GB2065169A - Electrocoagulation method and apparatus for dewatering of aqueous sludges - Google Patents

Abstract

An initial sludge from a waste water treatment system is treated to render the sludge more readily dewatered by a mechanical dewatering system, using an apparatus having spaced vertical parallel plates and by passing a current between the plates and through the initial sludge at a value of 10-80 ampere minutes per gallon, with a retention time of 0.5-5 minutes for the initial sludge to produce a readily dewatered electrocoagulated sludge material. The preferred apparatus for effecting the treatment has spaced plates (115) superimposed over a movable belt (105) of a mechanical dewatering system such as a filter press, with a reservoir (121) provided on the housing (113) for the plates to control flow of initial sludge through the spaced plates and along a belt (105) of the filter press. <IMAGE>

Description

SPECIFICATION Electrocoagulation method and apparatus for dewatering of aqueous sludges This invention relates to an electrocoagulation method and apparatus for dewatering of aqueous sludges.

In the processing of contaminated wastewaters, the separation of suspended solids must be effected in order to render the waters acceptable for disposal in the environment. Generally, such processing involves electrical or chemical treatment and agglomeration or flocculation of the solids and separation by flotation, gravity separation or filtration. The sludges resulting from such treatment generally contain from 1-40 percent solids, whereas the initial wastewater may have contained solids in only a parts per million range, albeit in toxic or other undesirable form. Mining wastewater, for example, may contain minor amounts of toxic metals which must be removed from the wastewater and disposed of safely, such as by collection and encapsulation in cement or other non-leachable landfill.

These initial sludges must thus be concentrated to raise the solids content thereof to provide for more ready disposal. Filters or other mechanical means for concentrating such sludges are often not adequate due to the gelatinous or non-crystalline nature of the flocculated solids, and dewatering of these initial sludges continues to be a problem.

The present invention in one aspect provides a method for treating wastewater to remove solids therefrom and produces a sludge, and dewatering of the sludge to produce a disposable sludge material, wherein the said wastewater is treated to produce an initial aqueous sludge containing 1-40 percent by weight of solids, comprising: passing the said initial aqueous sludge between spaced, parallel, vertically aligned electrodes; electrocoagulating the initial aqueous sludge by passing an alternating current between the said electrodes and through the initial aqueous sludge, the said current applied being at a value from 1 0-80 ampere minutes per gallon of initial aqueous sludge, with the said sludge having a retention time between the electrodes of 0.5-5 minutes, so as to produce an electrically coagulated sludge material; and mechanically dewatering the electrically coagulated sludge material.

The invention in another aspect provides apparatus for dewatering of an aqueous sludge, resulting from initially treating a solids-containing wastewater to coagulate solids contained therein, comprising: a housing having at least one inlet thereto and at least one outlet therefrom; a plurality of spaced, vertically positioned electrodes formed of electrically conductive material supported within the housing by means of non-conductive supports; means for passing the said aqueous sludge between the said spaced electrodes while maintaining a retention time therebetween of about 0.5 to 5 minutes; means for applying an alternating current to the spaced electrodes to provide a flow of current of from 10-80 ampere minutes per gallon of sludge passed therebetween; means for discharging sludge from the housing; and mechanically dewatering means for dewatering of sludge discharge from the housing.

Thus an initial sludge, that contains 1-40 percent solids and is produced by treatment of wastewater so as to coagulate solids therein, is treated so as to render the solids more readily dewatered by mechanical dewatering systems, by passing the initial sludge between spaced, parallel, vertical electrodes and electrocoagulating the same by applying an alternating current to the electrodes at a value of 10-80 ampere minutes per gallon of initial aqueous sludge, with the sludge having a retention time between the electrodes of 0.5-5 minutes, to produce an electrically coagulated sludge material, and then mechanically dewatering the material, preferably on a filter press.

The apparatus for dewatering of an initially treated aqueous sludge suitably comprises spaced, vertically aligned electrode plates supported in a housing by non-conductive supports, with means for passing the wastewater between the plates and means for applying an alternating current to the plates and through the sludge passing therebetween. Means are provided for discharging the sludge from the housing and for mechanically dewatering the sludge. Preferably parallel electrode plates are disposed in parallel relationship to a lower belt of a belt press, with the housing superimposed over a receiving end of the belt.A reservoir is preferably formed in the housing such that sludge fed to the reservoir may be fed, by an adjustable weir, to the belt at a steady rate, while the movement of the belt carries sludge between the spaced electrodes while the sludge is carried by the belt to the pressing section of the belt press.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of an apparatus for carrying out the method of the present invention; Figure 2 is a schematic illustration of another embodiment of an apparatus for carrying out the method of the present invention; Figure 3 is a side elevational view of a preferred apparatus of the present invention with portions removed for clarity; and Figure 4 is a top plan view of a half section of the apparatus illustrated in Figure 3.

