US2091631A - Sewage digestion - Google Patents

Description

1937. A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Original 'Filed May 31, 1934 7 Sheets-Sheet 1 Outlaw mokmw 5) (I'ttorneg Aug. 31, 1937. A. J. FISCHER ET AL SEWAGE DIGESTION "7 Sheets-Sheet 2 Original Filed May 31, 1934 1937- "A. J. FISVCHER ET AL 2,091,631

SEWAGE DIGESTION Original Filed May 51, 1934 7 Sheets-Sheet 3 g- 1937- A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Original Filed May 31, 1934 7 Sheets-Sheet 4 Gttomeg Aug. 31, 1937. A. J. FISCHER ET AL- SEWAGE DIGESTION Original Filed May 31, 1934 "7 Sheets- Sheet 5 FIG 3 n 1: en tors 4077/0 d. W509i attorneg I A. J. FISCHER ET AL 2,091,631

SEWAGE DIGESTION Aug. 31, 1937.

Original Filed May 51, 1934 7 Sheets-Sheef s lung L (Yttorncg Aug. 31, 193 7.

J. FISCHER ET AL SEWAGE *DIGES'I'ION Original Filed May 51, 1934 7 Sheets-Sheet 7 Patented Aug. 31, 1937 UNITED STATES FFICE Lund, Seaford, N. Y, assignors Company, inc, New York, N.

of Delaware Original application to The Dorr a corporation May 31, 1934, Serial No.

728,376. Divided and this application January 28, 1935, Serial No. 3,726 l 9 Claims.

In the treatment of sewage and analogous polluted waters, according to modern practice, it is quite common to effect an early separatingas by settling and withdrawal of the suspended solids of the sewage in the form of a sludge and to subject the withdrawn sewage sludge to what is known as sewage sludge digestion. Sometimes in this class of work the sewage sludge is obtained in a condition which is known as raw or untreated sludge.

According to other procedures either the sewage is chemically treated or the sludge may be treated whereby there may be obtained what may be termed a chemically treated sludge. The chemical treatment may have been provided to facilitate 5 the precipitation for the ultimate obtaining of the solids as sewage sludge. Also according to certain procedures the sewage or the sewage sludge is subjected to aeration which is employed as for example to facilitate the precipitation of the sludge 0 solids called activated sludge or in order to obtain a sewage sludge having certain characteristics which facilitate the process as a whole.

Raw or untreated sludge often has acid characteristics. Where the sewage is aerated the sewage 25 sludge obtained may have an acid characteristic particularly where the sludge is stored prior to digestion.

When sewage sludge is digested the acid characteristic has to be overcome and the sludge takes 0 on an alkaline characteristic which is desirable throughout the entire digesting period. This alkaline characteristic may be the result of.

anaerobic bacterial action which results in digestion which is duly initiated and which is main 35 tained. This digestion is the conversion by virtue of bacterial activity of certain of the sewage solids into some other physical form such as a liquid or a gas. Some sewage solids are digestible while others are not. Digestible sewage solids are N largely putrescible, so theirconversion by digestion thus eliminates from digested sewage sludge.

. The present invention is concerned with apparatus useful in connection with the digesting of 5 sewage sludges or mixtures thereof no matter how obtained, for example, in sludge digesters in which the sludge treatment carried out therein is by a method in which there is an anaerobic digestion of the sewagesludge as the result of biological or 50 bacteriological activity during which certain bothersome sewage solids undergo decomposition and distintegration, such digestion is accompanied with the production of a combustible gas containing methane as a substantial con- 55 stituent thereof.

their odoriferous nuisancev The present invention primarily relates to apparatus or instrumentalities usable for or in connection with the biological digestion of sewage sludge and its analogues and one of the objects thereof is to construct in commercial practical form an apparatus or instrumentality useful in the treating of the sludge and so that during said treatment the gas developed and given off as the process progresses can be collected and handled.

The apparatus which is the subject of the present invention embodies agitating or mixing devices which will aid mixing conducive to the seeding action referred to herein and which hastens the production of the methane gas due to the bacterial action which takes place as. the sludge digestion process goes on.

The invention according to one aspect there- .of relates to the providing for use in connection with a sewage sludge digester tank, of a vertically yieldable or vertically movable structure or unitary top assembly 4 comprising (a) a sheet metal gas holding type of top with depending marginal flange or peripheral wall; (b) a bracing and guiding means providing a centrally located vertically extending strut rigidly connected and depending from the sheet metal top and serving if desired as a vertically movable center guide element, and tie rods or connecting members extending from the marginal portion of the sheet metal top downwardly and inwardly tothe lower end portion of the center strut; (a) one or more ofi-center sludge stirring devices each extending downwardly into and through an inte mediate opening provided in a section of the sheet metal top member between the center strut and the marginal flange, and essentially comprising a vertically extending driven rotatable shaft carrying one or more sludge impelling or stirring elements, 2. base member normallyseated on the sheet metal top and having a gas-tight seal through which the shaft extends and serving as a closure cap for the corresponding intermediate opening; ((1) a steady bearing construction at and for the lower end of each shaft; and (e) an arm construction providing an arm for a co respond! ing sludge stirring device, each said arm extending outwardly, that is, horizontally from the lower end portion of the central strut, carrying at the outer end thereof a fixed member of the steady (bearing construction, deriving support from the strut because of being directly connected to the lower end portion of the latter, and preferably supported against downward deflection of the outer end thereof by a connecting brace extendingupwardly and inwardly from the outer end one is able to portion of the arm to central strut.

