GB2045803A - Electrolytic purification of effluents - Google Patents

Abstract

Effluent is treated by passing it through an electrolytic cell 12 having aluminium anodes 22. In the electrolysis, gas is evolved which carries certain of the particles in the effluent to the top of the cell to form a foam, and this foam overflows from the top of the cell. The electrolysis also produces a flocculating agent which causes other suspended particles in the effluent to coagulate and drop to the bottom of the cell. Clarified effluent is removed continuously from near the top of the liquid region in the electrolytic cell, and effluent to be treated is continuously fed into the cell. The effluent removed from the cell may be further treated by filtration before discharge. <IMAGE>

Description

SPECIFICATION The purification of effluents Background of the invention This invention relates to the purification of effluents and more particularly to the purification of effluents from textile plants such as a mixed effluent from raw wool scouring, dyeing and wet finishing.

Because of the problems encountered with such effluents raw wool scouring is being phased out in Japan.

Other plants have adopted very expensive processes involving hot acid flocculation.

In connection with effluent treatment in general metallic hydroxides have been used to assist in flocculation. Such hydroxides have been formed in situ by the comsumption of anodes in electrolytic processes. In one such process the anodes are made of aluminium.

Electroflotation is also known. In this process iron electrodes are immersed in an effluent and current passed between them. Hydrogen evolved in the process causes flocs lighter than water to float to the surface where they may be scraped off in the form of a foam. In applying the known process to textile plant effluents, the applicant has found that good results can be obtained when using a batch process. When an attempt was made to make the process continuous there was a carry-over of black liquid. It is thought that this is due to the fact that there is a sudden point of time at which clarification takes place in any batch. In a continuous process this point cannot be reached.

The applicant has now surprisingly found that with the use of aluminium electrodes continuous electroflotation is possible together with sludge precipitation.

Summary of the invention According to the present invention, there is provided a method of treating effluent, wherein the effluent is continuously passed through an electrolytic cell in which a DC current is caused to flow between electrodes and in which at least the anodes are made of aluminium, foam collecting at the top of the cell is removed, liquid is withdrawn from the cell, and the suspended solids are separated from the withdrawn liquid.

Prior to passing the effluent through an electrolytic cell, it is preferably stored to reach a chemical balance between its various constituents.

Flocculation takes place in the cell, and the flocs are separated, for example by straining or filtering.

If the anodes are cast from scrap aluminium, the process can be relatively cheap.

For reasons of economy, the voltage between electrodes should not exceed 5 to 7,5 V. The current density should also be controlled for the same reasons.

To simplify the electrical equipment, and at the same time reduce heavy conductor costs the highest safe voltage should be used.

The invention also provides apparatus for performing the above method, the apparatus comprising an electrolytic cell having a plurality of anodes and cathodes, at least the anodes being made of aluminium, a liquid inlet to the cell, a liquid outlet from the cell below the top of the cell which leads to a filter chamber, an inlet at the bottom of the filter chamber and an outlet above the bottom of the filter chamber with a filter medium between the inlet and outlet of the filter chamber.

A transfer chamber preferably communicates with the outlet from the cell and with the inlet to the filter chamber, and may contain a baffle for directing the flow of liquid from the cell to the filter chamber.

The filter medium in the filter chamber is preferably graded so that the coarser filter particles are at the bottom and the finer filter particles at the top.

A A drain valve may be provided at the bottom of the electrolytic cells and at the bottom of the transfer chamber.

The apparatus will preferably be surrounded by a drainage channel into which foam from the top of the cell can be overflowed.

The electrodes in the cell are preferably connected in a series parallel arrangement.

Brief description of the drawings The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram representing a method of treatment according to the invention; Figure 2 is a cross-section on the line Il-Il of Figure 3 through apparatus for carrying out the method of the invention; Figure 3 is a plan view of the apparatus shown in Figure 2; and Figure 4 is a schematic illustration of the electrode connections.

Description ofa preferred embodiment Effluent from the raw wool scouring, dyeing and wet finishing operations of a textile plant are fed to a common storage dam 10. Withdrawal from the dam commences after a chemical balance in the dam has been achieved.

Raw effluent from the dam 10 is fed to an electrolytic tank 11. In the tank 11, which will be described in more detail with reference to Figure 2, there are two electrolytic cells 12 and 13 each containing identical numbers of anodes and cathodes conveniently made of scrap aluminium. The cells are connected independently across a fixed DC supply.

Foam collecting at the top of the tank 11 overflows the low front wall 14, assisted by a blower if necessary, into channels 15 surrounding the tank. From the channels 15, the foam runs into a settling pond 16 (Figure 1).

