How To Handle Seepage From Farm Silos

Agdex#:	732
Publication Date:	November 2004
Order#:	04-031
Last Reviewed:	November 2004
History:	Replaces OMAFRA Factsheet, How to Handle Seepage from Farm Silos, Order Number 95-043
Written by:	Steve Clarke - Engineer, Energy and Crop Systems/OMAFRA; Robert P. Stone - Engineer, Soil Management/OMAFRA.

Table of Contents

1. Introduction
2. Why Silo Seepage Is An Environmental Problem
3. Rate and Volume of Seepage Flow
4. Managing Silo Seepage and Precipitation Runoff
5. Disposal of Seepage
6. Site Location for Seepage Collection Tanks
7. Sizing of Small Seepage Tanks

Introduction

Silage seepage presents two concerns for the agricultural industry - pollution of land and water may result from silage seepage, and the silage juices cause corrosion and deterioration of the silo. When silage is harvested and stored at low moisture contents less than 70% for horizontal silos and 60% for tower silos, there is minimal corrosion and pollution threat. Above this moisture level significant flow of silage juices (or seepage) from silos may occur (Table 1 and Figure 1).

Wet weather can force farmers into harvesting wet silage with resulting silage seepage, even when the greatest of care is taken.

Most of the environmental problems associated with silage/haylage seepage on farms result from improper or inadequate collection and retention of the seepage draining from the silos. An adequate collection and storage/treatment system is essential.

See Table 2 for information on the acids in silage seepage that cause silo corrosion. Detailed information on silo corrosion is available in OMAFRA Factsheets Order No. 90-236, Concrete Tower Silo Maintenance and Repair, and Order No. 08-057, Deterioration of Concrete Tower Silos.

Why Silo Seepage Is An Environmental Problem

During 1983 farmers in Ontario made 4.1 million tons of corn fodder, producing in the process approximately 20 million litres of silage seepage effluent. This effluent can be the most polluting organic surface discharge that occurs from farming. The potential oxygen-consuming capacity of effluent is measured by the biochemical oxygen demand test (BOD). Silage effluent in an undiluted form has extremely high BOD values, ranging from 12,000-90,000 mg/L (Table 3), which is approximately 60-450 times stronger than domestic sanitary sewage. A significant discharge of effluent into a watercourse can remove so much oxygen that fish and other aquatic creatures die immediately. For example, as little as one gallon of silage effluent can lower the oxygen content of 10,000 gallons of river water to a critical level with respect to fish survival.

Figure 1. Horizontal silo - seepage production based on silage moisture content.

There have been a number of fish kills from silage seepage in Pennsylvania, New York and Ontario. There are cases of silage seepage contaminating wells and ditches each year in Ontario and the United States.

Table 2. Aggressive Constituents of Silage Seepage

Lactic Acid: 4%-6%

Acetic Acid: 1%-2%

Butyric Acid: normally less than 1%

Acidity: 4

pH: 3.5-5.5

With respect to ground water quality, silage leachate contains nutrients, acids, minerals and bacteria. Nitrate-nitrogen is the most significant ground water contaminant from this group. The main constituents of silage seepage are listed in Table 3.

Rate and Volume of Seepage Flow

The addition of acid additives to silage combined with short chop silage lengths results in a higher initial rate of seepage flow. The greatest percentage of silage seepage is produced within 5 to 10 days of storage loading. The remaining seepage is produced within the next 30 days. The volumes produced are dependent on the vertical pressure in the silo and the initial moisture content of the crop (Figure 1). Seepage flow out of the silo will be greatest during the first month of storage and then taper off in silos with good internal drainage, i.e. network of floor drains to carry out leachate. Where internal drainage of the silo is poor, flow will occur throughout the total storage period as the silo is being emptied. Rainwater on uncovered silage can produce additional effluent.

For horizontal silos, the rain runoff or snow melt from the floor area inside the storage adds more effluent to the system. The highly polluted base flow will be augmented on occasion by the diluted flow from rainstorms and snowmelt. The first flush of rainwater runoff from the storage will contain higher levels of pollutants. It is important that all the base flow from the silo along with the first flush of precipitation runoff be collected and stored since this material is highly contaminated.

Storage and Treatment of Silage Seepage

The seepage and runoff may be stored in a small storage at the silo location and transferred to an outdoor liquid manure or runoff storage on the farm. Contain silage leachate only in an outdoor storage, because dangerous gases may be produced when the effluent and manure are mixed. Where outdoor liquid manure or runoff storages are not currently available on the farm, provide a separate storage to contain 240 days of seepage plus runoff material. During the cropping season this contaminated material can be spread regularly on land similar to manure application. If seepage is being applied to the land, the amount of material being applied needs to be accounted for in the Nutrient Management Plan.

Another means of handling and treating the effluent involves collecting and storing the low flow rates of concentrated leachate from the silo in a storage tank. The dilute high flow rates of material will overtop the collection area and flow to an approved vegetated filter strip (Figure 2).

Have a qualified person design a vegetated filter strip. The design must receive approval under the Ontario Water Resources Act through the Ministry of the Environment.

Source: Cornell University 1994 and OMAFRA

Figure 2. Horizontal silo front flow seepage system - diluted liquid to vegetated filter strip.

