CA2327065A1 - In-situ removal of arsenic from well water - Google Patents

Abstract

An in-situ well water system is described which removes arsenic from ground water contaminated by arsenic containing soils. A reactive, adsorptive barrier, consisting of iron containing material and filter media such as soils, sand and the like is described which surrounds the screened casing of the well. The system described can be constructed manually by relatively unskilled labour and most importantly does not produce a hazardous arsenic containing waste requiring disposal. The appropriate selection of materials, the physical dimensions, adjustment of the permeability through the use of dual media, reducing nature of the well water and the water head available determine the quantity and dimensions of the barrier.

Description

IN-SITU REMOVAL OF ARSENIC FROM WELL WATER
FIELD OF THE INVENTION
This invention relates to a process for the in-situ removal of arsenic from ground water, particularly the removal of arsenic from said water to provide potable drinking water, using an iron-containing material, and to apparatus and systems for carrying out said process.
BACKGROUND TO THE INVENTION
Arsenic drinking water adversely affects the health of many peoples throughout the world. For example, it affects up to 85% of the population of 120 million people in Bangladesh.
A United Nations survey found arsenic levels in drinking water supplies which approached 2 mg/L in northern regions, while the level of arsenic in drinking water which is considered safe has been set at 0.01 mg/L by the WHO. There are other parts of the world which face similar problems of arsenic contamination. Thus, there is a need for a water treatment system which will decontaminate water in a manner which is suitable for all peoples, particularly the citizens of the developing world.
Arsenic can be removed by a number of technologies, such as, for example, Reverse Osmosis, distillation, ion exchange ,chemical precipitation, and the like, but these technologies have inherent limitations that prevent widespread use in a developing country.
Such limitations, for example, are high capital cost, chemical usage, toxic waste byproducts including the arsenic removed from the water, and technology that cannot easily be understood, maintained, or operated by the local people. As an example, Reverse Osmosis, which is commonly used in North America and would be capable of providing arsenic-free water, requires an electrical supply. In the RO system, pressure is required to force pure water through the membrane, which leaves impurities in concentrated form in the residual liquid. Pumps to deliver the pressure are an integral part of the system. Thus, the RO technology requires electricity, high costs membranes, pumps and plumbing and has a concentrated residual waste stream which must be disposed of in an environmentally friendly manner.

