CONTENTS
Section Page
1.0 SCOPE ...5
2.0 REFERENCES...5
3.0 DEFINITIONS ...8
4.0 BACKGROUND AND SELECTION CRITERIA...10
4.1 BACKGROUND...10
4.2 SELECTION CRITERIA...11
5.0 SUMMARY OF BIOLOGICAL TREATMENT SYSTEMS ...11
6.0 ACTIVATED SLUDGE SYSTEMS...11
6.1 DESCRIPTION ...11
Process Microbiology...11
Reaction Kinetics and Fundamental Expressions...12
Substrate (BOD) Removal ...13
Temperature Effect / Importance ...14
Oxygen Requirements ...14
Process Variations...16
Clarification...17
6.2 DESIGN CONSIDERATIONS...18
Regulatory Effluent Requirements...18
Feed Characteristics: Equalization and Pretreatment Requirements ...18
Selection of Reactor and Clarifier Types ...20
Oxygen Requirements and Aeration Equipment...21
Need for Pilot Plant Data ...21
6.3 DESIGN PROCEDURE ...21
Quick, Rough Sizing Basis ...21
Standard Procedures - Design Conditions...21
6.4SAMPLE DESIGN PROBLEM ...27
Design Conditions ...27
6.5 OPERATING STRATEGIES AND ENHANCEMENTS ...31
Upstream Monitoring ...31
Equalization, Spill Diversion, and Pretreatment...31
Solids Retention Time ...31
Routine Operations Monitoring ...31
Operations Troubleshooting ...33
CONTENTS (Cont) Section Page 8.0 AERATED LAGOONS ... 34 8.1 BACKGROUND... 34 8.2 DESCRIPTION... 34 8.3 DESIGN CONSIDERATION... 34 Aeration Zone... 35 Settling Zone ... 35 8.4 DESIGN PROCEDURE... 36 Substrate Removal... 36 Temperature Effects... 36 Oxygen Requirements... 36
Power Requirements for Oxygen Transfer and Solids Suspension... 36
Sludge Accumulation... 36
8.5 SAMPLE DESIGN PROBLEM... 37
Temperature Effects... 38
Substrate Removal... 38
Oxygen and Power Requirements... 38
Sludge Accumulation... 38
8.6 OPERATING STRATEGIES AND ENHANCEMENTS ... 39
Oxygen Transfer... 39
Short Circuiting... 39
Algae and Suspended Solids Control... 39
Sludge Buildup and Removal ... 39
Biomass Return Options ... 39
9.0 AEROBIC ATTACHED GROWTH ... 40
9.1 DESCRIPTION... 40
Process Microbiology ... 40
Applications for Attached Growth Systems ... 41
Trickling Filters and Packed-Bed Biotowers ... 41
Rotating Biological Contactor Reactors (RBC) ...42
Aerated Biological Filters and Fluidized Beds ... 42
9.2 DESIGN CONSIDERATIONS ... 42
Additional Design Considerations for Trickling Filters ... 43
Additional Design Considerations for RBCs ... 43
Additional Design Considerations for Aerated Biological Filters and Fluidized Beds ... 43
Equipment Design for Trickling Filters... 44
Equipment Considerations for RBC... 46
9.3 DESIGN PROCEDURE... 47
9.4 SAMPLE DESIGN PROBLEM... 50
9.5 OPERATING STRATEGIES AND ENHANCEMENTS ... 52
10.0 ANAEROBIC SYSTEMS ... 52
CONTENTS (Cont) Section Page 12.0 NITROGEN MANAGEMENT ...53 12.1 DESCRIPTION ...53 Nitrogen Forms...53 Process Microbiology...54
Process / Reactor Variations For Biological Denitrification ...54
Alternative Reactor Designs ...54
Alternative Biological Processes...55
12.2 DESIGN CONSIDERATIONS...55
Effluent Nitrogen Limits...55
pH ...55
Temperature ...56
Dissolved Oxygen (D.O.) ...56
Mixed Liquor Recycle Rate and Recycle Ratio...56
Power Input to Anoxic Zone...57
Organic Substrate to Nitrogen Ratio ...57
Solids Residence Time (SRT) ...58
Hydraulic Retention Time (HRT)...58
12.3 DESIGN PROCEDURE ...58
12.4 SAMPLE DESIGN PROBLEM - PERFORMANCE REQUIREMENTS, EXTERNAL SUBSTRATE REQUIRED AND ROUGH SIZING...59
12.5 OPERATING STRATEGIES AND ENHANCEMENTS ...60
13.0 CHEMICAL ADDITION SYSTEMS...60
13.1 NUTRIENTS ...60
13.2 FLOCCULANTS ...60
13.3 pH CONTROL ...60
14.0 NOMENCLATURE...61
TABLES Table 1-1.A Characteristics of ExxonMobil Activated Sludge Units ...64
Table 1-1.B Characteristics of ExxonMobil Lagoon Systems...68
Table 6.1-1 Recommended Values of Ym and b at 20°C ...69
Table 6.2-1 Criteria for Pretreatment of Activated-Sludge Feed...70
Table 6.2-2 Relative Biodegrability of Certain Organic Compounds...71
Table 9.1-1 Characteristics of ExxonMobil Attached Growth Systems...72
Table 9.1-2 Sample Duty Specification Sheet for Attached Growth System ...73
Table 9.2-1 Comparison of Vendor Aerated Biological Filters ...75
Table 12.1-1 Wastewater Nitrogen Measurement Parameters...76
Table 12.2-1 Alkalinity Produced During Nitrate - Nitrogen Reduction ...76
Table 12.2-2 Mixing Requirements in Suspended Growth Systems...77
Table 12.2-3 Theoretical Substrate Requirements For Nitrate - Nitrogen Reduction ...78
Table 12.3-1 Performance of ExxonMobil Nitrification / Denitrification (N/DN) Systems ...79
Table 12.3-2 Sample Equipment List for Nitrification / Denitrification ...80
CONTENTS (Cont)
Section Page
FIGURES
Figure 4.2-1 Oxygen Demand / Dissolved Organic Reduction Decision Tree ... 82
Figure 4.2-2 Typical Wastewater Treatment Flow Plan... 83
Figure 6.1-1 General Schematic Of An Activated Sludge Process (Completely Mixed) Biological Treatment (BIOX) System... 84
Figure 6.1-2 Typical Plot Of The Relationship Between The Specific Growth Rate Constant And The Limiting Substrate Concentration... 84
Figure 6.1-3 BOD Removal And Sludge Growth Relationships... 85
Figure 6.1-4 Conventional Plug Flow Activated Sludge Process... 86
Figure 6.1-5 Typical Configuration Of A Step-Feed Aeration Activated Sludge Process... 86
Figure 6.1-6 High Purity Oxygen Activated Sludge Process ... 87
Figure 6.1-7 Completely Mixed Activated Sludge With A Selector ... 87
Figure 6.1-8 Two BIOX Configurations For Total Biological Nitrogen Removal ... 88
Figure 6.3-1 Determination Of Ym From Pilot Unit Batch Yield Test... 89
Figure 6.5-1 Succession Of Protozoa And Sludge Development ... 90
Figure 8.2-1 Aerated Lagoon Example Layout Of An Aerated Lagoon System ... 90
Figure 9.1-1 Films And Layers On Attached Growth Media ... 91
Figure 9.1-2 Schematic Of Trickling Filter Systems... 92
Figure 9.1-3 Typical Trickling Filter ... 93
Figure 9.1-4 Schematic Representation Of Rotating Biological Contactor System ... 93
Figure 9.1-5 Conventional And Submerged RBC Units... 94
Figure 9.1-6 Schematic Representation Of An Aerated Biological Filter ... 95
Figure 9.2-1 Surface Area Correction Curves For RBCs For Temperatures Below 55°F... 96
Figure 9.2-2 Fluidized Bed Flow Diagram (US Filter-Envirex) ... 97
Figure 9.2-3 Tricking Filter Media... 98
Figure 12.1-1 Simplified Nitrogen Cycle Within A Wastewater Treatment System... 99
Figure 12.1-2 Total Biological Nitrogen Removal Configurations ... 100 Revision Memo
12/01 Section 11.0 updated, Section 12.0 added with associated tables and figures. Old Sections 6 and 7 covering “Aeration Systems for Biological Treatment of
Wastewater” and “Clarification Systems for Biological Treatment of Wastewater” have been relocated to new DP Sections XIX-A6 and XIX-A7, respectively.
