THE EFFECTS OF AMMONIA NITROGEN ON THE
ANAEROBIC DIGESTION OF POULTRY MANURE
Leonard E. Ripley, Research Assistant
Nancy Mohr Kmet, Research Assistant
William C. Boyle, Professor
Civil and Environmental Engineering Department
James C. Converse, Professor
Agricultural Engineering Department
University of Wisconsin-Madison
Madison, Wl 53706
INTRODUCTION
The anaerobic digestion process suffers from the perception that it is easily upset by inhibitory or
toxic substances such as ammonia. Successful digestion of wastes rich in organic or ammonia nitrogen
requires identification of threshold ammonia concentrations at which methanogenic bacteria begin to
suffer. Dilution of the waste and other inhibition countermeasures such as supplementary buffering
or maintenance of a longer retention time may be necessary to guarantee stable operation of such a
digester.
Poultry manure (feces, urine, some broken eggs, and a few feathers) is a concentrated waste
with the potential for high energy yield through anaerobic digestion. The U.S. currently has approximately 280 million egg layers in production, each of which produces roughly 0.33 pound of manure
per day that goes to land disposal. A major obstacle to anaerobic digestion of poultry manure is seen
to be the high ammonia concentration of the waste ranging from 5000 to 12,000 mg N/1. This has led
to very conservative recommendations for low organic loading rates and for retention times up to
50 days [1].
One of the more frequently quoted criteria for onset of ammonia inhibition in anaerobic digestion
has been McCarty's [2] guideline of 150 mg/l free ammonia, a guideline which reflects the increasing
inhibitory effect of ammonia as pH increases. Parkin, et al [3] examined the effect of various industrial toxicants such as ammonia and demonstrated the importance of bacterial acclimation. Even with
step-acclimation, they reported significantly decreased performance of an anaerobic filter treating a
synthetic waste with ammonia concentrations of 7000 mg N/1 (free ammonia 125 mg/l). Lapp et al [4]
successfully operated swine manure digesters at ammonia concentrations as high as 3000 mg N/1 with
free ammonia concentrations of approximately 360 mg/l; however, a protracted acclimation period
was necessary. Kroeker, et al [5] studied ammonia inhibition in digesters fed different amounts of
urea and acetic acid. They reported a decrease in methane production as they increased the ammonia
concentrations from 2000 to 5000 mg N/1, but methane production did not cease until an ammonia
concentration of 6700 mg N/1 was reached. At their digester's pH, the resulting free ammonia concentration was greater than 300 mg n/1.
In a study of poultry manure, Gramms, et al [6] successfully operated bench-scale digesters at
ammonia concentrations as high as 3150 mg N/1 with a 130 mg/l free ammonia concentration.
Converse, et al [7] described a full-scale poultry manure digester operating with ammonia concentrations of 6200-8000 mg N/1 and resulting free ammonia concentrations of 260-340 mg/l. It was
unclear to what extent the digester's methane production rate was limited by the high ammonia
concentrations.
The purpose of this study was to quantitatively assess the effect of increased ammonia concentrations in continuous, completely mixed, mesophilic digesters while keeping the carbonaceous
organic loadings constant. Inhibition was evaluated by comparison of volatile solids (VS) destruction,
methane yield per volatile solids added, and volatile fatty acids concentrations.
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