Liquid is withdrawn from the tank 11 to a strainer or filter 17.

The effluent enters the cell 12 through an inlet 18. After electrolysis has taken place in the cell 12, the clarified effluent overflows from the cell 12 via a level control elbow outlet 19 to a transfer chamber 20. A drain valve 21 is provided at the bottom of the cell for the removal of a sludge from the bottom. The level at which effluent overflows from the cell 12 can be adjusted by means of a press fit stiff elbow on outlet 19so that the foam overflow is ensured.

Electrodes 22 are provided in cells 12 and 13. Only some of the electrodes are shown in Figure 3. The electrodes are in the form of rectangular plates which are supported at their ends in PVC tubes 23 with slots cut in them to receive corners of the electrodes and to provide a gap of 7 mm between electrodes. The electrodes are arranged in a V-formation as can be seen in Figure 2. In the centre of each cell, a baffle 24 is vertically mounted and extends from the top of the cell to the upper surface of the V formed by the electrodes, to cause the liquid in the cell to flow below the electrodes.

The transfer chamber 20 contains a corrugated asbestos baffle 25 so that the liquid flows up and over the baffle, thus allowing part of the carry over floc to settle. The floc which settles in the chamber 20 is removed periodically through a pipe 26, either by gravity or by a pump and can be recycled as indicated by dotted line 26a in Figure 1. A drain valve 27 is provided at the bottom of the transfer chamber to remove sludge and floc carryover when backwashing the strainer 17.

The liquid passes from the transfer chamber 20 into perforated pipes 28 at the bottom of the strainer 17, and passes upwards through a graded media filter bed 29. The bed 29 has granite graded 20 mm at the bottom and the size of the particles decreases in an upward direction to crusher dust fines at the top. The liquid level above the filter bed rises until it reaches the level of an outflow pipe 30 which is connected to a sewer or a recycle process water storage tank.

The electrodes in cell 12 are connected in series parallel so as to obtain high coulomb flow. To clarify an effluent with a conductivity of 100 mill Siemens/metre using a 7 kW,132 V, 53A, power pac and aluminium electrodes with aface area of 0,077m2will require 79 electrodes on each side (total 158 per cell) connected 26 electrodes in series and connected in 6 parallel groups (see Figure 4). When new, the electrodes are 15 to 16 mm thick, and the mean space between adjacent electrodes is 7 mm. The intermediate electrodes between connected anodes and cathodes are cathodic on one face and anodic on the other face.

To suit an effluent with a conductivity of 20 mS/m the connections would be 12 parallel groups of 13 electrodes in series.

The power pac consists of a 7,5 KVA 3 phase transformer, primary 380 volts, secondary 100 volts. The secondary is connected to a 3 phase bridge rectifier. The power factor is connected to unity at full load with a 2,5 KVAr capacitor, and the secondary is protected with H.R.C. high speed fuses. The rectifier output with this load is 53 amps at 132 volts. The conductivity of the effluent determines the method of connection of electrodes.

Although the foregoing description refers only to the cell 12, it is to be understood that the cell 13 will be constructed and arranged in the same way.

Operation In an example of the invention, the dam 10 has a capacity of 12000 cubic metres and has a retention period of seven days to ensure chemical balance of the various effluents.

The electrode cells 12 and 13 each have a capacity of 4,35 m3 in operation.

In operation the electrode cells are filled until overflow commences through the pipes 18. The power is switched on at the start of filling. Filling is stopped for 1/2to 2 hours after overflow begins depending on degree of clarification required. After this initial period the flow is increased to the desired rate, and the level in the electrode cells is adjusted by means of the press fit stiff elbows on pipes 19, so as to cause a foam sludge overflow into the drain channel 15.

The current flow from anode to cathode causes the water to electrolyze so that hydrogen is liberated at the cathode and aluminium goes into solution at the anode forming aluminium hydroxide. Bubbles of hydrogen gas carry all coagulated fats, oils, greases, greasy fibres and oily particles and other light suspended matter to the surface of the effluent where a dense stable foam is formed. This foam is removed by overflowing into the front drain channel 14. Over 90% of the suspended solids are removed in this foam.

The aluminium hydroxide formed is a very active flocculating agent and this promotes coagulation of all the remaining suspended solids and colloidal matter including dyes which coagulate and sink to the bottom of the electrode cell. Part of the aluminium hydroxide gel is carried over into the transfer chamber and trapped in the lower sections of the strainger 16 where it aids filtration.

The strainer 17 is backwashed when necessary by closing a valve in the outflow pipe 30 and allowing the liquid level to rise to a maximum, in this case 1 500 mm above floor level, and opening the drain valve 27.