upper portion of the The unitary top assembly is preferably made so as to permit ready removal and ready replacement of a sludge stirring device and therefore a sheet metal top is employed which is preferably provided with a depending intermediate flange along the marginal defines of an opening intermediate the central to is constructed so as to seal realized as by a and marginal wall portion of the top. The base member or cap referred provide a suitable gas gas sealing packing through which the stirring device shaft extends and this base or cap member operating parts and preferably carries the shaft is detachably secured to thesheet metal top thus permitting the sludge stirring device to be lifted as a whole from its normal position in respect to the sheet metal top. In

furtherance of a practical construction wherein the feature of ready removal and replacement may be realized together with the attainment of suitable guiding for the lower end of the stirring device shaft, the steady for, at portion of the shaft and and on the lower end liftable therewith and inverted cone shaped or a cone shaped section for directingand for aiding in the proper seating of the one member on the other as the sludge stirring device is lowered into operative position.

Further aspects relate to the simplifying of the construction of the digester; to the reducing of the cost of its operation; and tothe devising of means for preventing troublesome scum formation therein.

This application is a division of a parent application Serial No. 728,376 filed May 31, 1934, and which "eventuated into Patent No. 1,989,589

granted January 29, 1935. cation it was pointed can be depended upon for fresh sewage sludge with In the parent appliout that digester liquor seeding or inoculating anaerobic bacteria to initiate and to stimulate the bacteriological digestion thereof to a greater degree than can digester solids, when considered on a dry solids-content basis. 1 digested sludge solids to Formerly it was believed that the ratio of the raw sludge solids was the determining factor in calculating the volume of seeding 5 aforementioned discovery material required. With the in mind'one aspect of the apparatus of this inventionis to eflectonly such movements within the body of digesting sludge as will permit the heaviersludge solids to exist within a limited horizontal zone while otherwise the movement of the is circulated throughout the entire restricted and liquor is relatively unmass in the digester of which the apparatus of this invention cal element.

realize the constitutes an important mechani- By the apparatus of this invention selective and diflerential' circulation of digester liquor and digester solids.

According to one and in certain respects preferred arrangement (or arrangements) of apparatus for realizing the basic aspects of the invention there is employed a primary digester (or primary digesters) having a vertically yieldable or vertically movable top or gas holder construction. In any event in of the invention there are holder or top member or realizing certain aspects employed with the gas cover of the primary of digesting sludge ensures tion revolve about this construction of the gasholder and particularly about the construction of the yieldable top of the primary digester so that it will have the required movement. hlkaline condition essential for the carrying out of the sewage digestion by anaerobic bacteria has been once established that condition will not be upset by incoming fresh sludge, even though the incoming fresh sludge may have-an acid characteristic, because the immediate intimate mixing of the incoming sludge through the alkaline body the preponderance and prevalence of the alkaline characteristic and anaerobic bacterial activity.

The invention possesses other objects and features of advantage, some of which with theforegoing will be set forth in the following description.

In the following description and in the claims parts will be identified by specific names for convenience but they are intended to be as generic in their application to similar parts as the art will permit. In the accompanying drawings which constitute a part of this specification there have been illustrated the best and most characteristic embodiments of the invention known to the undersigned but such embodiments are to be regarded as typical only of many possible embodiments and the invention is not to be limited thereto.

Insaid drawings Fig. 1 is a diagrammatic of a sewage system embodying two sedimentation tanks either or both of which can be relied upon to receive a stream of sewage and from either or both of which at will sewage sludge derived as a corresponding to incoming sewage sludge as and when the latter is digester.

This figure yieldable peller element in submergence is rotated about an upwardly extending axis of a vertical shaft carrying the same.

Figures 3 and 4 are tional and plan views of a multiple digester.

Fig. 5 is a vertical sectional view partially broken away showing the relative arrangement and position ofcertainrparts of the primary digester in respect to; certain other parts of the secondary digester andmore particularly the relative position of certain overflow constructions provided for the respectively vertically secprimary digester in respect to 75 I certain overflow constructions provided for the secondary digester.

Fig. 6 is a perspective view illustrative of the overflow arrangements of Fig. 5. In Figs. 3 to 6 inclusive the overflow arrangements for the secondary digester are shown functioning in a horizonal path 180 degrees from the position which the same parts occupy in Fig. 1.

Fig. 7 is a vertical sectional view taken as on the plane indicated by the line ll-l of Fig. 5

looking in the direction of the arrows, this Fig. 7

being a vertical sectional view showing the overflow arrangement for the primary digester in respect to the inlet end of a pipe or conduit leading to the secondary digester.

Fig. 8 is a vertical sectional view taken as on the plane indicated by the line d--t of Fig. 5 looking in the direction of the arrows, this Fig. 8 being a vertical sectional view showing certain overflow arrangements for the secondary digester in respect to the inlet end of a pipe or conduit for conducting: the relatively clear effluent liquid to a place for the disposition of the same.

