Paper No. 94-25 60

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Introduction

Dairy farms are increasing in size as well as increasing their use of mixing wagons to feed the cows. This has caused the creation of more and bigger horizontal or bunk silos. The effluent or juice from these bunk silos can have a significant negative effect on the -local environment The juice produced is highly concentrated with potential pollutants. The relatively large surface area of the silo can also add runoff from rainfall events. This makes it especially hard to contain all the effluent There have been a number of fish kills from silage juice in both Pennsylvania and New York.

The solutions to this problem include: harvesting at the proper moisture content, diverting rainfall runoff and ground water away from the bunk silo, storage of all the effluent, and/or collecting the base flow from the silo while allowing the higher flows diluted by rainfall to be treated in a filter strip. There are several ways proposed to contain only the base flow. The best way is to have the flow collection system adjustable to allow for changing conditions and different flow amounts. The solution proposed is a combination of the experience in Pennsylvania and New York of Soil Conservation Service (SCS) field engineers with silage juice and barnyard runoff problems.

Silage, Effluent

The juice from a bunk silage can have variable amounts of pollutants in it. Dilution with rain water of varying amounts in the difficulty of predicting concentrations in the resulting runoff.

Typical characteristics of the effluent given by SCS's Ag Waste Handbook and Graves et, al. follow.

Typical Silage Effluent

The effluent is high in nutrients and has a high oxygen demand. For comparison dairy manure typical characteristics as given by SCS Ag Waste Handbook are as follows.

Typical Dairy Manure

Even though the dairy manure has a higher solids content and appears richer there is more nitrogen in Vocal silage juice. The high concentrations in the silage juice cause the burning of vegetation and the development of kill zones in the area the silage juice drains onto. Kill zones are areas where the vegetation has been killed because of exposure to high concentrations of pollutants. The impact of this juice can extend long distances. Usually the juice gets concentrated into a swale or ditch within a short distance. Without dilution these flows can travel quite a ways and still restrict vegetative growth and produce an unpleasant septic smell. This effluent seems to seal the soil surface since pools of it remain on the surface longer than clean water does. The effluent flowing from the farm in a ditch can be considered a point source of pollution and represent a liability to the producer. Any amount leaving the farm adds to the nonpoint pollution loading of the watershed.

Bunk Silos

Bunk silos and the use of other effluent producing commodities cause additional problems for producers. By-products from food processing plants are often used on a dairy farm as cheap feed. Wet by-products include sweet corn waste, apple pomace, and brewer's grain. These wet materials can produce additional effluent to the waste leaving the bunk area.

Bunk silos are exposed to rainfall. This additional water adds a varying amount to the volume of effluent leaving the bunk. The amount of runoff depends on the amount and rate of rainfall, the type and area of covering, if any, over the bunk, the area of the bunk filled with silage, and the amount of storage and the evaporation from the concrete surface. Small rainfall amounts coming at a slow rate produce much less runoff than a large intense storm. Well placed plastic covering in good condition can divert much of the rainfall off the bunk without getting it contaminated with the silage juice. The amount of direct runoff is proportional to the exposed of concrete. When the bunk is full most of the rainfall is absorbed into the silage. It may emerge later as silage juice. When there are large areas of exposed concrete the runoff amounts can be much higher. This exposure to the effects of rainfall makes the amounts and concentration of the combined runoff and silage juice flow variable.

The runoff has the potential to pick up and transport particles of silage. These solids drop out when the flow carrying them slows down. They can end up deposited off site in a depression or on a flatter area at the edge of the bunk. When left exposed to in a wet place the silage solids start to decompose and produce more effluent. These decaying solids can continue to pollute after the effluent from the silage in the bunk itself has stopped any significant flow. Although the amount of pollution from these displaced solids may not be a large amount it win have a significant effect on a grass filter.

Many bunk silos are built into the ground in a cut and fill operation on a side hill. This is done to reduce the cost of supporting the sides of the bunk and to facilitate loading by dumping incoming silage over the dug in end. By being in the ground drainage is often needed to intercept ground water. If drainage is not provided there is a potential for the ground water flow to mix with the silage in the bunk and add to the effluent. Bunk silos built on flat land often include a drainage system under the concrete floor. Some drainage systems also catch the silage effluent and transport it directly to a stream. This can be environmentally disastrous to the receiving stream when silage juice is added especially while the stream is flowing at its lower water levels. This often occurs during the late summer and, early fan when bunk silos are being filled.