1.0 SCOPE
➧ This section presents design considerations, recommended process design procedures, and certain mechanical design details for facilities to biodegrade wastewater containing organic and some inorganic contaminants to meet certain quality goals. Biological treatment facilities include: suspended growth, attached growth, or a combination of both. There are several varieties of suspended growth systems, the most common being activated sludge and extended aeration. Attached growth includes trickling filter, rotating biological contactors, and many others. This Design Practice includes directions for sizing the aeration basin / tank, calculating the aeration requirements, selecting aeration equipment and sizing the clarification systems. Currently, only the standard activated sludge, aerated lagoons, and aerobic attached growth systems (trickling filters, rotating biological contactors, etc.) are discussed in detail. Design information on other biological treatment technologies like sequencing batch reactors, anoxic systems, and others are not covered. Chemical addition systems and sludge management are briefly discussed. Biological treatment systems can be employed to meet complex effluent treatment requirements including aquatic organism toxicity reduction, selected heavy metals removal and nutrient management. For complex applications or where multiple water quality goals are required, it is recommended that ExxonMobil Engineering specialists be contacted. For reference purposes only, a table including the characteristics of ExxonMobil activated sludge units and aerated lagoon systems is provided in Table 1-1A and 1-1B, respectively.
The design procedures discussed in this section are set forth to give guidelines in developing biological treatment for wastewater. In all cases, consideration must be given to evaluating key parameters such as the potential to discharge a stream with low dissolved oxygen, oil sheen, toxicity, and/or high residual chemical oxygen demand to ensure compliance with appropriate environmental standards. Alternate treatment / configurations exist which can provide the necessary wastewater treatment.
2.0 REFERENCES 2.1 DESIGN PRACTICES
Section XIX-A1 Primary Oil / Water Separators Section XIX-A2 Flotation Units
Section XIX-A3 Media Filtration
Section XIX-A4 Chemical Flocculation / Specific Ion Removal and Clarification of Wastewater Section XIX-A8 Activated Carbon Treatment
Section XIX-A9 Water / Wastewater Chemical Feed Systems
➧ 2.2 GLOBAL PRACTICE
GP 3-2-1 Sewer Systems
➧ 2.3 EMRE WATER AND WASTEWATER DESIGN GUIDE (TMEE 080)
DG 11-1-1 Granular-Media Filters
DG 11-2-1 Fixed Bed Ion Exchange Water Treating Units DG 11-6-1 Chemical Feeders For Boilers & Deaerators DG 11-6-3 Chemical Feeders For Wastewater Treating DG 11-7-1 Wastewater Dissolved Air Flotation System DG 11-8-1 Gravity Belt Filter Press System
DG 11-9-1 Aeration Systems 2.4 OTHER REFERENCES
1. Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 2nd Edition, McGraw-Hill Inc., New York (1979)
2. Guidelines For Reducing Waste Treatment Cost, ER&E Report No. EE.48E.85
3. Grady Jr., C. P. L., Daigger, G. T., Lim, H. C., Biological Wastewater Treatment, Theory, and Applications, Marcel Dekker, Inc., New York (1999)
4. Eckenfelder Jr., W. W. and Grau, P., Activated Sludge Process Design and Control: Theory and Practice, Water Quality Management Library Volume 1, Technomic Publishing Co, Inc., Lancaster, PA, (1992)
2.0 REFERENCES (Cont)
6. Water Environment Federation, Clarifier Design - Manual of Practice FD-8, Lancaster Press, Lancaster, PA (1985) 7. Eckenfelder Jr., W. W., Industrial Water Pollution Control, 2nd Edition, McGraw-Hill Inc., New York (1989)
8. Hayes, B. E. and Cancellare, M. C., Industrial Water Pollution Control Technology Course - Eckenfelder's Method Compared to the Design Practices, 91 ECS2 191 (December 1991)
9. Altemoeller, P. H. and Goodrich Jr., R. R., Guidelines for Sizing Gravity Thickeners - Wastewater Treatment Sludge Applications, ER&E Report No. EE.24E.89 (October 1989)
10. Parker, D. S., The Case for Circular Clarifiers, WATER / Engineering & Management, April 1991
11. U.S. Environmental Protection Agency, Nitrogen Control Manual, EPA/625/R-93/010, USEPA, Cincinnati, Ohio (September 1993)
12. ENSR, Breakthrough Nite/Denite Process Purifies Wastewater and Saves Millions in Treatment Costs, ENSR Newsletter (1991) 800-722-2400
13. Great Lakes-Upper Mississippi River Board of State Public Health and Environmental Managers, Recommended Standards for Wastewater Facilities, Health Education Services, Albany, NY (1990)
14. Gerardi, M. H, An Operator's Guide to Protozoa and their Role in the Activated Sludge Process, PUBLIC WORKS, July, 1986
15. Wilkinson, J. B. and Palis, J. C., Trip Report: Filamentous Bulking in Augusta's Biox, 84ECD 213 (March 1984)
16. U.S. Environmental Protection Agency, The Causes and Control of Activated Sludge Bulking and Foaming - Summary Report, EPA/625/8-87/012, USEPA, Cincinnati, Ohio (July 1987)
17. Givens, S. W. and Grady, C. P. L. et al, Biological Process Design and Pilot Testing for a Carbon Oxidation, Nitrification, and Denitrification System, Environmental Progress Vol. 10, No. 2 (May 1991).
18. Water Pollution Control Research Series, No. 12020, 2/70, Petrochemical Effluents Treatment Practices Detailed, U.S. Department of the Interior, Federal Water Pollution Control Administration (February 1970).
19. Water Environment Federation, Manual of Practice No. 8 - Design of Municipal Wastewater Plants, WEF and ASCE, Book Press, Brattleboro, VT (1992)
20. Eckenfelder Jr., W. W., et al, Activated Sludge Treatment of Industrial Wastewater, Technomic Publishing Co, Inc., Lancaster, PA, (1995)
21. Mange, O., and Gros, H., Technical Advances in Biofilm Reactors, IAWPRC Congress, Nice, (1989)
22. Fiessinger, F., Water Treatment Technologies for the Challenges of the Nineties, in Water Treatment - Proceedings of the 1st International Conference, Elsevier Science Publishers Ltd, England (1991)
23. Sutton, P. M. and Mishra, P. N., Biological Fluidized Beds for Water and Wastewater Treatment, Water Environment & Technology, August 1991
24. Tsubone, T., Ogaki, Y., Yoshiy, Y. and Takahashi, M., Effects of Biomass Entrapment and Carrier Properties on the Performance of an Air-Fluidized-Bed Biofilm Reactor, Water Environment Research, Volume 64, Number 7 (Nov/Dec 1992)
25. Lessel, T. H., First Practical Experiences with Submerged Rope-Type Biofilm Reactors for Upgrading and Nitrification, Water Science Technology, Vol 23 (1991)
26. Robertaccio, F. L, Polyelectrolyte Guide, ER&E Report No. EE.20E.84, 1984.
27. Water Environment Federation, Guidance Manual for Polymer Selection in Wastewater Treatment Plants
28. Esler, John, Optimizing Clarifier Performance, CPE Services, Inc., New York State Department of Environmental Conservation, Albany, New York.
29. Water Environment Federation, Aeration - Manual of Practice FD-13, Alexandria, Virginia (1988).
30. Clesceri, L., et al, Standard Methods for the Examination of Wastewater, 17th Edition, American Publish Health Association, Washington, DC, 1989.
