US3764500A

US3764500A - Method and apparatus for electrolytic treatment of sewage

Info

Publication number: US3764500A
Application number: US00084839A
Authority: US
Inventors: F Gibson; B Halker; R Rhees; J Gibson
Original assignee: Pacific Eng & Prod Co Of Nevad
Current assignee: Pacific Eng & Prod Co Of Nevad; Pacific Eng & Prod Co Of Nevada us
Priority date: 1970-10-28
Filing date: 1970-10-28
Publication date: 1973-10-09
Anticipated expiration: 1990-10-09
Also published as: AU476567B2; AU5430873A

Abstract

APPARATUS AND METHOD FOR THE ELECTROLYTIC TREATMENT OF SEWAGE IS DISCLOSED. ACCORDING TO THE METHOD, ELECTROLYTIC TREATMENT OF A STREAM IN A SEWAGE TREATMENT PLANT IS PROVIDED AT ONE OR MORE SELECTED LOCATIONS. THUS IN AN OTHERWISE GENERALLY CONVENTIONAL SEWAGE TREATMENT FACILITY EMPLOYING VARIOUS WELL-KNOWN TECHNIQUES FOR THE REMOVAL OF SOLIDS AND ALSO WELL-KNOWN TECHNIQUES FOR THE BACTERIOLOGICAL DIGESTION OF CONTAINED ORGANIC MATERIALS, THE METHOD CONTEMPLATES THE ELECTROLYTIC TREATMENT OF THE FINAL EFFLUENT STREAM FOR THE PURPOSE OF DISINFECTION THEREOF. IN ADDITION, IN SYSTEMS WHEREIN ANAEROBIC DIGESTERS ARE EMPLOYED TO TREAT THE SLUDGE FROM PRIMARY SETTLERS, ELECTROLYTIC TREATMENT OF THE DIGESTER SUPERNATANT IS PROVIDED FOR THE PURPOSE OF DESTROYING ITS PUTRESCENT ODOR, IMPROVING ITS SETTELABILITY AND COLOR, AND REDUCING ITS BOD AND COD. THE APPARATUS EMPLOYED INCLUDES ONE OR MORE ELECTROLYTIC ASSEMBLIES EMPLOYING ANODES FORMED OF A COATING OF LEAD DIOXIDE ON A SUBSTRATE OF GRAPHITE OR TITANIUM, AND PREFERABLY INCLUDES A HOLLOW, CYLINDRICAL METAL TUBE WHICH FORMS THE CATHODE AND WITH THE ANODE CONTAINED THEREIN AND IN THE FORMS OF A CYLINDRICAL MEMBER WHOSE OUTER SURFACE IS EQUIDISTANTLY SPACED FROM THE INNER SUFFACE OF THE CATHODE CAN. ELECTROLYTE FLOWS THROUGH THE ASSEMBLY FROM AN INLET CONDUIT NEAR BOTTOM TO AN OUTLET CONDIUT NEAR THE TOP, AND ELECTRICAL CONNECTIONS ARE MADE TO THE EXTERIOR CATHODE CAN AND TO A PORTION OF THE ANODE SUBSTRATE WHICH PROTRUDES FROM THE TOP OF THE ASSEMBLY.

D R A W I N G

Description

Oct. 9, 1973 s JR, ETAL 3,764,500

METHOD AND APPARATUS FOR ELECTROLYTIC TREATMENT OF SEWAGE Filed 001;. 28, 1970 5 Sheets-Sheet 1 FIG. I Row Sewage Primary Effluent l lO I ll i2 2 15 Bar 7 Grli Primary Secondary screen Chomber Settler U 7 Treatment 36 Wote 2O I Electrolytic Anoeroblc Hypochlorinotor l9 -m Digesters Digester l sludge Discharge V Supernotant Electrolytic ln Stream Treatment Unit FIG. 6 Re-Use Water Seavaqe From Bar Screen 2 l8 Eectrovtic An GrltGhamber Cells Primary Settler JEA I v v No. l Electrolytic 16 N 2 :1- Cells Drying Beds l8 'Or Lagoons Return To Primary settler-I2 l8 l7 guperngtanDt From I naero ic igesters- 4 Electrolytic Head 1 C S Tank v Water FIG. '8. l9 t Supernatant Frorr Overflow Almerob'c R Digesters l7 i-- -l Electrolytic e :y c l e recited Fluid Cells IQ jjj Discharge To 6 Primary Settler '3 INVENTORS Fred D. Gibson, Jr. Raymond C. Rhees Su ernatant James I. Gibson 50 Tr ecitment Bruce B. Halker ATTORNEY 0d. 9, 1973 I GlBSON, JR" EI'AL 3,764,500

METHOD AND APPARATUS FOR ELECTROLYTIC TREATMENT OF SEWAGE Filed Oct. 28, 1970 5 Sheets-Sheet 2 FIG. 2.

1| ll i i 1 i I i i ://|8\ :I| I i A I i I l I II I l A [f M lm 25 53 FIG. I6

H eoT Exch0nger'39 ff" Pump-Z9 To Stream To 88 I Treated Brine Tank-36 D Recycle Ton k-38 INVENTORS Fred D. Gibson,Jr. Raymond C. Rhees James I, Gibson Bruce B. Holker 4W UM SM ATTORNEY Oct. 9, 1973 F. 0. GIBSON, JR., ETAL 3,764,500

METHOD AND APPARATUS FOR ELECTROLYTIC TREATMENT OF SEWAGE Filed Oct. 28, 1970 5 Sheets-Sheet 5 FIG. 4.