Referring now to the drawings, Figure 1 schematically illustrates a sludge dewatering electrocoagulation unit according to the present invention, wherein an electrocoagulation unit 1 comprises a housing 3 containing a plurality of spaced electrode plates 5 (only one shown in the end view). The plates 5 are maintained in spaced vertical relationship by walls 7 formed of a non-conductive material and are connected to a source of alternating current power (not shown). The housing 3 has a lower section 9 formed of downwardly depending bottom walls which provides a chamber 11 below the spaced plates 5 and an inlet 13 is provided through which sludge to be dewatered is introduced. The sludge passes upwardly between the spaced plates and overfiows end walls 7.After passage over the end walls 7, the sludge flows by gravity downwardly and is discharged from the housing 3 through outlets 15, such flow being indicated by the arrows in the figure.

In the embodiment illustrated in Figure 2, the sludge passes downwardly between the spaced plates. As illustrated, an electrocoagulation unit 22 comprises a housing 23 containing a plurality of spaced electrode plates 25 which are maintained in spaced, vertical relationship by end walls 27 formed of non-conductive material, the plates 25 being connected to a source of alternating current (not shown). The housing 23 has a lower section 29, formed of downwardly depending bottom walls, that provides a chamber 31 below the plates 25. In this embodiment, sludge to be subjected to electrocoagulation treatment is introduced into the housing 23 through inlets 33, the sludge flowing upwardly between the housing 23 and the end walls 27.After passage over the end walls 27, the sludge passes, by gravity, downwardly between the spaced plates 25 and into the chamber 31, as indicated by the arrows in the figure. The electrocoagulated sludge is discharged from the housing 23 through an outlet 35.

In the preferred embodiment illustrated in Figures 3 and 4, the electrocoagulation unit is an integral part of a mechanical dewatering system, such as a belt press. A sludge electrocoagulation unit 101 is supported by supports over the charging end of a filter press system. The filter press system, as illustrated, comprises upper and lower porous belts 103 and 105 which are directed by respective belt support shafts 107 and 109, which belts come together at a distance spaced from the electrocoagulation unit 101, to provide for pressing of sludge deposited thereon and mechanical dewatering of the same.The belt 105, at the position of the unit 101, is preferably inclined at an acute angle to the horizontal, as indicated in the drawing, generally at an angle of the order of 1 50. The belts 103 and 105 come in close proximity to each other at a position to the left of Figure 3, in the direction of the arrows shown on that figure. The top belt support shafts 107 of the belt press are adjustable.

Attached to the top belt support system is a support system 111 for the electrocoagulation unit 101.

A housing 113 is provided which contains a plurality of spaced, vertical electrode plates 11 5, the plates 115 being held in spaced relationship by non-conductive end walls 117. The plates 11 5 are connected to a source of alternating current (not shown) by means of terminals 11 9. The housing 113 has, adjacent an end wall 11 7, a reservoir 121 connected thereto, with an inlet 123 for conducting treated sludge from a treatment system into the reservoir. The reservoir 121 comprises a rear containment wall 125 and side containment walls 127, with an adjustable front weir 129 positioned opposite the rear containment wall 125. The housing 113 and reservoir 121 are positioned so as to be contiguous with the lower belt 105, with a seal 131 providing sealing between the reservoir and the movable belt.The lower belt 105, passing over the support shafts 109, is supported by non-conductive belt supports 133 in the region of the electrocoagulation unit 101.

