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Evaluation of feedstock combinations on the
composting process and quality L.
R. Cooperband, A. G. Stone, M. R. Fryda and J. L. Ravet Wisconsin
supports a strong timber industry, which generates a substantial amount
of sawdust throughout the state.
A survey by the USDA Forest Service reported sawdust production
in Wisconsin of more than 409,000 and 46,000 fresh tons for hardwood
and softwood species, respectively
(1). Presently,
a portion of the timber industry’s byproducts is used to produce animal
bedding, mulch or biofuels. Unfortunately,
these easily saturated markets for waste products are not readily accessible
to many of the waste producers around the state (2).
Recently, more stringent air quality standards limit the amount
of particulate emissions released into the atmosphere from burning or
stockpiling sawdust. Additionally, alternatives to landfilling are being sought
in response to the steadily increasing cost of maintaining and developing
landfills (3). These factors
highlight the need for the development of an environmentally sound and
economically feasible means of sawdust utilization.
Thanks to their physical and chemical characteristics, wood by-products can be co-composted with a variety of nutrient-rich waste products. Wisconsin has an abundance of industries that produce organic wastes that could be composted with sawdust. Many improvements to soil quality and crop yield accompany the application of compost in agricultural settings. The addition of municipal solid waste compost at rates of 15 tons per acre increased the rate of corn growth above the control in Wisconsin (4). Additionally, composts produced from bark yielded superior orchids, azaleas, camellias and rhododendrons (5). It is clear that composted organic wastes have chemical, biological and physical characteristics that benefit crops. The objectives of this research project were to evaluate the effect of different feedstock combinations on the composting process and to determine how feedstock combinations affect compost quality. Material and Methods This project involved the sampling and testing of four types
of composts. The composts
were subjected to tests that characterized the moisture content, biological
activity, pH, electrical conductivity and nutrient availability of each
type of compost. The temperatures
of the piles were taken in an attempt to determine when the piles reached
maturation. The four compost
piles under investigation were combinations of sawdust and other waste
materials produced in Wisconsin.
The compost windrows were built in October of 1997 at the University
of Wisconsin, West Madison Agricultural Experiment station.
The windrows were divided into three sections (replications)
and sampled on 11 occasions. It
should be noted that on 11/18/98 about 60 pounds of each type of compost
was collected in large bins and moved indoors.
These large samples were taken from each of the three replicates
of each of the four piles. This
was done to simulate the composting process under less severe conditions
and to avoid cooling the interior of the pile during the sampling process
in the winter months.
The bins of compost were incubated at room temperature and maintained
at constant moisture content.
The cannery waste, duck manure and potato cull composts were
maintained at 60% gravimetric moisture content.
It should be noted that due to the presence of rocks in the dairy
manure compost, we maintained this compost at a 50% gravimetric moisture
content. While the moisture level was maintained at a lower level, we
believe that the compost was still too moist.
Steps were taken to allow for the aeration of all composts, without
exposing the composts to overly drying conditions.
Throughout the first 2 months, we attempted to take samples every
7-10 days to characterize the changes that take place in an active,
thermophilic compost pile. A
sample, comprised of about two gallons of compost, was removed from
each bin and placed on a plastic sheet.
The compost was manually screened to remove large soil aggregates
and rocks. These materials
were not normally found in the compost and might throw off calculations
that involved fresh weight. Subsamples
were then taken to determine the moisture content of the compost, biological
activity, available nutrients, pH and electrical conductivity of the
sample.
We used a gravimetric method to determine the moisture content
of the compost. The subsample
of compost was weighed out immediately following the screening process.
The weighed sample was carefully placed into a 60 degree C oven and
dried for 48 hours and weighed.
The calculated moisture content was used to determine how much
water should be added to the bins in order to maintain the composts
at the optimum moisture level (50-60%).
The biological activity and nutrient availability of the compost
were investigated on the same subsample.
The calculated moisture content was used to determine the 10-gram
oven dried equivalent of each compost.
Duplicate samples were weighed into plastic cups.
Two resin membranes (cation and anion) were inserted vertically
into the compost. The cup containing compost was placed into a quart mason jar.
Additionally, a vial of distilled water and a vial of 20.0 ml
of 1.0 M NaOH were placed into the incubation jar with the compost.
This system was incubated for 7 days at 25 degrees C.
The membranes were removed from the compost and extracted on
a shaker with 1.0 M NaCl for 1 hour.
These extracts were placed into a freezer.
The analysis of these extracts will shed light on the available
nitrogen and phosphorous mineralized in the compost sample. The vials of NaOH were titrated with 1.0 M HCl to determine
the amount of carbon dioxide released by the sample. Additionally, a 15-gram oven dried equivalent sample of compost
was shaken with 150 ml of distilled water to determine the amount of
soluble nutrients in the sample.
This sample was frozen and later digested with the Kjehldahl
digestion method. These
samples will be later analyzed to determine the total nitrogen and phosphorous
in the distilled water extracts. Results
The data collected from the compost piles over time revealed
that the materials were decomposing.