3O 4O Fl0w Rore Supernatant Fiuid Feed To Unit (Goilons/Min.)

Sewage From Bar Screen-IO Primciry Effluent Primary And Grit Chcimber(See Fig. I.)

m Y OLS E TJ N mr m hse ww n 90 0% A d! 5 0. m s B mee e m nm m M c A r 31$ 0 ww .T F m w% W S n.m AD

.w n MB ll n m U W. u /k P m O Oct. 9, 1973 F. D. GIBSON, JR,

METHOD AND APPARATUS FOR ELECTROLYTIC TREATMENT OF SEWAGE Filed 001;. 28, 1970 5 Sheets-Sheet '4 Supernatant Treatment Results Chemical Oxygen Demand (MG/Ll Electrical Dosagel KAmp Hr/IOOO Gal.)

FIG. 8.

Typical Supernatant Treatment Results Current l-4 K Amp. Hr/lOOO Gal.

Treated(ppm) Decreased O 30 p pm Red uced lO-25% Reduced I5-30/ Decreased Medicinal Light Tan Reduced 3'll" o Reduced 2'l5 BY M V INVENTORS Fred D Gibson,Jr. Raymond C. Rhees James I. Gibson Bruce B, Halkei ATTORNEY Oct. 9, 1913 1 slBsON JR" ETAL 3 v 3,164,500 3 METHOD AND APPARATUS FOR ELECTROLYTIC TREATMENT OF SEWAGE Filed Oct. 28, 1970 5 Sheets-Sheet 5 FIG. 9. I FIG. 1/. Effluenl Trealmenl Results Effluem Treatment Results I C V 7a A 0 8O 3 z 11 3 0 00a o x E c o 8 40 30 g, 6 C D 3 20 o v 2O i C O 2.22. g BOD O 05 0.10 0.!5 0.20 o o Eleclrlcal DosaqelKAmp Hr/lOOOGal.)

o n O a 2 3 4 5 I 2 Electrical Dosage(KAmp Hr/IOOO Gal.)

FIG. IO.

- Reuse Effluent Data Untreated Current Treared MG/Ll (K Amp Hr/IOOO Gal.) (MG/Ll CL As NAOL 300'5OO 0.33 330 Cl. 0 0.33 lO-l8pprnllmmedialel 1 5-lOppm(Residuall GOD TO'IOO l.2 -80 BOD 22-27 O.l5 3'8 Baclerla |o,oao (-300 0.02 |oo* 7 Level Using Ghlorinaled lnfluenlAnd Hypochlorinaling Ta l8ppm INVENTORS Fred D. Gibson, Jr. Raymond C. Rhees James I. Gibson Bruce B. Halker BY m Sawwlv ATTORNEY United States Patent US. Cl. 204-452 17 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method for the electrolytic treatment of sewage is disclosed. According to the method, electrolytic treatment of a stream in a sewage treatment plant is provided at one or more selected locations. Thus, in an otherwise generally conventional sewage treatment facility employing various well-known techniques for the removal of solids and also well-known techniques for the bacteriological digestion of contained organic materials, the method contemplates the electrolytic treatment of the final effluent stream for thepurpose of disinfection thereof. In addition, in systems wherein anaerobic digesters are employed to treat the sludge from primary settlers, electrolytic treatment of the digester supernatant is provided for the purpose of destroying its putrescent odor, improving its settleability and color, and reducing its BOD and COD. The apparatus employed includes one or more electrolytic assemblies employing anodes formed of a coating of lead dioxide on a substrate of graphite or titanium, and preferably includes a hollow, cylindrical metal tube which forms the cathode and with the anode contained therein and in the form of a cylindrical member whose outer surface is equidistantly spaced from the inner surface of the cathode can. Electrolyte flows through the assembly from an inlet conduit near the bottom to an outlet conduit near the top, and electrical connections are made to the exterior cathode can and to a portion of the anode substrate which protrudes from the top of the assembly.

BACKGROUND OF THE INVENTION The subject of ecology is one of the foremost issues of the day, not only in the United States but throughout the world. Governmental bodies and industry are discovering that people everywhere are intensely interested in cleaning up the environment and in stopping the continuing pollution of rivers, streams,lakes, and oceans. It is now well-known that one of the greatest abuses of mans environment is the discharge of inadequately treated sewage from municipalities and industries into the very sources of water from which man must draw his essential water needs. Even in the United States, which is reputed to have what may be the highest standard of living in the world, many hundreds of communities discharge their sewage directly into the nearest available body of water without any treatment whatsover. In hundreds of other instances, the only treatment provided for sewage is to remove the largest solid matter which can be conveniently removed in settling tanks, but no other treatment is provided, with the result that a grossly contaminated efiluent is discharged into the most conveniently available body of water. Even in instances where secondary treatment of the effluent is provided, the resulting discharge from the system may be badly contaminated, particularly in instances where storm sewers are not separate from sanitary sewers with the result that the sewage plant becomes greatly overloaded in the event of a heavy runoff of surface water.

