NZ256941A - Extended aeration wastewater treatment with retention basin having two mixed liquor outlets - Google Patents

Description

New Zealand No. International No. 256941 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: Complete Specification Filed: 16.09.1993 Classification:(6) C02F3/02 Publication date: 22 August 1997 Journal No.: 1419 AMENDED under Section ift of the Patents Act 1953 from .frfti ASSISTANT COM^8H8flfy8Pt>ATENTS NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Mixed liquor wasting system and method Name, address and nationality of applicant(s) as in international application form: HAROLD J BEARD, of 1111 Colonial Drive, Baton Rouge, LA 70806, United States of America ttU iO/U/OUA DESCRIPTION MIXED LIQDOR WASTING SYSTEM AND METHOD TECHNICAL FIELD This invention relates generally to aerobic wastewater 5 treatment systems and methods and, more particularly, to aerobic wastewater treatment systems and methods which utilize a moving stream of mixed liquor in a retention basin, such as an oxidation ditch.

BACKGROUND ART In many aerobic wastewater treatment systems wastewat er flows into a retention basin, such as an oxidation ditch, where the wastewater is retained as mixed liquor. In a conventional oxidation ditch system, a continuous channel contains the flowing mixed liquor, which is aerated as it 15 flows around the channel to provide oxygen for microorganisms which decompose the waste in the channel. Thus, as waste is decomposed in the basin, suspended solids form in the mixed liquor. Clarifiers are used to clarify the mixed liquor by settling the suspended solids and removing clarified 20 wastewater from the system. Intrachannel clarifiers may be employed for this purpose. Examples of intrachannel clarifiers may be seen in Harold J. Beard's U.S. Patent Nos. 4,3 62,625; 4,383,922; and 4,457,844. Maintenance of the proper biological balance in the basin is essential for 25 proper operation of the system.

Various measures are used for maintenance of proper biological activity. One such measure is known as "sludge age". For a given concentration of waste entering the basin and a given concentration of waste within the basin, the 30 sludge age can be calculated. Thus, the number of pounds of waste within the basin divided by the number of pounds of waste per day entering the basin will yield a sludge age in days. It has been found empirically that a sludge age of 23-25 days will yield excellent biological activity within the 35 basin. While a sludge age of 23-25 days is preferred, the basin will possess adequate biological activity with sludge age in the range of 19-28 days.

Since only clarified wastewater is taken from the system, the solids concentration within the basi^i increases 25694 as wastewater continues to flow into the basin, increasing the sludge age. Varying concentrations of wastewater entering the basin over time also vary the sludge age. The retention of solids in the basin requires that nixed liquor 5 be wasted from the basin in order to maintain a desirable sludge age. Overwasting or underwasting can result in disastrous biological consequences. Prior art wasting was accomplished by observation of basin conditions and occasionally opening a waste valve or operating a waste pump, 10 guessing at the amount of solids removed from the basin. Prior art wasting methods require expert system operators. Further, variations in operating conditions often result in unforeseen variations in sludge age, regardless of the skill of the operator.

An additional problem is created when wasting mixed liquor. Mixed liquor exiting a basin has a very high water content. Thus, in order to dispose of a small amount of solids, a very large volume of mixed liquor must be removed. The wasted mixed liquor must be processed in digesters or 20 other similar sludge disposal systems.

DISCLOSURE OP INVENTION Therefore it is an object of this invention to provide a mixed liquor wasting system and method which maintains a constant sludge age in an aerobic retention basin. 25 It is another object of this invention to provide a mixed liquor wasting system and method which thickens the wasted mixed liquor before disposal.

