Effluent Characteristics
Silage effluent is 95% water. The solids portion is composed of soluble,
highly digestible nutrients (O'Kiely 1992). As a result, the effluent has a very
high oxygen demand when it enters surface waters. The biochemical oxygen demand
(BOD) of silage effluents is reported in the range of 12,000 to 80,000 mg/l (ADAS
1984). This compares to a BOD for typical domestic sewage of 200 to 500 mg/I.
Some other characteristics are found in the following table.
Composition of effluent - mean and range
| |
g(kg DM)-1
Mean |
Range |
| WSC |
100 |
(0-400) |
| Lactic Acid |
300 |
(0-400) |
| VFA |
70 |
(0-150) |
| Crude Protein |
250 |
(20-300) |
| Ash |
250 |
(20-300) |
| Other |
30 |
(0-100) |
| Dry Matter |
50g/kg |
(20-100) |
| pH |
4.0 |
(3.6-5.0) |
Adapted from O'Kiely (1992). WSC = water soluble carbohydrate VFA = volatile
fatty acids
To illustrate the strength of silage effluent, Bloxham reports that 1100 tons
of grass ensiled at 18% DM produces three times as much pollution as the manure
from 100 dairy cows, even though the cows will produce six times as much
material by volume.
The nutrient level, acidity, and odors associated with leachate are also
harmful to groundwater. Low pH levels can dissolve naturally occuring metals in
the soil.
The potency of uncontrolled silage effluent not only severely pollutes water;
it will also bum or kill vegetation if applied at full strength or allowed to
run directly onto crops from a leaching silo. Common disposal practices include:
1) diluting leachate with equal parts of milking center wastewater or barnyard
runoff water before using it for irrigation, and 2) diverting leachate to an
open-topped liquid manure Storage.
Effluent Storage
Do not add effluent to storage tanks, reception pits or sumps located inside
livestock buildings; other enclosed spaces; or any covered underground manure
storage. Mixing effluent and manure can result in the production and
rapid release of large quantities of hydrogen sulfide or other poisonous gases.
Death to humans and animals can occur very quickly from exposure to hydrogen
sulfide gases. People and animals die via direct poisoning, or when they are
overcome by fumes, fall into the storage, and drown. All openings and tanks
should be fenced off or guarded to prevent inadvertent entry by humans. Only
properly trained workers should be allowed to enter these tanks, and the use of
self-contained breathing apparatuses, rescue ropes, and harnesses must be
required for all workers. Warning signs should also be installed.
Effluent Quantities
The quantity of effluent from silage will vary with the crop, its length of
cut, moisture content, and the degree of packing, which depends upon the height
of the silo and packing methods. Pitt (1987) and Tang et al. (1987) provide
tables and equations that can be used to estimate effluent quantities from tower
silos. Uncovered, horizontal silos will generate extra effluent because
precipitation filtering through the silage produces leachate in addition to that
produced by silage. Various information exists for estimating effluent
production from horizontal silos. O'Kiely (1992) reports production in the range
of zero to 70 gallons per ton. Clark (1989) reports levels of 29 and 24 gallons
per ton respectively for 21.5 and 23.3% DM material: in a covered silo. After
filling the silo, daily flow rates peaked within the first week and decreased to
only a trickle after two months. He also states that uncovered, horizontal silos
may produce twice as much effluent as covered silos. ADAS (1987) reports
production of 270 liters per ton of silage for very wet grass (15% DM).
Structural Concerns
The corrosiveness of silage effluent has a negative effect on silos and their
appurtenances-especially exposed concrete and metal. Appropriate measures should
be taken to protect these critical materials and to minimize the severity and
duration of their contact with effluent. Effluent draining between staves will
structurally weaken the concrete and corrode exposed hoops. Effluent will also
rapidly corrode any exposed reinforcing rods in poured silos. Effluent flowing
over slab edges or foundations will weaken these structural components.
