CA2108981A1 - Removal of contaminants from water - Google Patents

Abstract

2108981 9219545 PCTABS00017 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

wO 92/19~ `~` 1 d S~J~1 PCT/GB92/00760 , .

Title RE~OYAL OF CONTA~INANT5 FRO~ WATER

This invention relates to a system for the treat~ent of contaminated water, particularly water that has been contaminated Rith an halogenated hydrocarbon.

It is a common practice, in treating water that has been conta~inated ~ith such conta~inants as carbon tetrachloride, to pass the uater 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 conta~inant is adsorbed onto the activated carbon.

In the conventional system, the conta~inant builds up ~ithin the body of activated carbon and periodically, eg every three ~onths, the activated carbon ~ust be replaced or must be treated in order to remove the accu~ulated contaminant.
Replacs~ent 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 uethods for cleansing and re-using the activated carbon involve driving the contaminant from the activatsd carbon, for exa~ple by means of a heating process.
In the conventional system, however, the contaminant, though separated fro~ the activated carbon, still remains intact.
.

W092/19~5 PCr/GB92/00760 .
: '-(, 3 8 ~ - 2 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 treat~ent 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 ~aterial contained therein, to the activated carbon cleansing treat~ent facility.

Generally, ~he treat~ent process carried out upon the adsorptive ~aterial is expensive and inconvenient; apart from the fact that the canistet has to be uncoupled and re~oved, also the activated carbon is saturated with the concentrated, hazardous, conta~inant.

BASIC FEATURES OF TH I~VENTION

In the invention, the halogenated hydrocarbon contaminants .
are cleansed fro~ the activated carbon by mixing the , !
3, activated carbon uith a metal. The ~ixture is maintained under the correct conditions of temperature, Eh, and pH, t' whereby the halogenated hydrocarbons undergo ch~mical breakdown, in the presence of the ~etal.

The conta~inants with which the invention is concerned are those which can get into groundwater fro~ acridental spills, W092/19~5 : . PCT~GB92~00760 ,~
, . .
~ ` !

or uhich ~ay be present in effluent. One main class of conta~insnts is the halogenated hydrocarbons, which include pesticides, solvents, chlorofor~, PCB, etc. These substances beco~e dissolved in water, in which state they can be hazardous in small trace quantities. Even the fluoride that is put into drinking uat~r for dental health purposes, and even the chlorine that is sdded to drinking water to kill bacteria, can give rise to hazardous hy~rocarbon subs~ances ~hich should be removed fro~ the water.

The invention is based on the fact that such halogenated hydrocarbons can be broken down che~ically by passing the~
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 che~ical breakdown: the chlorine or other halogen co~ponent of the contamin nt molecule may be converted, for example, into an appropriate solid, insoluble chloride, which may remain in solution, or ~ay precipitate out and be remoYable.
The chloride will in any case generally be har~less, at le~st in s~all concentrations. (A chlorinated hydrocarbon can be hazardous at tiny trace concentration levels, whereas the chloride can be harmless e~en at gross levels. It should be noted that the in~ention is not concerned essentially with totally eliminating the contaminant, but with reducing contaminant concentrations to tolerable levels.) r PGlJ~B ~ ~ ~ D ~ 7 611 !'' 10 AyGUST 1993 In one aspect or the invention, cont~in~nt-laden adsorpti~
~a~erial, or the kind uhich is conventionally and com~only produced ero~ uater treqtment pl nts, is cleansed by mixing the adsorptive~ ~aterial ~ith a ~etal. ~hen this is done, the contaminant is che~ically broken doun: t~e cont~minan~
disappears fro~ the adsorpti:~e ~aterial. There are tuo immediqte benefits to thls: one is that tne contaminant no longer e~ists, as a ~azard, and the other is that the ., :
(e~pensive) ~dsorptive ~aterial cqn be re-used.

