Other Types or Modifications
A fixed film digester vessel is filled with an inert medium or packing that provides a very large surface area for microbial growth. The influent (wastewater) passes through the media and anaerobic microbes attach themselves to it creating a thin layer of anaerobic bacteria called biofilm - this film gives the digester its name, fixed film. These microbes then continue to grow by removing material from the wastewater as it flows by. In most digesters the microbes are floating in the liquid and a portion of these active growing workers are continuously discharged with the effluent. In a fixed film digester the bacteria remain attached to the plastic media when effluent is discharged. The “bacteria are already at work” when new influent is added (Wilkie, AgSTAR Digest, Winter 2003). Fixed film digesters have smaller reactor vessels, shorter retention times and must be loaded with a feedstock that will readily flow through the media without clogging. Three to five day retention times are typical and digesters can be run at ambient temperatures in hot climates but are usually heated to mesophilic or thermophilic temperatures. Biogas produced by the digester is used to heat the digester to the desired temperature. Excess biogas can be used to run an engine generator. Heat can also be recovered from the engine generator and used for space or floor heating, water heating or steam production to offset the cost of purchased electricity, propane, natural gas or fuel oil used on the farm for daily operations.
A fixed film digester on the Farber farm operates with a four day retention time treating separated liquid manure from 100 cows. Odor control was an important reason for installing this digester. It was designed and extensive monitoring has been done by Dr. Stan Weeks. See the following two papers, Anaerobic Fixed-Film Digester System for Dairy Manure and Fixed Film Digester at Farber Dairy Farm: Case Study to learn more about this fixed film digester.
Another fixed film digester is being used by Dr. Ann Wilkie of the University of Florida, Gainesville at the Dairy Research Unit (DRU) as a model for the Florida dairy industry to treat diluted waste from dairies (<1 percent total solids). The fixed film digester constructed on the 500 milking cow Dairy Research Unit is filled with plastic media. The fixed film digester operates at an average ambient temperature of 86 °F during the summer and 68 °F during the winter months and has a three day retention time.
For more information on the fixed filmed digester see Fixed Film Anaerobic Digester by Ann Wilkie.
Temperature-Phased Anaerobic Digester (TPAD)
The temperature-phased anaerobic digester was developed by the Iowa State University. It is a two stage digester system with each digester operating at a different temperature. The first digester processes incoming manure at the thermophilic temperature of 135 °F. The second digester further processes the effluent from the first digester at a mesophilic temperature of 95 °F. The two phase system works best with a diluted dairy manure. The effluent, after having gone through two digestion processes typically has higher pathogen destruction which results in a high quality separated solid. These solids can be used as animal bedding, potting soil ingredient or as a soil amendment. Research has also demonstrated added volatile solids destruction, reduced foaming and greater odor reduction.
The first Iowa State temperature-phased anaerobic digester was built on Tinedale Farms in Wisconsin in 2001. The farm had 2,500 dairy cows and used a dry scrape manure management system. The digested solids were used for animal bedding and the biogas to heat parlor water. In July 2003 the digester was converted to run as a complete mix in the mesophilic range due to difficulties with the thermophilic phase.
For more information on Tinedale Farms TPAD see:
- Tinedale Farms Anaerobic Digestion, A Biomass Energy Project July 2002 (31 pgs.)