WO2007052085A1

WO2007052085A1 - Method for groundwater treatment

Info

Publication number: WO2007052085A1
Application number: PCT/GR2006/000059
Authority: WO
Inventors: Anastasios Zoumpoulis; Ioannis Katsogiannis
Original assignee: Aristotle University Thessaloniki
Priority date: 2005-10-31
Filing date: 2006-10-31
Publication date: 2007-05-10
Also published as: GR1005935B; GR20050100557A; EP1951629A1

Abstract

Method for treating underground waters (1) for removing arsenic. Said method is remarkable in that a biological oxidation of iron and manganese from the underground waters is performed during which the soluble iron and manganese are oxidized in the presence of iron and manganese oxidizing bacteria, characterized by the amorphous hydroxides and hydroxyoxides of iron and manganese which are formed and which on passage through a bed (4, 5) with a filter medium are removed from the water and gradually form a coating on the surface of the filter medium, thereby performing about a change in the properties of the filter medium which can simultaneously act as an adsorbent when it can also remove the arsenic by adsorption through a biological adsorption filtration.

Description

Method for groundwater treatment

Field of the invention

The present invention relates to a method for groundwater treatment, in particular for the removal of arsenic. The method is based on processing of underground waters which contain arsenic in combination with an elevated presence of iron (Fell) or manganese (MnII) or both cations.

Background of the invention

There are many conventional physicochemical treatment methods used for the removal of arsenic, the principal whereof consist of flocculation, coagulation-direct filtration, ion exchange, membrane filtration, removal by sorption on adsorptive media, i.e. granular ferric hydroxide adsorption on various adsorbent media, such as activated alumina, and further membrane methods. The conventional physicochemical methods are not efficient towards the removal of trivalent arsenic since they do not satisfactorily remove the trivalent arsenic encountered in underground waters.

All the aforementioned methods thus have the disadvantage that they do not satisfactorily remove trivalent arsenic which however is commonly encountered in underground waters due to the reducing conditions that prevail and resulting in the need to apply a preliminary stage of oxidation of the trivalent arsenic to pentavalent.

To overcome this disadvantage, chemical additives are usually used which increase the cost of processing and can create secondary problems due to the production of by-products or due to a residual effect. Finally they increase the complexity of the process, as a result whereof they usually require specialized personnel to operate properly and effectively. Aim of the invention

The present invention aims at remedying the aforementioned drawback, thereby providing an efficient method for treatment for the removal of arsenic from groundwater, more particularly during biological oxidation and removal of iron (II) and manganese (II). The goal of the present invention thus consists of the development of a simple method for the removal of arsenic - trivalent or pentavalent - without the use of chemical reagents for the oxidation of sorption of arsenic. To summarize said aim consists of applying a method which removes the arsenic -trivalent and pentavalent- without using oxidants and at an increased treatment speed.

Summary of the invention

The method proposed according to the invention is based on the treatment of groundwater which contains arsenic and elevated concentrations of iron Fe(II) and/or manganese Mn(II). Remarkably, the dissolved iron or manganese is oxidized in the presence of iron and manganese oxidizing bacteria by the dissolved oxygen, which is supplied to a treatment unit. The divalent iron and manganese cations are oxidized and transformed into iron and manganese oxides. The insoluble oxides are removed from water through filtration in filter beds packed with polysterene beads.

The present invention thus concerns a method for processing underground waters to remove arsenic including a biological adsorptive filtration without simultaneous use of additional chemical reagents which are usually used to oxidize or remove the arsenic. In particular, the present invention concerns the application of the method of biological oxidation of iron and manganese from the underground waters in the removal of trivalent and pentavalent arsenic.

The method according to the invention thus shows many advantages relative to the above-mentioned physicochemical processing methods for the removal of arsenic. Through applying this particular method, oxidation of the trivalent arsenic is catalysed by the presence of microorganisms and the presence of biogenic surface oxides of iron and manganese. In conjunction with aeration, this is achieved together with the other actions which take place during the processing. A further advantage of biological oxidation is the avoidance of use of chemical oxidants such as, for example, chlorine, ozone, hydrogen peroxide etc. The use of such reagents increases the operational costs and restricts the sustainability of the methods, so that said use should be circumvented.

