Spring 1993 (v5n3)
Martins, 0. and T Dewes
Bioresource Technology
42:103-111.1993.
It is well known that aerobic
composting of animal manures results in the loss of a portion of the nitrogen
originally contained in the manure. This loss can be ecologically significant
if nitrate leaches to groundwater or if large quantities of nitrogen gases
volatilize into the atmosphere. This paper quantifies these nitrogen losses
through research conducted under "semi-technical conditions."
Methods
The experiment utilized liquid
manures from two pig sources, three cattle sources and three poultry sources.
Each type of manure was mixed separately with chopped straw to form a
homogeneous mixture; a blend of all manure types was also used. The mixtures
were placed into individual composting tents in triangular heaps about
2.5 feet tall. The composters had drain pipes to collect the leachate
and air sampling tubes to measure gaseous emissions. The heaps were turned
when average temperatures fell below 30 C (86 F), which was three to six
times during the composting period. After analysis, the collected leachate
was sprayed back on the outer edges of the heaps to prevent drying and
to recycle leachate.
Results
Leaching. During the
composting period, about 10 to 20 percent of the initial nitrogen was
recovered as leachate. Over 70 percent of the total nitrogen that was
leached occurred within the first ten days (before the first rewetting).
As the composting period progressed, leaching generally occurred only
after the heap was turned. The total nitrogen concentration of the leachate,
and to a large extent the amount of nitrogen carried with it, was determined
by the nitrogen content in the initial material. The majority of the nitrogen
leached was ammonium-nitrogen; much less organic nitrogen and only minute
quantities of nitrate (0.1-2.2%) were recovered. The author implies that
this small amount of nitrate does not pose a significant threat to groundwater
quality. (Reviewer's note: In soils, ammonium is usually quickly converted
to nitrite and then to nitrate through the process of nitrification. Therefore,
leachate from composting operations, if allowed to drain into the soil,
may indeed pollute groundwater.)
Gaseous losses. Fifty
to 75 percent of the nitrogen in the original mixture was lost to volatilization
during the composting process. The gases consisted primarily of ammonia,
with very small quantities of oxides of nitrogen. The most important factors
that increased ammonia losses were high nitrogen concentration of the
initial material, high compost temperature, and more frequent turning.
Since the leachate was returned
to the compost heap, the nitrogen lost during the composting process was
solely due to volatilization. Although over half the nitrogen was lost
by this means, total nitrogen concentration actually increased. This apparent
nutrient enrichment was a result of about a 75 percent reduction in mass,
which was presumed to be caused by water loss.
Recommendations
The author notes that some
loss can be prevented by capturing the leachate and either using it to
rewet the heap (when it is protected from rain) or using it as a liquid
fertilizer. Emissions of ammonia and nitrous oxides are considered to
be a more serious ecological hazard from an air quality standpoint. Among
the preventive strategies for reducing gaseous losses are the appropriate
use of raw manure and composting at lower temperatures, although the latter
practice may allow weed seeds and pathogens to survive. The use of natural
materials with large adsorptive surfaces (e.g., bentonite) may also be
useful. Finally, reducing the use of protein-rich animal feeds could reduce
the amount of ammonia released. Other methods of reducing ammonia from
waste gases in central facilities are currently under investigation.
(CI-SWN.1 05)
Contributed by Chuck Ingels
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