WO2010041041A1

WO2010041041A1 - Method, system and apparatus for reducing oxyanion content

Info

Publication number: WO2010041041A1
Application number: PCT/GB2009/002439
Authority: WO
Inventors: Ewan Mcadam; Simon Judd
Original assignee: Cranfield Universtiy
Priority date: 2008-10-10
Filing date: 2009-10-12
Publication date: 2010-04-15
Also published as: GB0818698D0

Abstract

The invention provides methods for reducing the oxyanion (e.g. nitrate) content of an aqueous solution, for example drinking water. Such methods involve the use of at least one, preferably two, membrane bioreactors (MBRs) operated downstream of a conventional ion exchanger. Where two MBRs are employed, these are operated in series. In the methods described, spent brine produced following the regeneration of an exhausted ion exchange resin is treated in a first MBR operated under anoxic conditions to produce an effluent which has a low oxyanion (e.g. nitrate) content and which can either be disposed of or, more preferably, re-used in the ion exchange process as a regenerant for the ion exchange resin beds. In a further aspect, a second MBR which is operated under aerobic conditions is employed to treat the waste sludge from the first anoxic MBR. The effluent from this second MBR may also be recycled for use as a regenerant in the ion exchange process. The overall result is an essentially 'zero waste' process which is especially suitable for the treatment of drinking water.

Description

METHOD , SYSTEM AND APPARATUS FOR REDUCING OXYANION CONTENT

The present invention relates to improvements in and relating to processes for the removal of oxyanion contaminants from aqueous feed stocks such as domestic, industrial and agricultural water supplies, in particular from drinking water and brines, and to apparatus for use in such processes. More particularly, the invention relates to processes for the removal of oxyanions, especially nitrate, from drinking water with minimal generation of waste.

Borehole and surface waters sourced for potable water supply may be contaminated with nitrate and, in some cases, other oxyanions such as perchlorate, bromate and sulphate. The permitted level of nitrate in drinking water set by the EU and WHO is 50 mg/L as NO₃. However, in many waters, this level is exceeded. The removal of nitrate is conventionally carried out by ion exchange due to its simplicity, effectiveness and low cost. An adsorbent media (e.g. resin bed) is used for removing the nitrate. When the media becomes saturated with nitrate it is regenerated by flushing with a strong salt solution or "brine" (generally sodium chloride) which forms the waste stream from the process. This is an extractive process which does not destroy the nitrate, but simply removes it to a waste stream containing the pollutant in concentrated form.

There are several options currently available for managing the waste stream from an ion exchange process. These include: (1) direct disposal without recovery of the salt solution; (2) electrolytic reduction of the nitrate to recover the salt solution; and (3) biological denitrification of the nitrate to recover the salt solution. Direct disposal generally involves tankering waste off-site involving significant cost, not least because of the volumes involved; this can also lead to environmental problems. Both of the recovery processes (2) and (3) also present difficulties. Electrolytic reduction is energy intensive and subject to problems with the formation of scale on the surfaces of the equipment. Although the energy requirement is lower for biotreatment, the use of conventional open-flow bioreactors generally leads to contamination of the treated brine by bacteria and organic by-products from the denitrification process. Biological denitrification also has the disadvantage that this produces a sludge waste for which another disposal route must be identified. In order to reduce the requirement for salt and spent brine disposal in ion exchange, several procedures have been proposed for recycling spent brine. For example, non-membrane based biological treatments have been used for the removal of nitrate from spent brine; the refreshed brine is then recycled back to the ion exchanger. However, it has been found that these methods result in a significant loss in resin capacity when used repeatedly for regeneration of the resin beds.

To enable ion exchange to be applied more widely and more economically, these problems relating to the disposal and/or recycling of brines produced during regeneration of exhausted resins must be addressed.

Brines having a high concentration of oxyanions are also produced as waste in other industrial processes, such as armaments plants, and these must be disposed of without being considered a hazardous waste material. Alternative methods for handling these waste products are therefore also required.

We have now found that membrane bioreactors can effectively be used in methods of treating brine containing high levels of oxyanions such as nitrate, for example in methods of treating spent brine produced following regeneration of ion exchange resins used in the treatment of drinking water. Once treated, the 'regenerated' brine can either be disposed of by conventional means without necessarily being considered hazardous waste (due to its low oxyanion content) or, more preferably, this can be recycled (e.g. for use in regeneration of the exchange resins) thereby minimising the production of waste and consumption of salt.

Membrane bioreactors (also referred to herein as "MBRs") combine membrane filtration with biological processing to provide intensive biotreatment combined with highly effective clarification (i.e. solids and turbidity removal) to produce an effluent which is low in organic matter. In comparison to open-flow bioreactors, MBRs have the advantage of preventing the micro-organisms and micro-organism waste from contaminating the effluent (permeate). Whilst examples of biological denitrifi cation of potable water exist, including membrane biotechnology-based systems, to date these have generally been applied directly to the feed water stream, rather than the waste product from a standard physicochemical process such as ion exchange. The use of membrane-based processes to treat the main flow are effective, but made prohibitively expensive by the high cost incurred by the membrane at the fluxes involved (typically these are of the order of 10-25 L.m^.h^'1). As a result of the shear forces generated in the main feed stream, there is also increased risk of mechanical cell damage when using the bioreactor upstream of the ion exchange resin beds.