The soluble iron or soluble manganese is oxidized in the presence of iron and manganese oxidizing bacteria in combination with channelling of dissolved oxygen. The divalent cations are converted to insoluble oxides by the process of biological oxidation and are then removed from the water by filtration in suitable filter beds.

If arsenic is present in the form of arsenates, i.e. pentavalent, it will be subsequently removed by sorption on the iron and manganese oxides. In case arsenic is present in the form of arsenites however, i.e. in the trivalent form, it will be firstly oxidized under conditions, which prevail in the filter columns, and the pentavalent arsenic is subsequently removed by sorption on the preformed iron and manganese oxides.

The treatment method according to the invention shows several advantages in comparison to the conventional physicochemical treatment methods, used for the removal of arsenic. The application of the method according to the present invention enables the oxidation of arsenites by oxygen supplied during the pre- aeration in the aeration column, which is catalysed by the bacteria and the solid surfaces of biogenic iron and manganese oxides.

The present invention further relates to a system or device remarkably designed for implementing the method according to the invention which system consists at least of two columns which are advantageously be made of polythene which are filled with a suitable filter medium, such as polystyrene beads. These beds are fed with underground water which may contain iron, manganese and/or arsenic.

Before entering the beds, the water is subject to preliminary aeration with the aid of a suitable separate column which is necessary to grow the microorganisms so as to catalyse the oxidation of the substrates. The microorganisms which are used are native to underground waters and grow in the presence of iron and manganese on applying aeration.

According to an advantageous embodiment of the invention, said microorganisms used are Gallionella ferruginea and Leptothrix Ochracea.

Further preferred embodiments of the invention are directed to the optimum operating conditions for the method, which are related to the production of dissolved oxygen, the redox potential and the pH of the water.

According to a first preferred embodiment of the invention, there is supplied dissolved oxygen at concentrations greater than 4 mg/L, dissolved oxygen in underground waters is usually very low and usually does not exceed 1 mg/L. Effective application of this particular methodology requires a dissolved oxygen concentration of at least 2 mg/L for effective oxidation of the divalent iron while effective oxidation of the trivalent arsenic to pentavalent requires higher values of dissolved oxygen e.g. 4 mg/L, which is necessary for the application of this particular method.

According to a second preferred embodiment of the invention, the value of the redox potential is greater than 300 mV and less than 550 mV. This is the other highly significant parameter for effective application of this particular method, redox potential values. If it is less than 300 mV, the oxidation of the trivalent arsenic is not effective and as a result effective removal of total arsenic is not observed. It is not possible to apply the method at values greater than 550 mV since the microorganisms do not survive at such high values of redox potential.

According to a third preferred embodiment of the invention, the pH of the water fluctuates between 6,5 and 8. The pH of the water is also a very important factor for the application of the method according to the invention. At these values, the preliminary biological oxidation of iron is possible which is necessary for the co- removal of the arsenic. This is due to the fact that the microorganisms which are used work at these pH values, while their functioning is not possible in a strongly acid or alkaline environment. Further features and characteristics of the invention will be defined in the additional sub-claims.

Further details and particulars of the invention will be explained hereafter with reference to the description hereunder of an exemplary embodiment of the method and system according to the invention by means of the appended drawings.

Brief description of the drawings

Figure 1 is a diagrammatic representation of an underground water treatment unit according to the invention.

Figure 2 represents a scanning electron micrographs of respective microorganisms showing the existence of bacteria relevant to the method according to the invention.

Figure 3 represents an embodiment of filter media before and after the biological oxidation of iron in the method according to the invention.

Description

The method is generally based on processing underground waters which contain arsenic in combination with an increased presence of iron (Fell) or manganese

(MnII) or both cations in ascending flow beds filled with polystyrene beads. The arsenic, if it is in the form of arsenic oxyanions (pentavaient), is removed by adsorption on the iron and magnesium oxides. In the event that it is in the form of arsenous oxyanions (trivalent), it is oxidized under suitable conditions to pentavaient and then adsorbed onto the iron and magnesium oxides.

Figure 1 shows an experimental set up of biological removal of arsenic, iron and manganese as a biological adsorptive filtration unit. This apparatus consists of two columns, preferably made of PVC, which are filled with polystyrene beads used as filtration media. Iron or manganese and arsenic containing groundwater is forced to flow through the filter beds. Before entering the treatment columns, the groundwater is subjected to aeration. The aeration is performed in an additional separate column, before the primary filtration, in order to avoid the collision of bubbles with the deposited precipitates, which may cause disturbance of the system and increased iron concentrations in the effluent. Aeration is necessary for the growth of microorganisms, which catalyze the oxidation of dissolved iron and manganese.

