Fixed Film
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.
Examples
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
Ladders
and platforms more than eight feet high require railings, cages or fall
protection and should provide for exclusion of children and curious visitors.
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.
Examples
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.)
and
Agricultural Biogas Casebook – 2004 Update (69 pgs.)