All four of the piles were characterized by a marked reduction
in volume and by heat generation toward the interior of the pile.
After an initial peak, the temperature of the piles decreased
gradually throughout the study.
It should be noted that procedures followed to determine the
nutrient relationships, pH and electrical conductivity of the composts
proceeded as planned. However,
the samples will not be processed until a future date. The
titrations of the 1M NaOH traps with 1 M HCl indicated that the relative
amount of carbon dioxide generated by biological activity decreased
over time. The pile containing
the potato culls maintained a relatively high level (1.8mg CO2-C/g compost/day)
of carbon dioxide generation for the first 20 days. The duck manure
and cannery waste composts seemed to have experienced a moderate decrease
over the first 20 days (1.7mg down to 1.4mg CO2-C/g compost/day).
The data on the dairy manure compost indicates that the level
of carbon dioxide generated during the incubation decreased rapidly
during the same 20-day period (1.2mg down to 0.6mg CO2-C/gram of compost/day).
It should be noted that around 30 days from the start of composting,
there was a major decrease in the amount of carbon dioxide being detected
in all composts. The incubations
of the next set of samples yielded a significant increase in the amount
of carbon dioxide released. All
subsequent dates sampled for the duck, dairy and potato compost yielded
steadily decreasing carbon dioxide levels.
The cannery waste compost exhibited a gradual increase from day
60 until day 120 before a slight decrease.
In general, the duck manure, dairy manure and potato cull compost
behaved in a similar manner. The
cannery waste deviated from this in that it exhibited a gradual increase
in carbon dioxide generation after 60 days, while the others continued
to decrease. The cannery
waste, duck manure, and potato cull composts also tended to have a rough
similarity in the amounts of carbon dioxide released from the compost.
Discussion
While the decrease in temperature of the piles would be an indicator
of the maturation of the compost, the study took place during autumn’s
decreasing temperatures. Additionally, the piles would have an increasingly difficult
time maintaining a high temperature because of the reduction in volume
that characterizes composting.
Stable compost tends to be characterized by a decrease in biological
activity, which accompanies a reduced temperature.
The reduction in temperatures observed in these piles is likely
a combination of biological stabilization and a low ambient temperature. The carbon
dioxide generation data sheds light on the level of microbial activity.
The differential decreases in the amount of carbon dioxide being
released in likely a function of the materials being composted.
The abundance of oxygen, water, easily degradable carbon and
sufficient nitrogen would be conducive to a high rate of microbial respiration
and hence, carbon dioxide release.
The potato cull compost seems to supply its microbial community
with all the necessary ingredients needed to support the rapid mineralization
of carbon. The duck manure
and cannery waste manure compost start out by mineralizing carbon at
a slightly lower rate than the potato compost, and over the first 20
days the rates steadily decrease more quickly than the potato compost.
This is likely related to the decrease in amount of easily degradable
carbon sources or a decrease in the amount of nitrogen available for
microbial biomass production. The reduction in the amount of carbon dioxide released could
have been a combination of both of these factors.
The dairy manure compost stared out at a lower level of carbon
dioxide respiration and decreased rapidly during the first 20 days.
The low initial level might be due to the relatively high pH
of the compost, which may have inhibited the growth of some microbial
communities. The relatively
low level of carbon dioxide generation might also be explained by the
presence of a large amount of mineral matter in the compost. These tests were carried out on samples that were prepared
on a dry weight basis. The
large amount of ash in the samples would mean that there is comparatively
less oxidizable organic carbon to be mineralized to carbon dioxide,
relative to the other composts.
Analysis of the composts after 5 months also revealed that the
dairy manure compost had the highest carbon to nitrogen ratio, 21.2.
This is compared to the cannery waste compost (13.8), duck manure
compost (17.5) and the potato cull compost with a carbon to nitrogen
ratio of 12.9. The study of the composts over time revealed some significant
differences among the rates of carbon mineralization.
The carbon to nitrogen ratio is an important factor to consider
when applying organic wastes to agricultural land.
This must be taken into consideration to prevent nitrogen immobilization.
The mineralization of carbon and subsequent release of carbon
dioxide is a process that would reduce the carbon to nitrogen ratio. This loss of carbon takes place during to composting process
and reduces the amount of carbon available to the microbial community.
The limitation of the carbon source is important because if the
microbes are able to grow at a rapid rate, they will tend to use nutrients
that would otherwise be used by the crop for growth.
Composting is an effective means to reduce the carbon to nitrogen
ratio, thus reducing the biological instability of the organic waste.
The work being done to study the composting process and to
investigate other potential uses of organic wastes is quite important.
At this point their may be easier or more expeditious means to
dispose of these wastes, but a time will come when a more responsible
means of waste disposal will be dictated by market forces.
Composting of waste products helps divert material from landfills
and produces a soil amendment that may improve crop production and soil
quality. Increased use
of composting technology may result in beneficial uses for wastes while
employing an environmentally responsible and low-risk mechanism of waste
disposal. |
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Questions? Comments? Please contact Dr. Leslie Cooperband, UW-Madison |