CA1036948A - Sewage treatment system with reflux of sorbent made from sludge in a heated fluidized bed - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In a waste disposal system wherein the suspended solids containing organic materials are separated from the liquid carrier and thereby form a sludge having a relatively low moisture content, the improvement comprising the steps of drying the sludge to a moisture content below about 15%, the sludge drying step including final drying partially dried sludge by thermal means, passing the dried sludge to a fluidized bed for thermal treatment in a steam and low oxygen content fluidizing gas to form fine sorbent materials, separately heating a granular solid material, passing the hot granu-lar material to the fluidized bed to provide the heat for thermally treating the dried sludge, removing the fine sorbent materials from the fluidized bed entrained in the fludizing gases, passing the sorbent materials to the waste disposal system to remove solution contaminants from the liquid effluent in the system, and passing the separated gases from the fluidized bed to a combustion zone use in heating the granular solid material.

Description

'ase 3958 ~0369~
The present invention relates to the thermal treatment of waste materials and more particularly to an improved treatment of industrial and domestic wastes.
Many systems have been proposed and used in the treatment of domestic and industrial wastes, many using incineration or pyrolysis in the process to reduce odors and to condition the solid residue for eventual disposal. In the usual waste dis-posal system the waste solids are received in a liquid carrier in a very dilute form, and are then mechanically and/or chemi-cally treated before or after various forms of solids concentra-tion. The solids may thereafter be treated to form fertilizer or burned to remove the combustible materials therein with the inert solids or ash thereafter utilized for land fill or the like.
The gases leaving the system may or may not be scrubbed before discharge to the atmosphere, depending in part, on the solids content and the nature of the discharged solids. One of the end purposes of the process is ordinarily to reduce the liquid effluent to a more or less non-contaminated or relatively pure condition for discharge to streams or lakes.
In the present invention the raw sewage or waste is treated in any of the usual manners to concentrate the solids which are then transformed into a sorbent. The sorbent is highly effective in the removal of contaminating components from the liquid effluent and after use the combustible content can also be used as a fuel to produce at least a portion of the heat used in the process.
Of the drawings:
Fig. 1 is a diagrammatic showing of a waste treatment system constructed and arranged according to the invention; and Fig. 2 is a vertical section of the thermal treatment portion of the system shown in Fig. 1.
As shown in Fig. 1, the invention is illustrated as ~b ' Case 3958 ~ 03699ff incorporated in a typical sewage treatment system wherein the solids are concentrated for subsequent thermal treatment. As shown, the raw waste material is passed through a pipe lO, a comminutor 11 and then to a settling tank 12. The solids accumulate in the bottom of the tank 12 and are passed with en-training liquids through a pipe 13 to a separator 14 wherein the grits or oversized solids are removed and passed through pipe 15 to disposal. The liquid with entrained solids is then passed through pipe 16 to a mixing tank 17 where a recycled sorbent, as hereinafter described, plus fresh sorbent is mixed with the incoming sewage for delivery through pipe 18 to a treatment unit 20. The treatment unit may take many forms, but in the illustrated embodi-ment a polymer and a coagulant is added to the unit 20 through elements 21 and 22 respectively and the mixture subjected to mechanical agitation in the unit. After treatment in the unit 20, the liquids are passed through pipe 23 to a mixer 24 where a sorbent is added through pipe 25 and thence to a filter 26 with the effluent discharged to a lake or pond through pipe 27, with or without additional treatment.
As is customary, the filter 26 is provided with a back-washing arrangement. The backwash is recycled to mixing tank 17 via pipe 30. The concentrated slurry of solids from tank 20 are passed to a storage tank 32 where the sludge is desirably main-tained at a generally constant density. Sludge from the tank 32 is passed through a centrifuge 33 to remove a major portion of the water content thereof with the liquid from the centrifuge passed through a pipe 34 to the mixing tank 17. The slurry, which will now be in a semi-concen~rated form and preferably c~ntain fron 20 to 50 percent solids discharges from the centri-fuge through pipe 35 into a pugmill 36.
The semi-concentrated slurry is mixed in the pugmill with low moisture solids delivered thereto through a splitter 37, Case 395~