31. Stover, E. N., Kincannon, D. F., Rotating Biological Contactor Scale-Up and Design, WE&M - Reference Handbook (1980) 32. Thibault, G. T., Wastewater Treatment by Aerobic Biological Oxidation - Alternatives to Activated Sludge, ER&E Report
No. EE.62E.76
33. Lindquist, L. A., Performance of a Rotating Biological Contactor During Cyanide Shock Loading, ER&E Report No. EE.86E.80
34. Fort, L. R., Enhanced Water Treatment System (EWETS-PjBM/PSM), 92ECS2 104
2.0 REFERENCES (Cont)
36. U.S. Environmental Protection Agency, Upgrading Trickling Filters, EPA/430/9-78-004, USEPA, Washington D.C. (July 1978)
37. Neu, K. E., Upgrading of Rotating Biological Contactor (RBC) Systems to Achieve Higher Effluent Quality, Including Biological Nutrient Enrichment and Reduction Techniques, Wat. Sci. Tech, Vol. 29, No. 12, pp. 197 - 206 (1994)
38. Hao, O. J., et al, Biological Fixed-Film Systems, Research Journal WPCF, Vol. 63, Number 4, pp 388 - 394 (June 1991) 39. Ibrahim, A. A., et al, Biological Fixed-Film Systems, Water Environmental Research, Vol. 66, Number 4, pp. 336 - 342
(June 1994)
40. Galil, N., M. and Rebhun, A Comparative Study of RBC and Activated Sludge in Biotreatment of Wastewater From an Integrated Oil Refinery, 44th Purdue Industrial Waste Conference Proceedings, pp. 711 - 717 (1990)
41. Kigel, M. Y., Shultis, J. F., Wastewater Treatment Technologies Accomplished in a Pseudofluidized Bed Reactor, Wat. Sci. Tech, Vol. 26, No. 9 - 11, pp. 2501 - 2504 (1992)
42. Ro, K. S., Neethling, J. B., Biofilm Density for Biological Fluidized Beds, Research Journal WPCF, Vol. 63, Number 5, pp. 815 - 818 (July/August 1991)
43. Odegaard, H., Rusten, B., Westrum, T., A New Moving Bed Biofilm Reactor - Applications and Results
44. Missouri Basin Engineering Health Council for the U.S. Environmental Protection Agency , Waste Treatment Lagoons -State of the Art, Project #17090 EHX (July 1971)
45. Rich, L. G., Low-Maintenance, Mechanically Simple Wastewater Treatment Systems, McGraw-Hill, New York (1980) 46. Rich, L. G., Designing Aerated Lagoons to Improve Effluent Quality, Chemical Engineering, May 30, 1983, pp. 67 - 70 47. U.S. Environmental Protection Agency, Municipal Wastewater Stabilization Ponds - Design Manual, EPA / 625/1-83/015,
USEPA, Cincinnati, Ohio (October 1983)
48. U.S. Environmental Protection Agency, Retrofitting POTW's - Handbook, EPA/625/6-89/020, USEPA, Cincinnati, Ohio (July 1989)
49. Goodrich, R. R. and Urban, D. B., Refinery Process Unit Wastewater Load Factors-Final Report, EE.86E.86.
50. Burdick, C. R., Refling, D. R., Stensel, H. D., Advanced Biological Treatment to Achieve Nutrient Removal, Journal WPCF, Vol. 54, No. 7 (July 1982)
51. Christiansen, J. A., Kilgallen, P., Roy, S., Use of a Commercially Prepared Nitrosomonas and Nitrobacter Inoculum to Induce Nitrification in a Biological Treatment System
52. Hem, L. J., Rusten, B., Odegaard, H., Nitrification in a Moving Bed Biofilm Reactor, Wat. Res. Vol. 28, No. 6, pp. 1425-1433 (1994)
53. Johnson, W. K., Schroepfer, G. J., Nitrogen Removal by Nitrification and Denitrification, Journal WPCF Vol. 36, No. 8 (August 1964)
54. Kaczmarek, S. A., Biological Treatment for Upstream Concentrated Wastewaters, ER&E Report No. EE.46E.84 (June 1984)
55. Kaczmarek, S. A., Nitrification in Refinery Wastewater Treatment, ER&E Report No. EE.62E.85 (October 1985)
56. Picard, M. A., Faup, G. M., Removal of Nitrogen from Industrial Waste Waters by Biological Nitrification - Denitrification, Wat. Pollut. Control (1980)
57. Schmidt, E. L., Belser, L. W., Nitrifying Bacteria, Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties - Agronomy Monograph No. 9, 2nd Edition (1982)
58. Sekoulov, I., Addicks, R., Oles, J., Post-Denitrification with Controlled Feeding of Activated Sludge as H Donator, Wat. Sci. Tech, Vol. 22, No. 7/8, pp. 161 - 170 (1990)
59. Tiedje, J. M., Denitrification, Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties - Agronomy Monograph No. 9, 2nd Edition (1982)
60. Wilson, T. E., Pickard, D. W., Bizzarri, R. E., A Nitrogen Success Story, Wat. Envir. Fed. (September 1994)
61. Water Pollution Control Federation, Nutrient Control - Manual of Practice FD-7, Facilities Design, Alexandria, Virginia (1983)
62. Randall, C.W., et al, Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal, Water Quality Management Library Vol. 5, Technomic Publishing Co., Inc., Lancaster, Pennsylvania (1992)
63. U.S. Environmental Protection Agency, Nitrogen Control - Manual, EPA/625/R-93/010, USEPA, Cincinnati, Ohio (September 1993)
3.0 DEFINITIONS
Active Solids / Biomass - The portion of the solids in a biological system composed of microorganisms that are actively metabolizing the substrate (removing the organic or inorganic contamination). Non-biodegradable solids accumulate in the activated sludge system and reduce the percentage of active solids (biomass / organisms) in the system.
Aerated Lagoon - An oxidation pond with aeration devices. Mixing energy supplied to an aerated lagoon is usually insufficient to completely mix the system.
Aerobic - A system or process which is active in the presence of dissolved oxygen. In biological waste treatment, aerobic refers to a microbiological system in which microorganisms use dissolved oxygen in the metabolism of the substrate (remove contaminants).
Alkalinity - Alkalinity of a water is its acid-neutralizing capacity. Alkalinity is destroyed in the biological process of nitrification. Nitrification consumes 7.15 mg of alkalinity (expressed as CaCO3) per mg of ammonia-nitrogen nitrified. As a rule of thumb, 150 mg/L alkalinity measured as calcium carbonate is needed for nitrification.
Anaerobic - A system or process which is active in the absence of dissolved oxygen. In biological waste treatment, anaerobic refers to a microbiological system in which microorganisms metabolize the substrate in the absence of dissolved oxygen. Anoxic - A term frequently used to describe a system or process which is active in the absence of dissolved oxygen but in the presence of nitrate. In these systems, nitrate, not dissolved oxygen, acts as the terminal electron acceptor for the metabolism of the substrate.
Autotrophs - Organisms that obtain carbon for the formation of cell tissue from dissolved carbon dioxide.
Biochemical Oxygen Demand (BOD, BOD5, BODULT) - A general measure of organic material in wastewater samples that can be biologically degraded. It is the quantity of oxygen consumed during the biological decomposition (oxidation) of material in water. Certain inorganic compounds that exert an immediate oxygen demand (e.g., sulfite) will be detected in the BOD test. BOD is usually measured over a specific time period; a five-day period is commonly used, with the result expressed as BOD5. If the biological decomposition is allowed to proceed to completion, the quantity of oxygen consumed is termed the ultimate BOD, often designated BODULT and is normally measured over 20 days. In this case, some nitrogen compounds can be oxidized, a process called nitrification. BOD is normally expressed mg/L (ppm). BOD5 is typically 60 percent of BODULT. BIOX - Abbreviation for BIological OXidation and commonly used to describe an activated sludge system but can be used in reference to other aerobic biological oxidation processes used to treat wastewater.
Bulking - An upset condition in the settling basin of a biological oxidation system during which the bio-sludge doesn't settle, or settles slowly, and leads to floc or suspended solids carryover with the effluent. Bulking is usually associated with filamentous bacterial growths.
Carbonaceous Biological Oxygen Demand (CBOD) - A general measure of organic material in wastewater samples that can be biologically degraded; similar to BOD, but a nitrification inhibitor is used to eliminate the interference of nitrifying bacteria. Chemical Oxygen Demand (COD) - A measure of the amount of organic or reduced inorganic compounds in a sample that can be oxidized by a strong oxidizer, usually potassium dichromate and sometimes potassium permanganate. COD of a wastewater is generally greater than the BOD since the wastewater may contain oxidizable material that cannot be biologically degraded. The COD test is simpler and faster than the BOD test. Caution must be used when analyzing the COD in high salt (chloride) wastewater streams since the salts will interfere with the test results. COD is expressed as ppm or mg/L.
Denitrification - The process by which specific microorganisms biologically convert nitrate or nitrite into nitrogen gas. The principal biochemical pathway for denitrification involves using the oxygen bound up in the nitrate or nitrite, and not free oxygen, as the terminal electron acceptor in the oxidation of the organic compounds.
Dissolved Oxygen (DO) - Dissolved oxygen level, measured in mg/L (ppm), is an important monitoring parameter for biological systems and receiving water bodies. It indicates whether a biological treatment unit can sustain a healthy aerobic microbial population or whether the receiving water body can sustain microbial, aquatic fish, or plant life. A minimum DO value for healthy aerobic biological treatment systems is between 1 to 2 ppm (mg/L). The maximum concentration soluble in water under normal conditions (saturation concentration) is between 8 to 10 ppm (mg/L) and is a function of salinity and temperature. Higher DO levels are possible in high purity oxygen activated sludge systems.