High-quality concrete, properly placed and finished, and appropriate sealers or
coatings should be used. Regular inspections, maintenance, and repair are other
important factors in effluent management.
Control Methods
The three methods, other than collection and disposal, that are most often
cited for controlling silage effluent are: 1) Ensiling crops at lower moisture
contents (field wilting) , 2) Adding absorbents to silage, such as straw or dry
grain, to lower the moisture content, and 3) Feeding effluent to livestock. (Bloxham
1992 and O'Kiely 1989).
Effluent Collection and Containment
If effluent is produced as a part of the selected silage-making process, then
measures must be taken to protect ground and surface waters. The remainder of
this paper will review methods for collecting and containing silage effluent.
Location
The first step in silage effluent management is properly locating the silo.
Silos should be located as far as is practical from critical water resources. In
addition to controlling surface flow, attention should be given to wells, sink
holes, and other potential paths to ground water.
Precipitation and Ground Water Control
Precipitation can be controlled by properly installing permanent roofs or
temporary plastic covers. Surface water should be diverted away from silos and
silage handling areas.
Silos built partially or completely below ground are subject to groundwater
infiltration, especially by flows from subsurface springs. These seasonal flows
can last from several weeks to months and can cause long-term discharges of silo
effluent. Subsurface drainage should be installed to divert groundwater from the
silo.
Tower Silos
Effluent from tower silos may come from planned openings, such as drains or
weep holes, or it can escape randomly from silos doors and between staves. The
following figures illustrate methods for collecting effluent and directing it to
an appropriate collection tank or storage. If gravity flow cannot be achieved,
then a pumping station will be required. Sizing the pump and sump or collecting
tank will depend on expected effluent discharge rates. The pump and sump or tank
should be constructed of materials that will resist attack from acidic effluent.
Click Image for a Larger View (329 KB)
Horizontal Silos
Horizontal silos must consider controlling and handling effluent produced by
silage by precipitation that falls on the silage. Silos that are partially or
completely below ground level will also need to consider ground water exclusion
and control. Below are illustrations of methods for constructing water-tight
joints that may be required to prevent the flow of liquids either into or out of
the silo.
Click Image for a Larger View (79 KB)
Various methods can be used to collect leachate and direct it from the silage
to the desired location. Generally, floors are sloped toward both sides and one
end of the silo. Effluent buildup within the pile can be controlled by using
small drain gutters or drain tiles along the outside walls. If the floor is
sloped toward the center, then a collection gutter can be placed at the low
point. Various methods are available for constructing a gutter that will carry
effluent and not affect silage storage or equipment operation. Shallow gutters
with loose-fitting bricks or deeper gutters with railroad ties can be used. A
small gutter may be formed, from either half of a PVC pipe or a rain gutter, in
the bottom of the main gutter to provide a more durable surface for the
concentrated effluent flow. A large gutter with a railroad tie may be used for
the main collection channel across the front of the silo. During high-flow rates
after filling the silo, the ties can be removed. To prevent cracking and
structural failure at the gutter, the slab thickness should be increased and
reinforced as necessary. The following figures illustrate various gutter types.
Click Image for a Larger View (36 KB)
For bunker silos it may be necessary and easier to allow effluent to seep
between wall sections and through the floor to an outside collection gutter.
This collection gutter should be located where it will exclude clean surface
water and runoff water from the silo cover. A flat-bottomed gutter that is wide
enough to allow cleaning with a flat shovel is best.
Click Image for a Larger View (66 KB)
Effluent collection for horizontal silos becomes more complicated due to the
large area exposed to rainfall as the storage is being unloaded. Collecting all
runoff is usually undesirable due to the large volume resulting from the
essentially clean rainwater. A system that will collect and store the low daily
flows of effluent but discharge rainfall events over 1/4" is suggested. The
excess discharge either can be piped to a filter area or allowed to overtop the
collection system and move directly to a vegetated area. This water should not
flow directly to a road ditch or stream. The initial runoff cleans out the
leachate and the bulk of the contaminated material. The remaining water is
essentially clean and is treated in the vegetated area. The following figures
illustrate methods that can be used for separating high and low flows from
horizontal silos.