In September, 1993, the Environmental Protection Agency in the United States developed, with contractor support, a document entitled "Treatment and Occurrence-Arsenic in Potable Water Supplies". This document summarized the results of pilot-scale studies examining low-level arsenic removal, from SO parts per billion (ppb) down to 1 ppb or less. EPA convened a panel of outside experts in January 1994 to review this document and comment on the ability of the technologies to achieve maximum contaminant levels (MCLs) under consideration. Key findings of this report are summarized below. EPA is in the process of gathering new information with contractor support on these technologies to update the report. Information on prospective technologies were obtained from more recent studies and the results of the studies are also summarized below.
Coagulation/Filtration (C/F), according to laboratory and pilot-plant tests, is an effective treatment process for removal of As(V). The type of coagulant and dosage used affects the efficiency of the process. At either high or low pH ranges, the efficiency of C/F is significantly reduced. Alum performance is slightly lower than ferric sulfate, as are other coagulants.
Disposal of the arsenic-contaminated coagulation sludge may also be a concern, especially if nearby landfills are unwilling to accept such a sludge.
Lime Softening (LS) operated within the optimum pH range of greater than 10.5 is likely to provide a high percentage of As removal for influent concentrations of 50 micro;g/L.
However, it may be difficult to reduce as levels consistently to 1 micro;g/L
by LS alone.
Systems using LS may require secondary treatment to meet that goal. However, these systems are not appropriate for most small systems - in view of the high cost, need for well trained operators, and variability in process performance. Further, CF & LS alone may have difficulty consistently meeting a low-level MCL. Yet further, disposal of sludge may be a problem.
Activated Alumina (AA) is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride, and sulfate, if present at high levels, may compete for adsorption sites. AA is highly selective towards As(V); and this strong attraction results in regeneration problems, possibly resulting in 5 to 10 percent loss of adsorptive capacity for each run. Application of point-of use treatment devices would need to consider regeneration and replacement. This system also suffers from several disadvantage such as the lack of availability of F-1 alumina; testing of substitute not yielding same results; chemical handling requirements that may make this process too complex and dangerous for many small systems.
AA may also not be efficient in the long term, as it seems to lose significant adsorptive capacity with each regeneration cycle. Further, highly concentrated waste streams-disposal of brine may be a problem.
Ion Exchange (IE) can effectively remove arsenic. However, sulfate, TDS, selenium, fluoride, and nitrate compete with arsenic and can affect run length. Passage through a series of columns could improve removal and decrease regeneration frequency. Suspended solids and precipitate iron can cause clogging of the IE bed. Systems containing high levels of these constituents may require pretreatment. However, the following matters should be considered.
Highly concentrated waste by-product stream-disposal of brine may be a problem. Brine recycling might reduce the impact. Sulfate levels can affect run length.
Recommended as a BAT
primarily for small, ground water systems with low sulfate and TDS and as the polishing step after filtration for low-level options.
Reverse Osmosis (RO) provides removal efficiencies of greater than 95 percent when operating pressure is at ideal psi. If RO is used by small systems, for example, in the western United States, 60% water recovery may lead to an increased need for raw water.
Water recovery is the term used to mean the volume of water produced by the process divided by the influent stream (product water/influent stream). Discharge of reject water or brine may also be a concern.
Further matters to be considered are as follows.
Extensive corrosion control could be required for low-level option - ability to blend would be limited.
Water rejection (about 20 - 25 percent of influent) may be an issue in water-scarce regions.
Electrodialysis Reversal (EDR) may be expected to achieve removal efficiencies of 80 percent. One study demonstrated arsenic removal to 3 micro;g/L from an influent concentration of 21 micro;g/L. However, water rejection (about 20 - 25 percent of influent) may be an issue in water-scarce regions, and this technique may not be competitive with respect to costs and process efficiency when compared with RO and NF, although it may be easier to operate.
Nanofiltration (NF) is generally capable of arsenic removals of over 90%, with recoveries ranging between 15 to 20%. A recent study showed that the removal efficiency dropped significantly during pilot-scale tests where the process was operated at more realistic recoveries.
If nanofiltration is used by small systems in the western U.S., again water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.

Point of Use/Point of Entry (POU/POE). The 1996 SDWA amendments specifically identify Point-of Use (POU) and Point-of Entry (POE) devices as options that can be used for compliance with NPDWRs. POU and POE devices can be effective and affordable compliance options for small systems in meeting a new arsenic MCL. A Federal Register notice is being prepared by EPA to delete the prohibition (§141.101) on the use of POU
devices as compliance technologies, and because of this prohibition, few field studies exist on the application of POU and POE devices. One such case study was performed by EPA, in conjunction with the Village of San Ysidro, in New Mexico (Rogers 1990). The study was performed to determine if POU Reverse Osmosis (RO) units could satisfactorily function in lieu of central treatment to remove arsenic and fluoride from the drinking water supply of a small rural community of approximately 200 people. A RO unit, a common type of POU
device, is a membrane system that rejects compounds based on their molecular properties and characteristics of the reverse osmosis membrane. The RO units removed 96% of the total arsenic. However, adopting a POU/POE treatment system in a small community requires more record keeping to monitor individual devices than does central treatment. POU/POE systems require special regulations regarding customer responsibilities, water utility responsibilities, and the requirement of installation of the devices in each home obtaining water from the utility.
Waste Disposal - Disposal of the arsenic-contaminated coagulation sludge from the C/F
and LS technologies may also be a concern. For large treatment plants, a large body of water would likely be needed to discharge the contaminated brine stream from the RO/NF
technologies. Inland treatment plants would possibly need either some pretreatment prior to discharge or would need to discharge to the sanitary sewer due to the increase in salinity.
Discharge to sanitary sewers may required pretreatment to remove high arsenic levels. The waste streams produced by IE/AA technologies are highly concentrated brine with high TDS.
These brine streams may require some pretreatment prior to discharge to either a receiving body of water or the sanitary sewer.
It is clear from the aforesaid prior art technologies, that there is, therefore, a need for an arsenic-contaminant in ground water removal system readily adaptable for use in the developing world.

SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system to remove arsenic from ground water and other waters in-situ such that no hazardous waste is generated.
It is a further object to provide a system to remove arsenic from ground water and other waters in-situ such that water of drinking water quality is produced by the well.
It is a further object to provide a system to remove arsenic from ground water and other waters in-situ such that no chemicals or maintenance are required over the lifetime of the well.
Routine maintenance of a well pump which is independent of this well system may be required.
It is a further object to provide a system to remove arsenic from ground water and other waters in-situ such that construction of the system may be carried out manually.
It is a further object to provide a system to remove arsenic from ground water and other waters in-situ such that an existing well system may be used if a container or canister holding a reactive barrier material is used at the pump inlet. In this configuration less arsenic removal capacity would be available and replacement of the canister would be periodically required.
It is a further object to provide a system to remove arsenic from ground water and other waters in-situ such that other ground water contaminants such as anthropogenic chlorinated chemicals and heavy metals may also be removed.
Accordingly, in one aspect the invention provides a method for producing arsenic-free well water from arsenic-contaminated ground water, said method comprising passing said ground water through a well wall defining a well chamber and comprising particulate elemental iron; and removing said arsenic-free well water from said well chamber.
The term "passing in" in this specification includes the drawing of water through the well wall from the surrounding earth into the well chamber under the influence or effect of means of removing the well water by human, animal or mechanical bucket, siphon, pump or like means, which removal causes water permeation by the pressure of the ground water through the wall into the well chamber.
The system of the present invention uses apparatus to treat the ground water, particularly well water, which at depth is anoxic or in an anaerobic environment.

Preferably, the particulate iron is in admixture with a particulate, inorganic filler material, such as, for example, a mineral material, for example, a sand. The admixture, also known as a reactive, matrix, contains about 5 - 95 W/W % iron, and preferably 40 - 60 W/W
% iron.
The particulate materials are preferably retained as a well wall by use of a water permeable lining, such as, for example, a perforated polyvinyl chloride casing which lines the well wall. This enables the ground water to permeate through the reactive matrix and PVC lining into the well chamber.
In preferred embodiments, the admixture is retained within a particulate matter-retaining member having an inner wall and a water permeable outer wall, said walls define a particulate matter retaining chamber therebetween for retaining said admixture, and wherein said inner wall constitutes said water permeable lining.
The apparatus, thus, preferably comprises a tubular well wall enclosing the well shaft, bore, chamber and the like at a selected desired depth.
The shape, form and physical and chemical characteristics of a preferred hereinafter defined admixture are determined by the chemical and electrochemical characteristics of the ground water, and hydrological and physical characteristics of the well site.
Any source of particulate elemental iron, such as, for example, scale from a steel mill, turnings from a metal workshop, milled steel wastes or other wastes as well as iron filings can be used.
I have found that no sulphate is required in the process of the invention to produce a stable arsenic compound for removal from the ground water. Arsenopyrite (FeAsS) is not formed. The process according to the invention takes place under reducing conditions to provide co-precipitated compounds and iron arsenides (FeAs and FeAsz) depending on the pH and Eh characteristics of the water.
In a further aspect, the invention provides apparatus for producing arsenic-free well water from arsenic-containing ground water present in earth, said apparatus comprising a well wall within said earth and defining a chamber for receiving said arsenic-free water; said wall comprising a water permeable lining and elemental iron in particulate form in communication with said lining and said earth. It can be readily seen, that the well shaft, bore and associated wall can be designed to be of any practical user-friendly shape. Most preferably, the well is defined as a longitudinal, tubular hole by the wall lining. By the term "arsenic-free" in this specification is meant water having arsenic levels essentially acceptable for safe drinking by the WHO and EPA. It also includes well-water having reduced arsenic levels made by the process and apparatus according to the invention from ground water not meeting the WHO
and EPA
recommendations for having high levels of arsenic.
In a further aspect, the invention provides apparatus for use in a well shaft as a well wall, said apparatus comprising in combustion a container for retaining particulate matter comprising an inner water permeable wall defining an inner chamber; an outer water permeable wall spaced from said inner wall in opposing relationship to define therebetween an inter wall chamber; and particulate elemental iron within said inter wall chamber.
Thus, the process and apparatus according to the invention involve the use of an arsenic-removing chemically reactive wall or casing which results in keeping the arsenic contaminants below ground and avoiding the need for subsequent waste disposal. Thus, the system is adaptable to developing world conditions wherein it utilizes cost-effective iron scrap reactive materials, which can be manually constructed and does not involve dangerous chemicals additional to the arsenic contaminant.
The physical dimensions of the apparatus, placement depth and chemical constitution of I S the reactive absorptive matrix have been found to be readily determinable by the skilled person.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, a preferred embodiment will now be described, by way of example only, with reference to the accompanying drawings, wherein Fig. 1 is a diagrammatic vertical section of a potable water production system according to the invention;
Fig. 2 is a diagrammatic cross-section of a well casing and admixture apparatus along the line 2-2 of Fig. 1; and wherein the same numerals denote like parts.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
This embodiment describes the use of an in-situ reactive matrix bed of a particulate iron and sand admixture which absorbs, co-precipitates and filters out arsenic species from ground water to produce arsenic-free drinking water that meets United States EPA and WHO standards of 0.01 mg/L of arsenic in water for up to 20 years.