Fig. 9 is a perspective view of the vertically movable top member or gas holder used in the primary digester but it will be understood that the main features of this gas holder as embodied therein may also be embodied in the gas holder employed in the secondary digester. There is not included in Fig. 9 the mixing devices carried by the gas holder.

Fig. 10 is a view the mixing device and the manner in which it is applied to the gas holder of Fig. 9.

Fig. 11 is a partial sectional view taken as on the plane indicated by the broken lines ll-i.l of Fig. 3 looking in the direction of the arrows.

Fig. 12 is a vertical sectional view taken as on the plane indicated by the line l2-l2 of Fig. 11 looking in the direction of the arrows.

Fig. 13 is a plan view of an impeller element used in the mixing device of Fig. 10.

Figs. 5 to 13 inclusive are shown at a much larger scale than the same parts are shown in Figs. 3 is at a smaller scale than any of the other figures.

Fig. 14 shows a primary and secondary digester construction that functions practically the same as the digesters shown in Fig. 3 except that in Fig. 14 the gas holder of the primary digester is shown as a floating cover arrangement that is provided with a pontoon construction that defines the peripheral wall thereof. In Fig. 3 each of the gas holders is shown as resting on stopsto wit, a position below that which they occupy in normal operation.

Fig. 15 shows a construction wherein primary and secondary digesters are employed that function much the same as the digesters in the ar' rangement of Figs. 3 and 14 but in the primary digester of Fig. 15 the normal position for the yicldable top is that which it occupies when resting upon the stops. In other words the yieldable top of the primary digester of this figure leaves the stops only due to a decidedly abnormal condition existing within the digester and in which instance it is free to yield or move upwardly.

Reference will now be made to the drawings in detail.

Like reference characters indicate parts functioning the same or substantially alike wherever they appear.

In the arrangement indicated in Fig. 1 the incoming sewage supply is designated by S8. The

partially in section showingand 4 and it will also be noted that Fi 1 sewage from this supply is passed at will as along the fiath l to the clarifier, sedimentation tank, or sedimentation basin 2 or as along the path 3 to the clarifier d. Settled sludge is withdrawn at will from one or both clariflers as along the path 5 or the path 6 as the case maybe and is delivered intermittently by means of sludge pumps l and/or 3 ultimately along the path 9 that terminates in feed pipe M5 to and into the primary' digester iii) constituting part of a mul tiple digestion system that also comprises the secondary digester tank Ml arranged to receive intermittently from the primary digester sludge which is undergoing digestion and in an amount which is equal to the amount as and when fed into the primary digester for treatment. This tank ill and its equivalents, while herein are referred to as secondary digesters, are primarily for carrying out sedimentation and secondarily for carrying out digestion.

Each of these digesters comprises a digester tank and an associated gas holder or gas collector. The tank of the primary digester is designated by M2 and its associated gas holder or collector by M3. The tank for the secondary digester is designated by Hi l while its associated gas holder or collector is designated by M5.

As previously indicated piping or other passage is provided whereby partially digested sludge can pass from the primary digester to the secondary digester as is required incident to the supplying of the incoming sludge to the primary digester. Piping is also provided whereby the gas receiving spaces within the two digesters are in communication with each other during all normal periods of operation. These piping arrangementswill later be described more in detail.

Also as previously indicated the digester arrangement of the system of Fig. 1, to wit, the

arrangement. of the primary and secondary digesters thereof as constructed and connected, is essentially that of the arrangements of Figs. 3 to 15 inclusive. The early work in connection with the actual carrying out of certain basic aspects of sewage digestion and which led to the developmentof the present invention is illustrated in Figs. 2 to 5 inclusive of said already issued Patent 1,989,589.

On that early work there was employed sludge agitating mechanism which may be operated intermittently or continuously as desired. Such early mechanism comprised a rotatable vertical shaft preferably mounted off centerin respect impeller was such that it effected a non-hori-w zontal flow or current movement, to wit, movements which are not at right angles nor in a plane normal to the axis of the vertical shaft. The other impellers spaced along the shaft were located in a horizontal position and effect flow or current movements inplanes normal to the axis of rotation of the vertical shaft. The function of the impellers that were horizontally mounted, was to agitate the particles of the sludge in the tank in horizontal zones, while that of the top tilted impeller was to discourage if not to prevent the formation of scum in the zone of the liquid level in the digester by razzle-dazzle or circumnutatory flow or current movements which convectively carry scum .particles rising to and which would otherwise tend to float and collect at the top of the body of digesting sludge to submergence within the sludge whereby the scum particles ultimately become digested. In this way the collecting and formation of scum was eliminated and avoided. The effect of this agitation with its consequent hindering of the formation of a floating scum layer and a deposited sludge layer or blanket was to ensure that the entire sludge mass in the primary digester was brought into a relatively homogeneous condition by virtue of which portions of said sludge mass withdrawn from the said digester for further or dewatering treatment have on the average substantially the same proportion of liquid and solid constituents as said sludge mass.