Accordingly, in an extended aeration wastewater treatment system having wastewater flowing into a retention 30 basin so as to form mixed liquor within the retention basin, a mixed liquor wasting system, comprising a thickener tank, having a sidewall and a bottom; a waste line having a first end and a second end, the first end fluidly connected to the retention basin, and the second end fluidly connected to the 35 thickener tank; a first flow control device, such as a pump, for continuously removing a portion of the mixed liquor from the basin at a desired constant flow rate through the waste line, positioned in tha waste line; a sludge removal line having a first end and a second end, the first end fluidly connected to the bottom of the thickener tank, and the second end fluidly connected to a sludge disposal system; a second 5 flow control device, such as a pump, for removing a concentrated portion of the mixed liquor from the thickener tank, positioned in the sludge removal line; and a return line having a first end and a second end, the first end fluidly connected to the thickener tank above the bottom, and 10 the second end fluidly connected to the retention basin. A method for wasting the mixed liquor is provided, comprising the step of continuously removing a portion of the mixed liquor from the basin at a desired constant flow rate. DESCRIPTION OP THE DRAWINGS 15 Figure l is a schematic depiction of an embodiment of the system of the invention.

Figure 2 is a schematic depiction of another embodiment of the system of the invention.

Figure 3 is a perspective view of an embodiment of the 20 thickener tank of the invention.

Figure 4 is a curve showing the relationship of solids concentration versus retention time for mixed liquor in the thickener tank having a sludge volume index of 67-100.

Figure 5 is a curve showing the relationship of solids 25 concentration versus retention time for mixed liquor in the thickener tank having a sludge volume index of 130-155.

Figure 6 is a curve showing the relationship of retention basin sludge age versus pumping rate to the thickener tank.

BEST MODE FOR CARRYING OPT THE INVENTION As shown in Figure 1, a mixed liquor wasting system l is attached to an extended aeration wastewater treatment system 16. Treatment system 16 includes a retention basin 2, shown in the form of an oxidation ditch 3. Oxidation 35 ditch 3 is formed by a continuous outer wall 7 and a partition wall 8. Untreated wastewater enters the basin 2 through wastewater inlet 9 and is retained in the basin 2, where it is aerated by aerators 5. Collected wastewater in ditch 3 forms a mixed liquor 4, which is retained and circulated in the ditch 3 fox a desired period of time, allowing microorganisms to decompose the waste in the water. 5 Many different types of extended aeration systems 16 are used in the art. Most employ some sort of retention basin 2, to which the invention 1 may be attached. Without the intent of limitation, the invention 1 will be discussed herein in conjunction with systems 16 utilizing oxidation ditches 3. 10 Once the mixed liquor 4 is adequately treated, decomposed waste in the form of suspended solids must be removed through settlement, filtration or both. Separation between sludge (settled suspended solids) and clarified wastewater can be accomplished inside of oxidation ditch 3 15 through the use of an intrachannel clarifier 6, as shown in Figure 1. Intrachannel clarifier outlet line 60 removes clarified wastewater from the ditch 3, and sludge is returned to the ditch 3. An external clarifier 57, shown in Figure 2, may also be employed to accomplish the same task. Mixed 20 liquor 4 flows into external clarifier 57 from ditch 3 through external clarifier inlet line 58. Clarified wastewater exits clarifier 57 through external clarifier outlet line 59. Again, sludge is returned to the ditch 3 or occasionally wasted to a sludge disposal system. 25 As discussed above, one major problem associated with extended aeration wastewater treatment systems 16 is the maintenance of proper biological conditions. It is accepted that maintaining a constant sludge age is effective in maintaining proper basin biology. The inventor has 30 discovered that the continuous removal of a portion of the mixed liquor 4 from the basin 2, through waste line 10, at a constant flow rate will result in a constant sludge age within the basin 2. This is true on an average basis regardless of the concentration (BOO) of the incoming waste 35 or the flow rate of the incoming waste. As the concentration and/or flow rate changes, the constant withdrawal rate through waste line 10 will cause the ditch conditions to re VO 95/07862 PCl'/U S93/08760 establish equilibrium at the same sludge age. Equilibrium is reached when the mass of suspended solids 1 earing the ditch 3 through waste line 10 equals the mass of suspended solids entering the ditch 3 through wastewater inlet 9. If 5 the constant flow rate through waste line 10 is changed, ditch conditions will again re-establish equilibrium at a new constant sludge age associated with the new flow rate through waste line 10. Figure 6 depicts a design curve for maintenance of a constant sludge age for a one million gallon 10 per day oxidation ditch. Thus, in order to maintain a sludge age of 23 days, a constant flow of 30 gallons per minute per million gallons of daily basin capacity should be maintained through waste line 10.

