Maximizing Energy Recovery through Digester Gas Management System Enhancement

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ABSTRACT
Advanced anaerobic digestion practices and maximum energy recovery call for a digester gas system that reliably allows for complete biogas collection and utilization. This paper will describe the planning, design, and construction of a new digester gas management system at the East Bay Municipal Utility District’s (District) Main Wastewater Treatment Plant. The need for an upgraded system arose from the District’s digester modification program –including conversion from floating to fixed covers, the addition of digester mixing, modifications to the feed cycle, and addition of thermophilic temperature capability. Upgrades accommodated the District’s plan to be more energy self-sufficient and its Resource Recovery program, which dramatically increased the quantity and variability of digester gas production. The new system optimizes existing gas storage facilities and simplifies control. The design is complete, construction is underway, and operation will begin in fall 2006.

INTRODUCTION
The District’s eleven 2-million-gallon digesters vary in age from 30 to 55 years. The digesters were built in three phases – adding onto the gas management system each time. Three cogeneration engines were added to utilize the digester gas. The original system designs were based on lower digester feed concentrations, lower volatile solid fraction, and lower operating temperature. Digester gas production has increased with gradual variance in feed sludge characteristics. In January 2003, the increased rate and production of digester gas and high volatility of some wastes exceeded the capacity of the existing digester gas management system. This capacity issue, combined with the District’s initiatives to become capable of producing Class A Biosolids through thermophilic batch digestion, and to become more energy selfsufficient, resulted in the need for significant digester gas management system improvements. In response, the District proceeded to study and design required digester gas management system enhancements.

Design challenges included: unknown design criteria for future digester loading and gas production; future transition from continuous-feed mesophilic digestion to thermophilic batch digestion; two existing digester gas storage facilities located on opposite ends of the system – a membrane cover on one digester and floating piston gas holder; and phased construction of digester upgrades that would result in having both new, fixed covers and existing floating covers on the same system.

Design Goals and Objectives. The goal of the digester gas system enhancement is to fully utilize all of the gas in the District’s existing cogeneration facility. Achieving this goal will:

  • Increase on-site electrical generation and reduce power purchasing.
  • Eliminate uncontrolled release of digester gas to the environment and minimize frequency and quantity of digester gas wasted by flaring.
  • Better utilize gas storage facilities.
  • Simplify control.

BACKGROUND

The digester gas system collects biogas (primarily methane and carbon dioxide) produced in the anaerobic digestion process and manages the gas for combustion. The digester gas is used as fuel for the Power Generation Station cogeneration engines. Excess biogas is burned by the waste gas burners, and occasionally boilers are used as a backup heat source.

Digesters. The digester gas system improvements are part of the Digester Upgrades Project, which replaces four (of eleven) floating covers with fixed dome covers. The new fixed dome covers can operate at up to 18 inches water column (w.c.), but their pressure relief valves will be set at 10 inches w.c. until all of the floating covers are replaced. The fixed-cover digesters are equipped with mechanical draft tube mixers. The floating-cover digesters use gas bubble mixing. The new mixers will increase digestion efficiency.

Gas Storage. The MWWTP has two gas storage locations: a dual-membrane cover system (Dystor) and a floating piston gas holder (LPGH). The Dystor system was installed on Digester 12 after that floating cover failed. The Dystor cover holds approximately 160,000 cubic feet of digester gas with pressure regulated by air system and self-contained regulating valve. The LPGH is a constant-pressure system with about one-tenth the storage of the Dystor. The pressure in the LPGH can only be modified by adding (or removing) concrete ballast from the piston. The Dystor and LPGH are located on opposite ends of the digester gas system.

Piping. The piping of the digester gas system can be classified as part of the “header”, or as a “lateral”. The laterals connect the digester cover’s gas dome to the header. Each digester has its own lateral. The header is a common pipe, fed by all the laterals, that conveys the gas to the cogeneration system and the waste gas burners.

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