METHODS FOR TREATING WASTE ACTIVATED SLUDGE

Abstract

A method of treating waste activated sludge. More specifically, the disclosure concerns treating waste activated sludge by a membrane aerated sludge digester to reduce of volatile soluble solids (VSS) concentration in the sludge to obtain aerobically treated sludge.

Claims

1. A method for treating waste activated sludge (WAS), the method comprising: feeding WAS into a treatment tank, the WAS having an initial volatile suspended solids (VS S) concentration, the treatment tank comprising one or more oxygen-permeable water-impermeable membranes, said membranes being configured for supporting biofilm growth thereon; feeding an oxygen-containing gas into said one or more oxygen-permeable water-impermeable membranes to provide oxygen to at least a portion of the biofilm, while maintaining the WAS under anoxic condition; and mixing the WAS in the treatment tank to cause circulation thereof in proximity to the one or more oxygen-permeable water-impermeable membranes and retaining the WAS in the tank for a period of time permitting substantial reduction of said initial VSS concentration, thereby obtaining an aerobically treated sludge.

2. The method of claim 1, wherein the WAS is retained in the tank for a period of time of at least about 5 days or for a period of time between about 5 days and about 30 days.

3. (canceled)

4. The method of claim 1, wherein one or more membrane-aerated biofilm reactor (MABR) modules are positioned in the tank, each of said one or more MABR modules comprises one of said one or more oxygen-permeable water-impermeable membranes, and each of said MABR modules occupies a portion of the tank's volume.

5. The method of claim 1, wherein said mixing of the WAS is carried out within tank in at least a portion of the tank's volume and/or by diffusing air into the tank.

6. (canceled)

7. The method of claim 5, wherein the air diffusing is intermittent or periodical.

8. The method of claim 7, wherein air is diffused for about 10 to 120 seconds once every about 5 to 120 minutes.

9. The method of claim 6, wherein the air is diffused into the tank by at least one diffuser, wherein said at least one diffuser is positioned at a bottom portion of the tank.

10. (canceled)

11. The method of claim 9, wherein said at least one diffuser is positioned below the one or more oxygen-permeable water-impermeable membranes.

12. The method of claim 9, wherein one or more membrane-aerated biofilm reactor (MABR) modules are positioned in the tank, each of said one or more MABR modules comprises one of said one or more oxygen-permeable water-impermeable membranes, and each of said MABR modules occupies a portion of the tank's volume, and said at least one diffuser is positioned below the MABR modules.

13. (canceled)

14. The method of claim 1, comprising sensing at least one parameter of the content of the tank by at least one sensor positioned in the tank, wherein at least one parameter of the content of the tank is at least one of ammonia concentration, nitrate concentration and oxidation-reduction potential (ORP).

15. (canceled)

16. The method of claim 15, comprising modifying at least one mixing condition according to a sensed value of said at least one parameter, wherein said mixing condition is at least one of mixing frequency, mixing duration and mixing intensity.

17. (canceled)

18. The method of claim 1, wherein the WAS is received from a wastewater treatment system or process.

19. The method of claim 1, wherein the WAS is received from a primary clarifier and/or secondary clarifier.

20. The method of claim 1, wherein the WAS is concentrated or thickened WAS.

21. The method of claim 1, wherein said oxygen-containing gas is fed to said membrane at a pressure that is lower than the hydrostatic pressure of sludge in the tank.

22. The method of claim 1, also comprising collecting the aerobically treated sludge from a bottom part of the tank.

23. The method of claim 1, comprising gravitational thickening of the sludge within the tank.

24. (canceled)

25. The method of claim 22, wherein thickening and collecting are carried out continuously or periodically.

26. (canceled)

27. The method of claim 25, wherein mixing is carried out intermittently or periodically followed by time intervals in which no mixing is carried out, said thickening and collecting are carried out in said time intervals.

28. The method of claim 22, comprising dewatering the aerobically treated sludge after collection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0048] FIG. 1 is a schematic representation of a system for operating in a method according to an embodiment of this disclosure.