The problem is greatly compounded by the growth of ice population, particularly in urban areas whose facilities for the treatment of sewage cannot keep pace with the growthsin demand. The cost of building sewage treatment facilities where none now exist, the expansion of facilities Whose capacity has been exceeded, and the improvement of existing facilities which do not provide adequate treatment of the sewage so as to provide a safe effluent requires the expenditure of vast sums of money, amounting to many billions of dollars, and it is already evident that the urgent and immediate needs will not be met. This recognition that immense resources must be devoted to providing a solution to a critical problem coupled with the knowledge that such resources will not in many instances be available to the extent required, has generated intense interest in finding ways in which existing techniques for the treatment of sewage can be improved so that needed facilities can be provided at lesser cost than heretofore contemplated and also in order that existing facilities can be improved so as to increase their effective capacity, thereby making it possbile for an existing sewage system to discharge a safe effluent even though the facility is now required to treat an amount of sewage greatly in excess of its design capacity.

SUMMARY OF THE INVENTION The method and apparatus of this invention has been put to use and has been found to fulfill the foregoing requirements, i.e. by making possible the construction of a sewage plant of a predetermined capacity at lower cost than previously contemplated and the modification of existing plants so as to increase their effective capacity by a substantial degree. Described briefly, the method and apparatus of this invention comprises the judicious application, at certain strategic locations in a sewage treatment plant, of electrolytic treatment of a portion or all of the stream. This concept is to be distinguished from prior attempts at electrolytic treatment of sewage wherein it was sought to provide substantially the entire treatment of sewage, from the raw state to a final effluent meeting prescribed standards, via electrolytic treatment. It has been found that such a goal is an unrealistic one, at least for large plants which must be capable of treating hundreds of thousands or even many millions of gallons of sewage per day. Thus, the capital cost and also the operating cost of such an installation is so high as to become prohibitive. Moreover, in many of the prior art systems of electrolytic sewage treatment, it was found that the anodes available for this purpose were entirely unsatisfactory, requiring frequent replacement and being moreover generally quite inefiicient.

By means of the present invention, in contrast, it has been found to be entirely practical both from the standpoint of capital cost and operating cost, as related to the surprising improvement in results, to provide several installations in a single plant where electrolytic treatment of all or a portion of the stream takes place to supplement the customary sewage treatment methods.

One highly advantageous location for electrolytic treatment is in the treatment of the supernatant from the anerobic digesters. As is well known, such anaerobic digesters are often employed, and their function is to treat the sludge which is drawn from the primary settling tanks. The sludge, a great proportion of which is water, is pumped to digester tanks and circulated through such tanks for a period of as much as four weeks, under conditions which favor the growth of bacteria which, in effect, digest the organic matter in the sludge. The supernatant discharge from such digesters is ordinarily caused to flow back to the primary settling tanks, while the sludge which accumulates at the bottom of the digesters is periodically removed and separately processed or buried.

In a system employing such anaerobic digesters, it.may, be said that such digesters in elfect constitute a bottleneck in the system. Thus, thep roper operation of the entire sys tem depends upon the ability of the digesters to oxidize the dissolved and colloidal organic materials in the sludge; however, when the system becomes overloaded, the flow through the digesters becomes too great to permit the proper digesting action to occur, with the result that an inadequately treated supernatant flows back into the systern. When this occurs, one of the principal undesired effects is the generation of foul odors in the vicinity of the plant, causing great annoyance to the local population, particularly those residing downwind from the plants location. As will be explained in greater detail subsequently, we have found that even a relatively slight amount of electrolytic treatment of the supernatant from the digesters with the aparatus of this invention results in complete elemination oft he noisome odors. This not only puts an end to complaints from the surrounding population but also makes it possible for the supernatant to be subjected to various kinds of treatment under conditions of overload which would not otherwise be possible. For example, it then becomes feasible to pump the supernatant to a lagoon or the like in order to permit additional solids to settle out of the supernatant. Such settling out of solids is, we have found, greatly facilitated by the electrolytic treatment applied to the supernatant. More specifically, we have found that even the small amount of electrolytic treatment required to effectively kill all odor is also sufiicient to improve greatly the settleability of the supernatant and to decrease the volume occupied by the supernatant solids. Another very important improvement in the characteristics of the supernatant is a change in color which is brought about by even a small amount of electrolytic treatment. Thus, whereas the supernatant is ordinarily of a dark green to black color, being opaque by reason of the solids entrained therein, we have found that a small amount of electrolytic treatment will bleach the dark solids to a light tan and having a slightly medicinal odor instead of the vile smell ordinarily associated with supernatants.

We have additionally found that such electrolytic treatment of the supernatant by the apparatus of this invention brings about a significant reduction in both its BOD and COD. Moreover, there is a very substantial kill of bacteria, with the amount of bacteria kill being a function of the amount of electrolytic treatment applied.

In addition to treating the supernatant as aforesaid, we have also found that greatly improved operation of the plant can be obtained by treating the final efiluent. Where the primary objective is to provide disinfection of the plant efliuent, as is the case when the effluent is to be returned to a stream or the like, it is then ordinarily possible merely to treat the efiiuent by circulating a portion through a side stream which includes an electrolytic hypochlorinator unit. Hypochlorite can efficiently be produced in such a side stream unit and thereby provide a very high degree of bacteria kill in the final eflluent returned to the stream. In some instances, it may be desired to use at least a portion of the final efiiuent for re-use, as for power plant cooling, irrigation, etc. In that event, it may be desired to provide further treatment, over that which is provided by the hypochlorinator; thus, the efiiuent may be caused to flow through a chamber wherein there are disposed a number of the electrolytic cells of the invention. Each cell generates hypochlorite and also ozone and other oxidative species which are effective to a very substantial degree to disinfect the effluent and to provide it with a substantial chlorine residual. Such further treatment has the additional effect of bleaching and coagulating the suspended solids and decreasing the organic pollutants, thereby clarifying and purifying still further the re-use water which additionally is entirely odorless. Frequently, such further treatment can be accomplished with no additio f chemica s t9 the system since only a very low salt concentration is required to permit the desired electrolytic action.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a combination block diagram and flow diagram of a typical sewage system employing anaerobic digesters for the treatment of sludge and electrolytic treatment of the supernatant from such digesters, together with electrolytic production of hypochlorite j for disinfection of the final plant effluent and supplementary treatmentfor re-use; V