In the apparatus of the early work primary and secondary sludge digesters were employed and partially digested sludge from the primary digester was suitably conveyed to the secondary digester or digestion zone or tank. This secondary tank was of the sedimentation type since the function of this tank was primarily to thicken the sludge and to obtain a clarified supernatant therefrom and secondarily to finish the biological digestion of the sludge from the primary digester. The tank 2? was therefore some times referred to as a sedimentation tank or basin for in it there was maintained predominant essentially quiescent conditions as required for sedimentation without concurrent predominating conditions conducive to digestion. Accordingly, digesting sludge from the primary digester Was fed to the secondary digester through suitable piping.

In the early demonstration plant as to which the showing of Figs. 2 to 5 of said Patent 1,989,589 is illustrative the primary tank was 29 ft. in diameter and 15 ft. deep. The impellers were of the Weedless type and 20 inches in diameter. They were run 'at a speed of 120 R. P. M. although speeds up to 600 R. P. M. may be used.

In the primary digester, the lower impellers caused horizontal rotation or agitation of the sludge mass to produce thorough mixing thereof in and throughout the horizontal section or stratum affected thereby. The resulting turbulence causes an evolution of gas, which in rising to the top of the digester, tends to form scum adjacent the liquid level thereof. This tendency to form scum is particularly discouraged, if not overcome, by the action of a tilted impeller (typified by impeller 23 of Fig. 2) which set up a razzle-dazzle action or circumnutatory flow current in the digesting sludge as indicated in Fig. 2, and serves to beat back or diffuse the scum particles and any liquor which may rise to the top of the digester into the digesting and horizontally agitated mass or sludge.

Reference will now be made in detail to the later or other developments wherein the novel yieldable and movable top constructions embodying the present invention in association with digester tanks are employed. It will be understood that for an' amplification of any part or description which has been eliminated, foreshortened or amended that reference may be made to the disclosure in the specification of Patent 1,989,589 since the disclosure in the application of this patent is the same as the disclosure of the application Serial No. 728,376 from which said patent eventuated.

The construction of the associated digesters of Figs. 3 and 4 will now be described and more particularly in conjunction with reference to Figs. 5 to 11' inclusive. The tank N2 of primary digester H0 is made of concrete. It comprises a floor or bottom portion H6 and peripheral upstanding wall lll the latter of which carries at the upper interior portion thereof an inwardly extending stop ring or corbel I09 providing a stop for the gas holder or gas holder construction H3 and for supporting the latter in a lowermost position therefor at the upper interior portion of the tank. This stop ring or corbel I09 serves to prevent the loss of gas through the liquid seal around the holder by deflecting rising gas bubbles from the liquid sealing section. The tank H2 is also provided with a vertically extending centrally located guide post 8 which is fixedly secured at H9 relative to the floor of the tank structure.

The gas holder H3 is provided by a vertically yieldable and in fact a vertically movable structure which comprises a sheet metal top l2!) and a depending peripheral wall l2l, a vertically extending centrally located strut Q22 and downwardly inclined connecting members I23 which extend from the peripheral wall iii inwardly and downwardly to a place where they are connected to the vertically extending strut member H22. This vertically extending strut member I122 is preferably tubular in form and has a vertically sliding engagement with the guide post Mil. See in this connection the arrangement in Fig. 11 wherein the post M8 is shown as carrying anti-friction rollers M38 that engage an interior section of the strut or guide tube H22. This strut member i22 has a dual function, it serves in conjunction with guide post M8 to guide the vertical movement of the gas holder i l and to ensure that the top l2 thereof shall maintain a desired horizontal arrangement for all positions thereof. It also serves to reinforce and support the top 92d when the gas holder is resting in its lowermost position. This provides a construction for preventing the top lZt from buckling, bending or collapsing incident to external air pressure exerted on the gas holder.

The top member or portion 52d of the gas holder is in the form of an umbrella cover or inverted saucer. More specifically and more ac= curately it may be described as preferably in the form of a top section of a hollow sphere, the periphery of which section is in plan a true circle.

The gas holder however need not be circular in horizontal cross section as any other geometrical form which in horizontal cross section presents a symmetrical arrangement could be employed as for example one showing a hexagonal, octagonal or other symmetrical form. Moreover the top member or top construction of the gas holder need not be in the form of a sphere top, of an inverted saucer, or resembling that of an umbrella top since any suitable geometrical form could be employed, be it that of a cone, of a pyramid, a frustrum of a cone or a frustrum of a pyramid. An essential factor is that the top member or top construction of the gas holder shall be hollow so as to have a concave portion at the under side and which is well constructed to withstand either internal or external pressures as and when applied thereto.

The peripheral wall I of the top holder deof the latter.

pends downwardly from said concave top section or inverted saucer shaped top member". This peripheral wall may be defined as a peripheral ring or member which has a gas-tight connection or union with the top member whereby a gas-tight top construction for the holder is provided. This peripheral ring or wall lZl is of metal and is loaded with concrete ballast IN. This concrete ballast is relatively heavy and the function thereof is to place a relatively progressive and increasing load on the gas holder particularly as the gas holder rises sufliciently to lift the concrete ballast so that it projects above the sealing liquid whereby a progressively and substantially increasing load is imposed upon the movable holder according to the amount of ballast projected above the liquid level. In the lowermost position of the gas holder the lower edge portion of the peripheral wall or ring iZl rests upon the stop ring or corbel m9. An inspection of. the drawings will make it clear that this stop ring is positioned so that the gas holder even in its lowermost position is located adjacent to the upper portion of the primary digester and also that the vertical movement of the gas holder during any normal operation thereof is small or relatively limited particularly as compared with the vertical movement of the gas holder for'the secondary ently appear. As previously indicated, according to and within the broader aspects of the invention, it is feasible to construct the primary digester so that the 'gas holder thereof can have a long range of movement and so that the gas holding capacity thereof can be relatively large.