In one aspect of the invention, a process for reducing the oxyanion (e.g. nitrate) content of an aqueous solution, preferably drinking water, is provided. This process involves the use of at least one, preferably two, membrane bioreactors (MBRs) operated downstream of a conventional ion exchanger. Where two MBRs are employed, these are operated in series. In this process, spent brine produced following the regeneration of an exhausted ion exchange resin is treated in a first MBR operated under anoxic conditions (herein referred to as an "anoxic MBR") to produce an effluent (permeate) which has a low oxyanion (e.g. nitrate) content and which can either be disposed of or, alternatively, re-used in the ion exchange process (as a regenerant for the ion exchange resin beds). Recycling of the spent brine not only removes the need to tanker away the waste stream, but also prevents the loss of salt from the water treatment process which would otherwise need replenishing.

Viewed from a first aspect, the invention thus provides a method of reducing the content of at least one oxyanion present in an aqueous solution (e.g. water sourced for drinking), said method comprising the following steps:

(a) contacting said solution with an anion exchange resin having an affinity for said oxyanion whereby to produce an effluent having a reduced oxyanion content and an oxyanion-loaded ion exchange resin;

(b) contacting the oxyanion-loaded ion exchange resin with a regenerant solution (e.g. a brine) whereby to form a treated anion exchange resin having a reduced oxyanion load relative to the oxyanion-loaded ion exchange resin and a spent regenerant solution having an increased oxyanion content relative to the regenerant solution; (c) treating the spent regenerant solution in a membrane bio-reactor under anoxic conditions whereby to produce an anoxic treatment effluent having a reduced oxyanion content relative to the spent regenerant solution; and

(d) either disposing of or, more preferably, recycling the anoxic treatment effluent to said anion exchange resin for use as the regenerant solution (or as a component of the regenerant solution) in step Qa).

As a result of the treatment process in the first anoxic MBR a waste sludge (generated from the growth and death of the microbial community) is also produced. Preferably, this waste sludge is treated by means of a second MBR operated under aerobic conditions (herein referred to as an "aerobic MBR") in which the sludge is used as the feedstock for a second microbial community (which be the same or different to the microbial community used in the first anoxic MBR). In this second MBR the sludge is degraded thereby removing the requirement for tankering sludge away. Although not essential, the effluent (permeate) from this second MBR can also be recycled for use as regenerant (or as a component of the regenerant) in the ion exchange process. This results in an essentially 'zero waste' process.

Thus, viewed from a second aspect, the invention provides a method of reducing the content of at least one oxyanion present in an aqueous solution (e.g. water sourced for drinking), said method comprising the following steps:

(b) contacting the oxyanion-loaded ion exchange resin with a regenerant solution (e.g. a brine) whereby to form a treated anion exchange resin having a reduced oxyanion load relative to the oxyanion-loaded ion exchange resin and a spent regenerant solution having an increased oxyanion content relative to the regenerant solution;

(c) treating the spent regenerant solution in a membrane bio-reactor under anoxic conditions whereby to produce an anoxic treatment effluent having a reduced oxyanion content relative to the spent regenerant solution and a waste sludge;

(d) treating the waste sludge in a membrane bioreactor under aerobic conditions whereby to produce an aerobic treatment effluent having a reduced sludge content; and

(e) either disposing of or, more preferably, recycling the anoxic treatment effluent and/or the aerobic treatment effluent to said anion exchange resin for use as the regenerant solution (or as a component of the regenerant solution) in step (b).

In additional aspects the invention provides apparatus and systems suitable for use in carrying out the methods herein described. Such systems comprise an anoxic membrane bioreactor downstream of an anion exchanger. Optionally, an aerobic membrane bioreactor may also be embodied in such a system. Where an aerobic MBR is present, this is provided downstream of the anoxic MBR. Such systems optionally further comprise means for recycling effluent (permeate) from one or both of the MBRs to the anion exchanger, optionally via one or more holding tanks.

Viewed from a further aspect the invention thus provides a water treatment system for reducing the content of at least one oxyanion present in an aqueous solution, said system comprising an anion exchanger upstream of an anoxic membrane bioreactor, together with means (e.g. a conduit or pipe) for recycling at least a portion of the anoxic treatment effluent from said bioreactor back into the anion exchanger. Preferably, the system may further comprise an aerobic membrane bioreactor downstream of the anoxic membrane bioreactor and, optionally, additional means for recycling at least a portion of the aerobic treatment effluent from the aerobic MBR back into the anion exchanger. The means for recycling the effluent(s) may comprise piping and a pump (or pumps).