With reference to Fig. 1 this shows a well system generally as 10 comprising a well chamber 12 defined by inner wall 14 of a tubular casing 16. Casing 16 is within and coaxial with an outer tubular casing 18. Casings 16 and 18 are suitably perforated as hereinafter described and formed of polyvinyl chloride and together define an intercasing chamber 20 which contains a reactive matrix 22 of 77% W/W iron scale pellets and 23% W/W sand in admixture. Casings 16, 18 along their full height below the top of the matrix were perforated with a plurality of 0.5 mm diameter holes to permit ingress of ground water and egress of well water.
Above the matrix media, the PVC were non-perforated to be impermeable. Admixture 22 constitutes an adsorption, co-precipitation and filter unit. Lower end 24 of casings 16 and 18 are sealed with a 5 cm deep water-tight PVC seal in this embodiment. Well casings 16, 18 and, hence, the depth of the well run approximately 22 m from the surface to point "A", whereas the height of the "B" admixture column is about 8 m from the bottom of the well. The average ground water surface level is at a depth "C" of about 5 m. Well 10 is fitted with a Direct Action Tara Displacement Hand Pump 26, atop a concrete pad 28. At a flow rate of 1,000 L of water per day, the arsenic levels can be 1 S reduced from a maximum concentration of 2.4 mg/L in the ground water to a maximum concentration of 0.01 mg/L in the well water. The pressure created by the pumping action of the positive displacement pump 26 draws water from the surrounding aquifer into matrix bed 22 whereby arsenic-free water enters and is pumped up well chamber 12 and out of pump 26. The removal of arsenic was found to be rapid and efficacious.
As water is pumped from the aquifer, the original water table level is lowered in the vicinity of the well. This creates a cone of depression around the well, which cone is known as the area of influence. The distance between the original water table level and the lowered water table level is termed the drawdown. In radial water flow to a well, it is assumed that at a distance beyond about 150 m from the well the drawdown is zero . This radial distance is known as the radius of influence.
Drawdown calculations are needed to be performed to estimate the efficacy and efficiency of the filter at any given time when the pump is in use. The filter is working at its full potential when it is completely submerged. The worst-case scenario in terms of drawdown occurs when the top of the filter is at the same height as is the original water table level, because in this case any drawdown reduces the filter's effective area.

