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By Hany Gerges, Ph.D., PE P erformance of grit removal units at water resource recovery facilities (WRRFs) (also known as wastewater treatment plants) has been evaluated by many wastewater practitioners and the subject of many debates in the wastewater industry for the last two decades. Lack of understanding of the nature of the grit and its settling velocity has led to unsatisfactory performance of many installations in recent years. In many situations, litigation was the only way to resolve a performance dispute between equipment manufacturer, the consulting engineer, and the end user. While the wastewater industry has generally made great progress in the area of treatment technologies, very slow progress has been realized in the area of grit removal. Recent advancements in design of biological treatment systems have led to very low effluent nutrient concentrations from WRRFs, yet the industry is still struggling to define best practices for designing effective grit removal systems. There are many current grit removal systems in the marketplace that can remove a wide range of grit particle sizes, but their removal efficiencies vary widely. Some can effectively remove the very fine particles, while others remove the heavier particles but are less effective at removing the fine ones. The decision of which particle size to remove should be site-specific and depends on many factors, among which are the size of the WRRF, the variability in the incoming flows, the downstream liquid processes and their ability to capture escaping grit, and the adverse effect of uncaptured grit on equipment and liquid and solids handling processes. Selecting grit removal systems that remove the very fine particles without consideration of these factors will lead to over- building of unnecessary units that may not be utilized most of the year. On the other hand, selecting grit removal systems that remove only the heavy particles could lead to an undersized system and unsatisfactory performance most of the year. Getting Grit Right The most effective grit removal system for a specific WRRF removes only the grit that would cause problems for the downstream processes at this facility all year round. The first step in selecting this system is for the wastewater practitioners to determine the nature and quantity of the grit the facility receives. In other words, practitioners need to determine grit particle distribution and estimate the amount of fine grit versus heavy grit in the incoming flows to select the appropriate grit removal system. To determine the amount, nature, and distribution of grit particles, industry-standard grit sampling and characterization techniques should be applied. Until recently, there was no industry-standard, peer-reviewed, or widely accepted reference for techniques used in sampling and characterization of grit. In 2014, a task force was formed by the Water Environment Federation (WEF) to study the topic and make recommendations to address the issue. The task force developed and published the first grit sampling and characterization manual, Guidelines for Grit Sampling and Characterization, 2016. The publication was based 20 wateronline.com n Water Innovations Best Practices For Grit Sampling And Characterization The science behind grit removal is an all-important but underserved aspect of wastewater treatment operations — until now. The most effective grit removal system for a specific WRRF removes only the grit that would cause problems for the downstream processes at this facility all year round. What goes in has great bearing on wastewater treatment equipment.