As used herein, the term "aerobic" means the presence of air or oxygen. The term "anoxic" means that no oxygen or substantially no oxygen is present (e.g. the dissolved oxygen concentration is zero), but that an alternative electron acceptor (source of oxygen), e.g. nitrate or nitrite, may be present. In relation to the treatment of a regenerant solution (e.g. brine) under anoxic conditions, this means that the solution (e.g. brine) has zero or a minimal amount of dissolved oxygen in the solution during microbial treatment in the MBR. Anoxic conditions occur when there is less than 0.1 mg.L^"1 of dissolved oxygen (DO) in the MBR. Preferably, there is less than 0.1 mg.L^"1 of DO in a substantial part of the MBR.

The water treatment systems and methods herein described are capable of providing high-quality product brine combined with minimal, preferably zero, waste. They are particularly appropriate for remote sites with no facility for discharge to sewer and where all wastes must otherwise be tankered off-site. Moreover, the ability to run an ion exchange water treatment plant at zero (or near zero) waste is particularly important. A consequence of this is that the treatment plant has a far smaller carbon footprint than existing technologies.

The methods herein described are preferably operated on a continuous flow basis.

In the methods herein described the feed water supplied to the ion exchanger generally comprises water or waste water which is contaminated with at least one oxyanion compound. The methods may be used to treat water streams including ground water, drinking water, waste water, surface run off water, etc. However, preferably, the feed water comprises raw water for drinking water. Several oxyanions may be present and the processes and systems herein described may be used to reduce the concentration of one, or more than one, oxyanion present. Typically, the oxyanions present may be one or more ions selected from nitrate, nitrite, perchlorate, sulphate, cyanide, borate and bromate. These oxyanions are typically present at levels in excess of 1 μg.L^"1 (e.g. perchlorate), e.g. in the range 1 μg.L^"1 to 150 mg.L^"1. The methods and systems herein described are particularly suitable for the reduction of nitrate in aqueous solutions, e.g. in drinking water. A representative feed water may contain from about 1 μg.L^"1 to 150 mg.L^"1, e.g. 35 mg.L^to 150 mg.L^"1 nitrate.

The flow rates of feed water entering the IEX resin system during exhaustion (i.e. Cl^" is displaced from the resin and NO₃ ^" is taken up from the feed water onto the resin) are conventional. For example these may range from 5 to 25 bed volumes (BV) of water per hour, typically from 15 to 25 bed volumes (BV) of water per hour.

Resins suitable for use in water treatment units are well known and described in the art and may be selected based on the nature of the contaminant being removed. Any conventional anionic exchange resin that exhibits the desired affinity for the oxyanion (or oxyanions) of interest (e.g. nitrate, perchlorate, bromate, etc.) may be used in the invention. Anionic exchange resins having an affinity for nitrate, particularly those which are selective for nitrate, are particularly preferred.

Anionic exchange resins which may be used in the invention are those which contain ligands having cationic functional groups such as -NHR₂ and -NR₃ (where R is an organic group). Suitable cationic groups include, for example, diethylaminoethyl (DEAE), quaternary aminoethyl (QAE) and quaternary ammonium (Q). Of these, quaternary ammonium (Q) is particularly preferred. Particularly suitable for use in the invention are strong base resins based on various polymer structures such as cross-linked polystyrene with suitable active groups such as quaternary ammoniums. Representative resins that have Drinking Water Inspectorate approval in the UK include MIEX DOC Resin (Orica UK Ltd- Watercare Europe), Purolite A520E nitrate selective anion exchange resin (Purolite International Ltd), AMBERLITE PWA5, IMAC HP 336 and IMAC HP 555 (Rohm and Haas (UK) Ltd). These anionic resins can also remove dissolved organic carbon, colour, nitrate, arsenic, sulphide, bromide and chromium (IV).

Separation of the reduced oxyanion content product water from the oxyanion loaded ion exchange resin yields the desired product (i.e. an aqueous solution having a low oxyanion content). As the resin becomes partially exhausted its performance in water purification decreases. Often a resin can be considered "exhausted" when as little as 30 to 40% of the total available capacity has been used up. Once the resin shows breakthrough for the oxyanion (generally after 250 to 450 bed volumes of water) the flow of influent water is halted and the regenerant solution (e.g. brine) is added.

The ion exchange resins are regenerated by washing or rinsing with a solution (herein referred to as a "regenerant solution") containing a high concentration of ions capable of displacing or desorbing the captured oxyanions. The regenerant solution is one having an oxyanion desorbing salt content and may be any solution conventionally used in the regeneration of anion exchange resins. Generally, this will be a brine. As used herein, the term "brine" means any aqueous solution having dissolved therein a quantity of a monovalent alkali metal salt to provide a salinity of at least 3%. Concentrated brine, i.e. water which is nearly saturated with a salt is preferred for use in the invention. Due to its low cost, sodium chloride is particularly preferred. For example, the regenerant solution may be a brine having a sodium chloride content of at least 4% by weight, preferably from 5% to 20% by weight. A typical regenerant solution may comprise from 5 to 10 wt.% sodium chloride.

Regeneration may involve the use of one bed volume of regenerant solution (e.g. brine). This may be allowed to contact the resin for a period in the range of from 20 to 60 mins. More than one bed volume of regenerant can be used, e.g. typically 2.5 to 5 bed volumes, if desired. The resin contact time is determined by regenerant salt concentration and the nature of the resin used.