Amount of Effluent

The amount of effluent from silage can vary significantly. It is dependent p y on the moisture content of the silage and the pressure on the silage. The size of the silage particles and the speed of the ensilaging process effects the rate, of effluent production but may not have much effect on the total amount (Savoie, 1993). There are methods to predict the rate and therefore the amount of effluent coming from a tower silo. Savoie has developed an equation that predicts the rate of effluent production from a bunk silo. The parameters used include: initial moisture content or dry matter, a chopping factor, and a silage additive factor. This equation does not predict the effect of rainfall on the silage in the bunk or the runoff from the concrete area around the silage. Other silage effluent predictions only estimate the total amount. Estimates from SCS (Silage draft) are given below.

A recommendation of one cubic foot of storage for each ton of silage is given by SCS when including silage juice in manure storage systems (SCS Ag Waste Handbook).

The amount of silage effluent varies throughout the year. Ideally when a bunk silo is loaded effluent flow starts. It peaks from 5 to 10 days later and then dwindles to a minimum by 3 months. Rainfall complicates the process. Additional water added to the silage can produce additional effluent The runoff from the emptied bunk area from rainfall or snow melt adds another variation into the amount of effluent to be dealt with. The drainage water under or around the bunk can also add to the flows, especially during high ground water seasons. The concentrations of the pollutants in the silage juice vary over time depending on how much extra water is added.

This varying amount and changing concentration makes control of this effluent difficult. This is like the problem many municipalities are facing dealing with combined sewers. There is a polluted base flow that is augmented on occasion by the diluted flow from the storm drains. Designing a system to handle the total flow can be difficult and expensive. The storm drain runoff like the runoff from a bunk silo area contains more pollutants in the first flush than in the remaining flow.

Prevention

The amount of moisture in the silage as it is loaded in the bunk is the most important parameter to control to reduce the amount of silage juice produced. Dry matter above 30% should prevent any significant juicing. Dry matter of 30% is ideal for the ensiling process. Weather conditions and harvesting time constraints sometimes prevent these ideal dry matter percentages from being obtained.

Diverting all outside runoff, rainfall, and ground water will reduce the amount of effluent. This will require installing diversions to exclude the outside surface flows. Traffic patterns and adequate outlets need to be considered in locating the diversions. Roof runoff can be controlled with rain gutters or drip trenches. Ground water should be intercepted far enough away from the bunk so that no leachate can get to the drain tiles. Planning the layout of the bunk to avoid areas of ground water could eliminate this problem The rainfall falling on the bunk itself could be controlled with a roof. Black plastic covers on bunks are common. If water is directed over the sides of the bunk effluent production will be reduced as long as the water does not reenter the system at the base of the bunk wall. In practice it is difficult to place the silage and the plastic so the runoff does not go down the inside of the bunk.

Containment

Catching, storing and field applying the effluent, the runoff and the drainage water may be an overwhelming task on a farm. If a farm had a bunk silo with silage piled an average of 15 feet high that covered one acre, they would store about 21,500 tons of silage (Pitt, 1990). Using SCS's recommendation of providing storage for 1 cubic foot of leachate for each ton of silage 21,500 cubic feet of storage would be needed. In addition to the silage juice the farm would catch the runoff from the one acre of bunk area, any drainage water intercepted by the drains, and need to be prepared to store, the 25 year 24 hour storm (Environmental Protection Agency (EPA) 1993). A yearly average of only 45 percent of the precipitation falling on the bunk would result in runoff since some of the precipitation would be evaporated from the surface. (SCS Ag Waste Handbook) For an annual rainfall of 36 inches approximately 16.2 inches would runoff in New York State's climate. Over the one acre area this becomes 58,800 cubic feet. A 4 inch 25 year 24 hour storm would require a storage volume of 14,520 cubic feet. There could be additional water collected from the drainage system around and under the bunk. If the average flow from the drainage system was one gallon per minute, and flow occurred for a nine month portion of the year, the volume needed to store the drainage water would be 51,300 cubic feet. These volumes are summarized below.