Endogenous Respiration - The energy required for cell maintenance. Other factors such as cell death and predation are usually combined with endogenous respiration in a term called endogenous decay.
Enzymes - Biological catalysts. Several vendors offer these chemicals to enhance biotreatment. Facultative Bacteria - Bacteria that can grow in either an aerobic or an anaerobic environment
Filamentous Growth - Thread-shaped microorganisms sometimes found in activated sludge plants. Excessive filamentous growth is to be avoided because they are difficult to settle. Filamentous growth can be controlled by maintaining a proper F/M ratio or SRT. Severe filamentous growth may require drastic control, i.e., chlorination or peroxide.
3.0 DEFINITIONS (Cont)
F/M Ratio - Food to Mass ratio which corresponds to the amount of substrate (organic contamination) removed per day per total mass of activated bio-sludge in the aeration basin. F/M ratio is normally expressed as pounds of BOD5 or TOC removed per day per pound of MLVSS in the aeration basin. The growth and settling characteristics of the biomass in an activated sludge plant depend upon the F/M ratio. F/M ratio varies inversely with the solid retention time (SRT).
Heterotrophs - Organisms that obtain carbon for the formation of cell tissue from dissolved organic substrates.
Hydraulic Retention Time (HRT) - The length of time the influent wastewater is retained in the aeration basin (not including the effect of sludge recycle; i.e., aeration volume divided by influent volumetric flowrate).
Inert Solids - Those solids within the aeration basin that are not active. Inert solids can include non-degradable solids which enter with the feed, activated carbon, cell debris from dead organisms, and extra-cellular by-products produced by living organisms.
Mean Cell Residence Time (MCRT) - See Solids Retention Time (SRT).
Mixed Liquor - The contents of the aeration basin, consisting of the wastewater and cell biomass.
Mixed Liquor Suspended Solids (MLSS) - The concentration of total suspended solids in the aerated section of a biological treatment unit or lagoon. MLSS is normally expressed in units of ppm or mg/L.
Mixed Liquor Volatile Suspended Solids (MLVSS) - The portion of the MLSS which volatilizes at 1022°F (550°C). Biological solids (microorganisms) are the main contributor to MLVSS. For systems which add powdered activated carbon, a separate acid digestion step can be done to distinguish between MLVSS due to carbon and MLVSS due to biomass. MLVSS is normally expressed in units of ppm or mg/L.
Nitrification - Nitrification is a biological process where ammonia is converted / oxidized to nitrate. The process involves one microorganism species (Nitrosomonas) converting ammonia to nitrite and then a different microorganism species (Nitrobacter) converting the nitrite to nitrate. This term should not be confused with “denitrification," which refers to the biological process of converting nitrates to nitrogen gas.
Nutrients - Chemical elements such as nitrogen, potassium, phosphorous, sulfur, cobalt, zinc, and copper, which are essential for microbial growth. As a rule-of-thumb, a ratio of BOD5, nitrogen, and phosphorus (BOD5:N:P) = (100:5:1) is needed for biological activity.
Organic Loading - The daily rate of BOD5 applied per unit volume of attached growth system media. Organic loading is expressed in units of lb. of BOD5/d/1000ft3 of system volume or kg of BOD
5/d/m3 of system volume.
Oxidation Pond - A basin in which wastewater undergoes biological oxidation treatment by the action of algae and bacteria but without the aid of aeration devices, mixing devices, or sludge recycle.
pH - A measurement of the acidic or basic character of a solution at a given temperature. It is defined as the negative logarithm (to the base 10) of the hydrogen ion concentration (-log[H+]). Pure water is slightly ionized with a pH of 7, and at
equilibrium the ion product, Kw , is [H+][OH–] = 1.01 x 10–14 at 25°C. Generally, biological treatment systems operate best at
pH ranges between 6.5 to 8.5. Wastewater outside the 5.5 to 9.5 pH range (before being commingled in the mixing zone of the wastewater effluent and the receiving water body) can potentially cause harm to the receiving water aquatic life as well as the biological treatment microorganisms.
Recycle - The portion of solids which is taken from the clarifier underflow and routed back to the aeration basin to control the mixed liquor microbe population in the bioreactor, sometimes referred to as RAS, Recycle Activated Sludge.
Return Activated Sludge (RAS) - The portion of solids which is taken from the clarifier underflow and routed back to the aeration basin to control the mixed liquor microbe population in the bioreactor.
Sludge Blanket - The level of sludge in the settling basin (clarifier), usually expressed in ft (m).
Sludge Volume Index (SVI) - A measure of compaction of sludge after gravity settling. SVI is the volume in milliliters occupied by one liter of activated sludge after 30 minutes of settling divided by the MLSS concentration.
SVI = [(ml of settled sludge / liter of initial suspension) / (mg/L of MLSS)] x 1000
Normally, the SVI of the sludge varies with the F/M ratio and the SRT in the aeration basin. Values below 100 are generally indicative of good settling and compaction. Care must be taken in use of the SVI in systems with high MLSS levels, e.g., high purity oxygen systems or very high sludge ages.
Solids Residence Time (SRT) - The average length of time the solids are held in the system expressed in days, sometimes referred to mean cell residence time (MCRT).
SRT = (Total mass of solids in the aeration basin and clarifier) / (mass of solids lost both intentionally and unintentionally per day)
3.0 DEFINITIONS (Cont)
Substrate - Carbon and energy sources needed to promote the growth of microorganisms. The organic waste which is used as food by the microorganisms to produce more organisms, water, carbon dioxide, and intermediate by-products.
Total Dissolved Solids (TDS) - A measure of all dissolved material in a solution, including inorganic salts (e.g., NaCl, MgCl, etc.) that typically make up the bulk of the TDS measured in the standard lab test. TDS is used to determine the salt levels of wastewater. Measurement for TDS consists of passing a sample through a standard glass fiber filter, and the filtrate is evaporated to dryness in a weighed dish and dried to a constant weight at 356°F (180°C). The material remaining on the filter paper is the total dissolved solids and is reported in units of ppm or mg/L. Conductivity can be used for a quick substitute measurement for TDS (reported in units of micromhos or microsiemens). As a rule-of-thumb, for wastewater streams at pH 7, the TDS of that stream in ppm (mg/L) can be approximated by multiplying the conductivity in units of micromhos or microsiemens by 0.7.
Total Suspended Solids (TSS) - The amount of suspended matter removed by a 0.45 micron filter when a wastewater sample is dried at 217°F (103°C) and is reported in units of ppm or mg/L. Inorganic particles such as clay or grit as well as organic particles (biological solids including algae) contribute to the suspended solids concentration.
Theoretical Oxygen Demand (ThOD) - The oxygen required to oxidize all organics or reduced inorganic compounds to CO2, SO4, NO3, etc. In practice, the TOD and COD measure by analytical procedures will approach the calculated ThOD. ThOD is normally expressed as ppm or mg/L.
Total Organic Carbon (TOC) - The quantity of organically bound carbon in a sample. TOC is commonly used as a replacement for BOD since the test for TOC is significantly faster than the 5-day test for BOD, and the BOD test can sometimes give erroneous results. TOC is normally expressed as ppm or mg/L.
Total Oxygen Demand (TOD) - The amount of oxygen required to oxidize all oxidizable substances in a sample, including the biodegradable organic matter. It is measured using a special analytical instrument. TOD is normally expressed as ppm or mg/L.
Total Kjeldahl Nitrogen (TKN) - A measure of total organic and ammonia nitrogen (does not include nitrite, nitrate).
Volatile Suspended Solids (VSS) - Volatile suspended solids is the portion of the TSS (or MLSS) which volatilizes at 1022°F (550°C). Biological solids (microorganisms) are the main contributor to VSS. VSS is normally expressed in units of ppm or mg/L.
Waste Bio-Sludge - The portion of excess biomass that is intentionally discharged from the activated sludge system. Normally bio-sludge is intentionally wasted by purging a portion of the return activated sludge from the clarifier underflow or recycle line.
