GB2255088A

GB2255088A - Removal of contaminants from water

Info

Publication number: GB2255088A
Application number: GB9109085A
Authority: GB
Inventors: Robert Winston Gillham; Donald Lawrence Lush
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-04-25
Filing date: 1991-04-25
Publication date: 1992-10-28
Also published as: GB9109085D0; AU1668692A; CA2108981A1; WO1992019545A1

Abstract

In a water treatment plant, contaminants such as CTC are adsorbed onto activated carbon: the contaminants are removed by mixing granules of a metal, e.g. iron fillings, into the contaminant-laden activated carbon. The presence of the metal leads to the chemical breakdown of the contaminant. The cleansed activated carbon may be re-used, with or without separating the iron from the mixture.

Description

Title: REMOVAL OF CONTAMINANTS FROM WATER This invention relates to a system for the treatment of contaminated water, particularly water that has been contaminated with an halogenated hydrocarbon.

It is a common practice, in treating water that has been contaminated with such contaminants as carbon tetrachloride, to pass the water through an adsorption material such as activated carbon. Conventionally, the activated carbon is contained in a canister. The contaminated water is piped through the canister whereupon the dissolved contaminant is adsorbed onto the activated carbon.

In the conventional system, the contaminant builds up within the body of activated carbon and periodically, eg every three months, the activated carbon must be replaced or must be treated in order to remove the accumulated contaminant.

Replacement of the activated carbon entails the cost of the new activated carbon and the cost of disposing of the saturated carbon, and can be expensive, and cleansing and re-use of the activated carbon is preferred.

The conventional methods for cleansing and re-using the activated carbon involve driving the contaminant from the activated carbon, for example by means of a heating process.

In the conventional system, however, the contaminant, though separated from the activated carbon, still remains intact. The periodic cleansing of the activated carbon may or may not be carried out at the same site as the water treatment.

As far as the water treatment plant is concerned, cleansing the saturated activated carbon involves uncoupling the canister from the piping system at the plant, and #transferring the canister, with the contaminant-laden adsorptive material contained therein, to the activated carbon cleansing treatment facility.

Generally, the treatment process carried out upon the adsorptive material is expensive and inconvenient; apart from the fact that the canister has to be uncoupled and removed, also the activated carbon is saturated with the concentrated, hazardous, contaminant.

BASIC FEATURES OF THE INVENTION In the invention, the halogenated hydrocarbon contaminants are cleansed from the activated carbon by mixing the activated carbon with a metal. The mixture is maintained under the correct conditions of temperature, Eh, and pH, whereby the halogenated hydrocarbons undergo chemical breakdown, in the presence of the metal.

The contaminants with which the invention is concerned are those which can get into groundwater from accidental spills, or which may be present in effluent. One main class of contaminants is the halogenated hydrocarbons, which include pesticides, solvents, chloroform, PCB, etc. These substances become dissolved in water, in which state they can be hazardous in small trace quantities. Even the fluoride that is put into drinking water for dental health purposes, and even the chlorine that is added to drinking water to kill bacteria, can give rise to hazardous hydrocarbon substances which should be removed from the water.

The invention is based on the fact that such halogenated hydrocarbons can be broken down chemically by passing them over and through a body of a metal (in granular form). By what is thought to be a hydrolysis reaction, the halogenated hydrocarbon, in the presence of the metal, undergoes chemical breakdown: the chlorine or other halogen component of the contaminant molecule may be converted, for example, into an appropriate solid, insoluble chloride, which may remain in solution, or may precipitate out and be removable.

The chloride will in any case generally be harmless, at least in small concentrations. (A chlorinated hydrocarbon can be hazardous at tiny trace concentration levels, whereas the chloride can be harmless even at gross levels. It should be noted that the invention is not concerned essentially with totally eliminating the contaminant, but with reducing contaminant concentrations to tolerable levels.) In one aspect of the invention, contaminant-laden adsorptive material, of the kind which is conventionally and commonly produced from water treatment plants, is cleansed by mixing the adsorptive material with a metal. When this is done, the contaminant is chemically broken down: the contaminant disappears from the adsorptive material.There are two immediate benefits to this: one is that the contaminant no longer exists, as a hazard, and the other is that the (expensive) adsorptive material can be re-used.