Potential storage volumes per acre of bunk silo

The farmer could empty the storage periodically to reduce the amount of storage needed but room for the 25 year 24 hour storm event would always be required. Using a 3800 gallon tank spreader would take 260 tank loads to empty the storage if the 25 year 24 hour storm did not occur. This is a lot of storage. Most of the storage volume required is for relatively clean water. Separating the concentrated leachate from the other volumes of water would provide pollution control at a reasonable cost.

Low Flow Collection

Many different systems have been made in the past to collect the low or base flow from bunk silos. The ultimate system is yet to be proposed. It would consist of: a method to catch both the floating silage solids and any dirt washed in by the runoff events, a flow diverter to switch the flows to storage when the concentrations of pollutants were high and switch the flows to a filter area when concentrations were low, a storage area for the concentrated pollutants, and a low cost treatment system for the high volume low concentration effluent. The generic drawing at the end of this paper is the closest system to the ideal that is being proposed for use by SCS in New York and Pennsylvania.

Refinements Needed

Each of the parts of this system needs refinement. Solid collection with a small settling basin and a screen can work in most cases. The solid bottom on the screen if it fits tightly to the concrete floor can provide a shallow area of ponding to settle out denser particles. Because the amount of denser particles and the flow volume are variable from event to event and farm to farm it is hard to design a specific settling volume. 'Me screen to retain the silage solids will be a high maintenance item. It is expected that the various sizes of silage will clog the screen occasionally. Screen holes sized to stop the average cut length of 3/8 inch may not exclude enough smaller particles.

The flow diverter only separates the flow based on the volume flowing through the pipe. Lower flows drop into the pipe to the storage system while high flows with a higher velocity arc over the pipe. In general higher flow volumes will be more dilute but it would be better if the flow diversions were based on concentrations of pollutants and not on volume only. The early part of a runoff event will likely contain a higher concentration of pollutants dm the ending part even though they have the same flow volume. A sensor and valve that could detect pollutant concentrations and then direct the flows appropriately could be used both for bunk silage flows as well as urban storm water systems. The storage of the concentrated effluent is best included with the long term storage of manure on the farm. If the bunk is lower in elevation than the manure storage a pump will be required. Pumping only the low flows makes the pump sizing much easier. A pump that could handle the high flows from a 25 year 24 hour storm would be much larger and have extra capacity most of the time.

The treatment of the high flows that by pass the storage needs to be refined also. Since the amount of runoff, the concentration of pollutants, and the timing of these events are all variable it is hard to predict the treatment level in a filter strip. A biological indicator of healthy grass at the beginning of the filter strip would show that the concentrations of pollutants and the amount of time they were in contact with the grass would not damage a similar ecosystem downstream This does not meet EPA's proposed criteria of storage of the 25 year 24 hour storm, but a properly sized and well functioning grass filter strip for the least concentrated flows might provide close to the same protection to the environment as storage and then land spreading. If storage was required separating the flows and sending the event generated high flows to a constructed wetland might be one cost effective solution. Wetland treatment needs a relatively dilute waste to function effectively. The treatment provided by the constructed wetland may be sufficient for slow release to the environment

Existing System Advantages

The method of collecting the solids and separating the flows based on rate of flow does have advantages over any alternatives tried by SCS in New York and Pennsylvania. The solids collection at the beginning of the system prevents solids from entering the filter area during large runoff events. Without the screen silage would collect in the filter area and start to create its own juice. This juice would be beyond the low flow collection device and so would cause a kill zone in the vegetation downstream By making the screen an extension of the concrete curb on one side of the bunk it is easy to clean with the silage handling equipment. As the operator is loading the feed wagon she/he can see if any solids have collected in front of the screen and scrape them out of the flow path for disposal at a later time. Any juice they produce is still controlled by the low flow collection.

An adjustable collection pipe allows the manager to determine what flow rate will be stored and how much will be handled by the grass filter. Systems that collect all the runoff will leave the farmer with too much water to dispose of. Systems that use an orifice to only allow a fixed amount to go into storage are not flexible enough when conditions change in the bunk. If no liquid producing commodities are being used and after the silage has stopped producing juice very little of the effluent from the bunk needs to be retained. A good grass filter can treat most of the runoff. If high moisture silage has recently been put in the bunk or if a very wet commodity has been brought in more of the effluent needs to be retained. An orifice can't be adjusted to accommodate these different situations. Orifices will also continue to allow relatively dilute water to be sent to storage as the higher flows are being bypassed. By having the higher flows arc over the pipe to the storage the dilute water is excluded from the storage system.

