Oct. 15, 1946.- ~ _ ' ` w. M. FIAT-r l 2,409,585 vSUPERNATAN‘II‘> SELECTOR DEVICE FOR SLUDGE DIGESTION TANKS lOriginal Filed April 16, 1942 - Patented Oct. 15,1946 2,409,585 UNITED STATES- PATENT OFFICE 2,409,585 SUPERNATANT SELECTOR DEVICE FOR SLUDGE DIGESTION TANKS William M. Piatt, Durham, N. C., assigner to Paciñc Flush Tank Company, Chicago, Ill., a corporation of Illinois Continuation of application Serial No. 439,188, April 16, 1942. `This application July 21, 1945, Serial No. 666,412 3 Claims. (Cl. 210-2) 1 My invention relates to apparatus and method for the digestion of sewage sludge, and it has to do particularly with control of >the removal oi tank. On the other hand, the partially digested by anaerobic bacteria. In the digestion tank, the natant liquor containing least quantities of solids or undigested solids which are undergoing active digestion and consequently releasing gases which supernatant liquor from digestion tanks. ’ tend to rise vertically, will keep the upper por The present application is a continuation of 5 tion of the digestion tank more or less stirred up. my copending application Serial No. `439,188 ñled Depending upon the quantities of raw solids April 16, 1942. which are being added, the temperature of the In the operation of sewage treatment plants, digestion tank and other uncontrollable factors, it is quite customary to first introduce raw 'sew the digestion tank contents are rather unpre age into a preliminary settling tank wherein set l0 dictable as to the nature of the material which tleable solid materials are separated from the may be found at any one point in the tank and liquid, the liquid being discharged to other por it is for that reason, as above stated, that the tions of the plant for further treatment or dis location and quantity of supernatant liquor which posal while the solid material or sludge is trans may be suitable for. withdrawal is highly vari ferred to a digestion tank where it is acted upon . able and the selection and removal of the super presents a diiiicult problem. tions resulting in the destruction of the organic The usual method of disposal of supernatant portions of the sludge which have objectionable liquor is to return it to the inlet of the sewage qualities. In the handling ofv ordinary domestic v20 plant for treatment along with the raw sewage, sewage, between forty and'sixty per cent of the that is, either before or after the raw sewage has passed through the primary settling tank. solid matter in the raw sludge will be destroyed and released as gas during the digestion period.` In view of the fact that this supernatant liquid Raw sludge oi this character as added to the contains some solids from an anaerobic digester digester may contain between ninety-four and 25 in which the reactions take place in the absence ninety-eight per cent water, while the digested of oxygen, these solids have a very high afñnity and completely treated sludge, as withdrawn from ,for oxygen and will deplete oxygen in the raw sewage, thereby causing septic action to take the digester, will contain from about eighty-eight to ninety-four per cent water or moisture. In place which, when it occurs, disturbs the proper the digestion process there is naturally an in 30 functioning of the entire plant. Consequently, it is essential that the supernatant liquor sent back creased concentration of the solids in the di gested sludge as withdrawn from the digester into the plant contain as little solid material and, because of the considerable quantity of sol from the digester as possible so as to lessen this solids undergo decomposition, a series of reac ids which may be destroyed, as above explained, upsetting effect on the entire sewage treatment it will be appreciated that there will be large 35 process. quantities of liquid, usually termed “supernatant Heretoiore, digestion tanks have taken various liquor,” which must be withdrawn from the di gester and disposed of. This liquor will gener ally collect or separate to form substantially hori zontal strata or layers of clear liquid and, while forms and various means have been provided for supernatant liquor draw-off, all of which, how ever, are subject to objection from the foregoing standpoints. For example, digestion tanks have been employed ‘with either a iixed or a floating cover and they have been `provided with one or more pipes at the side of the tank for removing this stratification may occur at almost any level in the digestion tank, the clear liquid is usually found in the upper half of the contents of the tank although not at the very top. During the digestion action, those solids which the supernatant liquor from the digester. are more completely digested and are therefore quite important that a multiplicity of pipes be provided at diiierent elevations along the side not being acted upon by bacteria and releasing of the'tank so that the operator may at will select a point of supernatant liquor draw-ofi gases, >will tend to settle to the bottom of the digestion tank. These solids will normally have most of the organic material digested from them so that they will contain the inert and heavier portions of the solids, which will cause them to settle in the bottom of the tank, and as they set tle they will release some of the liquid by corn In the use of a floating cover type of tank, it is from that stratum where, in his opinion, the t best quality of liquor occurs. This same arrange ment may be employed with a fixed cover or, as in usual practice, a single point of take-off of material may be provided, with the material pacting and concentrating at the bottom oi the 55 merely overflowing from this point whenever sol . 