Reference numerals 1 represent a continuous flow of contaminated groundwater,

2 an air injection, 3 an aeration column, 4 a primary filtration, 5 a secondary filtration, and 6 the outlet sampling point respectively.

Preferably, the microorganisms used are the Gallionella ferruginea and Leptothrix ochracea. Figure 2 shows said (a) Gallionella ferruginea and (b) Leptothrix ochracea in samples collected from the backwashing sludge.

Figure 3 shows the modification of the surface of polysterene beads as filter medium by the continuous filtration of iron oxides, onto which arsenic can be removed by sorption.

After the aeration, the water is forced to flow in upflow mode through the filtration bed. In the beds, the surface of the filter medium, advantageously consisting of polystyrene beads, is covered with iron and/or manganese oxides, as shown in Figure 3. The microorganisms are entrapped in a thin layer of iron and/or manganese oxides which cover the surface of the medium. To avoid filter clogging, the filter beds are subjected to a preferably regular backwashing. The frequency of the backwashing depends strongly on the dissolved iron and manganese concentrations in the groundwater.

The conditions for optimum operation are dependent on the air flow and consequently on the dissolved oxygen concentration, on the redox potential and on the pH value of the waters. The concentration of dissolved oxygen exceeds 2 mg/L for an efficient iron oxidation. For an efficient trivalent arsenic oxidation, said concentration is set at approximately 4 mg/L.

The redox potential for an efficient As(III) oxidation should be higher than 300 mV and lower than 550 mV. If it is less than 300 mV, it presently appears to be not enough for As(III) oxidation. In case it is higher than 550 mV, it causes bacterial destruction and therefore, neither the oxidation of As(III) appears to take place efficiently.

The pH value of the water is also very important for enabling biological oxidation. It must be comprised in a range between 6,5 and 8,0 since only between these values, biological oxidation presently appear to be feasible. However, out of said pH range, the specific microorganisms do not presently appear to be active.

The remarkably exemplary working of said system device is set out hereafter. The experimental array includes a feed system which consists of the main underground water feed line into which the arsenic (III or V) solution is introduced and, as the case may be, solutions of divalent iron or manganese. Then follows the mixing of the various constituents in a mixing vessel and the uniform feed enters the aeration column into which air is channelled counter to the flow. Aeration takes place in a separate column and not in the filtration columns so as to avoid contact of the air with the deposited oxides of iron and manganese which would result in their detachment from the filter medium and ultimately to a rise in the Fe and Mn concentrations at the outlet (processed water). After aeration, the water enters the filtration columns with an upward flow. In these columns the filter medium is coated with oxides of iron and manganese, while the microorganisms are trapped and finally immobilized on the modified material.

For the experimental process to run smoothly, reverse washing of the beds is necessary. This is done by a downward flow of processed water. This is performed to remove the large percentage of oxides of iron and manganese which have been filtered and thus avoiding the blockage of the pores of the bed. The reverse washing is done at regular intervals according to the load of oxides of iron and manganese on the beds. Example

The tested groundwater contains dissolved iron at 2,8 mg/L and manganese at 0,6 mg/L, arsenic at 0,05 mg/L, including 0,03 mg/L as As(III) and 0,02 mg/L as As(V), and dissolved oxygen concentration of 0,7 mg/L having a redox potential value of -

150 mV. This underground water is subjected to aeration in the aeration column.

During this process, oxygen is dissolved in the groundwater and increases the respective concentration at least to 4 mg/L, whereas the redox potential increases up to +150 mV. Then the water is submitted to flow through the first filtration column with a maximum linear velocity of roughly 15 m/h.

In this stage, Fe(II) is oxidized to Fe(III) and iron oxides are formed which are removed from water through filtration. A small part of manganese is removed as well, up to 20%. The redox potential is then increased up to +350 mV, as a result of the oxidation process and As(III) is oxidized to As(V). In the first filter, the As(V) is removed by sorption.

Streamdownwardly from the first filter, the following concentrations are measured:

Fe(II) 0,2 mg/L, M(II) 0,5 mg/L, As(III) is oxidized to As(V) and As(V) partly removed.