1036'9~8 as hereln~f~t~r ~escribed.
In the present in~ention, it has been found desirable to dry the slurry prior to thermal treatment to a moisture content of less than approximately 15~ to minimize the size of the sub-sequent thermal treatment unit and maximize the formation of a useful sorbent. As shown in Fig. 1, the final drying of the slurry is accomplished in a flash dryer 38. In the form shown the dryer is formed as an upwardly elongated tube or cylinder supplied at its lower end with flue gas introduced thereto from duct 40. Some of the flue gas from duct 40 is also passed through duct 41 and is introduced as a fluidizing medium into a thermal treatment zone 70, as hereinafter described. The conditioned discharge from the pugmill 36 fiows into the dryer 38 at a pos-ition intermediate the height of the dryer. In addition, hot coarse solids from the thermal treating zone are also delivered through duct 43 into the dryer 38. Such hot solids are used to provide heat to the dryer and to reduce the percentage of weight of moisture present in the feed to the dryer.
Within the dryer 38 the flow of gases suspends the solids while the mixture is passed upwardly therethrough. Heat from the solids introduced through the duct 43 aid in the evapori~a-tion of moisture from the mixture deli~-ered from the pugmill 36.
Alternatively additional heat could be supplied by obtaining hot recycle gases from duct 5~ subsequent to the air heater 51 here-inafter described. The gases and solids leaving the flash dryer 38 are discharged to a cyclone 44 where the solids separated from the entraining gases are discharged downwardly through pipe 45 to the splitter 37. As hereinbefore explained some of such solids are discharged from the splitter into the pugmill to con-dition the dewatered sewage solids from centrifuge 33. Alter-natively, this conditioning could also be accomplished by recycle solids discharged from cyclone 93 hereinafter described. ~rom Case 3958 10369~B
splitter 37 an additional portion of the solids are passed through a pipe 46 to the thermal treatment zone 70, as herein-after describcd. The gases with fine particles suspended there-in leave the upper portion of the cyclone and are discharged through a duct 47, A portion of such gases and solids are passed through a duct 4~ into the thermal treatment device for a pur-pose hereinafter described. The remaining ~ases are passed through a duct 50 to combine with gases discharged ~rom the thermal treat~ng zone for delivery to and through an air heater 51.
In the air heater 51 a supply of combustion air passed through the heater by a fan 52 and connecting ducts discharges the pre-heated air through a duct 53 to the thermal treating zone. The heat exchanger also serves to reduce the temperature of the flue gases prior to delivery of such gases to a wet scrub-ber 55 through duct 54.
In the embodiment shown in Fig. 1 the scrubber is of the venturi type where the flue gases passing through the scrubber is intimately contacted by sprayed liquid introduced through pipe 56 and are accelerated in passing ~hrough the throat 57. The liquid and gas is intimately mixed and the entrained solids in the gas agglomerated to pass downwardly into a settling tank 58 where a major portion of the liquids and solids collect in the bottom of the tank 58 with the gases discharging ~hrough the duct 60. Some of the gases discharging '.hrough the duct 60 are passed to the atmosohere through duct 61 while another portion of the gases are passed through duct 62 and fan 63 into the duct 40.
Some of the liquids accululating in the tank 58 are with-drawn through pipe 64, and combined with make-up liquid from tank 65 for delivery to pipe 56 for spray introduction into the gases passing into the venturi throat 57. It has been found desirable to mix sodium carbonate with water in the tank ~5 for subsequent mixing with the liquid-solid mixture in the settling tank 58, for reasons hereinafter described.

