HU196154B - Process for elimination of arsene content from waters having big content of organic materials - Google Patents

Abstract

Coagulant and filtration is used to remove arsenic from drinking water and effluents. The coagulant (an iron or aluminium salt) and if needed a sec. coagulant is added. The floc contg. mixt. is prefiltered by upward flow through a 1.0-2.5 m thick layer of 0.5-25 mm dia. grain size. - Having removed the larger hydroxide flocks contg. adsorbed arsenic the finer flocs are removed by filtration through a 1.0-2.0 m thick layer contg. grains of 0.5-2.0mm dia.size. An upward stream of water and air is used to remove contaminant collected on the filter. The sludge is sedimented and removed.

Description

BACKGROUND OF THE INVENTION This invention relates to a process for removing arsenic from high organic waters (e.g., drinking water, wastewater), addition of inorganic coagulants (e.g., 5 iron salts, aluminum salts), and preferably an oxidizing agent (e.g., chlorine, hypo) and filtration.

The removal of arsenic by coagulants is based on the fact that iron and / or aluminum salts produce positively charged, high surface area hydroxide pellets in the pH range of natural waters which, on account of their physical adsorption, bind arsenite or arsenic in water. molecular ions. Among the two forms of arsenic, which are present in different proportions in water, arsenate is more adsorbed by the V-valent arsenic species.

Of the two types of coagulants, the iron salt is more suitable. Its advantage is that the iron (III) hydroxide resulting from the hydrolysis has a specific arsenic binding capacity higher than that of aluminum hydroxide and does not require the prior adjustment of the pH of the raw water due to optimal coagulation. It is also advantageous to remove arsenic by iron salt because the iron content of the raw water - groundwater - usually reaches or exceeds the permissible limit and must be removed. If this is not done, it adsorbs some of the arsenic remaining in the water, which is deposited in the reservoir or pipeline network, leading to secondary contamination.

Any iron (II), iron (IIl) and aluminum salts that meet the health requirements can be used as coagulants. The hydrolysis of the iron (II) salt at the pH of natural waters is incomplete and must therefore be oxidized or added together with the oxidizing agent before addition to the water. In many cases, the original iron content of the raw water can also be used effectively to reduce the arsenic content. 05

The morphology of the resulting hydroxide precipitate is significantly influenced by the quality of the raw water, primarily its organic matter content. The organic matter content also adversely affects the specific adsorption of arsenic.

The arsenic-containing hydroxide precipitate formed is separated off by separation using water technology. Organic and inorganic coagulants are widely used in water technology to promote clarification. Such a solution is described in U.S. Patent No. 613,917. Swiss Patent Application No. 5,684,128, wherein Fe (II) and / or Fe (III) salts are added as coagulants to increase clarification efficiency by the addition of oxidizing agents.

According to one of the prior art techniques (Röske, I. - Kormer, K., Acta Hydrocholine. Hydrobiol. 7, 115-121, 1979), iron (III) chloride is used to remove arsenic from mine water. Due to the low organic matter content of water, the required iron (III)

6-7 times the initial arsenic concentration. The iron (III) hydroxide flakes are separated by settling and filtration. This solution is not insignificant for high water containing 6-8 g / m ^{3 of} humic acid, because the resulting flakes are small and do not settle properly. According to our results, due to the adsorbent inhibitory effect of the humic substance, approx. 40 to 70 times the amount of iron (III) ion required for safe removal of arsenic (even higher when used with aluminum salt: gQ

70-120 fold). The iron (III) hydroxide formed from this amount of iron (HI) ion, due to the low sedimentation (20-25%), results in such a high filter load that an economical mode cannot be achieved due to frequent filter clogging.

Another technology developed for de-arsenic removal (Thematic Report No. 84/84 / / LXVIK by Survey and Soil Research Company) is to de-arsenic low organic matter water. Iron (III) chloride, sodium carbonate and KMnO _{4 are} added as chemicals. Sodium carbonate is required for subsequent pH correction. The iron (HI) hydroxide precipitate is separated by a single layer filter. The process is expensive because of the need to use sodium carbonate and potassium permanganate. Another disadvantage of humic acid-containing water is the use of a single-layer filter which has a low retention capacity of specific suspended solids and thus a short filtration cycle time.