While maintaining a constant flow rate through waste 15 line 10 maintains a constant sludge age, stabilizing basin conditions, something must be done with the suspended solids being removed from the basin 2 through waste line 10. Because of the large water content in the mixed liquor, it is desirable to thicken the sludge prior to further 20 processing in order to reduce handling costs and disposal volumes. In order to accomplish this goal a thickener tank 13 was designed to concentrate suspended solids for disposal while returning clarified wastewater to the basin 2 or to a clarifier outlet line 59,60.

As discussed above, waste line 10 has a first end 11 fluidly connected to basin 2 and a second end fluidly connected to thickener tank 13. A first flow control means 18 for continuously removing a portion of mixed liquor 4 from basin 2 at a desired constant flow rate, fluidly connected 30 to basin 2. Preferably, first flow control means 18 comprises a first pump 19, although gravity flow control could be utilized where feasible. A gravity flow device could be a flow control weir (not shown) connected to return line 28, thus establishing a substantially constant flow 35 through waste line 10 and thickener tank 13. First pump 19 may be set at a desired constant flow rate in order to accomplish the method of the invention.

WO 95/07862 PCT/US93/08760 6 As shown in Figure 3, thickener tank 13 is has a sidewall 14 (circular in the embodiment shown) and a bottom 15. Preferably, bottom 15 has a sloped portion 17 (preferably at an angle no less than thirty-five degrees with 5 horizontal), sloping inward from sidewall 14 to encourage sludge concentration at bottom 15. A sludge removal line 20 has a first end 21 fluidly connected to bottom 15 and a second end 22 fluidly connected to a sludge disposal system 23. Sludge disposal system 23 can include such conventional 10 components as a digester 24 having aerators 25 located therein. A second flow control means 26 is fluidly connected to thickener tank 13. Preferably, second flow control means 26 includes a second pump 27, positioned in sludge removal line 20. Pump 27 may operate intermittently or continuously 15 at a relatively slow rate to remove accumulated sludge from tank 13. A bottom valve 61 is provided in sludge removal line 20 for additional control. If gravity drain to sludge disposal system 23 is possible, means 26 may comprise only bottom valve 61, which may be intermittently or continuously 20 opened (either manually or electronically) to remove accumulated sludge. At the top of thickener tank 13 is a support structure 62, supporting a mixer 40, including a motor 41, a shaft 42 rotatably attached to motor 41, and a blade 43 fixedly attached to shaft 42 just above bottom 15. 25 Preferably, blade 43 extends to either side of shaft 42 as shown, and is oriented parallel to bottom 15. A resilient scraper extension 44, preferably made of pliable material, such as rubber, is fixedly attached to blade 43 and extends downward so as to lightly touch bottom 15 so as to eliminate 30 any dead zones beneath blade 43. It has been found that, by operating mixer 40 so as to turn blade 43 at a tip speed of twenty feet per minute (three revolutions per minute for a 1.208 foot blade radius), gas build-up from denitrification is released preventing sludge floatation from 35 denitrification. It is believed that the tip speed for blade 43 should be designed to be twenty to twenty-five feet per minute so as to achieve proper mixing and yet create minimum turbulence.

To dampen any turbulence produced within thickener tank 13, at least one baffle 46 is provided just above the entry 5 point of waste line 10 and below collector pipes 51. Although any arrangement which dampens turbulence will suffice, the X-shaped arrangement of baffles 46 shown in Figure 3 appears to be effective.