The invention lies in a procedure and an apparatus for cleansing the contaninant-laden adsorption material. The invention also lies in a procedure and an apparatus for treating water that contains halogenated hydorcarbon contaminants.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT By way of further explanation of the invention, an exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig 1 illustrates a water treatment plant, for treating contaminated water; Fig 2 illustrates a cleansing facility, for cleansing activated carbon which is laden with contaminant.

The procedures and apparatus shown in the accompanying drawings and described below are examples which embody the invention. It should be noted that the scope of the invention is defined by the accompanying claims, and not necessarily by specific features of exemplary embodiments.

In Fig 1, contaminated water is conveyed through an entry pipe 2 to a canister 3. The water is contaminated with a dissolved halogenated hydrocarbon, such as, for example, carbon tetrachloride.

Often, the reason for the treatment is that the water must be de-contaminated before being allowed to enter a drinking water supply. The water treatment plant may be placed near the source of the contaminant, or near the drinking water supply draw-off point, depending on the circumstances as to the manner in which the contaminant arose, how it came to be discovered, whether the source is identified, etc.

The water is piped through the canister 3 by means of a pump 4, or under a natural pressure differential if such is available. The treated water emerges from the outlet pipe 5, whence it passes to the drinking water supply, or is otherwise discharged.

The canister contains a body 6 of activated carbon in granular form. The quantity of activated carbon, in relation to the throughflow of water, is such that all the contaminant is adsorbed onto the activated carbon, or at least sufficient of the contaminant is adsorbed that only legally permitted traces remain in the water.

After a time, the body of activated carbon 6 becomes saturated with the adsorbed contaminant. The canister 3 is then disconnected or uncoupled from the water pipes 2,5 and the canister, with the activated carbon contained therein, is removed from the water treatment plant and sent to the activated carbon cleansing facility.

At the cleansing facility, as shown in Fig 2, a quantity of metal 7 is mixed with the activated carbon 6 from the canister in a treatment vessel 8, which contains water 9.

The mixture is stirred thoroughly, to ensure that the metal is well-dispersed through the body of activated carbon.

The mixture of activated carbon and metal is then left in the treatment vessel under such conditions, as will be described, and for a sufficient time, that the halogenated hydrocarbon contaminants undergo chemical breakdown.

The metal 7 is in granular form. For example, the metal may be iron filings or powder, of the kind that is produced as waste in such industrial processes as the grinding or fettling of iron castings. Or the metal may be in the form of cuttings from metal cutting machines.

Whatever its source, care should be taken that cutting oils, for example, are removed from the metal. Preferably, the metal should be washed in suitable solvents, to remove oils etc. Also, the metal may be etched slightly in an acid, to remove oxide and expose the metal.

The metal preferably is iron, since iron is widely available in granular form inexpensively as waste from many processes.

The grain size of the granules of metal should be as small as possible, in order that the granules may have a maximum reactive surface area. On the other hand, the metal should not be in the form of so fine a dust as would make it difficult to handle.

The metal need not be elemental, so that steel or cast iron granules may be used, rather than pure iron. The metal selected for use in the invention should not be of a very low electrochemical activity: silver or gold, for example, would not be effective. Metals such as zinc, iron, aluminum, are candidates for selection on the basis of their electro-chemical activity, and considerations of practical availability will usually favour iron, as mentioned.

The presence of oxide on the metal is generally detrimental, and pre-treatment, for example an acid wash, is usually to be recommended to remove at least some of the oxide and expose the metal.

It has been found that sometimes the speed of the reactive effect attributable to a metal may be affected by the presence of other electrochemically active metals: for example, if galvanized iron is used as the source of the granules, the breakdown rate can be expected to be slightly slowed by the presence of the zinc, as compared with iron by itself. Also, it has been found that granules of stainless steel are not so effective as granules of ordinary carbon steels.

In some cases, it has been proposed that certain pairs of metals, mixed or alloyed together, will out-perform a single metal in breaking down such contaminants as the halogenated hydrocarbons. It should be understood that the invention may be used to advantage when the metal in the mixture is in fact such a pair of metals.