In calculating groundwater drawdown, Darcy's Law is employed. Darcy's Law incorporates several assumptions in the equations for groundwater analysis that are integrated in the calculations for the drawdown as follows:-~ Flow in the aquifer obeys Darcy's Law ~ Flow is laminar ~ The aquifer is homogeneous, isotropic, of uniform thickness and of infinite areal extent ~ The aquifer's bottom is horizontal ~ The water table, without pumping, is horizontal ~ The soil type is silty sand ~ The hydraulic conductivity, K, (where the filter is located) of the soil is 1x10-5 m/s ~ There is no drawdown at a radius of influence (r°) of at least 152.4 m ~ The flow is horizontal and uniform everywhere in a vertical section ~ The community's water demand is 1,000 L/day ~ The wells will be in operation for 8 hours a day ~ The aquifer is unconfined, stretching from about lOm below ground level to 100m below ~ The filters are 0.2m in diameter and 8m in height ~ Groundwater flow is 5 m/year ~ The pump's capacity is 23 L/min During drought season, the water table may be 1 Om below ground surface Accounting for the above stated assumptions, and designing for a well with a filter height of 8 m and radius of 0.1 m, the drawdown will be 0.33 m which would provide about 10001 of water per well each day. In a worst case scenario where the wells are pumped at, say, the maximum rate of 23 L/min, the drawdown will be 3.7 m. This drawdown is accounted for in the design by lowering the filter so that the top of the filter will be 4 m below the water level during the drought season. This will ensure that the filter is always submerged and therefore is working at 100% capacity.
Flow to the well is another important factor in choosing the optimum dimensions of the wells. If the flow rate to the wells is slower than the withdrawal rate, more force will be required to pump the water out. The converse is true if the water flow into the well is greater than the pumping rate. Using a filter height of 8 m, provides a flow of 7.59x10-5 m3/s opposed to the withdrawal flow rate of 3.48x10-5 m3/s. It should be noted here that these calculations consider a worst case scenario, where the well is completely dry. In reality, this will not occur unless the drawdown exceeds the depth of the filter. This condition cannot exist, however, because the maximum drawdown that can be achieved using Tara Hand Pumps is 3.7 m.
Once the wells have fulfilled their useful lifetimes, they are sealed off.
Since the filters are not removed from the ground, there are no hazardous wastes produced during the lifetime of the well or after closure. Accordingly, the population is not exposed to arsenic through the well water, nor will the waste take up any space that could potentially be used for other purposes, such as agriculture.
In the preparation of the well according to the invention, manual, animal or mechanical means are used to drill the well hole. Sandy soils may be readily removed by means of an auger of, say, 20 cm diameter by human or animal power.
Once drilled, the double casings are lowered down the well to constitute the wall, which casings may already contain the particulate iron or admixture. Alternatively, the inter casing volume may be readily filed with the casings in-situ by simply pouring the admixture between the two casings.
EXAMPLE OF ARSENIC REMOVAL FROM DRINKING WATER
A conical 2L vacuum flask was filled with 1 litre of water containing sodium arsenate Na2Has04. 7H20 at the concentration of l3mg/L (arsenic).
A polypropylene tube with internal diameter of 0.55 cm and length 27 inches long was aligned vertically. The lower end of the tube was filled with 2.40 g of steel oxide fines obtained from grit blasting of pipes. A porous plug of polyethylene was pushed into the tube bottom to prevent the solids from falling out of the tube. A small quantity of calcium carbonate (0.31 g) was poured into the open end of the tube and allowed to fall down on top of the steel oxide fines.
This created a two layer structure with the steel fines pressed against the porous plug and the carbonate layer sitting on top of the steel fines. The tube was tapped several times to compact the materials but did not cause any mixing of the two components.
The tube was slowly inserted into the aforesaid arsenic-containing water to allow the water to percolate up through the steel oxide layer and then the carbonate layer. The movement of the rising water could be seen by a darkening of the solids due to wetting and finally by the formation of a water layer on top of the solids. The water that had percolated through the two layer bed was sampled from the top using a syringe from the open end of the polyethylene tube.
A sample of the arsenic-containing water was also taken for analysis. The two samples were analyzed by a technique known as Induced Coupled Plasma Spectroscopy, commonly employed in water analysis. The results from the analysis showed that the pre-treated water had 13 mg/L
arsenic, but that the water which percolated through the two layers of steel oxide fines and calcium carbonate had no detectable arsenic concentration. Arsenic at a concentration of greater than 0.001 mg/L would be detected by this technique.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to those particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.

