processes that is Nano filtration and Reverse osmosis. To remove constituents by chemical diffusion, while low-pressure, i.e. Micro filtration and Ultra filtration, they initially remove the constituents by the process known as sieving. The benefits of these two techniques is the tendency of removal of wider range of constituents when compared to others for example Micro filtration and Ultra filtration. Moreover, the increase in energy level and decrease in recovery are the main drawbacks of them.
The basic principle of MF and UF is physically separating them. The extreme to which dissolved solids and substances, turbidity and organisms like pathogens are removed is found out by the size of the pores of the membranes. Particles that are larger than the pores in the membranes are fully removed while particles that are smaller than the pores of the membrane are partially disposed off, focusing on the construction of a refusal layer on the membrane.
MF and UF are pressure interdependent processes, which will remove dissolved substances and other substances from water to a lesser extreme than NF and RO.
Application of Microfiltration in Water Treatment
Membranes of a pore size of 0.1 – 10 µm perform MF. MF membranes dispose off all bacteria like pathogens. Just only parts of the viral contamination is caught up in this process, all though viruses are smaller compared to the pores of a MF membrane. This is why viruses can attach themselves to bacterial surface or film.
MF can be applied in many various water treatment processes where particles with a diameter greater than 0.1 mm need to be removed from the sample or fluid.
Examples of MF uses are:
· Cold sterilisation of pharmaceuticals (preservation).
· Cleaning of fruit juice, wines and beer
· separating bacteria from water in wastewater treatment
· Effluent treatment or removal
· Separation of emulsions like water, oil and other emulsions and Pre-treatment of water for NF and RO.
While treatment of water or wastewater, where membrane technologies play a very crucial role. Different factors, like increase in the stringency of water quality applications that cannot be efficiently met by conventional treatment methods. As it reduces in quality freshwater supplies, improvement in membrane performances and lower costs. As a result of technological improvements and development have led to the use of membranes.
Due to high recovery (up to 99%) and low energy requirement, MF process has been used widely in water treatment. Membranes of MF have the largest pore size (0.1 µm to 10 µm). Removal of particles, pathogens like bacteria can be done by Dead-end MF with subsequent backwashing and it is preferred in the municipal applications due to the reason of reduced energy consumption and increased water recovery. But, since the deposition of materials on the membrane surface and also in pores which is otherwise known as membrane fouling, is the major disadvantage for its application. Moreover applying a pre-treatment one way to increase particle size and also to reduce membrane fouling. This technique will also improve the quality of water and its purity
Membrane processes can reduce or even dispose arsenic-bearing compounds through filtration and adsorption. Even though the size is a major important factor that influences rejection, studies have shown major rejection of arsenic containing compounds which are one or two orders of magnitude lesser than membrane pore size. Such inference or view indicate the presence of removal mechanisms rather than physical straining. The chemical properties, specifically charge and hydrophobic, of membrane material and source water constituents, play a crucial role in the removal mechanism by membranes. For example, a membrane with a fixed negative charge are found more efficient to dispose anionic arsenate more than a membrane that is uncharged, implying the inhibition of the passage of ions possessing the same charge. MF can remove particulate forms of arsenic, since this process cannot alone provide an efficient As removal unless a considerable percentage of As is in particulate form since Micro filtration performance is dependent on pore size. The strategy for As removal from bore well water was to incorporate As into ferric hydroxide pin flocs of suitable size to be removed with a microfiltration technique. This As removal increased with increase in coagulant dosage, but the increase in level of coagulant dose above 10 mg/L was very less effective. The size of As bearing flocs remained in the micron range at all coagulant doses, but only at highest coagulant dose that is (25 mg/L) significant portion of As removed was associated with floc size greater than 1 micron.
An investigation conducted experiments to evaluate the removal of As from groundwater by iron coagulation, adding it as ferric sulphate plus ferric chloride, and MF. The writers stated that the degree of rejections of precipitate particles by the membrane at a given ferric ion dose should increase with decrease in pH since increase in arsenic adsorption at low pH will lead to increase in particle size. They also observed a low residual turbidity in the presence of sulphate, suggested that the sulphate groups were involved in the coagulation reactions. The presence of sulphate groups can lead to larger flocs that can be more easily rejected by the membrane, when compared to chloride ions. But when applied flocculants (ferric chloride and ferric sulphate) before MF to remove As from drinking water, showed the membranes with a pore size of 0.22 µm remove more As than membranes with a pore size of 1.2 µm for a given ferric ion dose and ph.
After coagulating with ferric chloride, As removal efficiency was examined with various sizes of membrane filters (0.1, 0.5, and 1.0 µm) and without them. Remarkable decrease in As removal were observed with unfiltered samples, while variations in filter size had only minor effects on the removal of As.
For complete removal of viruses, UF is must. The pores of UF membranes can remove particles of 0.001 – 0.1 µm from liquids.
Examples of fields where UF is used are:
· dairy industry (milk, cheese, butter)
· food industry
· metal industry (oil/ water emulsion separation/removal units, paint treatment)
· textile industry or manufacturing unit
UF can also be used for pre-treatment of water for NF and Ro.
Pre-treatment of water is very crucial when these filtration technique are used, as membrane fouling can easily destroy the purification process. Pre-treatment is not only crucial for NF and RO process, but even also for the above mentioned Mf and UF processes. A pre-treatment needs to be found as soon as the composition of the wastewater is found out.
To prevent clogging or damaging of membranes by tough and sharp particles of the feed water, water needs to be pre-filtered before MF or UF processes takes place. The pores for the pre-filtration unit need to be of range 0.5 and 1.0 mm, depending on the composition of the wastewater, further pre-treatment will not be required when MF or UF is performed.
FURTHER TREATMENT IF REQUIRED
When wastewater is treated it needs to follow certain demands and rules. It depends on these demands if or not permeated needs additional treatments after MF or UF.
Purification processes that are suitable for additional treatment are:
· Quick filtration methods
·active carbon (C) filtration technique.
Since arsenic (As) has become a major problem for environment as well as living beings, it is necessary to remove them and have safe and healthy environment. For that we can use membrane filtration techniques as discussed above. This will improve the water quality as well as help in waste water treatment.