As a result thereof, the concentration of total arsenic does not exceed 0,015 mg/L streamdownwardly from the first column.

Subsequently, the water is submitted to flow through the second column. During this process an additional removal of Fe(II) will be achieved at least down to 0.05 mg/L, together with a removal of manganese at least down to 0,015 mg/L and an additional removal of arsenic down to 0,005 mg/L.

As a result of the implementation of this preferred method according to the invention, the treated water will comply with the Maximum Concentration Limits imposed by the EU directive 98/86 related to water intended for human consumption, at least concerning the existence and/or presence of arsenic, iron and manganese.

To summarise said example, tested groundwater containing 2,8 mg/l dissolved iron, 0,6 mg/L manganese and 0,05 mg/L arsenic, is subjected to aeration and is then forced to flow through the filtration beds, resulting in the iron and manganese oxidation and removal to concentrations below 200 and 50 μg/L respectively, which are the maximum concentration limits set by the European union. The part of arsenic, which is in the trivalent form is oxidized and then the total arsenic is removed from water to below 10 μg/L, which is the maximum concentration limit imposed by said EU directive.

The method was examined for one year period and showed no operational problems.

Claims

1. Method for groundwater treatment for the removal of arsenic, characterised in that it comprises the steps of a biological oxidation of iron and manganese from said groundwater, during which dissolved iron and manganese is oxidized in the presence of iron and manganese oxidizing bacteria and aeration, forming amorphous iron and manganese oxides, which coat the surface of the filter medium, thereby modifying its properties by a so-called biological adsorptive filtration method.

2. Method according to Claim 1 , characterised in that arsenates are subsequently removed from the groundwater by adsorption onto the biogenic iron and manganese oxides, which have covered the surface of the filter medium.

3. Method according to one of the claims 1 or 2, characterised in that arsenites are oxidised by the aeration in the presence of microorganisms and in contact with biogenic solid surfaces and are subsequently removed by adsorption on the iron and manganese oxides.

4. Method for the treatment of groundwater containing arsenic and high concentrations of iron Fe(II) and/or manganese Mn(II), in particular according to one of the preceding claims, characterised in that said method comprises the steps of supplying a determined quantity of dissolved oxygen and of iron and manganese oxidizing bacteria respectively, wherein said dissolved iron or manganese is put in the presence of iron and manganese oxidizing bacteria, wherein said dissolved oxygen oxidize the divalent iron and manganese cations, both of which are further transformed to iron and manganese oxides, wherein the insoluble oxides are then removed from water through filtration in filter media.

5. Method for processing underground waters according to one of the preceding claims, wherein arsenic is removed through biological oxidation of iron and manganese from the underground waters during which the soluble iron and manganese are oxidized in the presence of iron and manganese oxidizing bacteria, characterized by the amorphous hydroxides and hydroxyoxides of iron and manganese which are formed and which on passage through a bed with a filter medium are removed from the water and gradually form a coating on the surface of the filter medium, thereby performing about a change in the properties of the filter medium which can simultaneously act as an adsorbent when it can also remove the arsenic by adsorption by said biological adsorption filtration.

6. Method according to one of the preceding claims, characterised in that, if arsenic is present in the form of arsenates, said pentavalent arsenic is subsequently removed by adsorption on the oxides of iron and manganese covering the surface of the filter medium.

7. Method according to one of the preceding claims, characterised in that the trivalent arsenic is further adsorbed on the oxides of iron and manganese due to oxidation achieved by the iron and manganese oxidizing microorganisms, whereby in case arsenic is present in the form of arsenites, i.e. in the trivalent form, it is firstly oxidized under conditions prevailing in filter columns, and the pentavalent arsenic is subsequently removed by adsorption on the preformed iron and manganese oxides.

8. Method according to one of the preceding claims, characterised in that there is performed a combined aeration, which is catalysed by said bacteria and the solid surfaces of biogenic iron and manganese oxides, thereby enabling the oxidation of As(III).

9. Method according to one of the preceding claims, characterised in that there is performed a biological adsorptive filtration which enables the circumvention of chemical reagents for the As(III) oxidation, such as chlorine, hydrogen peroxide, ozone.