The strut member I22 heretofore described as a tubular member is provided with openings M6 at the top end thereof whereby any pocketing of gas that might otherwise affect the movement of the gas holder is avoided.

Mixing devices or sludge impelling mechanisms i243, frequently referred to as turbo-mixers, are carried by the gas holder so as to be vertically movable therewith. The arrangement of these mixing devices is clearly shown in Figs. 3 and 4 and the general construction thereof and mode of mounting the same is clearly shown in Figs. 10 to 16 inclusive. The mixing device (or devices) Hi l is similar in essential operative characteristics to the mixing device or sludge impelling mechanism previously referred to in connection with the early system of Figs. 2 to inclusive of said Patent 1,989,589. Each of these mixing devices i2 5 comprises a base H5 which is detachably secured to the top I20 as through the medium' of bolts I26 and in a manner whereby a gas-tight joint is provided as between the top i213 and the base i255. Upon this base H5 there is carried the housing E21 of an electric motor I28 the rotor of which is connected so to drive through the medium of suitable gearing and coupling ltd, a vertically extending shaft ltli of the mixing device so as to effect the normal rotary movement Suitable gas-tight packing itl is provided below said coupling 8%. In the immediate region of each mixing device a depending ring i32, which is circumferentially arranged with respect to the shaft I130, depends from the cover a sufficient distance to extend into the liquid or digesting sludge maintained up to a minimum level within the primary digester when the latter is functioning. The purpose of this depending ring I32 and liquid seal resulting from the use thereof is to provide an arrangement whereby, at will, the mixing device as a whole can be refor producing impeller The inclination of the body as it digester, as will pres-- moved from place without allowing the escape of gas from the holder by merely removing nuts from the bolts I26.

Each mixing device is provided with non-clogging impellers as I33 in vertically spaced arrangement. Each of these impellers comprises a hub section We, a body section l35 arranged in an inclined position in respect to the hub axis whereby when the impellers are in place the bodies are tilted at a slight angle from the horizontal and each impeller also includes vanes or blades H6 effects in directions away from the axis. rotates produces vertical components of flow movements while the vanes or blades produce horizontal components of flow movements and the functioning of these impellers produces what may be referred to as the razzle-dazzle or circumnutatory motion or mixing action within the liquid body afiected thereby. The functioning of these impellers, which located in respect to the general axis of the tank as is manifest from Figs. 3 and 4, produces what may be generally termed epicyclic type of currents or movements throughout the various sections of the liquid body.

Some or all of these impellers are frequently made and employed so that the body section thereof need not be inclined to the hub axis with the result that the body section and the impellers move in true horizontal paths thus avoiding most or much of the vertical components which are are also offset or eccentrically experienced where the body is inclined to its axis.

Impellers with bodies at right angles to their axes are useful in the lowermost layers of the sewage sludge but it will be pointed out that the impellers with the bodies inclined to their axes of rotation are particularly useful in the upper layers of the sludge since the operation thereof assists in breaking down scum and in the thorough mixing of the tank contents in such a manner that the accumulation of scum at the upper interior portion of the primary digester is avoided due to scum particles being swept by the razzle-dazzle currents back to submergence within the body of digesting sludge.

The mixing devices as illustrated herein have been described in considerable detail and as previously indicated function as non-clogging impellers or in other words are of the non-clogging type. It is feasible to construct each impeller so that it primarily comprises or consists of a body or hub portion and blade members, which body or hub portion is secured to the vertical extending propelling shaft to which the impeller corresponds and which blade members extend outwardly from said body or hub portion. Such blade members may be considered as compositely defined by the upper and lower blade elements heretofore described. The blade elements function to effect a forced or circulatory movement of liquid away from the shaft particularly since there is no peripheral restraint against outward flow because there is not any ring or other restraining member located at or in the immediate vicinity of the periphery of the impeller.

As to the impellers herein shown some have body members in effect provided by plates .that extend at right angles to the axis while some have body members in effect provided by plates that are inclined to the axis about which they are rotated. It is feasible, however, to construct impellers that will function along the same lines as the impellers herein illustrated and. this even though the plate portions just referred to are not necessarily employed. 18y bearing in mind the particular work to be done by a particular impeller it is merely a matter of design to construct the impeller to do the work desired therefor.