4.0 BACKGROUND AND SELECTION CRITERIA 4.1 BACKGROUND
Biological treatment systems are extensively used for the treatment of commercial, industrial, laboratory and municipal wastewaters. The main reason they are preferred by water pollution control professionals in government, academia and industry is that they generally are the most cost effective end-of-pipe systems to remove simple and complex organic and some inorganic contaminants. They are especially effective in removing several types of contaminants simultaneously, resulting in an effluent that can meet several types of water quality criteria, including oxygen demand, low concentrations of toxic compounds and low levels of aquatic toxicity. However, biological treatment may not be the most suitable technology for low volume, limited compound wastewater.
Biological treatment works by employing naturally occurring, living and reproducing microorganisms (bacteria, protozoa, fungi and algae) that use undesirable contaminants in wastewater as food, energy sources, and nutrients. The main difference between industrial wastewater biological systems and sanitary / sewage systems is that pathogenic or disease forming microorganisms are usually not present in significant numbers in industrial systems and the microbe populations are quite different. Hence, this is the reason why sanitary wastewater from human sources at the facility are usually treated separately from industrial wastewater and there is generally a much lower exposure risk to industrial treatment works operators by organisms that can affect human health. Biological systems, whether in the form of controlled bioreactors (activated sludge type) or simple, naturally aerated lagoon / ponds, require two key capabilities in design, the biological reaction zone and biomass / suspended solids settling zone. In many cases, settling of the biomass is the key and most difficult part of the process, since it is needed to meet effluent quality requirements for suspended solids and oxygen demand and different microbe populations exhibit different settling characteristics.
4.0 BACKGROUND AND SELECTION CRITERIA (Cont) 4.2 SELECTION CRITERIA
A description of the different types of biological treatment systems is provided in each of the areas of this practice that covers the technology. The first step in selecting a system is to evaluate the oxygen demand or dissolved organic reduction requirements for the particular application. Figure 4.2-1 provides a decision tree to assess general requirements. The next step is to see how the biological system would fit into the overall treatment plant wastewater system. Figure 4.2-2 provides an example; more details on wastewater treatment process selection can be found in Section XIX-A of the Design Practices. The next step is to evaluate the effluent requirements of the particular application and integrate the expected performance of the biotreatment with the rest of the treatment system. For the petroleum refinery and petrochemical plant applications, aerobic lagoon or activated sludge systems are the most prevalent systems in ExxonMobil plants. New types of bioreactor systems are being suggested and applied for selected applications in municipal and industrial wastewater treatment. Due to a general lack of data on the performance of these systems on petroleum refinery wastewaters, pilot testing would typically be in order for cases where the benefits of these systems over activated sludge are desired.
Anaerobic (without oxygen addition) biological treatment is known to have been applied to only one ExxonMobil location on a large scale, for a wastewater from a crude oil production treatment facility. This type of system was applied because of the concentrated nature of the produced water; a rule-of-thumb concentration for consideration of anaerobic treatment is over 3,000 ppm of oxygen demand. In this particular location, conventional aerobic activated sludge treatment followed the anaerobic treatment unit to ensure effluent quality requirements were met.
Fixed media or film bioreactors are being applied cost-effectively in selected applications for petroleum based wastewater. These are currently being evaluated for use on hydrocarbon contaminated groundwater and other low organic contaminated wastewater from the petroleum industry.
5.0 SUMMARY OF BIOLOGICAL TREATMENT SYSTEMS
The following provides a summary of the various types of Biological Treatment Systems and the sections within this DP covering each type:
• Aerobic Suspended Growth Systems
a. Activated Sludge (Section 6.0) b. Sequency Batch Reaction (Section 7.0) c. Aerated Lagoons (Section 8.0) • Aerobic Attached Growth (Section 9.0)
• Anaerobic Systems (Section 10.0)
• Anoxic Systems (Section 11.0)
6.0 ACTIVATED SLUDGE SYSTEMS 6.1 DESCRIPTION
Figure 6.1-1 presents a general schematic of the typical activated sludge process. Pretreated wastewater containing soluble organic and inorganic compounds is introduced into a reactor where an aerobic culture of bacteria and other microorganisms is maintained in suspension. The aerobic environment is maintained by the use of diffused or mechanical aeration, which also serves to keep the contents well mixed. The microorganisms utilize the dissolved substances to obtain energy and, in the presence of oxygen and nutrients, convert them to carbon dioxide, water, and more microorganisms. After a specified period of time, the mixture of microorganisms flow into a clarifier where the microorganisms are separated from the treated wastewater. The majority of the microbial cells (biomass) is returned to the reactor to maintain the desired microbial concentration, while the remainder is purged from the system. (Reference 1, 2)
Typically, most activated sludge systems are designed and operated as a continuous flow process. Facilities with small flows may use a sequencing batch reactor (fill-and-draw) approach. Reactors are often designed as completely mixed tanks, although plug flow designs are also common. While there are many variations in process design and configuration, fundamentally they are all similar as described above. (Reference 1, 5)
Process Microbiology
To design and operate an activated sludge system efficiently, it is necessary to understand the importance of the microorganisms in the system. The most important and abundant group of microorganisms is the bacteria. They are the
6.0 ACTIVATED SLUDGE SYSTEMS (Cont)
an important means of organic waste removal. However, they do predate on bacteria and remove excess, non-flocculated bacteria from the wastewater, helping to clarify the effluent. A third category are nuisance organisms, those that when present in sufficient numbers interfere with the proper operation of the process. Most problems arise with respect to sludge settling and are the result of filamentous bacteria and fungi. These organisms can reduce the specific gravity of the flocs so that the sludge is very difficult to separate by gravity clarification. Consequently, an effective design is one that allows rapid decomposition of the waste, fosters good microbial flocculation, and selects against nuisance organisms. (Reference 3)
There are two primary biochemical reactions that occur in biological wastewater treatment, and both involve the utilization of energy. A portion of the soluble organic matter in the wastewater is used by the organisms as food to obtain energy and the remainder of the organic matter is used for the synthesis into new microbe cells. In simplest form, synthesis can be expressed as: (Reference 1, 3, 5)
Organics + O2 + Nutrients → More (New) Microorganisms +
CO2 + H2O + Energy + Non-degradable Soluble Residue Eq. (6.1-1) The second reaction involves the energy required for cell maintenance. This is the energy needed to keep cells functioning even in the absence of growth. As the amount of organic matter is decreased, less energy will be available for new growth. When the point is reached at which the rate of energy supply (food or organic matter) just balances the rate at which energy must be used for maintenance, no net growth will occur because all energy will be used to maintain the status quo. If the rate of energy supply is reduced still further, the difference between the supply and the maintenance requirement will be met by the degradation of energy sources available within the cell, i.e., by endogenous respiration (the microbes will use each other for a food source). This will cause a decline in the mass of the culture. Simply stated, this reaction can be expressed as: (Reference 1, 3, 5)
Microbe Cells (C5H7NO2) + 5 O2 → 5 CO2 + 2 H2O + NH3 +
Non-degradable Cellular Residue + Non-degradable Soluble Residue Eq. (6.1-2) Contaminants other than organic compounds can also be biologically removed in the activated sludge process. For example, in the presence of two specific types of bacteria, ammonia can be removed from the wastewater through conversion to nitrate. This process is called nitrification, and can be expressed as follows:
Nitrosomonas
+
4
NH + 1.5 O2 New Cells + NO + H2− 2O + 2 H+ + Energy Eq. (6.1-3)
Nitrobacter
−
2
NO + 0.5 O2 New Cells + NO + Energy3− Eq. (6.1-4)
Carrying this one step further, under anoxic (without free oxygen) conditions, nitrate can be biologically removed from the wastewater by conversion to nitrogen gas in a process called denitrification:
Microorganisms
Organics + NO−3 New Cells + N2 + CO2 + H2O + OH– + Energy Eq. (6.1-5)
The microorganisms responsible for carrying out denitrification are similar to the ones responsible for the removal of organics as described in Eq. 6.1-1. However, the principal biochemical pathway for denitrification involves using the oxygen bound up in the nitrate, and not free oxygen, as the terminal electron acceptor in the oxidation of the organic compounds.
Reaction Kinetics and Fundamental Expressions
Biological oxidation of organic matter is a process governed by a multitude of reactions catalyzed by microbially produced enzymes. The rates of substrate removal and cell growth depend on the composition and concentration of both the organic material and the metabolizing microbial population, and on the temperature. While the heterogeneous nature of industrial wastewater and the activated sludge microbial community make modeling these kinetics somewhat difficult, some proven approaches have been developed.