FIG. 1A is a flow diagram illustrating a modification o a portion of the process illustrated in FIG. I;

FIG. 1B illustrates a further modification of the process of FIG. 1; i v I FIG. 10 illustrates schematically a 'typical'hypochlorinator for treatment of re-use water;-

FIG. 2 is a cross-sectional view of a typical electro-v lytic assembly of the kind preferred for usein the method and apparatus of this invention;

FIG. 3 is a view illustrating a plurality of-assemblies of the type of FIG. 2; I

FIG. 4 graphically shows the relationship between cell amperage and the flow rate of supernatants through a cell for various desired dosages of the supernatant;

FIG. 5 is a flow diagram illustrating still another modification of the process of FIG; 1 wherein all of the output from the digester's including supernatant and sludge is treated by the electrolytic cellsg' y FIG. 5A schematically illustrates apparatus for the arerangement of electrolytic cells to provide "additional water disinfection adequate to permit re-use of the final effluent;

FIG. 6 is a flow diagram of another modification of the process of FIG. lwherein the sludge fromthe primary settlers is fed directly to electrolytic cells for treatment instead of being first treated in digesters;

FIG. 7 graphically illustrates the relationship between chemical oxygen demand and electrical dosage of the supernatant; '1 j FIG. 8 presents in tabularformsomeof the changes in characteristics of the supernatant after electrical treatment;

FIG. 9 graphically demonstrates the variation in chemical oxygen demand of the effluent for various levels. of electrolytic treatment;

'FIG. 10 sets forth in tabular form some of the variations in characteristics of the plant efiluent in response to electrolytic treatment of said efiluent; and

FIG. 11 illustrates graphically the variation in percentage of bacteria kill of the efiiuent with variations in electrical discharge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a very common type of sewage treat.- ment system as modified to incorporatetherein-the elec trolytic treatment of the present invention. Thus, the raw sewage is shown as being subjected first to the treatment of a bar screen 10 whichhas the..function of removing large objects entrained in the sewage 'and which would otherwise prevent proper. treatment of the sewage if not first removed. Following removal of such solidparticles by the bar screen, the sewage is conducted to the grit chamber 11 which provides an opportunity fo'r additional heavy solid and suspended articles to settleout. Followq ing this, the sewage is conveyed to the primary settler. 12. V The function of settler 12, as is well known, is'to per: mit additional solid matter tofioat or settle out of the water. The liquid portion, termed the primary efiiuent, is subjected to further secondary treatment at, l3vwhich may include, for example, the well-known trickle filters. The sludge which settles put of the primary settlerf 12 is commonly treated in one ormore anaerobic digesters 14. In such digesters, thesludge is preferably kept at an ele vated temperature to. promote the growth of bacteria,and the sludge is maintained in such digesters for aperio l of from two to four weeks to provide the required bacteriological digestion of the suspended and colloidal'or: ganic material therein. The sludge which settles out of the digesters is ordinarily removed and disposed of by any one of several well-known methods, but the supernatant is generally caused to flow back to the primary settler 12 for further treatment;

As long as the system is treating a quantity of sewage which is within the plants design limits, and the plant is operating as designed, adequate digestion can take place in the anaerobic digesters 14 with the result that the supernatant can be discharged to the primary settler 12 without undue deleterious effect. However, in the event of a system upset or overload, bacteriological action or residence time in the anaerobic digesters is insutficient to produce an inert or stabilized discharge. The odor of the supernatant under such conditions becomes so vile that it cannot be discharged to the outside atmosphere without creating a serious odor problem. In addition, the only partially digested discharge has a very high Biochemical Oxygen Demand (B.O.D.) which increases the load on the rest of the sewage plant.

One solution to the foregoing problem is to increase the capacity of the anaerobic digesters, but this is a highly expensive undertaking and also. requires a considerable amount of time. Many communities have therefore been required to resort to extreme methods including, in many instances, the removal of the supernatant to a remote site in order to bury it. This, in itself, is a most unsatisfactory solution since it does not effectively and safely dispose of the supernatant and besides the mere handling of the supernatant creates real problems because the supernatant under these conditions tends to be a highly putrescent material, contaminated with fecal bacteria and high in organic contaminants.

FIG. 1 illustrates the process and apparatus of this invention for treatment of the supernatant. Thus, the supernatant is pumped from the digesters 14 to a recycle tank 15 where it rapidly mixes with previously treated supernatant. The oxidation species in the already treated fluid react rapidly with odorous compounds, quickly destroying the putrescent odor. The supernatant is caused to recycle by means of a pump 16 through a head tank 17 and then through electrolytic cells 18 back to the recycle tank 15. The head tank 17 has a suflicient height so that,

if kept substantially filled, the various electrolytic cells will also remain filled with supernatant as is required for proper cell operation. A brine tank 19 is provided in order that a predetermined minimum concentration of salt can be maintained in the supernatant. While it has been found through experience that the salt (particularly I sodium chloride) normally present in the supernatant fluid is enough to give suificiently high' conductivity for the desired electrolytic action, it has also been found that improved performance in the cells 18 can be achieved by addition of salt. A concentration of salt of two grams per liter has been found adequate, although higher con- 5 centrations do no harm. In order to provide this concentration, sodium chloride is placed in the brine tank 19 and water is caused to flow therein, and the resulting overflow of the sodium chloride solution then flows through an overflow line 20 to the recycle tank 15. The

resulting treated fluid discharge from tank 15 may then be fed back to the primary settler 12.