Mixed liquor 4 enters thickener tank 13 through waste 10 line 10 and is dispersed by inlet diffuser 47. Waste line 10 should enter tank 13 at a point above bottom 15 such that 30-40 percent of the volume of tank 13 is below diffuser 47 to allow for sludge accumulation. Diffuser 47 includes a top plate 48 and bottom plate 31. Baffles 50 are contained 15 between and around the periphery of plates 48,31 and serve to dampen inlet flow and allow sludge to begin to settle immediately upon entry into tank 13. Baffles 50 and holes 49 in bottom plate 31 allow mixed liquor to flow both horizontally outward from between plates 48,31 and downward 20 through holes 49 in bottom plate 31. Diffuser 47 is supported by support anqle 67, connected to waste line 10 and recycle line 39. When recycle line 39 is employed, it should enter tank 13 through diffuser 47 opposite waste line 10, as shown in Figure 3, allowing the opposing flows to dampen each 25 other. If recycle line 39 is not employed an additional plate (not shown) should be added to diffuser 47 in place of the termination of recycle line 39 to dampen flow from waste line 10.

When the portion of mixed liquor 4 removed from ditch 30 3 through waste line 10 enters thickener tank 13, sludge will immediately begin to settle, creating a concentrated, or thickened, portion 55 in the lower part of the tank 13 and a clarified portion 56 in the upper part of the tank 13, as shown. As the clarified portion 56 reaches the level of 35 perforated collector pipes 51, it flows out of tank 13 through return line 28. Return line 28 has a first end 29 fluidly connected to thickener tank 13 and a second end 30 8 fluidly connected to the retention basin 2. Collector pipes 51 connect to a header pipe 52, which connects to return line 28. Perforations 63 are included on the sides and bottoms of collector pipes 51, and are more numerous and of larger 5 diameter as the distance from return line 28 increases, encouraging an even flow into header pipe 52. Collector pipes 51 and header pipe 52 are supported by support structure 62.

As the concentration of solids in basin 2 changes, the concentration of solids in thickener tank 13 will also change, causing fluctuations in the height of concentrated portion 55. Tank 13 should therefore be sized such that the height of concentrated portion 55 does not approach collector pipes 51 during periods of high solids concentration. In order to properly size tank 13, the settlement characteristics of the particular mixed liquor should be analyzed. Prior research performed on particle settling velocity has shown that non-hindered particle settlement achieves a velocity estimated by the following formula: Velocity (ft./hr.) ■ 1200 [(SVI + 100) X MLSS]" where MLSS - mixed liquor suspended solids in parts per million for a 25 given sludge age and average incoming suspended solids through inlet 9 Once the settlement velocity is calculated, it is multiplied by the cross-sectional area of the tank 13 above diffuser 47, giving a flow rate in cubic feet per hour, which must equal or exceed the flow rate coming into tank 13 through waste line 10.

Figures 4 and 5 depict the relationship of retention time in tank 13 versus the concentration of solids at the 35 bottom 15. As shown, the concentration reaches a substantially constant value as the retention time in tank 13 increases, for a given sludge volume Index (SVI). As shown in Figure 4, for a mixed liquor 4 having an SVI of 67-100, an optimum concentration is safely reached at a retention time greater than 100 minutes. As shown in Figure 5 5, for a mixed liquor 4 having an SVI of 130-155, an optimum concentration is safely reached at a retention time greater than 225 minutes. The volume of concentrated portion 55 in cubic feet divided by the pumping rate of second pump 27 in cubic feet per minute will give a value for sludge retention 10 time in minutes. Thus, the rate of removal through sludge removal line 20 should be such that the optimum retention time is reached, maximizing the thickness of sludge removed and minimizing the amount of water removed by second pump 27 through sludge removal line 20. Most of the water is removed 15 in clarified portion 56. A means 53 may be provided for maintaining a desired level of concentrated portion 55 within tank 13. Means 53 may take any form known in the art, such as a level controller 54 having sensors 64,65 which sense when the level of concentrated portion 55 exceeds or falls 20 below a desired level. For example, when the level of concentrated portion 55 rises above upper sensor 64, controller 54 (such as a BT6 Model SMS-3000 Solids Monitoring System) will activate second pump 27. When the level of concentrated portion 55 falls below lower sensor 65, 25 controller 54 will deactivate second pump 27.