The reverse flow velocity is initially 20 to 30 times the up flow filtering velocity and effectively cleans the filter media. The electrodes are hosed down to remove loose scale when the sludge which collects at the bottom is removed through valves 21. The normal operating time would be 20 to 22 hours per day, and cleaning should be done at peak (factory) load periods so as to reduce kVA demand charges. After 100 to 120 hours operation electrodes must be removed for cleaning. The hard scale formed on the cathode face must be scraped off or preferably removed by sandblasting.

The electrodes closest to the connections are eroded away at a greater rate than the electrodes midway between connections, and thus after a short time there are thick and thin electrodes in the cell. The thin electrodes < 6 mm thick are replaced with new electrodes 15 to 16 mm thick. With random replacement the mean electrode thickness will be 10 to 11 mmandthemeangap between electrodes will be 12 to 13 mm.

In the following table, sample No. 1 is of untreated effluent and sample No. 2 is of the treated effluent.

Sample No. 2 was produced with a flow rate of 14000 litres per hour in a double flotation cell with straining of the solids.

In another example of the invention wet finishing effluent which is slightly polluted can be clarified in this process and recycled. As the wet finishing effluent has a conductivity of only 20 mS/m the electrodes should in this case be connected in 12 parallel groups of 13 electrodes in series.

TABLE Sample No. 1 2 pH 6,9 8,6 Conductivity in mS/m 115 86 Dissolved solids at 105 C in mg/l 1071 683 Suspended solids at 105 C in mg/l 426 20 Oxygen absorbed from 0,0125N Kin04 in 4 h as O2in mg/t 120 27 Chemical oxygen demand as 2 in mug/4 3056 340 Free and saline ammonia as N in mug/4 16,5 9,2 Nitrate as N in mg/4 Total hardness as CaCO3 in m9le 46 20 Calcium hardness as CaCO3 in mug/4 44 20 Total alkalinity as CaCO3 in mug/4 435 366 Chloride as Cl in mug/4 36 28 Sulphate as S04 in mug/4 4,0 4,0 Sodium as Na in mug/4 116 110 Oil and grease in m9le 296 47 Hexavalent chromium as Cr in mug/4 0,26 0 Calcium as Ca in mg/4 17,6 8,0 Magnesium as Mg in mug/4 o,4 0

Claims (15)

1. A method of treating effluent, wherein the effluent is continuously passed through an electrolytic cell in which a DC current is caused to flow between electrodes and in which at least the anodes are made of aluminium, foam collecting at the top of the cell is removed, liquid is withdrawn from the cell, and the suspended solids are separated from the withdrawn liquid.

2. A method as claimed in claim 1, wherein the effluent is stored until it reaches a chemical balance between its various constituents, prior to passing the effluent through the cell.

3. A method as claimed in claim 1 or claim 2, wherein the liquid withdrawn from the cell is filtered before being discharged.

4. A method as claimed in any preceding claim, wherein the voltage between adjacent electrodes is chosen according to the conductivity of the effluent and is between 5 and 10 volts.

5. A method as claimed in any preceding claim, wherein the current density flowing in the cell is controlled.

6. Apparatus for treating effluent, comprising an electrolytic cell having a plurality of anodes and cathodes, at least the anodes being made of aluminium, a liquid inlet to the cell, a liquid outlet from the cell below the top of the cell which leads to a filter chamber, an inlet at the bottom of the filter chamber and an outlet above the bottom of the filter chamber with a filter material between the inlet and outlet of the filter chamber.

7. Apparatus as claimed in claim 6, including two electrolytic cells and two filter chambers arranged side-by-side.

8. Apparatus as claimed in claim 6 or claim 7, wherein the electrolytic cell is surrounded by a drainage channel, so that foam overflowing from the top of the cell collects in the channel.

9. Apparatus as claimed in any one of claims 6 to 8, including a transfer chamber communicating with the outlet from the cell and with the inlet to the filter chamber.

10. Apparatus as claimed in claim 9, wherein the transfer chamber contains a baffle for directing the flow of liquid from the cell to the filter chamber.

11. Apparatus as claimed in any one of claims 6 to 10, wherein the anodes and cathodes are cast from scrap aluminium.

12. Apparatus as claimed in any one of claims 6 to 11, wherein the filter medium in the filter chamber is graded with coarser filter particles lying at the bottom and finer particles at the top.

13. Apparatus as claimed in any one of claims 6 to 12, wherein a drain valve is provided at the bottom of the electrolytic cell.

14. A method of treating effluent substantially as herein described with reference to the accompanying drawings.

15. Apparatus for treating effluent substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.