CA1074758A

CA1074758A - Sewage treatment

Info

Publication number: CA1074758A
Application number: CA255,134A
Authority: CA
Inventors: Peter F. Johnson
Original assignee: Dorr Oliver Inc
Current assignee: Dorr Oliver Inc
Priority date: 1975-07-11
Filing date: 1976-06-17
Publication date: 1980-04-01
Also published as: JPS6211602B2; JPS529944A; IN155727B; FR2317012B3; FR2317012A1

Abstract

ABSTRACT OF THE DISCLOSURE
A method for the treatment of sewage screenings which comprises passing sewage screenings together with water through a disintegrator or macerator to provide a slurry comprising mace-rated screenings and water and subsequently dewatering this slurry on a concave sieve bend screen.

Description

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This invention is concerned with improvements ln and relating to the treatment of sewage and, more particularly, is concerned wi-th the treatment of screenings produced in I sewage treatment.

Raw sewage or waste effluent, particularly municipal waste effluent, generally contains extraneous solid matter such as paper, cloth or like ~ibrous material or plastics material.
This ex~raneous solid material (which will hereinafter slmply be referred to as "screenings") if allowed to remain in the sewage during its subsequent treatment can lead to diff~culties of operatlon since it may create blockages in pipelines or in other treatment operations such as filter pressing, vacuum filtration, heat treatment or digestion. Accordingly, the raw sewage is generally first treated, before being passed to other treatment stages, to deal with the screenings problem.
Thus, for example, the whole of the sewage input may be passed through a comminutor in order to reduce the screenings to a ' suitable acceptable particle size but the shredded material produced thereby, especially in the case of the non-woven - 20 ~abrics which are coming increasingly into use as dishcloth~, tends to reform to create blockages in the pipelines in the treatment system and, further, there is very heavy wear on the cutting parts of the comminutor since the whole of the raw sewage, including grit, is passed through the comminutor.
Alternatively, the raw sewage may flrst be screened to remove the screenings (hencethe ~erm "screenlngs") and screens which may be used to achieve this include bar screen racks and drum screens. The screenings so produced may t~len be dealt with in one o~ kwo ways, namely they may be dlsposed off wholly _ I ~

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~Q7~7Si8 separately from the remainder of the sewage, for example by landfill or b~ incineration, or they may subsequently be macerated or disintegrated and the macerated or disintegrated material then returned to the flow of sewage for treatment therewith. Simply disposing of the screenings without return to the main sewage treatment system generally involves difficulty since the material produced is unsightly and com-prises the paper and rags etc. together with the unmacerated faecal matter and is, consequently, unpleasant to handle and difficult to dispose of. Where the screenings are macerated and returned to the main sewage treatment system problems of reformation of screenings with consequent blockage can occur, especially as indicated above, in the case of unwoven fibrous materials. It has further been proposed to discharge the macerated screenings direct to outflow sewers discharging into the sea but this is generally unwelcome or prohibited under more stringent effluent discharge regulations.
~ It is an ob~ect of the present invention to provide an ; improved method for the treatment and washing of sewage screenings and, in particular, it is an ob~ect of the present invention to provide a method of treating sewage screenings to provide them in a more acceptable form for subsequent disposal, for example by land-fill or by incineration.
Basically, the process in accordance with the present invention comprises passing sewage screenings together with water through a disintegrator or macerator to provlde a slurry comprlsing macerated screenings and water and subsequently dewatering this slurry on a concave sieve bend screen. ~he screen employed ln the method of khe invention, the so-called 107~L75~3 concave sieve bend screen, comprises a plurality of generally horizontally extending bars having generally flat faces and arranged to ~orm a concave surface with the tangent to the upper part of the screen being generally vertical. The spacing between the bars of the screen is suitably from 0.7 to 1.5, preferably about 1 mm, and the bars have a face width o~ from

