Water Online

JUL 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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on industry consensus and prepared and coauthored by industry experts representing utilities, consulting engineers, equipment manufacturers, and academia. It provides the wastewater practitioner with background information that is essential to understand the grit removal process, including removal from liquid streams, washing of grit slurry, and recovery of organics. It clearly identifies the roles and responsibilities of all parties interested in the characterization and sampling of grit, including WRRF staff, consulting engineers, testing companies, and equipment manufacturers. With full understanding of the roles and responsibilities of each party, future grit removal systems will be more efficient and cost-effective, and legal litigation could be avoided. The publication covers in detail all sampling techniques that have been used over the last decades. It provides a full description of each method, including safety requirements, testing preparation, sampling locations, sampling equipment, sampling procedures, and collection of samples. It also provides the practitioner with a thorough comparison between the different sampling techniques. In the characterization chapter, full description and discussion of both dry and wet sieve analysis are presented. The publication addresses all aspects of grit characterization, including pretreatment, determination of settling velocity using different techniques, and state- of-the-art techniques for characterization such as particle imaging. Out With The Old, In With The New Until recently, grit characterization was conducted by performing sieve analysis of dry samples of the grit (dry sieve analysis). A decade ago, wastewater practitioners didn't differentiate between dry and wet sieve analysis, and grit was characterized based on grit particle size (or mesh size), regardless of the settling velocity of the particle. It was assumed that grit particles are all round, have a specific gravity of 2.65, and that the settling velocity will depend on the size of the particle. Wilson (2007) and McNamara et al. (2009) challenged that assumption and argued that "actual" grit particles settle at much slower velocities than dry particles with a specific gravity of 2.65. The argument was that grit particles, while traveling in the collection system, entrain or get covered with a layer of fats and oil that cause the particles to be lighter and their specific gravity to be less than 2.65. However, when particles are placed in the oven as part of the dry sieve analysis procedures, the fat and oil layer is removed. Recent work at many treatment plants proved that the grit particles have a settling velocity slower than what had been predicted assuming a specific gravity of 2.65. The publication also includes equations for calculating the quantity of grit received and removed by the WRRFs for liquid and solids streams. It discusses the sand equivalent size of grit particles and conversion of actual size to equivalent size and its relationship to settling velocity. It also presents current challenges and knowledge gaps, future research, and requirements for grit removal. Guidelines for Grit Sampling and Characterization not only sets the rules for performing grit sampling and characterization, it also paves the road for achieving optimal grit removal through innovative technologies built on the accurate estimation of grit quantities and full understanding of its fate — making it a "must have" for every wastewater practitioner. n wateronline.com n Water Innovations 21 PRIMARYTREATMENT Hany Gerges, Ph.D., PE, has more than 26 years experience in wastewater treatment engineering. His experience has included evaluation, optimization, and design of wastewater treatment plants, including headworks and grit removal technologies. He has also served as HDR's technical advisor for many national and international wastewater authorities in the area of headworks and grit removal. Dr. Gerges is HDR's technical director of wastewater optimization services, based in the Walnut Creek, CA office. He was one of the primary authors of Guidelines for Grit Sampling and Characterization (WEF, 2016; www.wef.org). About The Author Input raw water pipeline travels to the solid contact clarifier tank.

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