FIG. 2. illustrates a typical one of the electrolytic assemblies-of the type which is preferred for use in connection with this invention. The anode 21 comprises a and impervious even in the presence of highly corrosive electrolytes. Such an anode is very efiicient in the conversion of sodium chloride into sodium hypochlorite, even at very low concentrations of sodium chloride. Ozone and other oxidative species are also produced, making the cell extremely effective in oxidizing contaminants present in the waste water being treated.

The cathode 24 is a metal can which may be of copper, stainless steel, iron, or other suitable metal, and serves the dual function of being both a cathode and a container for the'fluids being processed. The can 24 is Provided with fluid inlet 26 and (fluid outlet 32, thereby providing for the upward passage through the assembly of the fluid being treated. Electrical connections may be made to a source of direct current at 31 for the anode and at 33 for the cathode. While the preferred configuration is as described above, it must be understood that other anode cathode configurations can also be used. It is, for example, feasible to provide alternate flat anodes and cathodes assembled in sandwich fashion, and other arrangements are also possible.

Multiples of the anode-cathode assemblies of FIG. 2 may be provided as shown schematically in FIG. 3. The number of assemblies selected is dependent upon the power dosage required for the desired treatment and the fluid flow rate. The waste water to be treated enters the conduit 25 and then enters each assembly at its bottom, inlet end, flowing upwardly therein, and through a respective outlet 27 to a common discharge conduit 29. Each may be connected across the electrical supply as shown.

FIG. 1A illustrates a modified form of the process of FIG. 1. In FIG. 1A, the supernatant from the anaerobic digesters is all caused to flow to the head tank 17 and then through the electrolytic cells 18, and with the discharge from such cells being returned to the primary settler 12. In this modification, there is no need to provide the separate recycle tank and head tank 17; however, it must be remembered that flow rate through the cells is dependent on supernatant feed rate and is thus not an independent variable but is instead a function of the plant load, sewage quality, and other factors. Consequently, if all the supernatant is to flow through the electrolytic cells 18, special precautions must be taken to ensure that all the cells are at all times provided with a fluid flow therethrough during cell operation.

A still different modification of the process of FIG. 1 is shown in FIG. 1B. According to this modification, a portion only of the supernatant from the anaerobic digesters is caused to flow into the recycle tank 15. The combination of the recycle tank 15, pump 16, header tank 17, electrolytic cells 18, and brine tank '19 is the same as previously described in connection with FIG. 1. However, in FIG. 1B, only a portion of the supernatant is thus treated, with the remainder of the supernatant being supplied to a separate supernatant treatment tank 30. This treatment tank 30 also receives the discharge from the recycle tank 15, which discharge is overtreated so as to have a high concentration of hypochlorite, substantially in excess of the hypochlorite concentration when the embodiment of FIG. 1 is employed. However, such overtreated supernatant is then mixed with the supernatant directly supplied to tank 30 from the digesters with the result that the untreated supernatant is also subjeced to oxidation from the overtreated supernatant. The resulting discharge from treatment tank 30 is then fed back to the primary settler as before.

One advantage of operating in this manner is that the salt concentration in the recycle tank 15 and hence in the electrolytic cell 18 can be increased, thereby increasing the conductivity of the fluid being electrolytically treated and therefore increasing the efficiency of the electrolytic treatment but without increasing the salt concentration in the final treated discharge from tank 30. Another advantage is that in treating sludge or supernatant from two different sources (for example, supernatant and sludge from the anaerobic digester, and sludge from primary and secondary settlers), the two sources providing respectively different solids content, the one which contains less troublesome solids can be fed to the recycle tank for better operation. Still another advantage is for treating large volumes of fluid containing low solid s content since all of the fluid does not need to be increased in salt concentration for efficient treatment. I

FIG. 4 illustrates the relationship between the-flow rateof supernatant fluid feed to the unit as measured in gallons per minute compared to the cellamperes for diflFerent dosage values as measured in kiloamperes' per thousand gallons of treated supernatant. As anex'ample, it can be seen from FIG. 4 that, fora supernatant fluid feed of thirty-two gallons per minute, and fora desired dosage of 1.0 kiloampere per thousand gallons of supernatant, the'combined cell amperage must be approximately 2000. Similarly, for a fluid feed of supernatant at a level of 40 gallons per minute, and for a desired dosage of 4.0 kiloamperes per thousand gallons, it can be seen that the combined cell amperage must be adjusted to about 9400 amperes.

The shaded area of the graph of FIG. 4 illustrates the desired operating range for effecting the desired treatment of the supernatant in accordance with the process of FIGS. 1, 1A, and 1B. As a further example, assuming that the rate of supernatant flow is at about 32 gollons per minute, one can see from FIG. 4 that the desired cell amperage to stay within the shaded area of the graph extends from approximately 3600 'amperes to 9800 amperes, and that this magnitude of cell amperage will provide a dosage of the supernatant extending from somewhat less than 2.0 kiloamperes per thousand gallons for the lower current level to a level in excess of 5 kiloamperes hours per thousand gallons. The lower level of current dosage for the cells is the minimum level at which the odor of the supernatant is destroyed. The upper level of dosage as computed in accordance with FIG. 4 is approximately the level beyond which increased treatment will result in a sufiicient bacteria kill to result in a substantial reduction in the bacterial action taking place within the system.