Claims (26)

1. A method for producing arsenic-free well water from arsenic-contaminated ground water, said method comprising passing said ground water through a well wall defining a well chamber and comprising particulate elemental iron; and removing said arsenic-free well water from said well chamber.

2. A method as defined in claim 1 wherein said well wall comprises said particulate iron in admixture with a particulate inorganic filler.

3. A method as defined in claim 1 or claim 2 wherein said particulate iron is selected from steel mill scale, metal workshop turnings, milled steel wastes and iron filings.

4. A method as defined in claim2 or claim 3 wherein said filler is a mineral selected from a sand.

5. A method as defined in any one of claims 2 to 4 wherein said admixture contains 5 - 95 W/W % iron.

6. A method as defined in any one of claims 2 to 4 wherein said admixture contains 40 - 60 W/W % iron.

7. A method as defined in any one of claims 2 - 6 wherein said wall further comprises a water permeable lining.

8. A method as defined in claim 7 wherein said admixture is retained within a particulate matter-retaining member having an inner wall and a water permeable outer wall, said walls define a particulate matter retaining chamber therebetween for retaining said admixture, and wherein said inner wall constitutes said water permeable lining.

9. A method as defined in claim 7 or claim 8 wherein said lining is perforated and formed of a thermoplastics material.

10. A method as defined in any one of claims 1 - 9 wherein said arsenic-free well water is removed from said well chamber by pump, siphon or bucket means.

11. Apparatus for producing arsenic-free well water from arsenic-containing ground water present in earth, said apparatus comprising a well wall within said earth and defining a chamber for receiving said arsenic-free water; said wall comprising a water permeable lining and elemental iron in particulate form in communication with said lining and said earth.

12. Apparatus as defined in claim 11 wherein said well wall comprises said particulate iron in admixture with an inorganic particulate filler.

13. Apparatus as defined in claim 12 comprising a particulate matter retaining member having an inner wall and a water permeable outer wall, said walls define a particulate matter retaining chamber therebetween for retaining said particulate admixture and wherein said inner wall constitutes said water permeable lining.

14. Apparatus as defined in claim 11 wherein said water permeable lining is perforated and formed of a plastics material.

15. Apparatus as defined in claim 11 further comprising water removing means.

16. Apparatus as defined in any one of claims 12 - 15 wherein said admixture comprises 5 -95% ion.

17. Apparatus as defined in claim 16 wherein said admixture comprises 40 - 60%
iron.

18. Apparatus as defined in any one of claims 11 - 17 wherein said iron is steel mill scale, metal workshop turnings milled steel wastes and iron filings.

19. Apparatus as defined in any one of claims 12 - 18 wherein said inorganic particulate filler is selected from a sand.

20. Apparatus for use in a well shaft as a well wall, said apparatus comprising in combustion a container for retaining particulate matter comprising an inner water permeable wall defining an inner chamber; an outer water permeable wall spaced from said inner wall in opposing relationship to define therebetween an inter wall chamber; and particulate elemental iron within said inter wall chamber.

21. Apparatus as defined in claim 20 wherein said inner and outer walls comprise a pair of concentric coaxial tubular members.

22. Apparatus as defined in claim 20 or claim 21 wherein said inner and outer walls are perforated and formed of a plastics material.

23. Apparatus as defined in any one of claims 20 - 22 wherein said particulate ion is in admixture with a particulate inorganic filler.

24. Apparatus as defined in claim 23 wherein said filler is a mineral.

25. Apparatus as defined in claim 23 or claim 24 wherein said admixture comprises 5 - 9 W/W %iron.

26. Apparatus as defined in claim 25 wherein said admixture comprises 40 - 60 W/W % iron.