If the clarified portion 56 is of sufficient quality, flow through return line 28 may be diverted through a clarified fluid line 32, utilizing a flow selection means 33, 2 5 6 S 4 such as return valve 34 and clarified fluid valve 35. Flow selection means 33 may be controlled by a means 36 for determining clarity of fluid and a control means 37, such as a combination turbidity meter and automatic valve control 38, which will select a flow path dependent upon the clarity of fluid in return line 28.

As shown in Figure 2, sludge removal line 20 may be connected to a digester 24. The digester 24 may be connected to a decant tank 66. In decant tank 66, sludge is settled for removal, and clarified fluid is decanted and sent through recycle line 39 back to thickener tank 13 for further thickening, thus further minimizing the amount of water in digester 24.

As can be seen, the system 1 is extremely versatile. When used with an intrachannel clarifier 6 (shown in Figure 1), waste line 10 may be connected to intrachannel clarifier 6 so as to remove more concentrated mixed liquor 45 from within the clarifier 6. However, the system 1 may just as easily be used as shown in Figure 2, removing mixed liquor 4 directly from the basin 2. The system 1 and method described herein allows treatment plants to maintain a constant sludge age, stabilizing plant biology, while decreasing the water content of sludge removed from the plant. Other embodiments of the invention will occur to those skilled in the art, and are intended to be included within the scope of the following claims. 250 S

Claims (21)

CLAIMS |P£4/(/$ 93/03 760 0 S SEP 095 What is claimed is:

1. A method for wasting mixed liquor from an extended aeration wastewater treatment system having wastewater flowing into a retention basin so as to form mixed liquor within said retention basin, said mixed liquor containing suspended solids, said suspended solids having a sludge age, said mixed liquor being treated in said retention basin and then flowing out of said retention basin through a retention basin exit line, said method for wasting mixed 10 liquor comprising the step of substantially continuously removing a portion of said mixed liquor from said extended aeration wastewater treatment system at a substantially constant flow rate, said portion removed being in addition to said mixed liquor flowing through said retention basin exit line.

2. A method for wasting mixed liquor according to claim 1, further comprising the step of separating said portion into a concentrated portion and a clarified portion, said concentrated portion containing concentrated suspended solids.

3. A method for wasting mixed liquor according to claim 2, further comprising the step of returning said clarified portion to said retention basin.

4. A mixed liquor wasting system including a retention basin, a wastewater inlet fluidly connected to said retention basin and an external clarifier inlet line fluidly connected to said retention basin or an intrachannel clarifier outlet line fluidly connected to said retention basin, said system having wastewater flowing into said retention basin through said wastewater inlet so as to form mixed liquor within said retention basin, said wastewater being treated in said basin and then flowing out of said external clarifier inlet line or said intrachannel clarifier outlet line, a mixed liquor wasting system, comprising: a. a thickener tank, having a sidewall and a bottom; b. a waste line having a first end and a second end, said first end fluidly connected to said retention basin, and said second end fluidly connected to said thickener tank; tH-\ 93/0?76 12 IPEA/US 0 G 3£? , c. a first flow control means for continuously removing a portion of said mixed liquor from said basin at a desired constant flow rate through said waste line, fluidly connected to said retention basin; d. a sludge removal line having a first end and a second end, said first end fluidly connected to said bottom of said thickener tank, and said second end fluidly connectable to a sludge disposal system; e. a second flow control means for removing a concentrated portion of said mixed liquor from said thickener tank, fluidly connected to said thickener tank; and f. a return line having a first end and a second end, said first end fluidly connected to said thickener tank above said bottom, and said second end fluidly connected to said retention basin.

5. A mixed liquor wasting system according to claim 4, wherein said thickener tank further includes a baffle, positioned in said tank above said bottom.

6. A mixed liquor wasting system according to claim 4, further comprising a clarified fluid line, fluidly connected to said return line, and a flow selection means for diverting flow through said clarified fluid line, connected to said return line and said clarified fluid line.