It takes a period of typically a few hours, or even a few days, for the chemical breakdown and disappearance of the halogenated hydrocarbon, in the presence of the metal, to be completed.

It is important that atmospheric oxygen, or any other oxidising agent, be excluded from the body of the mixture during the treatment process. This is not difficult to ensure, because in order for the chemical breakdown process to occur, the metal and the activated carbon mixture must at least be wet, and preferably should be under water. When the mixture is under water, the condition of excluding oxygen from the mixture generally follows automatically, so long as the water is not aerated. The breakdown or cleansing treatment therefore preferably is carried out with the mixture immersed in water.

The mixture should undergo a stirring action, to ensure against leaving pockets of intact contaminant. One way of achieving stirring is to circulate the water vigorously through the mixture. In fact, if the water were not to be stirred or circulated during cleansing, there might be a danger that, when the water came to be dumped at the end of the cleansing process, that some remaining intact contaminant might be dumped along with the water.

It is important to ensure homogeneity of the distribution, in the mixture, of the metal granules and of the activated carbon granules, and it is also important to ensure homogeneity of the distribution of the (gradually disappearing) contaminant within the water. Stirring or swirling the mixture is therefore to be preferred.

The stirring or swirling action should not be done in such a manner that oxygen would be introduced into the mixture or into the water. Periodic agitations will generally be adequate: it is not necessary that the stirring or swirling be carried on continuously.

At first, the main chemical effect that takes place during the initial period after the mixture has been prepared, is the reduction of the initial oxygen content of the mixture.

Such oxygen content may be present as a result of whatever oxygen is dissolved in the water, whatever oxygen or oxyidizing agents have been introduced in transferring the canister from the water treatment plant to the cleansing facility, whatever oxygen was present in or on the metal and the activated carbon, and so on.

The oxygen content is measured by the Eh probe, and it has been found that the Eh voltage must drop below a probemeasured voltage of about -200 millivolts before the chemical breakdown of the halogenated hydrocarbons will start to take place at economical rates.

The pH of the mixture should be monitored, and pH modifying substances added as necessary to maintain a pH level that promotes a rapid breakdown of the contaminant, bearing in mind that water that is to be dumped must be within certain pH limits.

Once the Eh voltage is below -200 mv, and especially below -600 mv, the breakdown of the halogenated hydrocarbon contaminant proceeds at a rapid rate.

It may be preferred in some cases that the process of adding the metal to the activated carbon, and the process of mixing and dispersing the metal within the body of activated carbon, be carried out under conditions of oxygen exclusion, for example, as mentioned, by carrying out the mechanical mixing operations under water. On the other hand, it is acceptable for the contaminant-saturated activated carbon, and the iron filings, to be simply poured from the canister into the water in the treatment vessel; such a short exposure to the atmosphere would not be significant. The saturated activated carbon should not, however, be allowed to dry out and be open to the atmosphere for more than, say, overnight.

When the mixture has been cleansed of the contaminant, to whatever level as is legally or otherwise dictated, the cleansed activated carbon may be re-used. In one procedure for re-using the activated carbon, the metal is removed from the mixture. This may be done by means of magnetic separation, or other suitable means. The cleansed activated carbon, now separated from the metal, is put back into a canister, and piped back into the water treatment plant.

In an alternative re-use procedure, the cleansed mixture of the metal and the activated carbon is left as it is: the mixture is put into the canister, and is piped back into the water treatment plant with the mixture1 including the metal, intact. In this alternative, the metal is left in the mixture, and in future the contaminated water is passed through the mixture of activated carbon and metal: the benefit of this is that the cleansing of the activated carbon takes place at the water treatment plant continuously and simultaneously with the de-contamination of the water.

The mixture can therefore be expected to remain operational more or less indefinitely, without needing to be cleansed.

However, in some cases where a water supply is passed over a body of metal, the water can become tainted by the metal itself and be rendered unsuitable for drinking. In those cases, the metal should be removed from the activated carbon prior to re-use.