2,409,585 4 Figure I is a vertical, sectional view of one form of sewage sludge digestion tank embodying my ids are added to the tank. With these prior ar rangements, it is necessary to give very careful attention to manual control of the draw-off, ob invention; Fig. 2 is a separate fragmental View of one form of selector or strainer device that may be employed the material is of a character which will permit in carrying out my invention; and the return of the same to the treatment plant. Fig. 3 is a diagrammatic View of a sewage treat Even with very careful control, it is not possible _ to always return the best quality of material to .Y Ament system including the digestion tank shown serving all during the draw-off period whether the plant for retreatment, thereby resultingr in Y in Fig. 1. explained. Moreover, the withdrawal is done peri odically, rather than continuously throughout the In the drawing, I have shown my invention ap plied to sewage treatment system including a di gestion tank I0 of conventional form having an digestion process, so that the rate of return dur _ annular side Wall Ita and a bottom Iii". The tank an upsetting of the treatment process as-above 10 is provided with a 'so-called iioating cover II ing such periods is relatively high, thereby tend ing to upset the treatment process. Also, with 15 having the usual gas-collecting dome I2, gas pipe housing I2“ and gas discharge pipe I3. Raw, these prior arrangements, constant attention is undigested sludge from any suitable source, such required, thereby tending to increase operating personnel necessary for proper operation of the plant. as a primary clarifier 4, is fed to the digester lil through piping I4 including an inlet pipe Illa, and the digested sludge is discharged from the bottom of the tank through a draw-off pipe I5. It will be understood that, although I have shown my One of the objects of my invention is to provide improved sewage digestion means including im proved supernatant liquor draw-oiî means by invention as applied to a floating cover type of which manual control and guesswork are elim tank, it may be equally well applied to the Well inated in the selection oi the supernatant liquor to fixed cover or any other conventional type be drawn off, and by which all such liquor which 25 known of sludge digestion tank. Raw sewage may be is properly clariñed is drawn olf. supplied to the primary clariñer or settling tank Another object is to provide means for selec 4 by an inlet 5, and the liquid separated from tively discharging supernatant liquor from a sew the solids in the clariñer may be discharged by age digestion tank. outlet 6. Still another object is to provide improved 30 anIn carrying out my invention, I provide a se means for automatically and selectively discharg lector device which is disposed within the digest ing supernatant liquor from a digestion tank, at ing material and which is constructed and ar varying levels therein where the supernatant ranged to automatically draw off supernatant liquor is sufñciently clariñed and free of suspended liquor therefrom at any elevation therein where 35 solids that its oxygen demand is minimized and such liquor ordinarily collects and is clariñed or it may be retreated with raw sewage without up solids content reduced to a predetermined extent. setting the normal treatment functions of the More particularly, I provide a perforated or sewage plant as a whole. slotted selector device I6 (Fig. 2) which is An additional object is to provide means for vertically within the digestion tank and automatically selecting and discharging superna 40 mounted which extends from near the bottom of the tank tant liquor from a digestion tank, at varying levels to a point near the top of the material therein. therein, according to the solids content thereof, This selector device I6 takes the form of an elon whereby only that liquid which is clarified to a gated tubular member having its opposite ends predetermined extent and is substantially free of 45 blocked or plugged as at Il' and its intermediate suspended solids is discharged. portion perforated or slotted to provide annular A more speciñc object is to provide a digestion or circumferential series of superimposed elon tank having therein a vertically disposed selector gated slots or orifices I8. The particular strain discharge device which extends substantially er shown in the drawing (Fig. 2) may take the throughout such depth of the material in the tank Where the clearest liquor is most likely to occur, 50 form of a plurality of circumferentially-spaced vertical supports I9 having a rectangular, Wire which selector device is perforated or Islotted sub like member 20 wound spirally thereabout to stantially throughout its length so as to afford a provide the slots I8. The slots I8 may vary from straining action predetermined in character and ïlë" to %” in width, and I have