Spent regenerant solution is separated from the regenerated ion exchange resin to form a waste stream. This waste stream comprises spent ion exchange regenerant (typically NaCl) and displaced oxyanions. The content of these components will vary depending on several factors, including the initial salt concentration of the regenerant solution, the oxyanion content of the initial feed water, etc. However, in general the spent regenerant may be expected to contain levels of salt (e.g. sodium chloride) from about 2 to 20 % by weight, e.g. about 2 to about 12 % by weight and from about 0 to 500 mg.L^"1, e.g. from about 5 to about 100 mg.L^"1 oxyanions. A typical regenerant solution may, for example, contain from 5 to 10 % by weight NaCl and from 0 to 50 mg.L^"1 nitrate.

The waste stream from the ion exchanger flows directly into the anoxic MBR which produces a clarified brine product. Depending on the chloride concentration, this brine product may be disposed of as hazardous waste. However, it is preferred that this is recycled for use as the regenerant solution, optionally via a suitable storage tank. If required, the salt level of this solution may be adjusted to enable this to be used as a regenerant. Salt concentration may be adjusted either manually by the addition of salt to the solution or may be automatically controlled. In both cases, a conductivity measuring device may be used to indicate if salt needs to be added or not. The automatic system preferably uses a feedback loop between the dispensing equipment, e.g. a hopper feed or dosing pump and the conductivity measurement device.

Due to the high concentration of oxyanions (e.g. nitrate) in the regenerant stream, it is desirable to operate the anoxic MBR at relatively short solids retention times (SRTs). Suitable SRTs may readily be determined by those skilled in the art and can be adjusted by controlling the sludge wasting rate, but in general may range from 0 to 40 days, e.g. 12 to 72 hours.

Treatment in the anoxic MBR can also lead to the production of potentially large volumes of waste sludge (this includes a high proportion of dead cells). This may be tankered off-site for disposal. However, preferably, this waste stream (which is low in oxyanion content such as nitrate, high in salt and high in sludge) is fed into an aerobic membrane bioreactor in order to reduce the sludge content of the waste stream. In this unit, the sludge product is degraded aerobically, i.e. oxidised to carbon dioxide by micro-organisms fed with air. Ideally, this process is operated at long SRTs with minimal, preferably zero, waste sludge production. SRTs may be infinite (i.e. resulting in no waste sludge), but are typically in the range of from 0 to 200 days, e.g. 10 to 100 days.

The permeate from the aerobic MBR is a clarified solution (which is high in salt, low in oxyanion content such as nitrate, and low in sludge). Due to its low oxyanion content, but high chloride concentration, this may need to be disposed of as hazardous waste. However, this is also suitable for use as a regenerant solution for the ion exchanger. Thus, in a preferred aspect of the invention this permeate is recycled, optionally via a regenerant storage tank, for use as a regenerant solution for the ion exchange resin thereby further minimising losses from the process loop. Although fairly energy intensive in terms of aeration demand per unit volume of sludge feed, the contribution from this additional processing step to the overall costs per volume of treated water is minimal since the flows involved are low.

The membrane bioreactors (MBRs) used in the processes of the invention combine a membrane process, typically microfϊltration or ultrafiltration, with a suspended growth bioreactor thereby eliminating the need for any further clarification filtration. The advantages of MBRs over conventional processes include their small carbon footprint and the ease with which they may be used to retrofit and upgrade existing water treatment plants.

Two MBR configurations exist, both of which may be used in the context of the present invention. In the first configuration, the membranes are submerged in (i.e. immersed) and integral to the biological reactor. In the second, the membranes are provided in a separate membrane tank and an intermediate pumping step is required. These external or 'sidestream' MBRs are generally less preferred for use in the methods herein described due to the additional energy requirements involved in pumping the treated material to the separate membrane unit. Sidestream MBRs rely on pumping in order to create the necessary transmembrane pressure to achieve filtration. Not only does this incur a cost penalty, but increases the probability of breaking apart the microbial biomass floes that form in the MBR. Floe breakage increases the chance of membrane fouling which in turn reduces membrane efficiency. In contrast to sidestream MBRs, submerged MBRs use air to create the tangential sheer and operate at a much lower flux. The energy requirements are therefore much lower thereby leading to much lower operating costs.

Submerged MBRs comprise a plurality of ultraporous or microporous membranes submerged in a tank of wastewater with suction applied to one side of the membranes. In this way, clean water permeates through the walls of the membranes but bacteria and any suspended solids are retained by the membranes and remain in the tank to be biologically treated. Several different types of membrane are conventionally used which include materials such as ceramics or polymeric materials. Average pore sizes for microfiltration membranes generally range from 0.05 to 2 microns, preferably from 0.1 to 1 microns, more preferably 0.1 to 0.4 microns, whereas ultrafiltration membranes typically have an average pore size in the range 0.005 to 0.5 microns, preferably 0.01 to 0.1 microns, more preferably between 0.01 and 0.04 microns. Suitable membranes includes those supplied by Zenon Environmental, Inc. under the tradename Zeeweed. Other bioreactor membranes are available and are known to those skilled in the art. The membrane modules may be located in the middle of the tank or, alternatively, along one wall of the tank.