Design Considerations

The dimensions of the collection device, the amount of storage and the size of the grass filter are not easily sized. There are so many variables that are either site dependent or indeterminate that specific standards have not been developed. Clearly more experience with these systems with some monitoring is needed.

A weir is put in the low flow collection system to make sure extreme events are still sent to the grass filter. The flows to the grass filter should be kept out of watercourses as long as possible. A trapezoidal grass waterway can be constructed to direct the flows away from watercourses after they have gone through the grass filter. The trapezoid shape will spread the flow out better than a parabolic or vee shape to allow continued treatment. Any dense sod forming grass will work well in the grass filter. The grass should be adapted to the local conditions and be well established before being used as a filter. The adjustable collection pipe can be used to catch more effluent while the grass filter is being established.

Do not add the silage effluent to a storage system in a poorly ventilated building. The gases produced can be deadly. Adding the effluent to manure can increase the gas and odor production of the storage.

Operation and Maintenance

The outlet pipe may need a discontinuity to keep drips from running back up the outside of the outlet pipe and missing the collection pipe. A notch or screw set in the invert should prevent this from occurring.

The grass can act as a biological indicator. If the grass is of poor quality at the top of the filter area it indicates that too much of the highly concentrated effluent is getting through. The collection pipe should be adjusted to catch more of the effluent If the grass is in good condition the collection pipe may be moved to collect less effluent to keep the amount to be stored to a minimum.

These systems require some management If the farmer neglects them they either will collect too much effluent or too little. If poor management is a concern the orifice that continues to collect effluent may be best to prevent larger concentrated flows from injuring the grass filter or polluting downstream More storage will be needed if the orifice system was used. The storage needs to be emptied in a timely manner. It is best not to wait until the storage is full before emptying them. Poor weather may prevent emptying when scheduled. The solids need to be removed as they are collected. Leaving them at the screen will increase the chances that the screen will plug.

Conclusion

Although silage effluent is a potent pollutant it can potentially be controlled in a cost effective manner. Sending the highly concentrated low flows of silage effluent to storage for future land spreading while allowing the less concentrated high flows from runoff events to be treated in a filter system can be accomplished with this system. Commitment to managing and maintaining the system is important to the success of this system. A better flow separator, screen and grass filter design would improve the system. Monitoring of the existing systems would provide some of the information needed to develop standards that could be applied in the Northeast.

BIBLIOGRAPHY

Agricultural Waste Management Field Handbook. 1992. U.S. Department of Agriculture, Soil Conservation Service. NEH Part 65 1.

Cook, M.B. 1993. Water Quality Strategy For Animal Feeding Operations. Draft (Office of Wastewater Enforcement and Compliance, U.S. Environmental Protection Agency, Washington, D.C. 20460).

Cropper, J. and C. DuPoldt 1993. Silage Leachate, and Water Quality. Draft (USDA - SCSNNTC, Chester, PA).

Garthe, J.W. Conventional Harvesting Equipment. 1993. Silage Production: From Seed to Animal. Syracuse, NY: Northeast Regional Agricultural Engineering Service. NRAES-67.

Graves, R.E.and P.J.Vanderstappen. 1993. Environmental Problems with Silage Effluent.

Silage Production: From Seed to Animal. Syracuse NY: Northeast Regional Agricultural Engineering Service. NRAES-67.

Pitt,R.E. 1990. Silage and Hay Preservation. Ithaca NY: Northeast Regional Agricultural Engineering Service. NRAES-5.

Savoie,P. 1993. Probability Estimation of Silage Effluent From Horizontal Silos. Draft. Sainte-Foy Research Station, Agriculture Canada.

Savoie, P., D. Tremblay and L Wauthy. 1990. Novel Harvesting Equipment for Silage. Silage Production: From Seed to Animal Syracuse, NY: Northeast Regional Agriculture Engineering Service. NRAES-67.

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