found that eX whereby only clarified liquid substantially free of 55 cellent results may be obtained by using slots suspended solids is discharged. about- 1/8” in width. With this arrangement, the A further object is to provide a sewage treat orifices or slots I8 extend from near the bottom ment system for digesting sewage by anaerobic de Illb of the tank substantially throughout the composition, in which supernatant liquor evolved depth of the material therein so that supernatant in the digestion process is withdrawn from the digestion tank and introduced into the raw sew 60 liquor sufficiently clarified or free of suspended solids to pass through the oriñces I8, at any point age to be treated, the withdrawal being continu or level within the depth of the digesting mate ous throughout the process and limited to sub rial, will be drawn off. The slots I8 could, of stantially clear liquor whereby the tendency to course, be located only in the upper half of the deplete the oxygen in the raw sewage is held at 65 tank where the clarified liquor usually occurs in a minimum throughout the process. normal operation. The rectangularly shaped Still another object is to provide a novel method slots I8 have squared edges that are highly con of withdrawing supernatant liquor from a digesn ducive to an eifective straining action in that tion tank so that only the clearest liquor is with solids suspended in the supernatant liquor will drawn, the withdrawn liquor thus being of a char acter that it may be returned to the raw sewage 70 initially tend to collect and adhere around the edges of the slots, thereby reducing the area of without materially depleting the oxygen content the same and tending to provide a very effective thereof. straining action. That is to say, the solids that Other objects and advantages will become ap have collected around the edges of the slots pro parentas this description progresses and by ref 75 vide a medium which not only further restricts erence to the drawing, wherein: 5 'azioass's ' 6 the slots but also provides a medium to which the upsetting eiiect occasioned by intermittent solids in the material tending to pass through the return of poor supernatant liquor, the main units oi the treatment plant may be designed without the extra capacity which has been deemed slots will adhere so that the material which ac tually passes through the slots for draw-ofi pur poses will be free or practically free of suspended solids. Consequently, with this arrangement, au tomatic selection and draw-oli, without any manual control or guesswork, is provided, the best necess in order to take care or" the foregoing condition. This will naturally result in a con siderable saving of cost of plant structure in ad dition to the improved operation and eñiciency of the ‘treatment plant as a whole. In addition to supernatant liquor for return to the raw sewage for further treatment is discharged, and the like lihood of upsetting the treatment plant as a whole limiting the liquor returned to that which is sub is eliminated. Furthermore, the return of such liquor to the inlet 5 will be continuous throughout the digestion process without excessively de throughout operation of the plant at a relatively low »rate of flow so that the oxygen demand due stantially clear, the present invention provides means for returning the liquor constantly pleting the oxygen content of the raw sewage. 15 to such return is held at a minimum. I claim: ' It will be understood that, while I prefer to use i. In structure of the class described, a closed a 4selector device like that illustrated in Fig. 2, tank free of obstructions preventing horizontal other forms of slotted or perforated devices may ì flow of liquid material in said tank, said tank be employed. Also, while the slots I8 are prefer ably of uniform size throughout the tubular 20 being adapted to receive sewage sludge for di gestion thereof and without mechanical agita member, they may be varied somewhat, the main tion, means for passing sludge into said tank to purpose being to provide a selector device having be digested, and means for automatically dis a straining action which, cooperatively with the charging the supernatant liquor from varying material in the tank, will eiîect a properly clari ñed supernatant liquor discharge, 25 levels at which it may accumulate and substan tially free of suspended solids, including a tubu The selector device I6 is connected to a draw lar member disposed vertically in said tank and ofî pipe 2| which is adapted to return the super extending from near the top thereof downwardly natant liquor to the inlet 5, or ii desired to a point through at least the upper half of said'tank, beyond the primary settling tank- 4, where it is added to raw incoming sewage for retreatment 30 said member being provided with elongated slots of a width varying from île" to 3/8”, said slots along with the raw sewage in the usual manner. being located both circumferentially around and Or, if desired, this draw-oit pipe 2l may lead to longitudinally of said member whereby super any other point of final disposal permitted in the natant liquor at varying levels in said tank which operation of the sewage plant, as indicated by the branch pipe 2i). Also, in the use oi a iioating 35 is freed sufficiently