The activated sludge process has successfully been modeled under the assumption that the organic substrate, S, is limiting to growth. S is usually expressed in terms of BOD5, but COD, TOD and sometimes TOC can also be used. Experimentally, it has been found that the effect of a limiting substrate or nutrient can often be adequately defined using the following expression proposed by Monod. (Reference 3, 4, 5)
6.0 ACTIVATED SLUDGE SYSTEMS (Cont) ) S K ( ) S ( ) u ( u s m + = Eq. (6.1-6)
where: u = Specific microbial growth rate, (1/d)
um = Maximum specific microbial growth rate, (1/d)
S = Concentration of growth limiting substrate in solution (e.g., BOD or TOC), mg/L Ks = Substrate concentration at one-half the maximum growth rate, mg/L
The effect of substrate concentration on the specific growth rate is shown in Figure 6.1-2 (Reference 1, 3) Substrate (BOD) Removal
The rate of substrate removal is controlled by the growth rate of the microorganisms and is related to this growth by the microorganism yield coefficient. The relationship between the rate of substrate removal and the microbial growth rate can be expressed as follows:
Y u
q = Eq. (6.1-7)
where: q = Specific rate of substrate removal, (1/d) u = Specific microbial growth rate, (1/d)
Y = Microbial yield coefficient, mass microorganisms / mass substrate removed
The microbial yield coefficient, Y, is a function of the bio-sludge residence time (SRT) and can be described by the following equation: ) SRT ( ) b ( 1 Y Y m + = Eq. (6.1-8)
where: Y = Microbial yield coefficient, mass microorganisms / mass substrate removed Ym = Maximum yield coefficient, mass microorganisms / mass substrate removed,
measured when the microorganisms are in the logarithmic growth phase b = Endogenous decay coefficient, (1 / day)
SRT = Sludge retention time, d
For a continuous stirred tank reactor (CSTR) or completely mixed bioreactor at steady state, the solids leaving the system will be equal to the solids produced. Therefore, the growth rate and solids retention time of the microorganisms in the system are related by: b u SRT 1 − = Eq. (6.1-9)
where: SRT = Sludge retention time, d
u = Specific microbial growth rate, (1/d) b = Endogenous decay coefficient, (1/d) Combining Eqs. 6.1-7 and 6.1-9:
Y b SRT 1 q + = Eq. (6.1-10)
The specific rate of substrate removal can also be described, based on a mass balance on the rate of substrate removal:
) HRT ( ) X ( ) S S ( q v o − = Eq. (6.1-11)
where: q = Specific rate of substrate removal, (1/d) So = Feed substrate concentration, mg/L
6.0 ACTIVATED SLUDGE SYSTEMS (Cont)
Combining Eqs. 6.1-10 and 6.1-11 gives an expression for the concentration of active heterotrophic biomass in the reactor:
÷ ø ö ç è æ + − = b SRT 1 HRT Y ) S S ( Xv o Eq. (6.1-12)
Note: In Eq. 6.1-12, the term “b” is usually assumed to be zero. This is because a small change in this somewhat unreliable coefficient can result in significant changes to the calculated number. Experience has shown that dropping this term from this equation provides a good approximation of the actual MLVSS.
Lastly, the excess biological sludge production can be estimated by the following expression: (Reference 5)
Px = Q Y(So - S)[8.34 {(lb/Mgal)(mg/L)}] (Customary) Eq. (6.1-13)
Px = Q Y(So - S)(10-3 kg/g) (Metric) Eq. (6.1-13)M
where: Px = Net excess sludge production, lb/d (kg/d)
Q = Wastewater flow rate, Mgal/d (m3/d)
Y = Microbial yield coefficient,
removed substrate mass isms microorgan mass
So = Feed substrate concentration, mg/L
S = Effluent substrate concentration, mg/L
It should be mentioned that various other expressions have been used to describe the rates of specific growth and substrate utilization for the activated sludge process. (Reference 1, 3, 4, 5) What is fundamental in the use of any rate expression is its application in a mass-balance analysis. It does not matter if the rate expression selected has no relationship to those used commonly in the literature, so long as it adequately describes the observed phenomenon. (Reference 5)
In addition, where the removal of a specific organic constituent or nutrient (e.g., ammonia) is required, the rates of removal of such substances need to be considered. For example, where ammonia discharge is limited, the kinetics and expressions governing the rate of ammonia removal can be expected to govern the design of the activated sludge process. The model and equations described above for heterotrophic organic removal are also applicable to a system designed for ammonia removal, however, the kinetic coefficients may be different between heterotrophs and nitrifiers. (Reference 11)
Temperature Effect / Importance
The effects of temperature must also be considered in the design of biological treatment systems. A general rule of thumb is that the removal rate of organic matter is doubled every 18°F (10°C) temperature rise in the 50 to 105°F range (10 to 40°C). Temperature not only influences the metabolic activities of the microbial population but also has a profound effect on such factors as gas-transfer rates and the settling characteristics of the biological solids. The effect of temperature on the activated sludge process is usually expressed as follows: (Reference 5)
RT = (R20)[(1.08)(T–20)] Eq. (6.1-14)
where: RT = Reaction rate at T °C, also qT (specific rate of substrate removal at temp = T °C) (R20) = Reaction rate at 20°C, also q20 (specific rate of substrate removal at 20°C)
1.08 = Temperature activity coefficient for activated sludge processes T = Temperature, °C
Oxygen Requirements
The theoretical oxygen requirements can be determined from the BOD5 of the wastewater and the amount of biomass wasted
from the system per day. If all the BOD5 were converted to end products, the total oxygen demand would be computed by
converting BOD5 to the ultimate carbonaceous BODULT using an appropriate conversion factor. However, during the organic
removal, process oxygen is utilized in the process of providing energy for both the synthesis of new cell material (growth) and for basic cell maintenance (respiration). Since at steady state the new cells are wasted from the system, the BODULT of the
wasted cells must be subtracted from the total. The remaining amount represents the amount of oxygen that must be supplied to the system. From Eq. 6.1-2, it is shown that the BODULT of one mole of cells (approximate molecular weight = 113) is equal to 1.42 times the concentration of cells. Oxygen is also required for other oxygen demanding compounds not captured in the BOD test, such as nitrogen compounds.
6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Without Pilot Data
Therefore, the theoretical oxygen requirements for the removal of the carbonaceous organic matter in the wastewater can be computed using Eq. 6.1-15(Reference 5). Some contingency to the oxygen demand calculation is usually applied to ensure sufficient oxygen is available. If the design flowrate and influent substrate concentration is used in Eq. 6.1-15, a 25 percent contingency is recommended. If the average flowrate and influent substrate is used in Eq. 6.1-15, a 50 percent contingency is recommended. ) (P 1.42 f (8.34) S) (S Q /d O lb 2 = o − − x (Customary) Eq. (6.1-15) ) (P 1.42 f kg/g) (10 S) (S Q /d O kg x -3 o 2 − − = (Metric) Eq. (6.1-15)M
where: f = Conversion factor for converting BOD5 to BODULT (0.45 - 0.70) (this factor is wastewater dependent)
Q = Design wastewater flowrate, Mgal/d (m3/d)
So = Influent substrate concentration BOD5, mg/L
S = Effluent substrate concentration BOD5, mg/L
8.34 = Conversion factor [lb/Mgal-(mg/L)]
Px = Net excess sludge production, lb/d (kg/d) {calculated from Eq. 6.1-13.}
Y is calculated from Eq. 6.1-8, using Table 6.1-1 to for values of Ym and b.
When nitrification has to be considered, the total oxygen requirements can be computed as the oxygen for removal of carbonaceous matter plus the oxygen required for the conversion of ammonia to nitrate as follows: (Reference 5)
) 34 . 8 ( ) N N ( Q 57 . 4 ) P ( 42 . 1 f ) 34 . 8 ( ) S S ( Q d / O lb 2 = o − − x + io − i Eq. (6.1-16) 1 3 i io x 1 3 o 2 1.42 (P ) 4.57Q(N N) (10 g/kg) f ) kg / g 10 ( ) S S ( Q d / O kg − − − + − − = Eq. (6.1-16)M where: Nio = Influent TKN, mg/L Ni = Effluent TKN, mg/L
4.57 = Conversion factor for amount of oxygen required for complete oxidation of TKN f = Conversion factor for converting BOD5 to BODULT (0.45 - 0.70) (this factor is
wastewater dependent)
Q = Design wastewater flowrate, Mgal/d (m3/d)
So = Influent substrate concentration BOD5, mg/L S = Effluent substrate concentration BOD5, mg/L
8.34 = Conversion factor [lb/Mgal-(mg/L)]
Px = Net excess sludge production, lb/d (kg/d) - calculated from Eq. 6.1-13 using Y
which is calculated from Eq. 6.1-8 using values of Ym and b from Table 6.1-1.