Case 3958 ~0369~8 The solids and liquids accumulating in the bottom of the tank 58 are withdrawn through pipe 66 and pump 67 for discharge through pipe 68 and spray discharge into the upper portion of the thermal reactor shown in Fig. 2. The sodium carbonate in the bottom of the tank 58 will be present in the spray discharged from the pipe 68.
The thermal treatment device or unit 70 shown in the drawings and particularly in Fig. 2 is of the general type shown and claims in U.S. Patent 3,578,798, wherein superimposed annular fluidized beds 71 and 72 surround a central combustion tube 73.
With this construction the fluidizing flow to either bed need nvt be correlated with the combustion requirements in the combustion tube 73. While annular beds with a central tube -for combustion, as shown, forms an advantageous structure, it is possible to form the fluidized beds in separate structures and to also construct the combustion chamber as a separate element. However, the intercon-nection between the separate elements would be similar to perform the functions hereinafter described.
Referring specifically to Fig. 2 the unit 70 is con-structed with an exterior casing 74 which may be of upright cy-lindrical form constructed of ceramic material and/or metal, depend-ing upon the temperatures involved. A transverse partition 75 positioned intermediate the height of the casing divides the interior of the unit 70 into upper and lower portions 76 and 77, respectively.
The tube 73 extends upwardly through the bottom plate 78 to a position 80 spaced above the partition 75. The tube may be con-structed of ceramic and/or metallic materials and may be provided with water cooling provisions ~not shown).
Above the partition 75 a grill or grid 81 is positioned in generally parallel spaced relationship to define a plenum chamber 82 therebetween. The plenum 82 is provided with fluidizing gases specifically hot air from duct 53, through the inlet pipe 83 for Case 3~58 103694~
the distributed introduction of fluidiz:ing gases upwardly through the grid 81 to fluidize the solid particulate material forming the annular bed 72. In a similar manner the lower portion 77 of unit 70 is provided with a grid of perforated plate 84 spaced above the bottom plate 78 to form a plenum chamber 85 and is supplied with fluidizing gas through the inlet 41. The grid or plate 84 supports a fluidized bed of particular material 71 which is supplied with hot granular material, as hereinafter described, introduced to the lower bed 71 through a dipleg 86 which extends from the upper end of the fluidized bed 72, through the partition plate 75 into the fluidized bed 71. In addition, the lower fluidized bed 71 is supplied with low moisture waste solids through the pipe 46 which discharges directly into the fluidized bed 71.
With the construction described granular materials from the lower fluidized bed 71 are passed at a controlled rate from the fluidized bed into the combustion tube 73 through tangential openings 87 formed in the wall of tube 73 upwardly adjacent the grid 84. The rate of discharge through the orifice openings 87 is regulated by the separated introduction of fluidizing gas from pipe 41 and introduced through pipe 90 which enters an annular chamber formed by a cylindrical open upper ended baffle 91 en-circling the tube 73 and extending upwardly from the bottom plate 78 through the grid 84 into the lower portion of fluidized bed 71. :
The function of such structure is described in U.S. Patent 3,578,798 and is highly effective in regulating the rate of dis-charge of solid materials from the bed 71 into the combustion tube 73. The hot granular material delivered to the bed 71 through the dipleg 86 provides the heat to thermally treat the ~ -waste materials delivered to the plenum through pipe 46. With ~ :
the fluidizing gases provided to the plenum through pipe 41 con- : ?
sisting primarily of flue gases having a low oxygen content the thermal treatment in the bed 71 will char the organic matter in Case 3958 ~03694~
the waste materials. The finer particulate solids formed by decom-position of the waste materials will be entrained with the fluidizing gases leaving the lower portion of the unit 70 through pipe 92.
As shown in Fig. 1, such gases with entrained solids are passed to a separator 93 of the cyclonic type, with the separated gases passed through a pipe 94 to tangentially enter the lower end of the combustion tube 73 through a centrally located tube. (See Fig. 2).
The solids from the separator 93 are discharged into a tank 95, mixed with water and pumped as a slurry through pipe 96 for use as a sorbent in the waste treatment plant. As shown in Fig. 1, the sorbent is delivered to mixing tanks 17 and 24, via pipes 97 and 25, respectively.
The lower end portion 98 of the combustion tube 73 is supplied with auxiliary fuel through pipe 100 if needed~ hot air through pipe 53, and gaseous and solid fuels (from the pyrolysis of sewage) thru pipe 94. The burning gases swirl in their passage upwardly through the tube 73 and entrain particulate matter intro-duced thereto through the openings 87. The hot gases and heated particulate matter discharge into the space above the fluidized bed 72, with the solid particulates separated from the hot combustion gases by centrifugal forces,and change in gas velocity to deposit on the bed 72. The gases discharge into pipe 50 through pipe con-nection 101 for subsequent heat exchange with combustion air in the air heater 51 and passage through scrubber 55.
To fortify *he swirl in the combustion tube 73 and destroy and o~o~g~us constituents in the drying gases, the latter with any entrained solids are passed tangentially into the upper portion 76 of the unit 70 through pipe 48. Since the fluidizing gas introduced into the plenum chamber 82 through the pipe 83 in-cludes preheated air, sufficient oxygen will be present both in and above the bed 72 to burn combustibles present, thereby pro-viding some heat and destroying odors by exposure to high tempera-- "ase 3958 ~03~99~ j , tures before they are discharged from h~mber 76. Advantageously, slurry from tank 58 (~:ig. 1) is passed through pipe 68 to nozzles l02 positioned above the upper surface of bed 72 and arran~ed to spray slurry dowllwardly onto the granular material in the fluidized bed. The sodium carbonate in the slurry tends to agglomerate the fines present in and leaving the system.
Under the conditions described with temperatures over 2000F in the upper portion 76 of the unit 70, and with oxygen available in the fluidizing air passing upwardly through grid 81 from chamber 82, the combustibles present will be burned, with the remaining solids present in bed 72 being inert. This bed would normally be operated in the temperature range of ~ 1650-2000F
Excess ash or solids are withdrawn from the bed 72 through conduit 103. Since such solids are inert and sanitary they may be used for land fill or other purposes.
In the operation of the equipment hereinbefore described the waste material is treated in the usual manner, with a relatively concentrated waste material delivered to the tank 32. The waste material is further concentrated by removal of water, as for example in a centrifuge 33, and mixed in a pug mill 36 with relatively dry materials before discharge to the flash dryer 38. The solids from the flash dryer are separated in the cyclone 44 from the entraining gas and vapor and are delivered for the most part to the fluidized ~
bed 71 CFig. 2) with a moisture content of 3 to 15%. The bed 71 -will be maintained at a temperature of ~ 1000 to 1650F to convert ~`
the solids to a sorbent. With the 1uidizing gas formed of low oxygen content flue gases and heat supplied to the bed 71 by hot ~ granular materials through dipleg 86 the solids in the waste - 30 materials are converted to a sorbent. The sorbent leaves the fluidized bed in finely divided form as entrained in the gas leaving the bed and discharging through the pipe 92 to the separator 93.