7. A mixed liquor wasting system according to claim 6, further comprising a means for determining clarity of fluid in said return line, connected to said return line.

8. A mixed liquor wasting system according to claim 7, further comprising a control means for controlling said flow selection means, operatively connected to said means for determining clarity of fluid in said return line and said flow selection means.

9. A mixed liquor wasting system according to claim 4, further comprising a sludge disposal system fluidly connected to said sludge removal line. t ur/uo^/ 08/&U 2569 • H

10. A mixed liquor wasting system according to claim 9, further comprising a recycle line, fluidly connecting said sludge disposal system and said thickener tank.

11. A method for wasting mixed liquor according to claim 2, further comprising the step of transferring said concentrated portion to a sludge disposal system.

12. A mixed liquor wasting system according to claim 4, wherein said bottom of said thickener tank has a sloped portion, sloping inward from said sidewall.

13. A mixed liquor wasting system according to claim 4, further comprising a mixer including a motor attached to said thickener tank, a shaft rotatably attached to said motor and extending vertically downward into said thickener tank, and a blade fixedly attached to said shaft and positioned just above said bottom, said thickener tank further including a baffle, positioned in said tanlc above said blade.

14. A mixed liquor wasting system according to claim 12, further comprising a mixer including a motor attached to said thickener tank, a shaft rotatably attached to said motor and extending vertically downward into said thickener tank, and a blade fixedly attached to said shaft and positioned just above said bottom, said thickener tank further including a baffle, positioned in said tank above said blade.

15. A mixed liquor wasting system according to claim 13, further comprising a resilient scraper extension fixedly attached to said blade and extending downward so as to lightly touch said bottom.

16. A mixed liquor wasting system according to claim 14, further comprising a resilient scraper extension fixedly attached to said blade and extending downward so as to lightly touch said bottom.

17. A mixed liquor wasting system according to claim 4, wherein said first flow control means comprises a pump.

18. A mixed liquor wasting system according to claim 4, wherein said second flow control means comprises a pump. 19. A method for wasting mixed lfquor from an extended aeration wastewater treatment /system having wastewater flowing into a retention basin so/as to form mixed liquor flowing within said retention basing said mixed liquor containing suspended solids, said suspended solids having a sludge age, and said retention basin having an intrachannel clarifier positioned therein, said /intrachannel clarifier having concentrated mixed liquor therein, said mixed liquor being treated in said intrachannel clarifier and then clarified water flowing out of said intrachannel clarifier through a retention basin exit /ine, said method for wasting mixed liquor comprising the step of substantially continuously removing a portion of said concentrated mixed liquor from said intrachannel clarifier at a substantially constant flow rate. 20. A mixed /Liquor wasting system according to claim 4, further comprising a means for maintaining a desired level of said concentrated portion within said thickener tank. 21. A method for wasting mixed liquor according to claim 1, wherein y6aid predetermine sludge age range is 19-28 days END OF CLAIMS 18 JUN 1997 256>H + \ AS AiViEMDED '<• 14

19. A method for wasting mixed liquor from an extended aeration wastewater treatment system having wastewater flowing into a retention basin so as to form mixed liquor flowing within said retention basin, said mixed liquor containing suspended solids, said suspended solids having a sludge age, and said retention basin having an intrachannel clarifier positioned therein, said intrachannel clarifier having cohcentrated mixed liquor therein, said mixed liquor being treated in said intrachannel clarifier and then clarified water flowing out of said intrachannel clarifier through a retention basin exit line, said method for wasting mixed liquor comprising the step of substantially continuously removing a portion of said concentrated mixed liquor from said intrachannel clarifier at a substantially constant flow rate.

20. A mixed liquor wasting system according to claim 4, further comprising a means for maintaining a desired level of said concentrated portion within said thickener tank.

21. A method for wasting mixed liquor according to claim 1, wherein said sludge age has a predetermine sludge age range and wherein said predetermine sludge age range is 19-28 days.