2 to 4 mm, preferably about 3 mm. Commonly, the concave face o~ the screen wlll form an arc o~ a circle as viewed in cross-section and the screen preferably has a radium of from 2 to 7, preferably about 3 feet. Such screens are commonly described with reference to the an~les subtended by the radii extending to the upper and lower edges of the screen and the length of the arc subtended and, in acoordance with the present ~nvention, the screen is suitably one having such an angle of from 40 to 60 , preferably about 50, and an arc length of from 600 to 1,800mm, pre~erably about 800mm. Concave sieve bend screens of the men~ioned typ~ are sold under the trademark "DSM" and are descrq~ed, for example, in British Patent Specification No. 791,520.
It will, of course3 be appreciated that other types of concave sieve bend screens operating upon the same principle and having the same general construction, that is comprising a plurality of generally horizontally extending bars having flat faces and arranged to form a concave surface with the tangent to the upper part of the screen being ~enerally vertical~ may be employed. Thus, for example, there may be employed screens in which the concave surface does not form a Rmoo~h curve, e.g. does not form an ~rc o~ a olrole as ln .

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the so-called "DSM'I screen, but is formed of a series of flat portions angularly connected to form a generally concave sur-face, for example the screen described and shown in Uni~ed States Patent No. 3g815~740~
In order to achieve the best possible separation of the macerated screenings on the concave sieve bend screen it is desirable that the rate of flow of slurry over the screen be ad-~usted to the optimum value and in the case of a screen hav-ing the preferred dimenslon indicated above this is generally about 50 gallons per minute per foot width of screen.
The method in accordance with the lnvention may be operated using any suitable form of disintegrator or macerator capable of reducing screenings to a particle slze o~ the order of quarter of an inch, e.g. in the range 1/8th to one-half inch. Such disintegrators may, for example, be of the shroud ring type.
In accordance with a preferred feature of the invention, only a part of the slurry from the disintegrakor is fed ko the concave sieve bend screen, the remainder being recycled to the disintegrator and this is conveniently achieved by recycling the slurry to a screenings sump receiving the screenings from the influent raw sewage.
Recycle of a part of the slurry flow from the disintegra-tor makes it possible to operate the disintegrator at a steady throughput thereby achieving the best performance from the disintegrator and the recycled slurry may also be used, as descrlbed below, to carry the screenings ~rom the primary screening operation, especially when the latter is carried out using a bar rack screen.
The screenlngs may be removed from the influent raw sludge by any suitable means ~or example by means of a so-called bar .

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rack screen or a ~ine drum screen. The former ~orm of screen generally comprises a curved screen ~ormed of generally h~ri-zontal bars having a spacing o~, for example, three-quarters of an inch and through which the whole o~ the ln~luent raw sewage is passed, the large sized screenlngs being retained upon the screen and periodically removed there~rom by the actinn of a raking or scraper blade, generally mechanically operated, which deposits the screenings into a trough whence the screenings are conveyed, under the influence o~ a stream o~ water, to a screen-ing sump and thence, in accordance with the present invention~to a disintegrator. In this case, the stream of water used to wash the screenings from the trough to the screening sump may suitably be a part of the recycled slurry from the disintegrator.
The second form of screen, the fine drum screen, generally comprises a per~orate rotating drum partially immersed in a trough into which the influent raw sewage is fed, the raw sewage not retained on the outside of the drum being removed axially there-of and the screenings retained on the outside o~ the drum being scraped off therefrom by the action o~ the outside o~ the drum in proximity with a part of the trough in which the drum is mounted.
In either case, the screenings comprise generally solid material above a certain partic~e size and will generally com-prise, in additlon to rags, paper, etc., faecal matter, grease and the like. It is an important advantage of the method of the present invention that during passage through the dislnte-grator such material, i.e. ~aecal matter or grease, is thoroughly disintegrated or emulsified so that when the slurry containing the macerated screening is dewatered on the concave sieve bend screen sucb unwanted matter is washed ~rom the solid constl-..