Also known is a technology which uses a dual-layer filter (Patent Application No. 1508/83). Iron (III) chloride and hypo are added before the first filter. The amount of iron (II) ion added is 6 g / m ³ . Prior to the second filter layer, which layer is coated with manganese dioxide, KMnO4 is added. This technology can be reduced to a concentration of 0.09 to 0.12 g / ^m3 to ³ Arsenic 0.03 to 0.05 g / m. The process is costly to maintain the manganese dioxide coated charge in a constant "active" state with continuous addition of KMnO4. Measurements show that a filtration cycle of about 1.0 to 1.5 hours is required for the arsenic content of the filtered water to reach 0.05 g / m ³ . It also makes the process expensive to re-apply the lower filter layer \ once. Here, too, the standard top-down filter is a problem because of the low specific suspended solids retention. The procedure is only low or low. suitable for reducing the average arsenic content. '

A 3476/82 is known. U.S. Patent Application No. 4,123,129, wherein adsorption is provided by a layer of iron (III) hydroxide on alumina granules. This is primarily suitable for the post-purification of pre-treated water, which is already degassed, de-ironed and manganese-free, as water contamination reduces the adsorption surface. A further disadvantage of the process is the removal and costly regeneration of the expensive dye, the carrier and the adsorbent layer.

These technologies are either unsuitable for the removal of high humic water content of arsenic by coagulants, or are otherwise achieved by reducing the amount of coagulant required, but by increasing the specific cost.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process which overcomes the above-mentioned disadvantages by allowing the removal of arsenic from waters with high organic content of natural origin.

The present invention is based on the discovery that phase separation is accomplished by adding polyelectrolyte and bottom-up filtration equipment, which is problematic in the technologies listed and also allows for the treatment of water with a higher initial concentration of arsenic. Due to the presence of hymen substances in the groundwater, most of the hydroxide precipitates are in the form of microparticles, which makes it difficult to separate the upstream phase

196.154 - sedimentation - and their removal by filtration.

This problem can be overcome in the present invention by the addition of polyelectrolyte. Polyelectrolytes are mixtures of polyocrylanyl acrylic acid prepared by polymerization from acrylamide 5 monomer followed by partial hydrolysis of the polymerized mixture. For example, the Purifloc A-23 polyelectrolyte has an acrylic acid amide to acrylic acid ratio of 3: 1 and a degree of polymerization of ca. 9,800 polymer units per acrylic acid amide / acrylic acid ratio of 19: 1 in the Purifloc N-17 polyelectrolyte, and a degree of polymerization of 700 polymer units. Polyelectrolytes promote competitive adsorption, that is, they serve to suppress the adsorption activity of humic substances. With a suitable concentration of polyelectrolyte - g 0.05-0.5 g / m ³ - a flake size suitable for the pre-phase separation can be achieved without significantly reducing the specific adsorption capacity of iron (III) hydroxide. The flakes thus expanded can be well separated by pre-phase separation, preferably with a bottom-up filter, thereby significantly reducing the load on the post-filter floating material.

Therefore, it is advisable to use pre-oxidation prior to the addition of the iron or aluminum salt, also in order to save coagulant. Only chemical oxidants such as chlorine, hypo, chlorine dioxide, potassium permanganate, ozone peroxide are suitable for this purpose. Oxidation directly by air blasting is small and slow, while with these oxidants the reaction is rapid and takes place within a few minutes. Pre-oxidation is important here as well, because the III. a valence form of arsenic is more toxic. '40

The process according to the invention comprises adding an inorganic coagulant and, depending on the organic matter content of the high organic water content, 0.05-0.5 g / m ³ polyelectrolyte (eg Flocouit, Purifloc) to the water to be treated, followed by fold water into a 0.5- to 25-mm particle-sized upstream filter with a 1.0 to 2.5-meter filter layer, removing the chemical suspended solids and the arsenic adsorbed on the hydroxide flakes. The pre-filtered water is filtered through a post-filter with a particle size of 0.5-2.0 mm and a filter layer of 1.0-2.0 m to remove fine suspended material and drained of arsenic-free water. The suspended solids filtered through the filters are washed with a bottom-up stream of water-air, the slurry is sedimented in a basin, and the settled arsenic sludge is removed from the settler.

A further advantageous effect is achieved by the addition of an oxidizing agent (e.g., hypo, chlorine) together with the coagulant.

The oxidizing effect of the disinfectant enhances the adsorption of arsenic on the surface of the hydroxide flakes formed in the contact filter layer, and the contact filter layer catalyzes the flocculation of the continuously added flocculants and the regeneration of the contact filter layer.

Further, it is advantageous to use closed filter containers as a filter.

Z Sludge from filter backwash is settled in a closed container and the decanted water is returned to the beginning of the process.