The lower ends of the mixer shafts i139 are provided with removable steady bearings each of which includes a bearing member 43? that is retained upon the shaft ldd and which is removable therewith because of spaced collars l38-l3t which are secured to the shaft; In this connection see Figs. 10, 11, and 12. Each bearing member Ed? has an outer section N5 in the shape of a frustrum of a cone whereby it readily sets or positions itself in place on the conical seat sec= tion I353 which is carried at the outer end of the corresponding extension arm construction 1 39 that includes parallel channels Ml and M2. The intermediate sections of these channels are secured in any suitable manner M3 to the vertically extending strut member l22. Braces l l l are relied upon for firmly supporting the outer ends of the arm construction Md against downward deflection. It will be noted that the outside diameter; of each impeller H33 and of the removable steady bearing member Mill is less than the inner diameter of the depending ring I132 with the results that the mixing device 92d as a whole can be readily placed at will solely by the operator removing 0 nuts from the bolts l2 and vertically moving the of which extends into the mixing devices from or towards its normal position in respect to the gas holder top.

The primary digester receives the sewage sludge to be digested in any suitable manner as through the feed pipe M5 which it will be noted according to the arrangement shown delivers the incoming or influent sewage sludge into the upper interior portion of the primary digester tank, preferably through a section of the feed pipe M5 which is located somewhat below and which extends past the supporting surface at the top of the stop ring or corbel me.

Sewage sludge, which has been treated, passes from the primary digester through a discharge section M8 into a discharge pipe or conduit Ml! having a valved branch M8 and an tending branch M9 the latter of which terminates in an adjustable overflow or weir construction it!) that is relied upon for ensuring that the minimum level of the body of sewage sludge retained in the primary digester shall always be at least up to the height determined by the overflow. For the details of the overflow arrangement of both the primary and secondary digesters see particularly Figs. 5 to 8 inclusive.

In the normal functioning of the primary digester the valved branch M8 is closed whereby all of the sludge passing from the primary digester must pass through the overflow branch I49. It will also be here noted that as sludge is supplied to the primary. digester for treatment therein a passes therefrom through the overflow branch I 49 on its way to a secondary digester as will hereinafter more clearly appear.

The primary digester also has a gas oiftake piping or conduit I5I the upper intake end I52 gas receiving space which is always maintained within the upper interior portion of the primary digester, to wit, into the gas receiving space at the under side of the gas top I20 on the one hand and the top of the body of sewage sludge maintained within the primary digester on the other hand. This gas pipas by welding at upwardly exing l5l extends into the secondary digester and. terminates in a pipe section #153 the upper end of which extends into a gas space provided in the upper portions of the secondary digester with the result that the digesters the gas spaces of both digesters are in constant communication. The upwardly extending delivery end portion or pipe section I53 derives support from the floor of the tank of the secondary digester througlr the medium of supporting rod l53'.

The primary digester is also preferably provided with .a normally closed depending pipe 554 (see Fig. 3) that always dips into the sludge within the primary digester but which is arranged to provide a sampling opening by which ready access for sampling purposes is realized.

The primary digester is provided with heating coils E55 which are useful in ensuring sufficient heat to the sewage sludge to facilitate'the digestion that is carried out therein. The gas holder is secured against horizontal rotationrelative to the tank through the medium of chains '556 and which are referred to as tangential guides. T

The secondary digester ill is shown larger than the primary digester and comprises a sec- -ondary digester or sedimentation tank Ht provided witha downwardly and inwardly inclined bottom Me and upstanding peripheral wall ill carrying spaced stops Mill. The secondary di- The peripheral the latter is in I M6 for the secondary digesterand inwardly and ly located strut corresponding thereto. The

lower edge portions of the peripheral ring ltd are 50 construction the secondary gas holder is guided throughout the entire range of its normal vertical movement and the strut construction also v serves to interiorly support the plate top section of the digester against internal buckling or collapsing should there be realized a decrease in pressure within the tank and an excessive unbalanced pressure on the outsideof the tank.

Reference has heretofore been made to the valved branch I48 of the primary digester.

sewage solids disposal as along the pathSB to a sludge bed. The tank of the secondary digester has a solids 75 during the normal functioning of 5 I10 leading to a valved sludge pipe or conduit "I which terminates in a T pipe fitting I69. It will therefore be apparent that the piping which includes the pipe I41, cross fitting I69 and pipe I1I when the valves are open can be employed for the direct passage of sewage sludge from the primary digester to the secondary digester. Normally the valves are closed so this direct transfer cannot take place. Upon the opening of either the valved pipe I41 or the valved pipe I1I, it will -be manifest that sludge can be pumped at will from the primary digester or secondary digester to and through the cross fitting I69, thence along the path I90 by a pump as 1 or 8. See Figs. 6, 4, 3, and 1. It would also be possible to effect a reverse pumping action to unplug the valved pipes I01 or I1I should occasion require. The secondary digester normally functions over an extended period without the withdrawing of sludge and then at the proper time the deposited solids remaining after the digestion and sedimentation carried out therein are removed through the pipe III. This removal of sludge solids of course effects a lowering of the normal water level within the secondary digester and this is accompanied by a lowering of the gas holder. In order to avoid lowering of the liquid within the secondary digester in such a manner as to unduly reduce the gas pressure therein whereby too great a differential as between the exterior air pressure and the internal pressure will take place-so great a differential as will tend to cause the gas holder to collapse or fail-there are provided certain ordinary check arrangements as follows: a sludge check or ball valve I12 that is carried by chains I13 is provided and which sets itself in the solids discharge section I10 slightly before or .at least by the time that the secondary gas holder reaches its lowermost position; also a gas check comprising a valve or pipe closing member I19 supported at the upper end of rod I'I5 arranged to effect a closing of the upper end of the gas pipe or conduit I53 at or by the time the secondary gas holder reaches its lowermost position with the result that communication between the digesters is shut off and also with the result that further withdrawal of gas from the secondary digester cannot take place. A sealing ring I60 depends from the top member I53 of the gas holder for the secondary digester (see Fig. This sealing ring dips into the sludge liquid within the secondary digester when the gas holder is in its lowermost position and provided the liquid is up to the minimum height as determined by the overflow weir construction for the secondary digester. Under such conditions access to the valve I14 is afforded without all-owing the escape of gas from within the secondary gas holder. Likewise a similar sealing ring I59 is provided at the under side of the top portion of the gas holder for the primary digester. The sealing ring I59 dips into the sewage sludge when the gas holder is in its lowermost position. Under the conditions specified-with the gas holders in their lowest position-the flow of gas therefrom into the gas pipe I5I is prevented.