10. Device for performing the method according to one of the preceding claims, characterised in that it comprises at least two columns, which are filled with filtration media through which iron or manganese and arsenic containing groundwater is forced to flow.

11. Device according to the preceding claim, characterised in that said filtration media consist of filter beds, preferably packed with polystyrene beads.

12. Device according to one of both preceding claims, characterised in that said columns are made of PVC.

13. Method according to one of the preceding claims 1 to 9, characterised in that the aeration is performed in an additional separate column (6), before the primary filtration, in order to avoid the collision of bubbles with deposited precipitates.

14. Method according to one of the preceding claims 1 to 9 or 13, characterised in that catalyzing means are supplied for catalyzing the oxidation of dissolved iron and manganese.

15. Method according to the preceding claim, characterised in that said catalyzing means are selected from among microorganisms and in that a thorough aeration is performed for the growth of said microorganisms, which catalyze the oxidation of dissolved iron and manganese.

16. Method according to one of both preceding claims, characterised in that Gallionella ferruginea are selected as said microorganisms.

17. Method according to one of the claims 14 or 15, characterised in that Leptothrix ochracea are selected as said microorganisms.

18. Method according to one of the claims 1 to 9 or 13 to 17, characterised in that after the aeration, the water is forced to flow in upflow mode through said filtration media.

19. Method according to one of the preceding claims 1 to 9 or 13 to 18, characterised in that in said filter beds, the surface of the filter medium is covered at least partially with iron and/or manganese oxides.

20. Method according to the preceding claim, characterised in that said surface of said filter medium is covered with a thin layer of iron and/or manganese oxides.

21. Method according to the preceding claim, characterised in that the microorganisms are entrapped in said thin layer of iron and/or manganese oxides covering the surface of the medium.

22. Method according to one of the preceding claims 1 to 9 or 13 to 21 , characterised in that the filter beds are subjected to a backwashing to avoid filter clogging.

23. Method according to the preceding claim, characterised in that said backwashing is performed regularly, the frequency whereof is set in accordance with the concentrations of dissolved iron and manganese.

24. Method according to one of both preceding claims, characterised in that said reverse washing of the beds is performed by a downward flow of processed water, wherein the large percentage of oxides of iron and manganese which have been filtered is removed, thereby avoiding the blockage of the pores of the bed.

25. Method according to one of the preceding claims 1 to 9 or 13 to 24, characterised in that the concentration of dissolved oxygen is set in excess of 2 mg/L, thereby enabling an efficient iron oxidation.

26. Method according to the preceding claim, characterised in that the concentration of dissolved oxygen is set at approximately 4 mg/L, thereby enabling an efficient trivalent arsenic oxidation.

27. Method according to one of the preceding claims 1 to 9 or 13 to 26, characterised in that the redox potential is comprised in a range between 300 mV and 550 mV for an efficient As(III) oxidation.

28. Method according to one of the preceding claims 1 to 9 or 13 to 27, characterised in that the pH value of the water is set in a range comprised between 6,5 and 8,0, thereby enabling a biological oxidation.

29. Method according to one of the preceding claims 1 to 9 or 13 to 28, characterised in that groundwater containing dissolved iron the concentration whereof is measured at 2,8 mg/L, manganese the concentration whereof is measured at 0,6 mg/L and arsenic the concentration whereof is measured at 0,05 mg/L, is subjected to an aeration and is then forced to flow through the filtration beds, which generates said oxidation of the iron and manganese and the removal thereof to concentrations below 200 and 50 μg/L respectively, which are the maximum concentration limits set by present standards, wherein the part of arsenic which is in the trivalent form, is oxidized and wherein the total arsenic is subsequently removed from water to below 10 μg/L, which is the maximum concentration limit.

30. System for underground water processing by means of a method according to one of the preceding claims, including a device as defined in one of the claims 10 to 12, characterised in that it includes a feed system which consists of a main underground water feed line into which the arsenic (III or V) solution is introduced and, as the case may be, solutions of divalent iron or manganese, in that then follows the mixing of the various constituents in a mixing vessel and the uniform feed enters the aeration column into which air is channelled counter to the flow, in that aeration takes place in a separate column so as to avoid contact of the air with the deposited oxides of iron and manganese, in that after aeration, the water enters the filtration columns with an upward flow, and in that the filter medium is coated with oxides of iron and manganese while the microorganisms are trapped and finally immobilized on the modified material.