Prepared by the Operation of Municipal Wastewater Treatment Plants Task Force of the Water Environment Federation

Transcription

1 Operation of Municipal Wastewater Treatment Plants WEF Manual of Practice No. 11 Chapter 19 Primary Treatment Sixth Edition Prepared by the Operation of Municipal Wastewater Treatment Plants Task Force of the Water Environment Federation Mike Nelson, Chair Richard Atoulikian G. Michael Coley John Donnellon John Meader Daniel Nolasco Holly Tryon Greg White Under the Direction of the MOP 11 Subcommittee of the Technical Practice Committee 2005 Water Environment Federation 601 Wythe Street Alexandria, VA USA 1

2 Water Environment Federation Improving Water Quality for 75 Years Founded in 1928, the Water Environment Federation (WEF) is a not-for-profit technical and educational organization with members from varied disciplines who work toward the WEF vision of preservation and enhancement of the global water environment. The WEF network includes water quality professionals from 79 Member Associations in over 30 countries. For information on membership, publications, and conferences, contact Water Environment Federation 601 Wythe Street Alexandria, VA USA (703) IMPORTANT NOTICE The material presented in this publication has been prepared in accordance with generally recognized engineering principles and practices and is for general information only. This information should not be used without first securing competent advice with respect to its suitability for any general or specific application. The contents of this publication are not intended to be a standard of the Water Environment Federation (WEF) and are not intended for use as a reference in purchase specifications, contracts, regulations, statutes, or any other legal document. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof 2

3 by WEF. WEF makes no representation or warranty of any kind, whether expressed or implied, concerning the accuracy, product, or process discussed in this publication and assumes no liability. Anyone using this information assumes all liability arising from such use, including but not limited to infringement of any patent or patents. Copyright 2005 by the Water Environment Federation All Rights Reserved. Water Environment Research, WEF, and WEFTEC are registered trademarks of the Water Environment Federation. 3

4 Manuals of Practice of the Water Environment Federation The WEF Technical Practice Committee (formerly the Committee on Sewage and Industrial Wastes Practice of the Federation of Sewage and Industrial Wastes Associations) was created by the Federation Board of Control on October 11, The primary function of the Committee is to originate and produce, through appropriate subcommittees, special publications dealing with technical aspects of the broad interests of the Federation. These publications are intended to provide background information through a review of technical practices and detailed procedures that research and experience have shown to be functional and practical. Water Environment Federation Technical Practice Committee Control Group B. G. Jones, Vice A. B. Pincince, Vice-Chair S. Biesterfeld G. T. Daigger R. Fernandez L. Ford S. S. Jeyanayagam Z. Li M. D. Moore M. D. Nelson S. Rangarajan J. D. Reece E. P. Rothstein A. T. Sandy J. Witherspoon 4

5 Contents Chapter 19 Primary Treatment Introduction Description of Process Settleable Solids Removal Removal of Floatables Variables Affecting Settleable Solids Removal Hydraulic Loading Parameters and Tank Configuration Wastewater Characteristics Freshness Particle Characteristics Temperature Industrial Contribution Variables Affecting Removal of Floatables Tank Configuration Wastewater Characteristics Removal Mechanisms and Conditions Interaction with Other Unit Operations Upstream Units Downstream Units Secondary Processes Solids Handling Systems Recycle Flows Description of Equipment Fine Screens Sedimentation Tanks Rectangular Tanks Circular Tanks 5

6 Square Tanks Pumping Appurtenances Chemical Addition Expected Performance Process Control Sludge Handling Sludge Thickness (Solids Concentration) Sludge Quantity Collection Sludge Pumping Pumping Appurtenances Floatable Solids Skimming Hydraulic Considerations Recycle Flow Management Odor Control Housekeeping Chemical Addition Process Control Data Suspended and Settleable Solids Total and Volatile Solids in Wastewater Total and Volatile Solids in Sludge Biochemical Oxygen Demand ph Fats, Oils, and Grease Troubleshooting Characteristics of Collected Sludge Design Shortcomings Operational Problems and Solutions Maintenance Planned Preventative Maintenance Program 6

7 Emergency Maintenance Spare Parts Safety Records and Reports Computer Systems Primary Sludge Fermentation Alternative Primary Treatment Methods Enhanced Primary Treatment Fine Screens Dissolved Air Floatation Primary Systems Combined Grit Removal/Primary Treatment Processes References 7

8 LIST OF TABLES 19.1 Characteristics of raw sludge from plain sedimentation tanks Common design shortcomings and ways to compensate Troubleshooting guide for primary sedimentation problems Troubleshooting guide for maintenance of primary sedimentation process Test results to be recorded for primary sedimentation. LIST OF FIGURES 19.1 Section of a single rectangular tank for primary sedimentation Typical fine screens: internally fed rotary wedge wire screen (left) and externally fed rotary wedge wire screen (right) Section of a circular sedimentation tank Rectangular primary sedimentation tank with wood or nonmetallic flights mounted on parallel chains Rectangular primary sedimentation tank with traveling bridge sludge collector Circular sedimentation tank with sludge-scraping mechanism Scraper arms move the settled solids to the center of the tank Circular sedimentation tank showing grease skimmer and effluent weirs Estimated removals of suspended solids and five-day BOD (BOD 5 ) in primary basins at various hydraulic loadings (without chemicals) An example of deep primary tanks (activated primary tanks) An example of a fermenter that can be designed as a static fermenter or a complete-mix fermenter An example of a separate fermentation type system that can also include both a complete-mix fermenter followed by a static-type sludge thickener. 8

9 Preface This sixth edition of this chapter was produced under the direction of Mike Nelson, Chair. The principal author of this chapter is Ken Schnaars. There have been several full-scale and current operational modifications and improvements to the primary treatment process since 1996 that have been included in this latest edition of the chapter. The latest upgraded primary treatment chapter includes new expanded discussions on primary sludge fermentation, enhanced primary treatment, dissolved air flotation primary systems, and combined grit removal/primary treatment processes. This revised chapter also provides a brief description about the ways that computer systems can assist with the operation of the primary treatment process. The primary treatment process appears to be a simple process to operate compared with other processes within a wastewater treatment plant; however, new advances to the primary treatment process are being made to improve removal efficiencies within the primary tanks and improve process reliability in downstream biological processes. For example, modifying the primary treatment process to a primary sludge fermentation system can increase the production of volatile fatty acids, which are known to play an important role in the success of the biological phosphorus removal process. Authors' and reviewers' efforts were supported by the following organizations: Montgomery Watson, Cleveland, Ohio New York State Department of Environmental Conservation, Albany, New York Nolasco & Associates, Buenos Aires, Argentina Philadelphia City Government, Philadelphia, Pennsylvania Weston & Sampson Engineers, Inc., Peabody, Massachusetts 9