FIG. 7 illustrates graphically the relationship between the amount of electrical dosage applied to the supernatant and the reduction in chemical oxygen demand of the supernatant. Thus, in a typical situation, the supernatant prior to any electrolytic treatment may have an oxygen demand in excess of 13,000 milligrams per liter,

but even a relatively small amount of electrolytic treatment of, for example, less than two kiloampere hours per thousand gallons results in a reduction in oxygen demand to a level of less than 11,000 milligrams per liter. FIG. 7

shows that the oxygen demand decreases with increasing electrical dosage, and becomes less than half of the original value when the electrical dosage is somewhat in excess of 20 kiloampere hours per thousand gallons. Of course, such a high electrical dosage is often not practical, but even the substantially lower reduction in oxygen demand,

in the order of 10-25% which results with an electrical dosage not exceeding five kiloampere hours per thousand gallons is very beneficial in the overall plant operation.

FIG. 8 illustrates in tabular form some of the additional changes in the characteristics of the supernatant following electrolytic treatment. Thus, it is shown that for even relatively moderate dosages, i.e. a 'cellcurrent of between 1 to 4 kiloampere hours per thousand gallons, the chlorine in the form of sodium chloride is decreased,

the residual chlorine is increased and may be in the range of zero to 30 parts per million dependent upon the amount of the current dosage, both the C.O.D. and B.O.D. are reduced, the COD. being reduced from 10-2'5%" and the B.O.D. from 15-30%, the putrid septic odor is completely eliminated so that the supernatant is left with a slightly medicinal odor, and the color is transformed ing-re s: Accordingjtol FIG. 5, the electrolytic cells 18.receive both the supernatantand the sludge from the anaer bic digesters-14.This type of operation is practical only where .the mechani cal separationof solidstrom thesewage is -sufliciently effectiyegtoensure that ,all large particles are removed since it will be apparent that under these circumstances all-of .the fluid discharge from :the digesters, including-the sludge, is required to flow through the cells, and the rather limited anode-cathodespacing will not permit the introductiomof large solid matter n cel st a: --FIG. 5 further illustrates; that the outflow from the cells 18 is caused to; flow to drying beds or lagoons and that preferably two ormore suchdrying bedsj-orlagoons are employed. The cell outflow is first caused toflow to one of the drying bedsand with the .overflowtherefrom being returned to the primary settler 12. In such drying bed, the solids tend to settle out, and whenthe maximum amount of solids has. accumulated on thebottom, the outflow from the cells 18 is divertedto the QtherLdrying bed, with the settled-out sludge then being .pe'rmitted :to dry in the first bed and being thereafter removed and disposed of in any-desired manner. F

In employing the modification of FIG. 5, it is desired that the electrolytic dosage be at-the higher end of the scale, i.e. at the upper end of the shaded area oflFIG. 4. A still further embodiment of the invention .is shown in FIG. 6. In this embodiment, anaerobic digesters are not employed and instead the sludge which settles out in the primary settler 12 is applied directly to the electrolytic cells. Here, again, as in connection with FIG. 5, it is im"- portant that all large solids be first removed so that they will not tend to clog the cells. Also, as was described in connection with FIG. 5, alternately-used drying beds or lagoons are employedfor receiving the effluent from the electrolytic cells. The overflow from the drying'bed's or lagoons is returned to the primary settler as shown, and the mixture'of this return flow and the raw sewage may, of course, after passing through the primary settler, be given any of various kindsof secondary treatment as is Well-known in the art including, for example, the common trickle filters as already"described in connection with FIG. 1. V I

FIG. 1 illustrates not only the electrolytic treatment of the supernatant from the anaerobic digesters 14, but also shows that the efliuent from the secondary treatment 'at 13 may also be subjected to electrolytic treatment. Preferably, an electrolytic hypochlorinator 36' which also makes use of the electrolytic cellsas describedherein is employed to treat a side stream portion of the efiiuent; in some instances, as, for example, when the efiiuent is extremely hard, it may be preferable to feed fresh water or even softened water to the hypochlorinator to avoid .the deposition of scale in theapparatus. The hypochlorinator may comprise a brine tank from which an overflow having a high concentration of sodium chloride discharges to a recycle tank 38 as in FIG. 1C. The solution in the recycle tank 38 is continuously pumped by pump'40 through cell 18 and-thenthrough heat exchanger 39 andfinally back to"tank 38, with the overflow; from tank 38 discharging to the, stream to be-treated. Thejfunction of heat exchanger 39 is to prevent a build-up of heat in the electrolyte' as a result of the cell action which would form; reactions tending to produce chlorates instead of hypochloriteg; preferably the electrolyte is maintained; at ,a temperature not exceeding 37 C. 1 a

At times, it is desired, that at least a portion of the plant efliuent be made available for're use. -In that event, additional treatment may be reguired'over and above that'proyidedby the side-stream electrolytic hypochlori- :nator' 36;This may be provided by causing the entire effluent which is to receive such further treatment to now through an'assemblage of the cells 18 as shown in FIG. 5A. FIG. 9 illustrates graphically the relationship between chemical oxygen demand of the plant effluent and the amount of electrical dosage of the efiiuent, and it can be seen that even a modest electrical dosage of somewhat in excess of one kiloampere hour per thousand gallons will result in a reduction of C.O.D. from a level in excess of 70 milligrams per liter to a value less than 50 milligrams per liter, with increasing electrical dosage resulting in even lower values of GOD.

above. The current in the unit was 1800 amperes, the voltage was 13.0 volts, the effluent feed rate was 8.4 gallons per minute, and thesalt connection was 1.0 gram perliter. Samples of the supernatant were tested after passage through one, three, and finally all five assemblies of each bank.