~ O ~ g ~ Case 3958 As hereinbefore pointed out, the coarser portion of the sorbent material is slurrled in the tank 95 and passed to the mixing tanks 17 and 24 to remove soluble contaminants from the waste liquid so as to discharge high quality effluent water from the system through the pipe 27. Simultaneously, the gaseous materials and entrained solids from the separator 93 and the by pass are passed to the com-bustion tube 73 through pipe 94 to provide the main source of fuel.
As is well known in the waste disposal art, the composition of the waste varies widely depending on the source. For example, domestic sewage waste from different towns will vary through a wide range of compositions, including the organic carbon content, depend-ing upon the quantity and nature of waste from small industries which will use the domestic sewage system. In some test work in con-nection with the present invention it was found the raw sewage from several communities having populations between 25,000 and 250,000 contained up to several hundred parts per million of total organic carbon, and the other solid and solution contaminants also varied widely. Apparently, some minimum amount of organic material should be present in the waste material treated according to the invention to form an effective sorbent. Test work with different waste materials to produce sorbents in the apparatus and according to the procedure herein disclosed has included sorbents having mostly upward from 8 weight percentage of carbon, with the remainder ash. All of the sorbents tested have been effective in sorbing solution pollutants in sewage effluent, and in fact have proven to be equal to or better than the usual commercially available activated charcoal, when com-pared on a carbon content weight basis. Apparently, some of the ash (otherwise considered inert~ in the sorbent has become "activated", at least insofar as removal of solution pollutants are concerned.
The reason for this is not fully understood.
h particular sorbent was produced as described with an ash content of 86.9 weight percentage. The ash in the sorbent has -the following composition, as determined by spectrographic analysis:

Case 3958 ~0 ~ ~ 4~
Ash Analysis -_Spectro~rap'nic SiO2, ~ 38.
A123 8.o Fe203 6.o Ti2 -5 CaO 17.
MgO 4-5 Na 0* 0.7 K20 1.1 . P2 5 :10.
S03** 3.
*By flame photometer **By Gravimetric chemical analysis ACTIVATION CONDITIONS AND ANALYSIS OF SOME ACTIVATED SORBENTS
Sorbent 2 Sorbent 3 Source Material Dried Primary Sludge Dried Primary Sludge Activation Temp. 1640 1510 F - :
Steam Content ~ 8* 16*
20 Inert Gas % 92 84 Sorbent Analysis:
: .
Moisture % 0.7 o.8 BTU/~ (dry) 1370. . 1300.
Carbon % &.9 8.4 Hydr~gen ~ o;4 o.4 Sulphur % 0,9 0.7 Ash 9O 4 Oxygen + Nitrogen %
(by difference) 0.0 . o.6 .

Case 3958 .

si~in~ ~0369~8 thru ~30 Mesh 100.0 100.0 7 94~7 83.2 100 87.o. 70.9 200 66.6 52.1 325 50.2 36.8 *Sewage solids as fed to fluid bed reactor had ~ 3 wt % moisture.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a waste disposal system wherein solids contain-ing organic material and soluble contaminants suspended in a liquid carrier are treated to form a sludge, a method of treating the sludge to form products useful in a waste treatment process which comprises the steps of drying the sludge to a moisture con-tent between 3 and 15 percent, passing the dried sludge to a fluidized bed of hot inert granular solid material maintained at a temperature between 1000 and 1650°F to form reaction products including gases and solid sorbent materials containing carbon and ash from the sludge, heating the inert granular solid material in suspension in contact with hot flue gases formed by combustion of fuel in a combustion zone separate from the fluidized bed by passing granular solid material from the fluidized bed to the combustion zone, separating the inert granular solid material from the hot flue gases and returning the separated granular solid material to the fluidized bed, passing some of the flue gases to the fluidized bed to fluidize the granular solid material and dried sludge therein to char organic material in the sludge, removing reaction products including coarse and fine solid sorbent materials entrained in gases from the fluidized bed, separating the coarse solid sorbent materials from the last named gases in a separating zone, passing some of the coarse sorbent materials to the liquid carrier to absorb the contaminants in the liquid carrier, and passing the separated gases from the separating zone to the combustion zone for combustion therein.

2. The system of claim 1 wherein the drying step includes suspension drying the sludge in direct contact with hot gases from the combustion zone.

3. The system of claim 1 wherein the fluidizing gases utilized in producing the sorbent materials contain low quantities of oxygen, and steam, which is formed from moisture in the fluidized bed dried sludge, is used in the reaction to form the sorbent material.

4. The system of claim 1 wherein the hot inert granular solid material is heated in the combustion zone in direct contact with the hot gases produced in part by combustion of carbon con-taining sorbent solids.

5. The system of claim 1 wherein the solid sorbent material contains less than 25 weight per cent of carbon.