It hasheretofore been pointed out how the sewage which has undergone treatment in the primary digester passes therefrom by the overflow I50 as ,sewage sludge to be treated is supplied to the primary digester through the feed pipe I45. I

An inspection of Figs. 6 to 8 inclusive will make it clear that the digesting sewage sludge which discharge section rangements passes the overflow I50, enters the receptacle or trough I16 and flows therefrom through a pipe I11 that delivers the sewage to the secondary digester and more particularly into the lower section of the liquid body maintained within I the secondary digester.

These figures also show a second overflow pipe or conduit I18 leading from the primary digester and terminating in the adjustable overflow section I19 that is somewhat higher than the overflow I50. This second overflow: I18 and I19, is provided in case the overflow branch I49 which terminates in overflow I50 becomes plugged or unduly loaded, thereby avoiding any undue excess of liquid level within the primary digester during the normal operation thereof.

The secondary digester has an overflow construction relatively remote from the place where sewage sludge is introduced into this secondary digester as will be seen from an inspection of Figs. 3 to inclusive. The overflow arrangement for the secondary digester comprises an offtake or outflow pipe or conduit I80 leading from below but near the upper normal water level for the body of liquid within the secondary digester as is determined by the overflow arrangement for this digester. This pipe or conduit I80 leads through the wall of the digester and termimates in an adjustable overflow or weir section IBI for determining the maximum normal water level within the digester. The upwardly extending intake end section of the pipe or conduit I80 derives support from the floor of the tank of the secondary digester through the medium of supporting rod I80. In case this pipe construction I80 or I8I should become plugged or ineffective there is provided an auxiliary overflow or outflow pipe I82 that terminates in an adjust able overflow or weir I83 the latter of which is somewhat higher than the overflow I8I. For convenience and comparison the overflow arof the primary and secondary digesters have been brought into close relationship in Figs. 5 and 6. Overflows I8I and I83 of the secondary digester deliver into a tank or receptacle I84 from the bottom of which there extends a pipe or conduit I05 for conducting the relatively clear delivered effluent to such place of use or disposal as is deemed desirable. The normally functioning overflow I50 for the primary digester occupies a higher elevation than the normaloverflow IOI or even the overflow I83 for the secondary digester and it will be apparent that when the liquid level within the secondary digester is up to the normal maximum level as determined by overflow I8I that a feed of sewage into the primary digester simultaneously effects a flow of a corresponding amount of treated sewage from the primary digester into the secondary digester and a corresponding flow of relatively clear efiiuent from the secondary digester past the overflow I8I thereof. The secondary digester is preferably provided with sampling pipes I86, IBI, etc., where samples from the various levels within the secondary digester can be readily obtained. The secondary digester has any suitable sway bracing and guides such as are provided for example by sway chains I98 and I89 and by the vertical guideways indicated at I90.

In certain localities where freezing may take place arrangement is made for employing a reprimary digester or at 94 for the secondary digester.

The combination of primary and secondary dgesters as just described in detail particularly as shown in section in Fig. 3 typifies the construction of the primary and secondary digesters employed in the system of Fig. 1. A somewhat which appears in As previously stated the sewage supply to be treated passes from SS along a path l or 3 as the case may be, to a clarifier or thickener 2 or d wherein the sewage sludge is settled out and from which the sewage sludge thus settled out is withof either the raw sewage prior to sedimentation in the clarifier 2 or therefrom can be carried out.

The efiluent from the clarifier is passed therefrom as along the path 2A or 4A for such disposition as may be desired for a particular plant. Any scum collecting at the top of the clarifier may be passed therefrom as along the path 23 or 4B to a point where it will join the sludge from the clarifier 1 Sludge pumps 7 and 8 are connected in the piping system that includes the pipe lines 5 and 6 whereby at will either one of the pumps can be operated to function in the pumping of sludge from either of the clarifiers or whereby if desired one pump can be pumping from one clarifier while u the other is pumping from. the other clarifier or whereby one of the pumps may be pumping from one clarifier while the other of the pumps may be pumping sludge solids from the secondary digester as will hereinafter more fully appear.