' The following results were noted:

After treatment Before treatment 1 cell 3 cells 5 cells Residual chlorine, p.p m 0.0.. 0.0- 0.33.

,mg. r ,800- 11,900- 11,100 11,300. Bacteria count, No./ml.. 17,400 15,000 7,500 430. pH.- 6.3 .1. 6.1- 5.7. Odor Putrid Mildly putrid- Medicinal"... Medicinal. Color Dark paque.. Brown Light brown. Light brown.

Additional advantages resulting from the electrolytic Example 3 treatment of the efiluent are set forth in tabular form in FIG. 10. The most notable results are the increase in residual chlorine coming about from even a very small current dosage of approximately one-third of a kiloampere hour per thousand gallons producing a residual chlorine of to parts per million, and a reduction in bacteria from a value of greater than 10,000 to less than FIG. 11 shows graphically the relationship between bacteria kill and electrical dosage, illustratingclearly that even very small amounts of electrical dosage bring about a high percentage of bacteria kill.

SUPERNATANT TREATMENT Example 1 The supernatant treatment unit was operated in accordance with the diagram of FIG. 1. The electrolytic cells included 40 anode-cathode assemblies each comprising an anode having a coating of lead dioxide on a substrate of graphite and a cylindrical copper can as a cathode. Supernatant flow was through all of the assemblies in parallel. Electrically, the forty anode-cathode assemblies were connected in two groups of twenty anode-cathode assemblies in parallel. The two groups, or cells, were connected in series. The current through each cell was 5000 amperes, and the voltage of each cell was 16 volts. The feed rate of the efiluent from the digester 14 into recycle tank 15 (FIG. 1) was 38 gallons per minute, and the salt concentration in the recycle tank was maintained at 1.7 grams per liter. Each anode had an outside diameter of 4.125 inches, and an oyerall length of 45 inches, 4% inches at the upper end being uncoated and extending exteriorly of the surrounding cylindrical cathode can to permit the making of electrical connections to the anode. The cathode can was formed of copper and had an inside diameter of 4.5 inches.

Before After Odor Septic-putrid Slightly medicinal. Color Greenish-blaek Light brown. Turbidity.. Opaque Less turbid. COD mg./1 Percent volatile solids pH In addition, the suspended solids settled more rapidly and occupied less volume after treatment than before.

Example 2 Operation of the unit was'as shown in FIG. 1, using the anode-cathode assemblies as described in Example 1, except that 10 anode-cathode assemblies grouped in 1 cell, was made to discharge the treated fluid from the cell to a foam disengagement chamber which consisted of a 20-inch diameter pipe with a foam discharge spout Before treatment After treatment Putrid- Medicinal.

Color Black opaque--. Light brown. COD, mg [1 13,500 10,100.

BOD, mg./ 1 9,120- 8,380. Volatile solids, percent 62.5 43.5.

Settling characteristics of the solids were improved.

HYPOCHLORINATION UNIT Example 4 A hypochlorination unit as shown in FIG. 1C was operated to produce hypochlorite electrolytically from a brine solution. Salt was placed in the brine tank and water fed to the bottom of the salt bed. The brine approached saturation before overflow from the brine tank to the recycle tank, and thereafter additional water was fed to the recycle tank at a rate to maintain the salt concentration at a predetermined level. The brine solution is continually recycled from the recycle tank to the electrolytic cells and back to the recycle tank.

The electrolytic cells included 20 anode-cathode assemblies, each the same as described in Example 1 above and grouped in cells of 10. The flow rate and average residence time of the fluid in the cells wasselectedto produce hypochlorite at a predetermined level for disinfec- Percent Overflow rate from Percent conversion unit (gaL/min.) NaCl, g.ll. Clz, g./l. C.E. to C1: to C theicells. -Each of these. anode-cathode. assemblies -was. oi

t e t e eeeti z dfi n es n with pl 1 above- THe'26-anode-cathode assembliesin eachmajor cell was electrically connected in. parallel-across "the power source; and each assembly was also connected in parallel from the, viewpoint of solution flow. The four-major cells were electrically connected in series. The actual flow through the overall unit was approximately 3,000,000 gallons per day; Average voltage was 58 volts and cell amperage was 3440 amperes. The following results were observed:

Before After treattreatment ment Chloride concentration, mg./l 265 Residual Ch, p.p.m 0. 2 6. 4 Chlorine demand, p.p.m (break point) 4. 6 COD, mgJl 73 66 BOD, mg./l 16.1 5.6 E. coli, MPN [100 ml 5, 160 1. 7 pH- 7. 4 7. 2

Exmple 6 time . A I 12. quickly oxidize the malodorous compounds and/ or rapidly oxidizable components, and the further step of returning at least. apart of the thus electrolytically treated portion to the stream upstream of the place where such biological treatment step is effected. i

2. The process of claim 1 wherein the sodium chloride concentration of the liquid in said electrolytic cell is '-maintained at about at least two grams per liter.