The invention is illustrated by way of example with reference to FIG.

Λζ Figure 1 is the. Schematic flow diagram of a possible embodiment of the method.

250 For raw water containing 0.15 g / m ³ arsenic and 4.0 g / m ³ humic acid with coagulating agent membrane 1 chemical agent, 250 g / m ³ iron (TII) chloride (5.2 g / 111 ³ iron salts), 0.15 Purifloc was added as a g / m ³ polyelectrolyte and 5 g / m ³ hypo was added as an oxidizing agent and then mixed in 2 chemical mixers. The chemical water is introduced into the upstream closed filter 3, where three layers - 0.4 m thick with 6-15 mm grain size, 0.6 m thick with 3-5 mm grain size and 0.8 m thick with 1-2 mm grain size - removing the arsenic adsorbed on the bridged hydroxide flakes and the coarse suspended solids on the contacted 4 filter layers. The pre-filtered water is introduced into a closed post-filter 5 with a layer thickness of 0.6-1.2 mm with a sand layer of 0.6-1.2 mm, where the remaining floating material of the water is filtered off and the arsenic-free water is discharged for use. The suspended solids filtered through filter 3 and after filter 5 are washed with a stream of bottom-up water, the resulting slurry water is discharged to a settling tank 7, from which decanted water is pumped to the front of the system, and the precipitated arsenic sludge is removed from the sanitary sludge.

In order to support the process of the invention, we present our measurement results:

a) Example of arsenic removal of low humic acid water by addition of iron salt only plus yas salt and polyelectrolyte

raw water

As Huminic acid mg / 1 mg / 1

Chemical treatment

Cl ₂ dose Fe dose Pe dose mg / 1 mg / 1 mg / l

Treated water As mg / l

0.16 0.05

0.16 0.05

4.2 0 0.030

2.6 0.15 0.020

196 154

b) Example of de-arsenic depletion of high humic acid water only with iron salt and iron and with different amounts of polyelectrolyte added

raw water

As Huminic acid mg / 1 mg / l

Chemical treatment _ Treated water

Clj dose Fe dose Pe dose As mg / 1 mg / I mg / 1 mg / l

0.155 6.00 5.74 0 0,135 0.155 6.00 5.74 0.05 0,056 0.155 6.00 5.74 0.2 0,033 0.155 6.00 5.74 0.5 0,016

Advantage of the method of the invention:

- also allows for the purification of higher - 0.25-0.35 g / m ³ - wells with arsenic content - higher specific surface loads at lower concentrations of arsenic - the suspended suspended material is located in a wide filter band due to the particle size distribution of the charge, adsorption time. The latter improves the specific adsorption of arsenic, which also results in savings of coagulant, - simpler operation, - easy to automate, - integration into a closed process chain, which results in significant operating cost savings (pump operation), does not require an intermediate technology transfer as a result, disinfection begins with the addition of chlorine or hypo at the start of the system, which also improves the efficiency of arsenic removal, - no prior chemical treatment of the filter layer is required; flocculator, - the water loss of the process due to closed sludge treatment is low, the amount of arsenic sludge generated is low, - its investment cost is lower.

Claims (4)

A process for removing the arsenic content of waters with a high content of organic matter (e.g., drinking water, wastewater) by adding inorganic coagulants (e.g., iron salt, aluminum salt) and suitably an oxidizing agent (e.g., chlorine, hypo), after the addition of the inorganic coagulant, depending on the organic content of the water to be treated 25 0.05-0.5 g / m ³ polyelectrolyte is mixed, preferably using a chemical stirrer (2), into the water to be treated, and then the chemical water is provided with a filter layer (4) having a particle size of 0.5-25 mm, is passed through an upstream filter (3), through which the suspended solids and the arsenic adsorbed on the hydroxide flakes are removed through the filter, then the filtered water has a particle size of 0.5-2.0 mm and a filter layer of 1.0-2.0 m (6) filtering through fine filter (5) to remove fine suspended material and draining arsenic-free water, through filter (3) and after filter (5) The filtered suspended solids are washed from bottom to top with a stream of water and air, the slurry is settled in a settling tank (7), and the settled arsenic slurry is removed from the system. Method according to claim 1, characterized in that the filter (3 and 5) is a closed container. Method according to claim 1 or 2, characterized in that the slurry formed when the filters (3 and 5) are returned is settled in a closed settling tank (7). 4. Method 45 according to any one of claims 1 to 4, characterized in that the decanted water is returned from the settling tank (7) to the front of the system by the pump (8).