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Am they all they're cracked up to be?
Developed in Germany, egg-shaped digesters are reported to exhibit a number of advantages over conventional digesters, including minimum grit accumulation, reduced scum and foam formation, high mixing efficiency, low operating costs, and efficient land use. Now that a number of egg-shaped anaerobic digester facilities are oper- ating in North America, researchers decided to survey operators to determine whether these digesters measure up to their claimed operating advantages.
Researchers therefore prepared and distributed a survey questionnaire to 11 North American wastewater utilities that own and operate egg-shaped digesters:
* Appleton (Wis.) Wastewater Treatment Plant,
* Back River Wastewater Treatment Plant (Baltimore, Md.),
* Brawley (Calif.) Wastewater Treatment Plant
* Cumberland (Md.) Wastewater Treatment Plant,
* Deer Island Wastewater Treatment Plant (Boston, Mass.),
* Oceanside Water Pollution Control Plant (San Francisco, Calif.),
* Port Dalhousie (Ontario, Canada) Wastewater Treatment Plant,
* Port Dover (Ontario, Canada) Wastewater Treatment Plant,
* St. Charles 011.) Wastewater Treatment Plant,
* Terminal Island Wastewater Treatment Plant (Los Angeles, Calif.), and
* Theresa Street Wastewater Treatment Plant (Lincoln, Neb.).
They requested information on size and configuration, mixing and heating systems, loading rates and performance, operating characteristics, and costs. They also obtained design information for the Los Angeles Hyperion wastewater treatment plant digesters, which were under construction when the survey was issued. (Key operating parameters are summarized in the table on p. 30.)
The response rate was 100%. Survey data were compiled in a database and follow-up telephone calls were made, when appropriate, to complete the database as much as possible.
Cylinder or Egg?
For years, wastewater treatment plants have used anaerobic digestion to stabilize biological solids because the process can cut solids volume in half and significantly reduce or eliminate pathogens and odors, creating biosolids that can be landapplied as a beneficial soil amendment. Digesters operated with a mean cell residence time, or "sludge age," of at least 15 days at 35degC (95degF) typically will meet 40 CFR 503 requirements for Class B biosolids. The process also converts organic matter to usable methane gas.
Conventional digesters. In North America, anaerobic digesters traditionally have been designed as concrete cylindrical tanks with flat or mildly sloping bottoms and fixed or floating steel or concrete covers. The...