3. The process of claim 1 wherein the cell cathode comprises a hollow metal tubular can and the cell anode 4. The process of claim 3 in which said liquid in said electrolytic cell is caused to flow upwardly in said cell Ten anode-cathode assemblies were used, connected in two banks each including five assemblies. Each assembly was of the type described in Example 1 above. The water thereby treated was pumped to a head tank and then from the head tank through the two banks of cells in series. Electrical connection was in parallel for all the assemblies. The cascade flow through the five assemblies in series of each bank permitted sampling after any one of the five, with each of the five assemblies receiving essentially the same electrical dosage. The flow rate was 2.2 gallons per minute, the cell voltage was 14.2 volts, and cell amperage was 750 amperes. The following results were observed:

After treatment through,

assemblies- Before 1 2 3 4 5 Chloride, mg./l 257 237 225 202 181 167 Residual chlorine, p.p.m 0 23 27 82 120 155 COD, mgJl 60 42 BOD, mgJl 5 Bacteria count, No./m1 5 pH 7. 0

' TN C=(Too numerous to count).

The above test was repeated, but with a higher flow rate 'and a higher cell amperage. More specifically, the flow rate was 10 gallons per minute and the cell amperage was 900 amperes, with the cell voltage equalling 14 volts.

I Whatis claimedis:, p A

1. In a process for the treatment of a stream of waste *water containing malodorous compounds and/or other rapidly oxidizable components which includes at least one biological treatment step, the improvement which com lprisesthe'fstep' of electrolytically treating at least ,a portion of the stream in a cell havingelectrochemically stable electrodes and with an electrode current density "'sufficiently high teprcsatice bactericidal "products which '75 in the annular anode-cathode spacing,

said cell being so positioned that said tubular cathode can is substantially vertical.

5. The process of claim 12 in which the relationship between the cell amperage and the flow rate of supernatant being treated in such as to lie within the shade area of the graph of FIG. 4.

6. The process of claim 12 in which the supernatant from the anaerobic digester is transported to a recycle tank, and the liquid in the recycle tank which comprises supernatant mixed with electrolyzed supernatant is continuously recycled through said electrolytic cells.

7. The process of claim 6 in which brine is added to said recycle tank to maintain the salt concentration in said recycle tank at about two grams per liter.

8. The process, of claim 12 in which thesupernatant from the anaerobic digesters is transported through said electrolytic cells and thence directly to said primary settler. p I

9. The process of claim 12 in which the supernatant from the anaerobic digester is transported through said electrolytic cell and then to drying beds, and the overflow from said drying beds is returned to said primary settler.

10. The process of claim 12 in which a'portion of the supernatant is transported to a recycle tank and a portion is transported to a treatment tank, the supernatant in the recycle tank is continuously circulated through said cell and back to said recycle tank and the overflow from said recycle tank is transported to said treatment tank.

11. The process of claim 10 in which the treated fluid which is discharged from said treatment tank isdischarged to said primary settler. v

12. The process of claim 1 wherein the portion of the stream which is electrolytically treated comprises the supernatant from an anaerobic digester receiving the Sludge from a primary settler,

said electrolytically treated part of the sewage stream being returned to said primary settler. 13. The process of claim 12 wherein the supernatant from the anaerobic digester is circulated through a loop including a recycle tank and at least one electrolytic assembly, the overflow of said recycle tank being transported to said primary settler.

14. The process of claim 1' in which said'electrolysis takes .placein an electrolytic assembly comprising an anode formed of a dense hard coating of lead dioxide on a substrate.

15. A process -for the treatment of sewage comprising the steps of:

settling the sludge from the incoming sewage stream in a primary settler, electrolytically treating at least a part of the sludge from the primary settler in a cell having an anode and a cathode which are energized from a directcurrent source and in which the sodium chloride concentration of the cell electrolyte is maintained at at least about two grams per liter, the anode being formed of an electrochemically stable material and the current density of the cell electrodes being sufficiently high to produce bactericidal agents which quickly react with the rapidly oxidizable components of the sewage,

returning the electrolytically treated effiuent to the sewage stream,

and biologically treating the sewage stream downstream of the location at which the electrolytically treated effiuent is returned to the stream.

16. A process for the treament of sewage comprising the steps of:

screening from the raw sewage substantially all large solid particles,

transporting the screened sewage to a primary settler,

allowing smaller solid particles to settle out in said primary settler as sludge,

flowing said sludge through at least on one electrolytic cell comprising a metal cathode and an anode having an exterior coating of lead dioxide,

transporting the discharge from said cell to a lagoon,

conveying the overflow from said lagoon to said primary settler, and

biologically treating the mixture of treated effiuent and raw sewage from said primary settler.

17. A process for the treatment of sewage in a treatment plant having a primary settler and an anaerobic digester for digesting comprising the steps of,

conveying both the sludge and the supernatant from the anaerobic digester to at least one electrolytic cell having at least one anode and one cathode respectively connected to the opposite terminals of a directcurrent source,

thereafter transporting the discharge from said electrolytic cell to a lagoon,

conveying the overflow from said lagoon back to said primary settler, and biologically treating the effluent from said primary settler.

References Cited UNITED STATES PATENTS 1,079,377 11/1913 Swinburne 204l49 1,131,067 3/1915 Landreth 204l49 1,194,000 8/1916 Dobyns et a1 204277 X 2,997,430 8/1961 Foyn 204151 3,035,992 5/1962 Hougen 204--149 2,872,405 2/ 1959 Miller et a1. 204290 3,543,936 12/1970 Abson et a1. 204l49 X JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner US. Cl. X.R.