[0049] FIG. 2 shows ammonium (NH.sub.4.sup.+) and nitrite (NO.sub.3.sup.−) concentrations in the treated medium during a digestion process of WAS in a method according to an embodiment of this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0050] Turning to FIG. 1, shown is a system in which a method according to the present disclosure may be operated. System 100 comprises a treatment tank 102, which may be rectangular or cylindrical tank of any suitable material such as steel, polymer or concrete.

[0051] Waste activated sludge (WAS), such as WAS from primary and/or secondary wastewater treatment systems, is fed into the treatment tank via WAS inlet conduit 104. At least one MABR module 106 that contains an oxygen-permeable, water-impermeable membrane 105 is positioned in the tank 102. The number of membranes/modules may be determined by the VSS load in the WAS, the amount of WAS to be treated, and/or by oxygen transfer rate properties of the membrane(s).

[0052] Oxygen requirement for removal of VSS from WAS may be estimated according to established methodologies in the field; however, a common rule of thumb is that oxygen requirement is approximately 2 mass unit per each mass unit of VSS removed. The oxygen permeability of the membrane in terms of g/d/m.sup.2 is used to determine the surface area of membrane required to provide the oxygen demand produced by endogenous decay of solids in the sludge.

[0053] The volume of tank 102 may be determined according to established methodologies in the field of wastewater treatment. The WAS fed into the tank is retained in the tank for a period of time sufficient to induce sufficient hydrolysis and digestion of the WAS, thus reducing the VSS concentration below a desired level. In the methods of this disclosure, the retention time is at least 5 days, typically in the range 20-30 days for about 40%-45% reduction in VSS concentration, depending on temperature.

[0054] Table 1 shows an example for calculation of the required tank volume, the required membrane area and the volume fraction of the tank occupied by the MABR modules.

TABLE-US-00001 TABLE 1 Example for a calculation of membrane volume ratio A) Required Tank Volume Q (m.sup.3/day) 100 VSS in (mg/L) 20,000 Temp (° C.) 25 SRT × T(° C.) 475 SF 10% SRT (days) 20.9 Calculated Tank Volume (m.sup.3) 2,090 B) Oxygen Requirements VSS reduction requirement 45% VSS removed (mg/L) 9,000 VSS removed per day (kg/day) 900 Specific oxygen req. (kgO.sub.2/kgVSS) 1.5 Oxygen demand (kg O.sub.2/day) 1,350 C) Membrane fill ratio calculation membrane permeability (g/d/m.sup.2) 12 required membrane surface area (m.sup.2) 112,500 membrane packing factor (m.sup.2/m.sup.3) 110 membrane volume (m.sup.3) 1,023 Fill ratio 49% SRT = solids retention time

[0055] The mechanism of oxidation within a tank is typically explained as follows: oxygen from a oxygen-containing gas that is fed to the membrane in the MABR module 106 through oxygen-containing gas line 114, which may be coupled to an air delivery means, such as a blower or a suction system (not shown). The oxygen diffuses through the membrane, to support development of a biofilm on the water-facing surface of the membrane, oxidizing ammonium compounds present in the WAS; biomass remaining in the WAS use the nitrate produced in the nitrification process to oxidize organic material. Thus, the concentration of ammonium compounds and organic material is reduced.

[0056] Biological activity in tank 102 is associated with the endogenous decay of the bacteria in the WAS. Sufficient membrane surface area is provided to satisfy the endogenous respiration rate of the WAS. The endogenous respiration rate can be measured in a respiration test such as described in “Respirometry in Control of the Activated Sludge Process: Priciples” by Spanjers, Vanrolleghem, Olsson, Dold (IAWQ Task Group on Respirometry) 1998. Alternatively, the predicted endogenous respiration rate and corresponding oxygen requirements can be calculated according to parameters known in the field of wastewater treatment, such as published in Metcalf & Eddy (2003) Wastewater Engineering: Treatment and Reuse, pp. 1535-1539, 4th Edition, McGraw-Hill, New York.

[0057] The WAS is mixed during its retention in the tank by air diffusers 110, that are positioned at the bottom of tank 102. Air may be supplied to diffusers 110 through piping connected to a pressurized air source (not shown), such as a blower. The diffusers 110 may be operated intermittently, for example by turning the blower on and off or by operating a valve connected to a source of pressurized air, or by means of an air accumulator.