DETAILED DESGRIPTION OF PREFERRED E~BODIME~T
. ~

: By uay or rurther explanation of the in~ention, an exe~plary e~b~diment of: thè in~ention ~ill noR be described with ~ .
rererence `to the aoco~panYing dr~wings, in uhich:

Fig l illustrates a ~ater treat~ent plant, ror tre~ting conta~inated Rater;

.~
' ~ ~.
Fig ~ illustr~tes a cleansing facility, for cleansing f~ J~ rJ~";~ ')t~ S~ S~ET
~C;I JI~L~rna~iOnal Appl~catlon l _ , :~, '

2 ~ n ~ ~. 3 ~ PCI/GB92/00760 activated carbon ~hich is laden with contaminant.
A~

The procedures and apparatus shown in the acco~panying drawings and described below are examples which e~body the invention. It shol~ld be noted that the scope of the invention is defined by the acco~panying 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 tetrachlorids.

Often, the reason for the treatment is that the ~ater must be~de-conta~inated before being allowed to enter a drinking water supply. The water treat~ent plant ~ay be placed near the source of the conta~inant, or near the drinking water supply draw-off point. depenting on the circumstances as to the ~anner in whioh the conta~inant arose, how it came to be discovered, whether the source is identified, etc.

The water is piped through the canister 3 by ~sans of a pu~p 4, or under a natural pressure differe`ntial if such is , . ~
available. The treated water energes fro~ the outlet pipe 5, nencs it passss to the drinking water supply, or is ~- otherwise discharged.

;; The canister contains a body 6 of activated carbon in WO92~19545 PCT/GB92/00760 ~ 1 17~ 9 X ~

granular form. The quantity of actlvated carbon, in relation to the throughflow of water, is such that all the contaminant is a~sorbed onto the acti~ated 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 acti~ated 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 acti~ated carbon contained therein, is remoYed from the water treatment plant and sent to the activated carbon cleansing facility.

At the ~leansing facility, as shown in Fig 2 J a quantity of etal 7 is mixed with the activated carbon 6 fro~ the canister in a treatment vessel 8, which contains water 9.
The ~ixture LS stirred thoroughly, to ensure that the ~etal is well-dispersed through the body of activated carbon.

The ~ixture of acti~ated carbon and ~etal is then left in the tr~atment vessel under such conditions, as will be described, and for a sufficient ti~e, that tho halogenated hydrocarbon contaminants undergo chemical breakdown.

The ~etal 7 is in granular for~. For example, the metal may be iron filings or powter J of the kind that is produced as waste in such industrial procosses as the grinding or fettling of iron castings. Or the ~etal m~y be in the form W092/19545 2 ~ O `. '~; i PCT/GBg2/no76o _ 7 _ ~-of cuttings from metal cutting machines. I
~, ~- ~ Whatever its source, care should be taken that cutting oils, -~:
~ for example, are removed from the ~etal. Preferably, the , etal should be Qashed in suitable solvents, to remove oils eto.; ~Also~,~the~motal~may be~ etched~slightly in an acid, to re~ov~e oxid~e~;and~expose the~metal.

~- The ~etal preferably is ironJ since iron is widely available ;
in granular for~ inexpensively as waste from many processes.
The~grain~s~ize~o~f ~the~granules of~hetal should be as s~all s`ibl-,~in~ord~er~that~the~éranulès may have a Daximu~
-reactive~-surface~area.~ Qn~the other hand, the ~etal should ~- no~t be~in~the~orm~of~ s~o fine a dust~ as would make it ~e ~to handl~

he-^~metal need`~not~be~el;emental, so that;steel or cast iron ~granules~may`~bè~used,~rather~than pure~ iron. The ~etal cted~foir~use~in~the~in~ention shoùld not be of a very low`~electroche~ical acti~ity: ~silver or gold, for example, , would ~no~t be effective. Metals such as zinc, iron, alu~inu~, are candidates for selection on the b~sis of their lectro~-ohe-ioal aotivity, and oonsiderations of practical availability will usually favour iron, as nentioned.