The process of denitrification in the anoxic MBR involves the reduction of nitrate to nitrogen gas through a series of intermediate gaseous nitrogen oxide products:

NOf -> NO₂ ^" → NO → N₂O → N₂

This may be expressed as a redox reaction:

2NO₃ ^" + 1Oe^" + 12H⁺ → N₂ + 6H₂O

The nitrogen gas produced is released to the atmosphere and thus removed from the water. Denitrification occurs where oxygen, a more energetically favourable electron acceptor, is depleted such that the bacterial biomass respires nitrate as a substitute electron acceptor. Anoxic conditions are therefore required to encourage the growth of the bacteria.

The conversion of nitrate to nitrogen gas is effected using heterotrophic bacteria which require an organic carbon substrate for growth. Both monocultures and mixed cultures of bacteria may be used to facilitate denitrification and these may include, for example communities developed from samples of salterns, salt lakes and marine sediments. Due to the high salinity of the waste stream, these must be capable of tolerating such an environment, i.e. halotolerant. Halophilic bacteria are an example of such organisms. Because these live in media with very low or zero oxygen, they possess a mechanism of electron transport to acceptors other than free oxygen, for example the bound oxygen in nitrate. This capability renders these suitable for the denitrification of brines.

Halotolerant cultures have been developed which are able to survive in low to moderate salinity environments (1-3 wt.%) equivalent to those of seawater (~3 wt.%). Such cultures may be used in the methods according to the invention, however, this generally necessitates dilution of the feed water (i.e. regenerant solution) prior to biological treatment and subsequent amendment post-treatment with salt (NaCl) to approach an appropriate concentration for ion exchange regeneration. As an alternative to this, adaptation of halophilic micro-organisms (i.e. ones which are still active at high salinities) is possible. Halophiles have been reported to survive in salinities up to 18 wt.% and are therefore preferred for use in the invention. These micro-organisms are often characterised by their slow growth rates and cases of 'washout' have been reported. However, using membrane technology provides complete retention of the microbial community countering growth rate problems associated with halophiles and the process influent variation observed in this application due to the subsequent intensification. Suitable halophilic micro-organisms which may be used to denitrify brine in the anoxic MBR include Halomonas denitrificans, Halomonas campisalis and many other heterotrophic and halotolerant microorganisms.

Since denitrification involves the reduction of nitrate to nitrogen gas, an electron donor is needed. Typically, this will be an added electron donor such as an organic carbon substrate, e.g. methanol, ethanol, acetic acid, acetate, glucose, etc. Particularly preferred for use as an organic carbon source is ethanol which is applied as the electron donor to counter nitrate reduction:

5C₂H₅OH + 12NO; → 6N₂ + 1 OCO₂ + 9H₂O + 12OH-

In addition to the electron donor, the MBR will generally further contain standard nutrients suitable for the growth of the biomass. Typical examples of such nutrients include trace metals, phosphate, etc.

Typical organisms for use in the aerobic MBR include bacteria with a rapid growth rate and which are capable of consuming carbon aerobically. Suitable bacteria may readily be determined by those skilled in the art.

A key operating parameter of any MBR process is the flux, i.e. the rate at which water flows through a unit area of membrane surface. The flux is always limited by fouling, which is the accumulation of materials on the surface of the membrane and/or in the pores which limit its permeability thereby inhibiting the flow of water through it. A number of methods are used to deal with fouling, including both mechanical and chemical cleaning, such as reverse rinsing, chemical cleaning using sodium hypochlorite, etc.

In a conventional submerged MBR the membrane is scoured with air bubbles in a constant or near constant manner, to maintain the flux at a reasonable level. This procedure may be employed in relation to the aerobic treatment tank herein described. However, such ameliorative action is not appropriate for a denitrification or anoxic MBR because anoxic (low-oxygen) conditions must be maintained for the biological conversion of nitrate (NO₃ ^") to nitrogen gas. A number of methods have been proposed for dealing with this problem, including the supply of large scouring bubbles to clean the membranes. It has been found that such coarse bubbles (typically having an average diameter of greater than 1 mm) do not transfer sufficient oxygen to the feed water to create aerobic conditions throughout the reactor. Sensors within the tank may be used to measure the level of dissolved oxygen (DO) and may be used to control the aerating bubble supply.

As described above, submerged MBRs are preferred for use in the methods according to the invention. The use of a submerged anoxic MBR under highly saline conditions forms a yet further aspect of the invention.

Viewed from a third aspect the invention thus provides a method of reducing the content of at least one oxyanion (e.g. nitrate) in an aqueous solution having a high salinity, said method comprising the step of treating said solution in a submerged membrane bioreactor under anoxic conditions. Suitable solutions which may be treated according to this particular aspect of the invention are brines having a salinity greater than 3%, preferably from 3 to 20%, more preferably from 3 to 15%, e.g. 3 to 10%.