[0058] One or more sensors may measure various parameters in the tank, such as ammonia concentration, nitrate concentration and oxidation-reduction potential (OPR) and the readings may be transmitted though communication line 116 (which may be wireless or wired) to control unit 118. Control unit 118 may process the readings and issue commands to modify one or more mixing parameters—the commands being delivered to the diffuser 110 through communication line 120 (which may be wireless or wired). Thus, the mixing conditions may be modified throughout the treatment process in order to obtain a desired VSS reduction and/or to maintain or stabilize various conditions in the treatment tank.

[0059] Aerobically treated sludge is discharged from the tank via outlet conduit 112. In case thickening is carried out within the tanks 102, thickened sludge is discharged through outlet conduit 112 and supernatant is discharged from an upper supernatant outlet 108.

[0060] Aerobically treated sludge may be dewatered in a separate process unit such as a centrifuge or belt press (not shown). Dewatered sludge is discharged for reuse or additional processing according to local Regulations. Supernatant water from the dewatering process is typically returned to the wastewater treatment process from which the sludge was drawn for treatment.

[0061] Thickening is typically performed in either a gravity thickener or a mechanical thickener. Optionally, chemicals are added to aid in the thickening such as flocculating agents or coagulants, as part of the thickening process.

EXAMPLE

[0062] WAS from the bottom of a secondary clarifier was fed continuously by a pump from a feed tank into a treatment tank at a feed flow rate of 175 liter/day. The tank had a volume of 3.4 m.sup.3, with water depth of about 2.5 m. The tank was fitted with a spirally wound MABR module with surface area of 475 m.sup.2. Air diffusers were disposed in the tank below the MABR module to enable mixing of the WAS; mixing was carried out by intermittent air sparging from a blower through the coarse bubble diffusers every 30 seconds, 4 times per hour (every 15 minutes). Low pressure air from a fan was provided continuously to the MABR to provide air to the membrane. Retention time of the WAS in the tanks was about 20 days in continuous feed mode.

[0063] Samples of the WAS from the tank inlet and samples of the treated sludge from the tank's outlet were taken 2-3 times per week. TSS (total soluble solids) and VSS (volatile soluble solids) analysis was carried out according to Standard Methods for Examination of Water and Wastewater, 23rd edition, Author: E. W. Rice, R. B. Baird, A. D. Eaton, Publisher: American Public Health Association, American Water Works Association, Water Environment Federation. The analysis included filtering of pre-determined volumes of the taken sample, the filter cakes were dried at 105° C. to determine TSS, then cooked at 550° C. to determine VSS by difference. The % VSS removal was calculated according to: (VSS.sub.IN-VSS.sub.OUT)/VSS.sub.IN. The test results are shown in Table 2. The test was run for several months to obtain steady state conditions, and the results presented in Table 2 were obtained after about 3 months of continuous operation.

TABLE-US-00002 TABLE 2 test results VSS Day of TSS IN VSS IN TSS OUT VSS OUT reduc- operation [mg/l] [mg/l] [mg/l] [mg/l] tion 112 27,322 23,816 9,644 7,706 68% 114 29,988 25,840 8,750 6,749 74% 115 27,916 24,200 9,352 7,416 69% 119 30,337 26,277 9,311 7,400 72% 121 29,157 25,115 9,602 7,540 70%

[0064] The test results show that VSS reduction in the range 68-74% was obtained at about 20 days retention time in continuous feed mode. Comparing to the regulatory requirements in the USA (according to 40 CFR part 503), which requires minimum of 38% VSS reduction to permit safe release to the environment, the present results significantly exceed (in the specific example in about 2-folds) the % reduction of VSS required by the Regulation.

[0065] FIG. 2 shows ammonia and nitrate concentrations in the filtered water from the treated sludge during the same period as the example, as well as later results obtained at a later time without a MABR in the tank. It can be seen that in January, with the MABR in the tank, both ammonia and nitrate concentrations are low, indicating that hydrolyzed nitrogen was substantially removed. On the other hand, in April without the MABR in the tank, only part of the ammonia was oxidized to nitrate as a result of which both constituents are much higher.