The prosence of oxide on the ~etal is generally detri~ental, and pre-treat~ent, for exa~ple an acid wash, is usually to be recom0ended to re~ove at least so~e of the oxide and .,.,..,.. ~ . ~. ~, !.' W092/1954~ PCT/GB92/00760 -`

3~

expose the ~etal.

' It has been found th~t sometimes the speed of the reactive effect attributable to a ~etal ~ay be affected by the presence of other electrochemically active ~etals: for exa~ple, if galvanized iron is used as the source of the granules, the breakdown rate c n be expected to be slightly slowed by the presence of the zinc, as co~pared with iron by itself. Also, it has been found that granules of stainless steel are not so effecti~e as granules of ordinary carbon steels.

In some cases~, it has been proposed that certain pairs of metals, ~ixed or alloyed together, will out-perform a single metal in breaking down such contaminants as the halogen~ted hydr~ocarbons. It should be understood that the invention may be used to advantage when the ~etal 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, folr the che~ical breakdown and disappearance of the halogenated hydrocarbon, in the presence of the metal, to be `~ , , 1:'`
co~pleted.
~, : ~i.

- It~is i~portant that at~ospheric 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 W092/1954~ PCT/GB92/00760 b ~

g to occur, the metal and the activated carbon mix~ure must at least be wet, and preferably should be under water. When the mixture is under water, the condition of excluding oxygen fro~ the mixture generally follous automatically, so long as the water is not aerated. The breakdown or clean-ing treat~ent therefore preferably is carried out uith the ~ixture 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 ~ixture. In fact, if the uater were not to be gtirred or circulated during cleansing, there might be a danger that, when the water caIe to be dumped at the end of the cleansing process, that so~e re~aining intact conta~inant might be dumped ~long with the water.

It is i~portant to ensure ho~ogeneity of the distribution~
in the mixture, of the metal granules and of the activated carbon granules, and it is also i~portant to ensure homogeneity of the distribution o~ the (gradually disappearing) contaminant within the water. Stirring or swirling the mixture is therefore to be preferred. l`~
~.

The stirring or swirling action should not be done in such a ~nner 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 WO92J19~ PCT/GB92/00760 ,~

~,1Dg~

be carried on continuously.

At first, the ~ain chemical effect that takes place during the initial period after the ~ixture has been prepared, is the reduction of the initial oxygen content of the ~ixture.
Such oxygen content may be present as a result of whatever oxygen is dissolved in the water, uhatever oxygen or oxyidizing agents have been introduced in transferring the canister fro~ ~he water treatment plant to the cleansin~
facility, whatever oxygen uas 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 ~oltage must drop below a probe-measured voltage of about -200 millivolts before the che~ical breakdown of the halogenated hydrocarbons will start to take place at econo~ical 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 conta~inant, bearing in mind that water that is to be dumped must be within certain pH limits. 1-~nce the Eh volta~e is below -200 ~v, and espe~ially below -6~0 mY, the breakdown of the halogenated hydrocarbon conta~inant proceeds at a rapid rate.

W092/19~5 PCT/GB92/00760 2 1 3.~ ' ' 3 It ~ay be preferred in some cases that the process of adding the ~etal to the activated carbon, and the process of mixing and dispersing the ~etal within the body of activated car~on, be carried out under conditions of oxygen exclusion, for exa~ple, as mentioned, by carrying out the ~echanical mixing operations under water. On the cther hand, it is acceptable for the conta~inant-saturated activated carbon, and the iron filings, to be si~ply poured from the canister into the water in the treat~ent vessel; such a short exposure to the at~osphere would not be significant. The~
saturated activated carbon should not~ however, be allowed to dry out and be open to the at~osphere for more than, say, overnight.

~hen the ~ixture has been cleansed of the contaminant, to whatever level as is legally or otherwise dictated, the clsansed activated carbon may be re-used~ In one procedure for re-using the activated carbonl the metal is re~oved fro~
the mixture. This ~ 3y be done by ~eans of magnetic separation~ or other suitable means. The cleansed activated carbon,~ now separated from the ~etal, is put back into a c~nister, and piped back into the water treatment plant. r In an alternatiYe re-use procedure, the cleansed mixture of the ~etal and the activated carbon is left as it is: the ~ixture is put into the canister, and is piped bac~ into the water treat~ent plant with the ~ixture, including the ~etal, intact. In this alternative, the ~tal is left in the WO92/19~ PCT/GB92/00760 --9 ~ 12 -~ixture, 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 uater.
The mixture can therefore be expected to remain operational more or less indefinitely, uithout needing to be cleansed.