In operation of this embodiment, the aqueous initial sludge is charged to the reservoir 121 through the inlet 123, with a supply of sludge maintained within the reservoir, as illustrated by a sludge level line 1. The level I is maintained by raising or lowering the adjustable weir 129. Sludge from the reservoir 121 passes under the weir 129 at a rate determined by travel of the belt 105 upon which the sludge rests, while movement of the belt in the direction away from rear containment wall drags a portion of the sludge from the reservoir 121 between the spaced electrode plates 11 5. During passage of sludge between the plates 11 5, the sludge is subjected to an alternating current. The retention time of the sludge between the spaced electrode plates is controlled by the speed of travel of the lower belt 105.After passage through the electrode plates 11 5, and electrocoagulation thereof, the electrocoagulated sludge remains spread on the belt 105 for travel towards, and pressing by, the upper belt 103 to mechanically dewater the electrocoagulated sludge by pressing of the belts together and forcing water through the porous belts 1 03 and 105. The pressing of the electrocoagulated sludge to remove water is accomplished by conventional means and is not illustrated herein.

In the present method, sludges resulting from wastewater treatment are subjected to an electrocoagulation step in order to enhance the dewatering characteristics of the sludge under mechanical dewatering conditions. The sludges that are treated according to the present method can comprise sludges resulting from biological treatment of organic-containing wastewaters or those resulting from treatment of primarily inorganic constituent containing wastewaters such as those resulting from treatment of mineral or mining wastewaters. These sludges contain about 1-40 percent by weight of solids, with the former generally containing of the order of 1-10 percent solids and the latter generally containing of the order of 10 40 percent solids. Such sludges must be mechanically dewatered, such as by vacuum filtering or press filtering, in order to further decrease the water content thereof prior to disposal or subsequent processing to prevent contamination of the environment.

The treatment of the wastewater that produces such sludges can involve various processes, but generally coagulants or polymeric flocculating agents are used to flocculate and separate the solids initially present in the wastewater so as to form a sludge that may be dewatered mechanically. Such sludges are difficult to mechanically dewater, however, due to the sometimes gelatinous or non crystalline nature of the solids content thereof.

In the present method where a wastewater is treated to produce an initial aqueous sludge containing 1 40 percent by weight of solids, the initial sludge is rendered more acceptable to mechanical dewatering by electrocoagulation of the initial sludge under certain conditions.

The initial aqueous sludge is passed between spaced vertically disposed, parallel electrodes, preferably plates. The electrodes may be those which are used often in the electrocoagulation of wastewaters, for flotation purposes, such as aluminium plates. During passage of the initial aqueous sludge between the spaced electrodes, an alternating current is applied between the spaced electrodes and through the intermediate initial sludge. The application of the alternating current, in some manner, causes the solids content of the sludge material, and thus the sludge itself, to be more readily dewatered in a mechanical dewatering apparatus.

The alternating current must be applied at a value such that an application is achieved that will provide 1 0-80 ampere minutes per gallon of the initial sludge passed between the spaced plates, while the retention time of the sludge intermediate the plates must be between about 0.5 to 5 minutes.

Preferably, the alternating current is applied at a value of between about 20-60 ampere minutes per gallon for most efficient operation. The application of the alternating current at these values, and at these retention times for the initial sludge between the spaced electrodes, in some manner alters the characteristics of the sludge to produce an electrocoagulated sludge that is more readily subject to a mechanical dewatering and to dewatering to a lower water content. The electrocoagulated sludge is then mechanically dewatered to produce a material for disposal, incineration, or encapsulation in insulative material, for example.

In addition to rendering the sludge more susceptible to mechanical dewatering, the practice of the present method permits the use of less polymer in the initial dewatering step while still providing excellent dewatering properties. In this context, polymeric surfactants which include cationic, anionic and nonionic polymeric materials which are added to the initial wastewater may be added in lesser amounts and a portion thereof added with mixing, just prior to or immediately following, the electrocoagulation of the initial sludge to provide better dewatering characteristics with the total amount of such coagulants being less than the amount normally used in the initial wastewater treatment.

As an example of the present method, a wastewater from a mining operation was subjected to treatment to coagulate the solids content thereof, which solids were to be disposed of through encapsulation in cement, due to the toxic nature of some of the metallic content thereof. A series of five aluminium plates, as electrodes, were positioned vertically with a spacing of 7/8 inch between the surfaces of the plates. The initial sludge resulting from the wastewater treatment, which contained 1.8 percent solids by weight, was passed upwardly between the spaced plates and over the two nonconductive end sections of an apparatus similar to that illustrated in Figure 1. Occasional raking of the sludge was effected. After applying the current described in Table I and using the flow rates indicated therein, the electrocoagulated sludge had added thereto a cationic polymer, Dow 700. Samples were collected and pressing of the electrocoagulated sludge effected in a bench scale sludge press simulator.

