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Dual Zone Mixing System Anaerobic Digester Facilites

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Egg-Shaped (ESD) anaerobic digesters have been utilized in the United States for over 30 years. During that time we have seen both large and small communities utilize the technology because of the life cycle benefits it presents over conventional anaerobic digester vessels. Enhancements to the original systems developed in Europe have been introduced over the years. However, two issues of major concern to some have been the mechanical mixers on large volume vessels and the inability to operate the vessel under varying liquid levels.

Many people within the industry prefer to eliminate the need for moving equipment within the anaerobic digester. As the ESD volume increases it becomes impractical to rely on liquid recirculation systems due to the physical size of the pumps, piping and valves.

The inability to operate the ESD under varying liquid levels becomes an even bigger issue when considering 2-stage digestion processes. Typically, the first stage reactor of a 2-stage process requires a very specific and narrow range of detention time to ensure acid production is contained within the first stage of the process. Typically the actual operating conditions of a 2-stage digestion process will be a fraction of the design conditions during the initial phase of operation. This is due in part to the 20-year design conditions used during the design phase of the project.

At the Customer's request, CB&I proposed and supplied a Dual Zone jet pump mixing system for the Sand Island ESD project in Honolulu, Hawaii. The Dual Zone jet pump mixing system replaced a mechanical draft tube mixer with a separate external recirculation system for heating the vessel. The Dual Zone jet pump mixing system incorporates external liquid recirculation with two internal draft tubes in a 2.4 million gallon ESD vessel. The 2.4 million gallon vessel represented the first ESD vessel of its size in the United States that did not utilize a mechanical draft tube mixer. Benefits to the Dual Zone jet pump mixing system include:

No moving parts within the digester

Combined heating and mixing system

Elimination of a mechanical mixer on top of the ESD vessel

Reduction in maintenance and operating costs

Flexible operation of the vessel operating liquid level

Start up of the first Dual Zone mixing system in the United States began in the spring of 2006. Shortly after, a second Dual Zone mixing system was furnished for the City of Lakeland, FL. This second system was started up in the fall of 2007. It is used as the mixing system for a 264,000 gallon Acid Reactor at the Glendale WWTP in Lakeland. The Acid Reactor is part of a 2-stage digestion process designed to provide Class A Biosolids. The Acid reactor is used in combination with Lakeland's existing digesters as part of the IDI 2-Pad digestion process. As part of the process parameters, the Acid reactor is required to maintain a specific detention time, even though the current sludge flow is about 2/3 of the design flow. The Dual Zone jet pump mixing system allows the plant operator to vary the liquid level in the reactor and maintain proper mixing.

This paper will review the principles of the Dual Zone mixing system, its application at both facilities and the process results associated with each operation. In each case the Dual Zone mixing system was utilized to address Owner concerns with moving parts within the digester, provide a cost effective operation and maintain mixing energy within the system even when the liquid level is varied within the vessel. Process results from each facility reflect a well maintained digestion process.

The utilization of the Dual Zone jet pump mixing system has provided another enhancement to the ESD Technology while addressing two of its major concerns within the industry.
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Keywords: Dual Zone Mixing; Egg-Shaped Digesters; Jet Pump Mixing; anaerobic digestion; varying liquid level

Document Type: Research Article

Publication date: 2009-01-01

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