However, in some cases uhere a water supply is p~ssed o~rer a body of metal, the ~ater can become tainted by the ~eta:L
itself and be rendered unsuitable for drinking. In those cases, the ~etal should be removed from the activated carbon prior to re-use.

But in ~hose case~ 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 ~ithout separating the ~ixture of actiYatsd carbon and metal. ~ow, the contaminated uater thereafter is treated directly, at the water treatment facility, by the mixture of activa~ed carbon and metal, and thus the activated carbon is cleansed at the same tine as the water is de-contaminated.

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

In so~e pre~ious ways of treating the activated carbon, the W092/19~5 ~ 1 f, S 9 ~i) 1 PCT/GB92/00760 , ' 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 h~drocarbons, being generally volatile, are stripped from the activated carbon granules and enter the stripping air. Bu~ this process leaves the contaminants intact, si~ply 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 ~aste.

The new syste~ as described is quite the opposite of aerating the activated carbon: in the new system air (oxygen) is excluded fro~ the cleansing process to such an extent that the Eh voltage drops to -200 mv, and below, under which conditions the che~ical breakdown of the contaminants can proceed. Under negative Eh conditions, the halog~nated 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 ~`
acti~atPd c~rbon, but the mixture can also be used directly t~ treat the contaminated water. If the cont~minated water that is being treated is already anaerobic, or substantiallY
so, the direct treat~ent can be very economical. One preferred area of application of the direct treatment, ~v~f~ U ~
' ~ AUGUST 1993 3 1 !
1~
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 contami~ated 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.

Preferably the canister, during use, is capable of being maintained in an airtight condition, and the apparatus is so arranged that oxygen, including atmospheric oxygen, and other oxidising agents, are excluded from contact with the mixture.

PC-I- intcrna~ional Applicatlon ~ Y

Claims (8)

Claims
1. CLAIM 1. 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.
2. CLAIM 2. Procedure of claim 1, wherein the metal is iron, and the iron is in the form of filings or cuttings produced as waste from a metal cutting machine.
3. CLAIM 3. Procedure of claim 1 wherein the adsorptive material is activated carbon.
4. CLAIM 4. Procedure of claim 1, 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.
5. CLAIM 5. Apparatus for use in the treatment of contaminated water, wherein:
   the apparatus includes a canister through which the water containing a contaminant may be passed;
   the canister contains a mixture;
   the mixture comprises a body of adsorptive material in granular form and, physically dispersed therethrough, a body of a metal in granular form;
   the nature of the contaminant and the nature of the metal are such that the contaminant breaks down chemically when brought into, and during the course of, prolonged contact with the particles of metal;
   the nature of the adsorptive material is such that the contaminant is adsorbed out of solution onto the particles of adsorptive material upon the contaminated water being passed over and through the permeable mixture;
   the adsorptive capacity of the adsorptive material is such that the velocity of the contaminant passing through the permeable mixture is, in substance, very much more retarded than the velocity of the water passing through the permeable body;
   whereby the contaminant, being retarded on and by the particles of adsorbent material, is held physically adjacent to the particles of metal for a very much longer period of time than the passing water, and is so held long enough for breakdown of the contaminant to take place.
6. CLAIM 6. Apparatus of claim 5, wherein the metal is iron.
7. CLAIM 7. Apparatus of claim 5, wherein the adsorptive material is activated carbon.
8. CLAIM 8. Apparatus of claim 5, wherein the canister, during use, is capable of being maintained in an airtight condition, and the apparatus is so arranged that oxygen, including atmospheric oxygen, and other oxidising agents, are excluded from contact with the mixture.