But in those cases where tainting of the water by the metal is not significant, or not a consideration, it is preferred that the activated carbon be re-used without separating the mixture of activated carbon and metal. Now, the contaminated water thereafter is treated directly, at the water treatment facility, by the mixture of activated carbon and metal, and thus the activated carbon is cleansed at the same time as the water is de-contaninated.

When the activated carbon and the metal remain mixed together for re-use, the expense of separating them is of course also avoided.

In some previous ways of treating the activated carbon, the contaminants have been removed, at least partly, by the action of aerating the activated carbon, ie by bubbling air through the activated carbon. The halogenated hydrocarbons, being generally volatile, are stripped from the activated carbon granules and enter the stripping air. But this process leaves the contaminants intact, simply transferring them to the air, and it is becoming increasingly unacceptable simply to dump such contaminants into the atmosphere. Therefore the air-stripping process is no longer favoured: although air-stripping does cleanse the activated carbon, it does not solve the problem of disposing of the hazardous waste.

The new system as described is quite the opposite of aerating the activated carbon: in the new system air (oxygen) is excluded from the cleansing process to such an extent that the Eh voltage drops to -200 mv, and below, under which conditions the chemical breakdown of the contaminants can proceed. Under negative Eh conditions, the halogenated hydrocarbons can be broken down economically.

As mentioned, the new process of mixing metal granules with the activated carbon can be used not only to cleanse the activated carbon, but the mixture can also be used directly to treat the contaminated water. If the contaminated water that is being treated is already anaerobic, or substantially so, the direct treatment can be very economical. One preferred area of application of the direct treatnent, therefore, is in the de-contamination of ground water that is in its native aquifer, because groundwater generally contains very little dissolved oxygen. In contrast, when the contaminated water to be treated is, for example, effluent from a pesticide manufactory, or other surface water, the water can be expected to contain a high content of dissolved oxygen, and it takes time for this to disappear, which it must before the chemical breakdown of the contaminant can commence.

Claims

CLAIM 1. In a procedure for treating contaminated water,

wherein the water is passed through a body of an adsorption material; the contaminant is an halogenated hydrocarbon; the adsorption material is in granular form; wherein the improvement comprises the step of introducing metal in granular form into the body of adsorptive material, and of dispersing the metal within the adsorptive material.
CLAIM 2. Procedure for cleansing a body of adsorption material, which contains a contaminant adsorbed thereon, the contaminant being an halogenated hydrocarbon, and the adsorption material being in granular form; wherein the procedure includes the steps: of providing a body of a metal, the metal being in granular form; of making a mixture of the granules of metal and the granules of the adsorption material; and (a) of providing the metal in such quantity, in proportion to the quantity of adsorption material; (b) of so dispersing the granules of metal through the adsorption material; and (c) of maintaining the mixture under such conditions of temperature, Eh, and pH, and of so maintaining the mixture for a sufficient period of time, that the halogenated hydrocarbon contaminant undergoes chemical breakdown.
CLAIM 3. Procedure of claim 2, wherein the metal is iron, and the iron is in the form of filings or cuttings produced as waste from a metal cutting machine.
CLAIM 4. Procedure of claim 2 wherein the adsorptive material is activated carbon.
CLAIM 5. Procedure of claim 2, wherein: the adsorption material is contained in a canister; the canister is coupled to a water conveying means for conveying the contaminated water that is to be treated into the canister; the procedure includes the steps: when the adsorption material has become laden with the contaminant, of uncoupling the canister, and of detaching and removing the canister and the contaminant-laden body of adsorption material contained therein, away from the said water conveying means; and of mixing the body of metal granules into the body of adsorption material, and of maintaining the mixture under the said conditions.
CLAIM 6. Apparatus for use in the treatment of contaminated water, wherein: the apparatus includes a canister which, during use, is capable of being maintained in an airtight condition, and through which the water may be passed; the canister contains a mixture; the mixture comprises a body of adsorptive material in granular form and, dispersed therethrough, a body of a metal in granular form.
CLAIM 7. Procedure for treating contaminated water, comprising passing the contaminated water through a mixture of activated carbon and a metal.