With Pilot Data
There are two approaches for estimating oxygen demand if pilot plant data are available. The approach used depends on the amount of the data available. For the first method, the oxygen requirements can be calculated using Eq. 6.1-17 if this data is available. Contingency depends on the characteristics of the flowrates and influent substrate concentration plugged into the equation. Add 25 percent contingency when maximum values are used and 50 percent contingency when averages are used. When a large amount of data is collected either with a pilot unit or an existing full scale unit that covers both peak flows and substrate influent concentrations, the recommended approach to calculating oxygen demand is by calculating the daily oxygen demand by Eq. 6.1-15 or 6.1-16 (pairing flow with contaminant level, organics, nitrogen, sulfite, etc.) and using a probability plot. The 95th to 99th percentile of the oxygen demand is normally chosen for design specifications, but the ultimate decision is left up to the designer.
6.0 ACTIVATED SLUDGE SYSTEMS (Cont) V V ) X ( b ) S S ( Q a Rr = ′ o − + ′ v Eq. (6.1-17)
where: Rr = Oxygen utilization rate, (mg/L-d)
V = Aeration tank volume, Mgal (m3)
a′ = Oxygen utilization coefficient for synthesis, lb (kg) O2/lb (kg) organics removed
b′ = Oxygen utilization coefficient for endogenous respiration, lb (kg) O2/lb (kg) VSS-d
Q, So, S and Xv as described previously
The total oxygen requirement term Rr is usually determined from the following equation:
Rr = KLa (Cs - C) Eq. (6.1-18)
where: KLa = Overall oxygen transfer coefficient, (1/d)
Cs = Saturation oxygen concentration, mg/L
C = Actual oxygen concentration, mg/L Process Variations
The activated sludge process is very flexible and can be adapted to address many types of biological wastewater treatment problems. Diagrams of BOD removal and bio-sludge growth relationships for the process are given in Figure 6.1-3. (Reference 5) The most common configurations are the completely mixed single stage and conventional plug flow designs, and most of the process variations are modifications of these two basic approaches. Each variation tends to offer its own unique advantages. A few of the more common variations of the activated sludge process are discussed below.
Completely Mixed Activated Sludge (Figure 6.1-1) - By definition, the contents of a completely mixed reactor are thoroughly uniform. Pretreated influent wastewater is rapidly distributed throughout the basin and the operating characteristics of the system (e.g., MLSS, substrate concentration, oxygen uptake rate, etc.) are relatively constant. As the concentration of substrate in the reactor is the same as in the effluent, only a very low level of food is generally available at any time to the large mass of microorganisms. This characteristic is cited as the major reason why, when compared to plug flow systems, completely mixed systems are better capable of handling surges in organic loading and toxic shocks without adversely affecting effluent quality. This increase in process stability accounts for the reason why so many activated sludge systems have been designed as completely mixed reactors. (Reference 2)
Extended Aeration / Nitrifying Activated Sludge - Extended aeration systems are usually complete-mix designs where the microorganisms are kept in the endogenous respiration phase of the growth curve. The attributes of this system are a long aeration time (> 18 hr), a long sludge retention time (> 20 d), and a low food-to-microorganism ratio (< 0.1 (1/d)). This requires the design have a large reactor / hydraulic residence time, and sufficient clarification capacity to handle a relatively high MLSS (3500 + mg/L) While extended aeration requires more air than other activated sludge processes, it results in very low production of waste bio-sludge. These process conditions are also required if ammonia removal (nitrification) is desired. This is because the two microbial species responsible for nitrification have relatively low specific growth rates. Other advantages are a very low effluent BOD (90 to 95% reduction), and resiliency to upsets due to the buffering effect of the large biomass volume. For most cases, this is the preferred fundamental process configuration.
Extended aeration processes can be sensitive to sudden increases in flow due to resultant high MLSS loading on the final clarifier. Other potential problems with this system, especially at very high SRTs / low F/Ms, are the selection of filamentous bacteria, and the occurrence of pin-floc (dead cell bodies) in the final effluent. (Reference 2, 3, 5, 7)
Conventional Plug Flow Activated Sludge (Figure 6.1-4) - The conventional plug flow configuration has a high organic loading at the influent end of the basin. Loading is reduced over the length of the basin as the organic material in the wastewater is assimilated. Rapid microbial growth and substrate utilization occurs at the influent end of the reactor, while at the downstream end oxygen consumption primarily results from endogenous respiration. Air application is generally uniform throughout the tank.
The advantage of a plug flow design is that the high organic loading at the inlet of the process selects against filamentous bacteria growth and often improves sludge settling beyond that realized from a complete-mix reactor. Another advantage is the configuration is flexible and as discussed below, lends itself to modification. (Reference 2)
6.0 ACTIVATED SLUDGE SYSTEMS (Cont)
Step-Feed Aeration Activated Sludge (Figure 6.1-5) - In a step-feed aeration configuration, influent wastewater is split and introduced into the aeration basin at different points. This provides a more even distribution of oxygen demand, a more equal distribution of the F/M ratio, and helps lessen the effects of shocks. This configuration is restricted to plug-flow reactors, or to multiple completely mixed reactors in series. An attractive attribute of this variation is its flexibility. The ability to control the feed addition points (and often the distribution of aeration in the basin) allows various portions of the reactor to be adjusted to control nuisance organisms, or to provide for denitrification. (Reference 2, 3, 5)
High Purity Oxygen Activated Sludge (Figure 6.1-6) - In this variation, high purity oxygen (HPO) is used instead of air. Oxygen is diffused into a series of covered completely-mixed aeration tanks and is recirculated, while a portion of the gas is wasted to reduce the concentration of carbon dioxide. Data and reports on the benefits of oxygen systems have been under review over the years. (Reference 2) The most significant benefit appears to be conferred for high strength wastewater where normal diffused air aeration is insufficient to transfer the required oxygen. The amount of oxygen provided by a high purity oxygen system is about four times greater than the amount that can be added by conventional aeration systems. (Reference 5) Also, because of these high transfer rates, the necessary reactor size can usually be significantly reduced. In general, these savings in volume must be sufficient to offset the additional oxygen cost associated with this system. Additionally, the well-aerated mixed liquor selects against nuisance organisms thereby tending to produce a compact sludge. Solids levels usually range from 4000 to 9000 mg/L depending on the BOD of the wastewater. ExxonMobil has one of these systems at a chemical plant.
This system, however, has some issues that must be considered in design. As a result of the buildup of carbon dioxide, pH control tends to be more difficult than in air systems. There is also a need for careful selection of materials used for construction (due to the corrosive atmosphere), and the potential exists for presence of combustible conditions within the reactor vapor space. Additional sensors and control loops on the off-gas are standard design components in HPO systems. (Reference 2, 3, 5, 7)
Completely Mixed Activated Sludge with a Selector (Figure 6.1-7) - If the control of nuisance organism is expected to be of significant concern, then consideration should be given to installing an upstream contact or selector tank ahead of a completely mixed aeration basin. In this manner the environmental conditions of the tank (most importantly the dissolved oxygen content) can be controlled to select against filamentous microorganisms. Readily degradable wastewater such as food processing wastes will tend toward filamentous bulking in a completely-mixed system. Complex refinery / petrochemical wastewaters do not typically support filamentous growth and completely-mixed systems without selectors work very effectively. (Reference 2, 7) A disadvantage of using a selector reactor configuration is the potential for odors to occur.
Total Biological Nitrogen Removal Activated Sludge (Figure 6.1-8) - Total biological nitrogen removal involves three distinct processes: (1) the conversion of organic nitrogen compounds to ammonia, (2) the nitrification of ammonia to nitrate, and (3) the denitrification of nitrate to nitrogen gas. The first two steps are aerobic processes, the third occurs in the absence of free oxygen (without added aeration).
In the first configuration, a small denitrification reactor is installed upstream of the extended aeration / nitrifying activated sludge tank. The reactor is not aerated, and the nitrified effluent from the activated sludge tank is recycled back into this reactor to provide the nitrate source. As denitrification requires organic carbon as an energy source for the microorganisms, this process actually reduces both the organic and oxygen demand loads on the downstream activated sludge. This has the benefit of reducing energy costs and potentially providing a more stable activated sludge operation.