Click Image for a Larger View (342 KB)
Maintenance and Operation
Operation and maintenance of effluent control systems is important. A plan
for observation and maintenance should be developed for any system installed.
Collection tanks must be monitored and pumped out at appropriate intervals.
Gutters and pipes must be kept clean of debris and obstructions that can block
flow or encourage ponding, which can possibly create insect breeding areas.
Stone-filled infiltration trenches used to collect effluent from stave silos
require periodic cleaning or replacing of the top layer of stone to prevent
plugging by fine material.
Pumps and sumps must be checked for proper operation, and any debris clogging
them removed. Workers should not enter sumps or tanks that contain effluent
or effluent residue unless they are using proper procedures and equipment for
working in a confined space. Systems that use small orifices and screens to
control flow rates and separate solids must be cleaned as needed.
SUMMARY
Silage is a nutritious food for animals. When silage or silage effluent is
allowed to flow into ground or surface waters, pollution will result. Simple
steps can be taken during the design, construction, and operation of silos to
minimize effluent production and to keep it from polluting water resources. It
is the responsibility of the operator of any silo to prevent water pollution.
Silage effluent in tanks and storages, especially if mixed with manure ' can be
hazardous to humans and animals: In addition to drowning, hydrogen sulfide and
other gases can be released in large quantities from even small amounts of this
material. Designers, installers, and operators must be aware of these dangers
and take proper steps to protect people that may be around the systems.
References:
ADAS. Silage - avoiding pollution, Booklet 2429. Ministry of
Agriculture, Fisheries and Food, Alnwick, Northumberland NE66 2PF, Canada. 1984.
ADAS. Managing silage effluent. Ministry of Agriculture, Fisheries and Food,
Alnwick, Northumberland NE66 2PF, Canada. 1987.
Clark, J. J. Silage effluent production. Farm Building Progress (89) July
1987. pp. 23-27.
ESCA. Siting of silage pits, Advisory leaflet 128. The East of Scotland
College of Agriculture. March 1983.
Higgins, K. P. Silage pits - construction and repairs. Farm Building Progress
(84) April 1986. pp. 7-8.
Bloxham, P. "The management of silage effluent." Farm Buildings and
Engineering (9) 1, 1992 . pp. 21-23.
Johnston, C. A. Construction of concrete floors for silos. Technical Note
Number 153. The West of Scotland Agricultural College. April 1982.
Johnston, C., J. J. Clark, K. L. Machattie and G. A. L. Watson. Silage
clamps: design and construction. Scottish Farm Buildings Investigation Unit.
1985.
Kammel, D. and N. Houtman. Environmental concerns associated with feed
storage. Environmental Resources Center, College of Agricultural and Life
Sciences, University of Wisconsin-Madison. December 1989.
O'Kiely, P. "Silage production as a pollutant: new ways to reduce its
environmental impact." Biotechnology in the Feed Industry. T. P. Lyons,
ed. 1992. Alltech Technical Publications. pp. 151-163.
O'Kiely, P. "Two alternative ways to deal with silage effluent." Land and
Water Use. Dodd & Grace, eds. Balkema, Rotterdam. 1989. pp. 349-352.
Pitt, R. E. Preventing effluent from silage, EF-13. Agricultural Engineering
Facts. College of Agriculture and Life Sciences, Cornell University, Ithaca, NY.
1987.
Tang, Juming, J. C. Jofriet and B. LeLievre. "Juice flow from silages."
Canadian Agricultural Engineering. 1987. pp. 99-106.
ESCA. Earth-wall silage pits, Technical note 260. The East of Scotland
College of Agriculture. January 1981.
Welkin, P. "Concrete silage pits." Power Farming. May 1986.
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