Whereas previously proposed methods involving the use of various non- membrane based biological brine regeneration treatment processes have been found to result in a significant loss in resin capacity when repeatedly regenerating ion exchange resin beds, it has been found that the application of a membrane bioreactor retains the halophilic bacteria and some of the macromolecular species evolved from the bacteria. Although it is observed that a proportion of lower molecular weight organic carbon passes through the membrane, the impact of this on the resin is, surprisingly, limited. Although not wishing to be bound by theory, it is believed that any small molecules that are formed during the various treatment processes and which pass into the permeate from the treatment tanks have limited binding opportunities to the resin beds due to their low surface charge affinity. Fouling of the membranes is therefore reduced which, in turn, provides a higher final regenerant quality.

For example, when compared to a control (ion exchange resin bed regenerated with fresh brine), the resin capacity available prior to regeneration as a result of the process herein described was very similar. This is demonstrated in accompanying Example 1 and Figure 6. Whilst it is seen from Figure 6 that brine treated in accordance with the process herein described does lead to some loss in efficiency of the resin process (specifically a loss of capacity for nitrate), this loss is finite. This is in contrast to conventional biological IEX regeneration methods wherein the % loss of resin capacity increases during operation.

The methods herein described find particular use in the treatment of spent regenerant from an ion exchange process. However, the combined use of an anoxic MBR and an aerobic MBR also finds wider use in removing oxyanions from other waste streams having a high salt content, such as may be derived from other industrial processes including electrodialysis and reverse osmosis.

Viewed from a fourth aspect the invention thus provides a method of reducing the content of at least one oxyanion present in an aqueous solution, said method comprising the following steps:

(a) treating said solution in a membrane bioreactor under anoxic conditions whereby to produce an anoxic treatment effluent having a reduced oxyanion content and a waste sludge; and

(b) treating the waste sludge in a membrane bioreactor under aerobic conditions whereby to produce an aerobic treatment effluent having a reduced sludge content. Apparatus suitable for use in carrying out such a method forms a yet further aspect of the invention.

Viewed from a further aspect the invention thus provides apparatus for reducing the content of at least one oxyanion present in an aqueous solution, said apparatus comprising an anoxic membrane bioreactor and downstream thereof an aerobic membrane bioreactor.

Whilst the processes and apparatus herein described are primarily intended for use in the treatment of drinking water, these have wider applicability. For example, these can be applied to other aqueous solutions such as those produced as waste streams in other industries, such as in the generation of power, e.g. nuclear fuel reprocessing or in the maintenance of marine environments (aquaculture).

Certain preferred embodiments of the processes and apparatus of the invention will now be described further by way of the following non-limiting Example and with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of one embodiment of the invention in which an ion exchange regenerant solution is recycled;

Fig. 2 is a schematic representation of an ion exchanger which may be used in the embodiment of the invention shown in Fig. 1;

Fig. 3 is a schematic representation of another embodiment of the invention in which a dual tank system is used to treat an influent having an elevated oxyanion content;

Fig. 4 is a schematic representation of another embodiment of the invention which illustrates the use of a submerged membrane bioreactor in treating an influent having a high salt concentration;

Fig. 5 is a schematic representation of the experimental set-up used in Example 1; and

Fig. 6 is a graph showing the ion exchange breakthrough curves observed when using fresh brine and brine treated according to an embodiment of the invention.

With reference to Figure I₅ there is shown a dual tank system for ion exchange regenerant recycling which comprises a first anoxic membrane bioreactor 1 and a second aerobic treatment tank 2 connected in series. Influent 3 containing elevated levels of nitrate is pumped to an ion exchange chromatography resin 4 which is selective for nitrate ions. The reduced nitrate flow-through from the ion exchanger 4 is discharged as effluent 5. When the resin has reached capacity, it must be regenerated. This is achieved by adding a concentrated (e.g. 5-10 wt.%) NaCl solution such that the Cl^" ions displace the resin-bound nitrate. The regenerant solution 6 which leaves the ion exchange resin 4 is high in nitrate and high in salt. The volume of the regenerant solution 6 is expected to be approx. 2-4% of that of the influent 3. The regenerant solution 6 is pumped to the first anoxic membrane bioreactor 1 which is connected to a carbon source 7 (which acts as the electron donor) and a nitrogen source 8 (for scouring the membrane). Optionally, the regenerant 6 may be diluted prior to entering the bioreactor 1 (not shown in Fig. 1). The permeate 9 from the first bioreactor 1 is a clarified denitrified brine product (low in nitrate, high in salt) which is recycled as ion exchange resin regenerant via a brine holding tank 10. If desired, the salt concentration of the permeate 9 can be amended (in order to ensure that it is suitable for resin regeneration) prior to circulation to, or in, the brine holding tank 10. Suitable means for salt amendment may be adding salt manually or using automated machinery to measure and dispense salt as required based on solution conductivity measurements (not shown in Fig. 1). A by-product of the denitrification process is waste sludge 11. This is pumped to a second membrane bioreactor 2 which degrades the waste sludge 11 aerobically to produce carbon dioxide which is vented to the atmosphere. In order to maintain the aerobic conditions in this second bioreactor 2, it is provided with a supply of air 12. The permeate from the second bioreactor 2 is a clarified brine which is also fed to the brine holding tank 10. As required, the brine product or regenerant 13 is recirculated from the brine holding tank 10 to the ion exchange resin 4 whereby to regenerate the resin beds.