A second common configuration involves installing the anoxic denitrification reactor downstream of the activated sludge tank. This configuration negates the need for effluent recycle back to the front end. However, it has the disadvantage of requiring the addition of a carbon source (methanol is often used).
Clarification
The clarification of activated sludge is discussed in detail in Design Practice Section XIX-A7, Clarification Systems for Biological Treatment, and therefore will only be briefly mentioned here.
A very important aspect of the activated sludge process is the ability to separate and return the biomass to the main reactor. Efficient separation is important for meeting effluent requirements for TSS, BOD, and COD, and for maintaining an adequate concentration of biomass in the bioreactor part of the entire BIOX system. Clarification also acts to reduce the water in the sludge, as the first part of the biomass sludge dewatering system to reduce the volume of the waste sludge. As discussed under Process Microbiology, the type and abundance of the microorganisms and the point on the microbial growth curve at which the system is operating greatly influence the sludge settling characteristics.
As the flocculated sludge of a properly operating activated sludge process will have a specific gravity greater than 1.0, most clarifiers are designed as gravity settlers. For difficult to settle sludges (such as during an upset) gravity settling can be
6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 6.2 DESIGN CONSIDERATIONS
In the design of the activated sludge process, considerations must be given to: (1) compliance with regulatory effluent requirements, (2) feed characteristics and pretreatment requirements, (3) selection of the reactor and clarifier type, (4) oxygen requirements and aeration equipment, and (5) need for pilot plant data. Each of these considerations are discussed below. Regulatory Effluent Requirements
The determination of performance objectives is fundamental to selecting an appropriate design basis. Most importantly, the selected design must be capable of complying with anticipated discharge standards in a very reliable way. Hence, consideration must be given to the design of systems that can meet the effluent targets under maximum expected load conditions and perhaps 99+ percent of the time. Consideration should be given to both existing / near term requirements, as well as potential future requirements. If more restrictive requirements are anticipated in the future, or if new processing units are expected to come on-line, consideration should be given to designing the plant to meet these requirements and/or the ease of modification or retrofit.
Consideration should also be given to the existing or future need to reduce water usage and/or wastewater discharges. The need to produce an effluent capable of being reused within the process operation can affect the selection of wastewater to be fed to the BIOX reactor as well as the treatment required, and hence the design.
In addition to a liquid discharge, wastewater treatment via activated sludge inevitably produces both air and solid waste emissions. If air emissions are a current or future concern, consideration should be given to upstream reduction of volatile contaminants, selection of aeration equipment with lower stripping potential, and possibly covered basins. The ease and cost of disposal of the waste byproduct bio-sludge must also be considered. Restrictive regulations and high waste disposal costs may justify investment in a design which minimizes the production of waste sludge.
The design may also be influenced by a need or desire to protect groundwater quality. If leakage to the groundwater is prohibited or not desired, consideration should be given to above ground tankage for the activated sludge reactor. If lined impoundments can be used in lieu of tankage, consideration must be given to selection of the aeration equipment to ensure the integrity of the liner is maintained.
Feed Characteristics: Equalization and Pretreatment Requirements
The activated sludge process is capable of treating a wide variety of wastewaters. However, in order to ensure an effective and efficient operation, the characteristics of the feed must be understood and factored into the design.
Prior to treatment in an activated sludge system, most wastewaters require some form of pretreatment. One of the most important pretreatment considerations is equalization. Generally speaking, an activated sludge process performs best when fluctuations in the feed to the reactor are dampened and smoothed. It has been found that the microorganisms are quite capable of acclimating to a rather wide range of wastewater characteristics when given time to adapt. This holds true for conditions such as organic loading, temperature, pH, salt content, etc. However, rapid changes in feed characteristics and reactor conditions, even if within the tolerance range of the microorganisms, should be avoided as they can destabilize the system. A minimum of 8 hours equalization at maximum loading is recommended.
Table 6.2-1 gives criteria on the equalization and pretreatment requirements of activated sludge feed. A few of the more important feed characteristics are discussed below.
1. Oil Content
Free or emulsified oil in the feed can cause several problems. While dissolved oils can be biodegraded by the activated sludge microorganisms, the breakdown of larger oil molecules is limited by their low solubility and relatively small available surface area. As a result, these oils tend to build up in the system and impart a buoyancy to the floc, causing the sludge to settle poorly or to float. The oil can also coat the microbes, hindering diffusion of oxygen.
If the oil content of the influent wastewater is greater than 50 mg/L on the average or 100 mg/L at the maximum, upstream pretreatment to remove free and emulsified oils is likely required. Oil content can be measured using the Standard Methods Analysis on a filtered sample. Pretreatment can be induced air / gas flotation, dissolved air flotation, chemical flocculation, media filtration, or similar suspended oil removal technology. For selection of appropriate pretreatment technologies, see DP Section XIX-A, Guidelines for Selecting Wastewater Treatment Systems.
6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 2. Ammonia, Phosphorus, and Micronutrients
Nitrogen is an important microbial nutrient and must be present in the system. At a minimum, five parts of N are usually required for every 100 parts of BOD5. Refinery wastewater generally contains excess nitrogen, and pretreatment of
selected streams (usually steam stripping of sour waters) is often used to minimize the ammonia content. Traditionally, pretreatment has been used as a means of controlling gross discharges of ammonia into and from conventional non-nitrifying activated sludge systems. Normally, maximum ammonia levels of about 200 mg/L in the activated sludge influent can be tolerated without upsetting the process of organic carbon removal. Free (not dissociated) ammonia will have an effect on the organic carbon removal and therefore the maximum tolerable level of ammonia really depends on the pH of the wastewater.
As many regulatory agencies now severely restrict the amount of effluent ammonia and/or total nitrogen, many activated sludge systems must now be designed for ammonia removal (nitrification) and possibly total nitrogen removal (denitrification). In this regard, pretreatment may be necessary to keep ammonia levels low enough to prevent toxicity to the nitrifying bacteria. For example, non-ionized or free ammonia (NH3) may begin to inhibit nitrifying bacteria at a total ammonia concentration (at pH 7.0 and 20°C) of 20 to 50 mg/L in the aeration basin. With proper acclimation and buffering, however, much higher concentrations of ammonia can often be treated in a nitrifying system without microbial inhibition. (Reference 11)
Pretreatment is also advantageous to reduce the volume of the nitrifying activated sludge reactor. While high concentrations of ammonia (200+ mg/L) can be converted to nitrate in suspended growth nitrifying systems, this usually requires hydraulic residence times of two or more days necessitating large reactors. (Reference 11, 12). The additional reactor costs need to be compared to the cost for the pretreatment.
Phosphorus is another important nutrient which must be present in the wastewater feed. Although phosphates may enter refinery and petrochemical plant wastewater from boiler and cooling tower blowdown and from spent phosphoric acid catalyst from polymerization units, most refining wastewaters are deficient in phosphorus and additions are required. One part P is usually required for every 100 parts BOD5, but enough should be added to provide a residual phosphate
concentration of 0.1 to 0.3 mg/L in the effluent. Phosphoric acid is generally used for this addition. Adequate pumping capacity and control are a part of the design process.
Micronutrients are also required for a healthy and efficient biological population. These are typically available in sufficient quantities in refinery or chemical plant wastewater. Micronutrient requirements are listed below (Reference 20):
MICRONUTRIENT AMOUNT, mg/mg BOD
Ca 62E-04 Co 13E-05 Cu 15E-05 Fe 12E-03 K 45E-04 Mg 30E-04 Mn 10E-05 Mo 43E-05 Na 50E-06 Se 14E-10 Zn 16E-05 3. Hydrogen Sulfide
Streams containing hydrogen sulfide should be pretreated (such as the steam stripping of sour waters) so that the concentration in the reactor feed is < 50 mg/L. Levels above this can be inhibitory to non-acclimated activated sludge microorganisms. In addition, hydrogen sulfide exerts an immediate oxygen demand which can dramatically lower the dissolved oxygen content in the reactor. Even if the dissolved content in the bulk liquid is in the 1 to 2 mg/L range, the conditions within the sludge floc may in fact be anaerobic which can select for nuisance filamentous organisms. Sources of hydrogen sulfide within the refinery include sour water condensates, spent sulfidic caustic, and desalter wash water and tank bottom waters in warm and salty water environments. Sources of H2S within chemical plants include steam cracking sulfidic treatment, merox units, and other sulfidic treating processes. (Reference 7)
