Winter 2003
Ammonia accumulation in settling basin liquids
by
Kris Kohl ISU Extension Ag Engineering Field Specialist, Jeffery Lorimor
and Patricia Jaranilla, Department of Agricultural and Biosystems Engineering
Ammonia in surface waters is a serious pollutant because both fish and vegetation are very sensitive to it. Fish kills have been reported below small feedlots with settling basins when the basins have not drained completely following precipitation events and when retained liquid is flushed into nearby streams. The inability to maintain grass directly below settling basin outlets is also thought to be a result of high ammonia concentrations. Ammonia concentrations are believed to increase with time in undrained settling basins, but little data exists on how quickly the ammonia accumulates.
Dewatered solid settling basin below beef feedlot.
Procedure and results. A study, conducted by the authors in summer 2003, examined how quickly ammonia accumulates, and factors that influence the ammonia accumulation. In laboratory tests, initial solids and nitrogen concentration in the manure, the pH of the slurry, and the temperature of the slurry were studied. Liquid:manure solids ratios of 5:1, 10:1, 20:1, and 40:1 were tested in warm (60-70F) and cold (40-50°F) conditions. Both beef and swine manure solids were tested.
For the beef manure in warm conditions, there was an initial doubling of the ammonia concentration from 200 to 400 mg/l, then an increase 600 mg/L more than 7 days for the 5:1 liquid :manure solid ratio, (Fig. 1). Ammonia concentrations in that same mixture (5:1) exposed to cold temperatures remained relatively constant (Fig 2).
The 10:1 ratio increased similarly from a lower initial concentration up to 300 mg/L, and the 20:1 ratio increased to a concentration of 200 mg/L. Data on the 40:1 showed no significant increase in the ammonia concentrations at either temperature. Swine manure reacted the same as beef. In general, the concentrations quickly doubled, then increased three to five times the original concentration within 6 to 7 days.
At cold temperatures, ammonia concentrations remained at approximately the initial concentration.
A field study was done for both warm and cold temperatures. The average temperatures were 84oF during the day and 66°F at night. The test showed that the ammonia-N concentration increased similar to the laboratory results, increasing from 75 to 450 mg/L in 8 days (Fig. 3).
In all the tests pH remained essentially constant at approximately 6.8 through the duration of the tests.
Swine manure is similar to beef manure but converts to ammonia at a faster rate making it more important to dewater before the conversion occurs. The warmer the temperature the faster the conversion occurs. There is also more clean-water to dilute the liquid from a solids settling structure immediately after an event when the concentration is the lowest.
Results for the cold temperature study showed that the concentration remained constant and only slightly increased with time.
Conclusions. Several conclusions were derived from this study. Our findings indicated that:
- Ammonia concentrations at warm temperature increased to 3x the original concentration before stabilizing.
- The conversion to ammonia is slower atcold temperatures.
- For the warm temperature, the 5:1 ratio stabilized at 7 days, the 10:1 ratio stabilized at 4 days and the 20:1 ratio stabilized at 3 to 4 days. Unfortunately, data on the 40:1 was not enough to sufficiently conclude the exact number of days the manure ammonia concentration stabilized.
- Chemical analyses done on the 5:1, 10:1and 20:1 ratios showed that total Kjeldahl nitrogen, total solids and total volatile solids had a direct correlation with the concentration of manure to water.
- It is important to dewater settling basins as quickly after rainfall as possible, before the conversion to ammonia occurs.