Figure 2 shows a conventional ion exchange resin bed. Feed water high in NO₃ ^" is passed through the resin bed, Cl^" ions are displaced from the resin into the water and NO₃ ^" ions are taken up onto the resin. The effluent from the process is the treated water which is low in NO₃ ^" compared to the feed water. When the ion exchange resin is nearly exhausted, the bed is regenerated by using a brine solution (high concentration of Cl^" ions), this process produces a waste stream (spent regenerant) that is high in salt and nitrate concentrations. The spent regenerant constitutes the feed to the MBR system of the invention.

Figure 3 shows a dual tank system for treating an influent 14 having a high nitrate and high salt content which comprises a first anoxic membrane bioreactor Ia and a second aerobic treatment tank 2a connected in series. Influent 14 is fed to the first anoxic membrane bioreactor 1 a which is connected to a carbon source 7a (which acts as the electron donor) and a nitrogen source 8a (for scouring the membrane). The permeate 15 from the first bioreactor 1 a is a clarified product having a reduced nitrate content. A by-product from the first bioreactor 1 a is waste sludge 16. This is pumped to a second membrane bioreactor 2a which degrades the waste sludge 16 aerobically to produce carbon dioxide which is vented to the atmosphere. In order to maintain the aerobic conditions in this second bioreactor 2a, it is provided with a supply of air 12a. The permeate or effluent from the second bioreactor 2a is a clarified product 17.

With reference to Figure 4 there is shown an anoxic membrane bioreactor Ib for treating an influent 18 having a high nitrate and high salt concentration. Influent 18 is fed to the bioreactor Ib which is connected to a carbon source 7b (which acts as the electron donor) and a nitrogen source 8b (for scouring the membrane). The permeate 19 from the bioreactor Ib is a clarified product having a reduced oxyanion (e.g. nitrate) content. A by-product from the first bioreactor Ib is waste sludge 20.

Example 1 - Ion exchange regeneration with biologically denitrified brine

Figure 5 is a schematic representation of the configuration of a membrane bioreactor used in this Example for the biological denitrification of brine. Denitrification was carried out in a mixed vessel (75 litre volume) within which the membrane module (Zenon ZW-10 PVDF) was sited. The membrane module had a surface area of 0.93 m² and a nominal pore size of 0.04 μm. A temperature controlled jacket was fitted around the outside of the denitrification vessel.

An analogue feed with an influent feed concentration of 500 mgN.L^"1 (2214 IHgNO₃-X^"1) was fed into the reactor. The hydraulic residence time within the reactor was 18.5-21 hours. The SRT (solids retention time, i.e. wastage rate from the denitrification reactor) was 10 days. The stirrer speed was set to a shear intensity, G = 12.8 s^"1, providing sufficient mixing within the reactor to suspend the biomass. Temperature was controlled at approximately 20⁰C. Three solids retention times were passed before sampling began.

Nitrogen gas, used to scour the membrane surface, was produced from a nitrogen selective hollow fibre membrane fed with high pressure air. Nitrogen gas intermittency was controlled by a solenoid valve connected to a timer relay.

Permeate (effluent) was extracted from the membrane using a piston pump which delivered the permeate to a backflush tank with a 20 litre capacity. Permeate from this tank drained by gravity to a holding tank (T₂). When the fluid level in the holding tank (T₂) exceeded a set height, a conductivity probe connected to a relay set two pumps to: (1) drain fluid back to the main feed holding tank (T₁); and (2) dose potassium nitrate (KNO₃) to adjust the permeate concentration up to a set nitrate concentration. Where additional fluid was required in the loop, a solenoid valve was opened on tank T₁ to permit the inflow of tap water amended by NaCl and KNO₃.

In a series of experimental runs, the ion exchange resin was subject to exhaustion and subsequent regeneration. In the first series, brine which had been biologically denitrified (i.e. regenerated) in accordance with the procedure set out above was used. In the second series, fresh brine was used (on fresh resin).

Figure 6 shows that the impact of the biologically regenerated brine is on the upper portion of the exhaustion curve. When the area of anticipated regeneration is compared (marked by the oval), both curves are identical. Further resin regenerations by the biologically regenerated brine produced the same shaped resin exhaustion curve indicating that no further impact occurred. This confirms that the presence of organic materials derived from the brine regeneration process has negligible impact on the ion exchange uptake.