The simulator is limited by the fact that it dewaters across only one cloth surface, not two. In most runs material was pushed through the cloth. Filtrate quality was not determined but all runs were evaluated on cake dryness, determined on an Ohans moisture tester. The results were as follows: TABLE I Amperes Flow Rate Ampere-Minute (AC) (gpm) Gallon % Solids 0 1.6 0 5.5 50 1.6 31.2 6.8 100 1.6 62.5 7.3 50 2.6 19.2 6.9 100 2.6 38.5 8.5 50 3.6 14.0 8.0 100 3.6 27.8 8.3 120 3.6 41.7 14.4 The electrocoagulation treatment of the sludge, as is seen by the results, increased the solids content of the sludge upon mechanical dewatering.

Claims (16)

1. A method of treating wastewater to remove solids therefrom and produce a sludge, and dewatering of the sludge to produce a disposable sludge material, wherein the said wastewater is treated to produce an initial aqueous sludge containing 1-40 percent by weight of solids, comprising: passing the said initial aqueous sludge between spaced, parallel, vertically aligned electrodes; electrocoagulating the initial aqueous sludge by passing an alternating current between the said electrodes and through the initial aqueous sludge, the said current applied being at a value from 1 0-80 ampere minutes per gallon of initial aqueous sludge, with the said sludge having a retention time between the electrodes of 0.5-5 minutes, so as to produce an electrically coagulated sludge material; and mechanically dewatering the electrically coagulated sludge material.

2. A method as claimed in Claim 1, wherein the said electrodes are in the form of vertically aligned, parallel, spaced plates.

3. A method as claimed in Claim 2, wherein the said plates are of aluminium.

4. A method as claimed in any of Claims 1 to 3, wherein the said current is applied at a value of 20-60 ampere minutes per gallon of initial sludge.

5. A method as claimed in any of Claims 1 to 4, wherein the said electrically coagulated sludge is mechanically dewatered by passage through a filter press.

6. A method as claimed in any of Claims 1 to 5, wherein the said wastewater is biologically treated wastewater.

7. A method as claimed in any of Claims 1 to 5, wherein the said wastewater is mining wastewater containing trace metals.

8. A method according to Claim 1, substantially as herein described with reference to the accompanying drawings.

9. Apparatus for dewatering of an aqueous sludge, resulting from initially treating a solidscontaining wastewater to coagulate solids contained therein, comprising: a housing having at least one inlet thereto and at least one outlet therefrom; a plurality of spaced, vertically positioned electrodes formed of electrically conductive material supported within the housing by means of non-conductive supports; means for passing the said aqueous sludge between the said spaced electrodes while maintaining a retention time therebetween of about 0.5 to 5 minutes; means for applying an alternating current to the spaced electrodes to provide a flow of current of from 10-80 ampere minutes per gallon of sludge passed therebetween; means for discharging sludge from the housing; and mechanically dewatering means for dewatering of sludge discharge from the housing.

10. Apparatus as claimed in Claim 9, wherein the said electrodes comprise plates supported by non-conductive end walls and wherein the said housing has downwardly depending bottom walls forming a chamber below the electrodes.

11. Apparatus as claimed in Claim 10, wherein an inlet is provided in the said bottom walls and including means for passing sludge into the said chamber and upwardly through the spaced plates, with overflow, from the end wall supports for the plates, discharged from the housing.

12. Apparatus as claimed in Claim 10, wherein an outlet is provided from the said chamber in the said bottom walls and inlets are provided in the housing walls above the level of the chamber and means are provided for passing sludge into the said housing and upwardly therein for flow over the end wall supports, with the said sludge then passing downwardly through the spaced plates and into the chamber for discharge through the outlet.

13. Apparatus as claimed in any of Claims 9 to 12, wherein the said mechanically dewatering means has a travelling porous belt and wherein the said housing is superimposed above a receiving end of the said belt.

14. Apparatus as claimed in Claim 13, wherein the said electrodes comprise spaced plates aligned in parallel relationship to the direction of travel of the said belt.

1 5. Apparatus as claimed in Claim 14, including a reservoir formed by an electrode support wall and a spaced wall, with an inlet provided to the reservoir, and an adjustable weir provided intermediate the reservoir and ends of the spaced electrode plates.

16. Apparatus according to Claim 9, substantially as herein described with reference to, and as shown in, the accompanying drawings.