of suspended solids to pass through said slots will pass into said tubular cover, it is preferably provided with a selector member, and means including a generally hori housing Ila which accommodates the selector I6 zontal closed conduit connected to said tubular as the cover is raised and lowered within certain member between its ends for discharging the limits. In the use of a selector device I6 such as de 40 liquor from said tubular member to a point of disposal. scribed, there may be times when the slots I8 may tend to become clogged to an extent impairing 2. In a structure of the class described, a di gestion tank for receiving sewage sludge and in the draw-off capacity of the device. To take which digestion of said sludge takes place with care of that condition, I provide means for back íiushin-g or backwashing the device to free the 45 resultant formation of variably located horizon tal strata of sludge and supernatant liquor hav slots that may become clogged. This backwash ing is accomplished by feeding a washing fluid ing suspended solids content which vary through backwardly through the discharge pipe 2l. To this end, I provide a backwashing conduit 22, out said several strata, means including a con duit extending into said tank for admitting which may normally be closed by a valve 23, and 50 sludge to be digested, means including a conduit extending from said tank for withdrawing digest a pump 24 which may be employed for forcing» ed sludge therefrom, and means for continuously the backwashing iiuid or liquid through the pipe and automatically selecting and drawing off su 2l and into the device is. In some instances, pernatant liquor from those strata, wherever when a proper head pressure is available, the located, wherein the solids contents have been pump 24 may be dispensed with and the fluid fed reduced to a predetermined extent, said last backwardly by gravity. The backwashing ñuid named means including an elongated, hollow se may be ordinary city water or it may take the lector member extending in a generally vertical form of any reasonably clear fluid that may be direction in said tank, said selector member hav available in a sewage system. I have found that excellent results may be obtained by forcing the 60 ing a plurality of elongated slots therein of a width vvarying from à” to %" and disposed in backwashing fluid through the device at a pres _ sure of approximately ñfty pounds per square a longitudinally extending, closely spaced series inch, but this pressure may, of course, be varied without departing from my invention. I believe that the operation and advantages of my invention will be well understood from the foregoing description. By this arrangement one projecting to beyond the upper and lower limits of the strata from which supernatant liquor is to be drawn off, said openings providing inlets into which strata duced which supernatant liquor will pass from those in which `the solids content has been re to said predetermined extent and through can secure the removal of the best supernatant the solid material in the remaining strata liquor from a digestion tank with a minimum of diiiiculty and a minimum of operating control. 70 will not pass but will build up on the exterior of said selector member at said openings to form If the liquor is returned to the inlet of the plant with the adjacent imperforate portions oi said for retreatment with raw sewage it will not cause selector member a substantially enclosed dis an upsetting of the plant operation as a whole, thereby insuring the most eiîective use of the charge conduit leading from the portion or por actual plant treatment units. By eliminating 75 tions of said selector member into which super 2.409,5@ natant liquor is drawn and an imperforate draw off conduit connected to said selector member at a predetermined height therein and extending therefrom and out of said tank. 3. In a method of treating sewage wherein raw sewage is passed to a primary settling zone and the settled sludge from said primary settling zone is passed to a digestion zone wherein the sludge is digested by anaerobic decomposition during 8 natant liquor from said digestion zone through oriñces of a width varying from 11g" to %” formed in an elongated selector member extend ing in a generally vertical direction in said diges tion zone, said oriñces being disposed in a longi tudinally~extending, closely-spaced series pro jecting to beyond the upper and lower limits of the strata from which supernatant liquor is to be drawn off, whereby supernatant liquor is which decomposition horizontal strata of sludge 10 drawn into said orifices at said one stratum and and supernatant liquor are formed, the super the solids contents of the remaining strata block natant liquor at one stratum, at least, having a ofi” the oriñces at those strata, withdrawing the suspended solids content which will pass through liquor from said selector member through an im oriñces of from île” to %” in size, which stratum perforate draw-off conduit connected to said se of supernatant liquor is in condition to be with 15 lector member, and delivering such liquor to the drawn and passed to the raw sewage for retreat ment therewith, the steps of withdrawing super raw sewage passing to said settling zone. WILLIAM M. PIATT.