Water Online

July 2015

Water Online the Magazine gives Water & 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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U s i n g E m p t y - B e d C o n t a c t T i m e ( E B C T ) T o Using Empty-Bed Contact Time (EBCT) To D e s i g n B i o l o g i c a l O d o r C o n t r o l T e c h n o l o g i e s Design Biological Odor Control Technologies Odor control systems have come a long way, but certain hurdles remain. Improving EBCT design criteria will help engineers and utilities reach the finish line. By Jim Joyce T here are many different treatment processes available to remove odor from foul air. Some processes force the air through a vessel containing a bed of activated carbon pellets where the odor compounds are adsorbed onto the carbon and clean air is released. Other processes force the air through a scrubber vessel, which acts like a "chemical shower" where the odor compounds in the air react with chemicals in the "shower" and are oxidized and removed before the clean air is released. Still other processes force air through biologically- active compost or a vessel containing plastic biological media where the odor compounds are captured and biologically oxidized for food, releasing clean air to the environment. These are only three common technologies for odor control, but virtually all other odor treatment processes also require a vessel containing a bed of media or a reaction chamber where the removal process takes place. There are also design criteria used by engineers to design these different odor control processes. These criteria establish the minimum and maximum dimensions of the vessels, volumes of media, airflow rates, odor concentrations, and many other design conditions recommended by manufacturers or shown to be successful in past applications. While each different technology has its own specific set of design requirements, there are common design criteria used by engineers to select and size odor control treatment processes. In most cases these criteria are founded upon scientific and engineering principles, historical performance, and years of routine testing that can be directly measured. Each major odor control technology has its own set of common design criteria, ranging from the oxidation-reduction potential of chemical scrubbers to the inlet humidity of activated carbon scrubbers. Engineers use these common design criteria to select, size, and design odor control treatment processes. One of the most common design criteria is called "empty-bed contact time" or EBCT (also called "empty bed residence time" or EBRT). No single design criteria has been more used, and potentially misused, than EBCT. EBCT simply refers to the amount of time (usually in seconds) that air must spend in contact with the media or the contact chamber inside the process vessel. With no real basis in science or technology, this rule-of-thumb design criteria has persisted in odor control processes due to its simplicity. EBCT is calculated without regard for the volume of the physical media, hence the name "empty bed." The fact that the volume occupied by the media is not considered in the EBCT calculation makes EBCT a convenient but inaccurate design criteria. Different media with different densities and shapes will perform differently although the EBCT is identical. But how much inaccuracy is built into the EBCT varies by process. Three different, common odor control technologies are evaluated for EBCT inaccuracy in the following discussion. Activated Carbon Scrubbers Figure 1 illustrates the basic components of an activated carbon odor control scrubber. The volume of carbon media which represents the "empty bed volume" is shown. Figure 1: Typical Activated Carbon Odor Control Vessel The required carbon media volume is determined based upon the balance of several factors including vessel diameter, 10 wateronline.com ■ Water Innovations

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