7~7S8 tuents of the screenings~ i.e., the rags, paper or the like.The liquor from the dewatered screenings will generally be re-turned to the influent sewage sludge either before or after pri-~ary screenlng thereof. By dewatering the macsrated screenings on the concave sieve screen, the water content of the dewatered screenings can be reduced to give a material having a sollds content of, for example, about 9% by weight and this material may itself already be suitable for further treatment, for example land-fill or incineration, especially the latter. However, if desired, the dewatered macerated screenings may be further de-watered, for example using a screw press, to give a material having a solids content of from 20 to 40% by weight, which material is entirely suitable for land-~ill or ~ncineration.
Where such further dewatering is employed the liquid removed durlng the dewatering step will commonly be returned to the in-fluent sewage, either before or after primary screening.
In order that the invention may be well understood, refer-ence wlll now be made to the accompanying drawings, in which:
Figure 1 ls a diagrammat~c representation of a sewage screening and dewatering lnstallation operating in accordance with the invention;
Figure 2 is a diagrammatic representation of an alternatlve form of sewage screening and dewatering installation operating in accordance with the invention, and Figure 3 is a partial cross-section (on an enlarged scale) of a concave sieve bend screen.
; As shown in Figure 1, a screenings treatment installatlon comprises a raw sewage influent channel l havlng mounted therein a bar rack screen 2 provided with scraper arm 3 which serves to , :

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scrape screenings retained on screen 2 into a ~creenings trough 4.
Trough 4 is connected3 via line 5~ to screenings sump 6 which is, in turn, connected via line 7 to the input side of a dis~nte-grator 8, the output side of which is connected via line 9 to recycle line 10 (for recycling slurry from disintegrator 8 to screenings sump 6), recycle line 11 (for recycling slurry from disintegrator 8 to trough 4 to wash screenings contained there-on, via line 5, to screenings sump 6), and screen line 12.
Screen line 12 is connected via pump 13; to the input side of a ; concave sieve bend screen 14. The undersized material discharged from screen 14 is connected to line 15 by means of which the un-dersized material or ~iltrated is returned to the stream of raw ; influent sewage, either upstream or downstream of bar rack : 15 screen 2. The oversize material from screen 14 (dewatered sludge) is passed, via line 16, to a screw press 17, the liquor from which is passed, via line 18, to the stream of raw influent sewage, either upstream or downstream of bar rack screen 2. The dewatered sludge from screw press 17 is then passed for fur-. 20 ther treatment, e.g. either land-fill or incineration.
In the installation shown in Figure 2, the in~luent raw sewage is screened by means of a rotating drum screen~20, the ;~
screened sewage being withdrawn axially of the drum and the screenings being deposited in screening sump 6a which is connected via line 7 to the input side of disintegrator 8. The output side of disintegrator 8 i.s connected via line 9 to recycle line , 10 (whereby effluent slurry from disintegrator 8 is recycled to ,: :
,~ sump 6a) and screen line 11 whence the slurry is passed, via pump 13, to concave sieve bend screen 14, the remainder of the installation being eæsentially the same as that shown in Figùre 1.

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- ~Lll74';~S8 As shown in Flgure 3, the concave sleve bend screen 14 com-prises a plurality of spaced bars 21 of generally pentagonal cross-sections. It will of course be understood that the bars may be o~ other cross-sections, e.g. quadilateral with the two sides of the bar adjacant that forming the surface of the screen tapering towards the said latter side.

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Claims

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

1. A method for the treatment of sewage screenings which comprises passing sewage screenings together with added water through a disintegrator or macerator to provide a slurry comprising macerated screenings and water and subsequently dewatering at least a portion of this slurry on a concave sieve bend screen.

2. A method as claimed in Claim 1 in which the bars of the concave sieve bend screen have a spacing of from 0.7 to 105 mm.

3. A method as claimed in Claim 2 in which the bars of the concave sieve bend screen have a spacing of about 1 mm.

4. A method as claimed in Claims 2 or 3 in which the bars of the concave sieve bend screen have a face width of from 2 to 4 mm.

5. A method as claimed in Claim 2 or 3 in which the said bars have a face width of about 3 mm.

6. A method as claimed in Claims 2 which the concave sieve bend screen has a subtended angle of from 40 to 60°C.

7. A method as claimed in Claim 6 in which the disin-tegrator or macerator is one capable of reducing screen-ings to a particle size of one-eighth to one-half inch.

8. A method as claimed in Claim 1 in which only a part of the slurry is fed to the concave sieve bend screen, the remainder being recycled to the disintegrator.

9. A method as claimed in Claim 8 in which the dewatered macerated screenings separated from the slurry by the concave sieve bend screen are further dewatered.