Either or both of the pumps '1 and 8 can be relied upon as desired to pass sewage sludge into the primary digester through the feed pipe section I45 thereof or at will either or both of the pumps can deliver liquid pumped thereby along the path 9B Inthe normal functioning of the system this line I00 is closed and the normal functioning of the pump or pumps then is to deliver raw sewage 75 sludge into the primary digester thereby forcing indicates a valve 7 4 or of the sewage sludge digester and which cross member 569. The gas piping or conduit which includes the parts E52, 95! and I53 previously referred to (and in this connection see Figs. 1, 3, 4, and 6) places thegas receiving portions of the primary and The gas will be employed for heating by passing the same along the path 504 through condensate trap CT past meter water boiler 205 or dihot water boiler includes a heating coil H0 which is in a circulatory system that includes the heating coils l 55 of the primary may also include the removthe sheet metal top buoyant peripheral or roof portion 2|3 thereof a ring provided by the hollow vated position approximately that the dotted lines.

- to the arrangements shown and tions depending from struction provided by the hollow annular section 2 described is sufficiently heavy to effect a material loading of the topwhereby any extended lifting movement of the and incident to fluctuation in gas pressures during normal operation is avoided.

In Fig. 15 there'are shown a primary digester and secondary digester similar 'in many respects described in conjunction with Figs. 3 and 4. According to the construction of Fig. '15, however, the gas holder of the primary digester is vertically yieldable but it is loaded so as to remain during the normal operation of the digester in the lowermost position therefor, to wit. at rest upon the stop ring or corbel I09. In this figure the gas holder as a whole is designated as 3"). It comprises the umbrella shaped sheet metal top member or roof portion 3H! and has a peripheral ring 3", part of which ring depends from extends above the peripheral edge of the umbrella shaped portion 3l3. At the exterior top peripheral portion of this gas holder or top member 3"], as at 3H3, there is placed suitable weighting material as concrete which ensures weight somewhat in excess of that required for normally holding the cover in lowermost position on the stops and which functions in opposition to any buoyant effect of the sludge liquid within the tank H1 and against the normal gas pressures experienced within the digester. Otherwise than as indicated the arrangement of this Fig. 15 operates substantially along the lines described in connection with the arrangement of Figs. 3 and 4.

We claim:

1. A gas holder construction for use in conjunction withv a sewage sludge digesting tank comprising in combination atop member havin intermediate the central section and the periphery thereof apertured sections for receiving agitating means, a depending portion providing a peripheral flange having a gas-tight connection with the top member, intermediate flange porthe'intermediate apertured sections and having gas-tight connection with said sections, a vertical strut member extending downwardly from the central section of the top member, connecting members extending downwardly from the region of said peripheral flange by said arms in the regions .ceiving and holding to the lower end of said strut member, for. each of said apertured sections an agitating means comprising a vertically extending thereon and a seating member at the'lower end thereof, arms outwardly extending from and carried by said strut, and seating members carried directly below the apertures of said apertured sections, which last mentioned seating members are provided for rethe corresponding seating members of theagitating means.

2. A gas holder construction as defined in and by claim 1 according to which the intermediate flange portions extend downwardly to an elevation which is lower than that of the lowermost edge portions of the peripheral flange.

3. A gas holder construction as defined in and by claim 1 according to which a hollow gas-tight annular pontoon is provided which includes the depending peripheral flange as a portion thereof. 4. A gas holder construction as defined in and by claim .1 according to which an annular contop relative to the liquid and part of which ring gal type,

shaft, impellers I aperture.

' by claim 1 according to which the agitating means are removable and replaceable by vertical movement, according to which the seating member on each arm and the seating member carried by the shaft of the corresponding agitating means are constructed so as to provide conical seating surfaces for self-adjustment and automatic alignment of the associated shaft as introduced in place by vertical movement, and also according to which the seating member carried by its shaft is mounted so that vertical relative movement between it and its shaft can take place as the shaft is moved downwardly and incident to the last mentioned seating member coming into operative seating engagement with the corresponding seating member carried by the arms.

8. A construction as defined in and by claim 1 according to which the agitating means comprises a motor, according to which the vertically extending shaft is actuated through the medium of speed reducing gearing operatively located between therotor of the motor on the one hand and the shaft on the other hand, according to which the shaft has thereupon at different elevations along the same impelling devices of the centrifuand according to which the seating member on the shaft is mounted so as to permit not only rotary movement but also vertical movement as between the shaft and the seating member.

9. A gas holder construction for use in conjunction with a sewage sludge digesting tank comprising in combination a sludge stirring device; a sheet metal top member having intermediate the central section and the periphery thereof an intermediate aperture for receiving the sludge stirring device, 9. depending portion providing a peripheral flange having a gas-tight connection with the top member and an intermediate flange portion depending from the section defining the intermediate aperture and having gas-tight con nection with said section; a vertical strut member extending downwardly from the central'section of the top member; connecting members extending downwardly and inwardly from the region of said peripheral flange to the lower end portion of said strut member; said sludge stirring device comprising a vertically extending shaft driven from above the top member depending downwardly through the intermediate aperture and carrying sludge impellers located at the underside of the sheet metal top member; an arm construction providing an arm extending from said strut;,and a steady bearing for and associated with the lower end portion of the shaft and carried by said arm in the region directly'below the intermediate ANTHONY J. FISCHER. inns B. LUND.

to which the strut is a cen-

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