Claims

Claims:

1. A method of reducing the content of at least one oxyanion present in an aqueous solution (e.g. water sourced for drinking), said method comprising the following steps:

(c) treating the spent regenerant solution in a membrane bio-reactor under anoxic conditions (herein referred to as "anoxic MBR") whereby to produce an anoxic treatment effluent having a reduced oxyanion content relative to the spent regenerant solution; and

(d) either disposing of or, more preferably, recycling the anoxic treatment effluent to said anion exchange resin for use as the regenerant solution, or as a component of the regenerant solution, in step (b).

2. A method as claimed in claim 1 , said method comprising the following steps:

(c) treating the spent regenerant solution in a membrane bio-reactor under anoxic conditions (herein referred to as "anoxic MBR") whereby to produce an anoxic treatment effluent having a reduced oxyanion content relative to the spent regenerant solution and a waste sludge;

(d) treating the waste sludge in a membrane bioreactor under aerobic conditions (herein referred to as "aerobic MBR") whereby to produce an aerobic treatment effluent having a reduced sludge content; and

(e) either disposing of or, more preferably, recycling the anoxic treatment effluent and/or the aerobic treatment effluent to said anion exchange resin for use as the regenerant solution, or as a component of the regenerant solution, in step (b).

3. A method as claimed in claim 1 or claim 2 which comprises recycling the anoxic treatment effluent and/or, where present, the aerobic treatment effluent to said anion exchange resin, optionally via one or more storage tanks, for use as the regenerant solution, or as a component of the regenerant solution.

4. A method as claimed in any one of claims 1 to 3, wherein the anoxic treatment effluent comprises a clarified brine, the salt level of which is adjusted prior to use as the regenerant solution, or as a component of the regenerant solution.

5. A method as claimed in any preceding claim, wherein the aqueous solution contains one or more oxyanions selected from nitrate, nitrite, perchlorate, sulphate, cyanide, borate and bromate.

6. A method as claimed in claim 5, wherein said oxyanions are present in the aqueous solution at levels in excess of 1 μg.L^"1, e.g. in the range 1 μg.L^"1 to 150 mg.lΛ

7. A method as claimed in any one of claims 1 to 4, wherein the aqueous solution contains nitrate ions.

8. A method as claimed in claim 7, wherein said nitrate ions are present in the aqueous solution at a concentration of from about 1 μg.L^"1 to 150 mg.L^"1, e.g. 35 mg.L^ to 15O nIgX^'1.

9. A method as claimed in any preceding claim, wherein the aqueous solution is ground water, drinking water, waste water or surface run off water, preferably drinking water.

10. A method as claimed in any preceding claim, wherein the regenerant solution is an aqueous solution having dissolved therein a quantity of monovalent alkali metal salt to provide a salinity of at least 3%.

11. A method as claimed in claim 10, wherein the regenerant solution comprises sodium chloride, preferably in an amount of at least 4% by weight, more preferably from 5% to 20% by weight.

12. A method as claimed in any preceding claim, wherein the anoxic MBR is operated at relatively short solids retention times (SRTs), preferably from 0 to 40 days, e.g. 12 to 72 hours.

13. A method as claimed in any one of claims 2 to 12, wherein the aerobic MBR is operated at relatively long solids retention times (SRTs), preferably in the range of from 0 to 200 days, e.g. 10 to 100 days.

14. A method as claimed in any preceding claim, wherein the anoxic MBR and, where present, the aerobic MBR comprise submerged membranes.

15. A method as claimed in claim 14, wherein said membranes are microfiltration membranes with an average pore size in the range from 0.05 to 2 microns, preferably from 0.1 to 1 microns, or ultrafiltration membranes with an average pore size in the range 0.005 to 0.5 microns, preferably 0.01 to 0.1 microns.

16. A method as claimed in any preceding claim, wherein the anoxic MBR contains a culture of heterotrophic bacteria, preferably halophilic bacteria, e.g. bacteria selected from the group consisting of Halomonas denitrificans and Halomonas campisalis.

17. A method as claimed in any preceding claim, wherein the anoxic MBR further contains an electron donor, preferably an organic carbon substrate.

18. A water treatment system for reducing the content of at least one oxyanion present in an aqueous solution, said system comprising an anion exchanger upstream of an anoxic membrane bioreactor, optionally together with means (e.g. a conduit or pipe) for recycling at least a portion of the anoxic treatment effluent from said bioreactor back into the anion exchanger.

19. A water treatment system as claimed in claim 18 which further comprises an aerobic membrane bioreactor downstream of the anoxic membrane bioreactor and, optionally, means for recycling at least a portion of the aerobic treatment effluent from the aerobic MBR back into the anion exchanger.

20. A method of reducing the content of at least one oxyanion (e.g. nitrate) in an aqueous solution having a high salinity, said method comprising the step of treating said solution in a submerged membrane bioreactor under anoxic conditions.

21. A method of reducing the content of at least one oxyanion present in an aqueous solution, said method comprising the following steps:

(b) treating the waste sludge in a membrane bioreactor under aerobic conditions whereby to produce an aerobic treatment effluent having a reduced sludge content.

22. Apparatus for reducing the content of at least one oxyanion present in an aqueous solution, said apparatus comprising an anoxic membrane bioreactor and downstream thereof an aerobic membrane bioreactor.