Reduction of Seepage

• Harvest silage/haylage at low moisture:
  
  • < 65% moisture content for tower silos less than 40 ft. deep
  • < 60 % moisture content for tower silos over 40 ft. deep
  • < 70% for horizontal silos
• Planting shorter season varieties of corn will result in a drier crop; therefore, lower seepage production.
  

Bunker Silo Sealing Systems

• Reduce silage infiltration by air and water.
• Traditionally, a sealing system consists of a layer of white or black plastic used as a cover and seal. Old tires are placed edge to edge over the surface of this plastic to help in sealing.

New Silo Sealing System, "no tires used"

• Traditional plastic sheeting is covered with an additional cover. Instead of tires, sausage-bags, which are filled with sand or gravel, anchor the cover in place (Figure 3). The advantages of this system are the added protection, improved sealing, flexibility, and ease of installation and storage of the sandbags.
• A polyethylene sleeve holds together several of the sausage-bags across the width of a silo. This product reduces the chance of air infiltration between the sausage bags. Figure 4 shows sausage bag placement.
• The sausage bags can be used directly on the silo plastic. This reduces the cost, and replaces the use of tires. This is a good solution if birds or animals tear the plastic seal.

Figure 3. Tarpaulin and sausage bag system for silage protection.

Figure 4. Sausage bag placement.

Adding absorbents designed to take up excess moisture will result in very low or no seepage production. Useable materials include oatmeal, dried sugar beet pulp, dried corncobs, ground corn and hay cubes. To be effective, enough material must be added to absorb the anticipated seepage.

On many occasions it may not be possible to wilt the forage adequately or harvest at the desired dry matter content. If the forage is too wet, then seepage is likely. Absorbent materials can be added to "absorb" this seepage. Table 4 lists the water holding capacity of various materials.

1. Materials are on an air-dry basis
* 10% moisture content
** 12% moisture content

Source: University of Minnesota (1980)

Managing Silo Seepage and Precipitation Runoff

• Cover the silos - this prevents precipitation from entering and leaching through the silage/haylage.
• Divert all surface water away from the silo site.
• For new silos install a seepage collection and storage system as shown in Figure 2, Figure 5 or Figure 6.
• Inspect the interior silo surface each time the silo is empty for signs of corrosion. Whenever corrosion is severe, recoat the inside of the silos.
• For existing horizontal silos, place a 4 in. tile drain on the floor where the wall meets the silo floor (Option A, Figure 7), or for new silos form holes in the wall to drain silo seepage to an outside drain (Option B, Figure 7. Caution: Provide protection of steel from silage acids with adequate concrete cover (i.e. min. 3 in.)
• For existing or new horizontal silos with good floor drainage to the front of the silo, a catch basin that collects seepage and drains to a long-term storage tank will be suitable (Figure 8 and Figure 9). Flow will occur throughout the total storage period as the silo is emptied. Diluted flow can by-pass the storage tank and overflow to the approved vegetated filter strip (Figure 2).

Figure 5. Tower silo seepage storage system.

Figure 6. Horizontal silo seepage floor drain collection system.

Notes for Figure 6:

• Cross drains 3 in. x 3 in. on 20 ft. spacing. Filled with 7/8 in. clear stone. (Drain to pick up seepage and first flush of rain runoff.)
• Header drain 4 in. x 4 in. to drain cross drains to storage tank.
• Rain runoff from top of storage may be considered as clean water and will not reach the collection system.
• Rain runoff from inside of storage should be collected, stored and spread on cropland.
• Diluted rain runoff may be treated by an approved vegetated filter strip.

Figure 7. Outside drain collection system for existing horizontal silo.

Notes for Figure 7:

(A) 4" diameter tile drains placed on silo floor.

(B) Holes in silo walls to an exterior covered drain. Rain runoff from inside storage should be collected, stored and spread on cropland.

Diluted rain runoff may be treated by an approved vegetated filter strip.

Figure 8. Low flow collection system. (Source AEM)

Figure 9. Low flow collection system.

Caution: Never mix silage effluent in enclosed tanks, especially tanks within barns because silage effluent mixed with manure slurry will accelerate the release of hydrogen sulphide gas. Add seepage only to uncovered outdoor storages.

Disposal of Seepage

Dilute the concentrated seepage with the same amount of water (1:1) and spread this material on land using liquid manure spreading guidelines. Seepage is a nutrient; therefore, the amount of seepage being applied needs to be accounted for in the Nutrient Management Plan. Seepage also may be used as a supplementary feed. Fresh effluent may be fed to pigs and cattle or one may feed "stored effluent" if collected in closed drains and stored in airtight containers. High potassium and nitrate levels can cause problems, therefore, feed with expert advice only. Some research in Europe indicates that feeding of silage seepage to dairy cows increased milk yields, protein levels and fat levels. Treat dilute material with an approved vegetated filter strip.

Site Location for Seepage Collection Tanks

The Environmental Farm Plan recommends, as a good management practice, locating seepage collection tanks at a separation distance of 200 ft or greater from surface water, i.e. streams, ditches, ponds or tile inlets, and separation distances between seepage tanks and wells at 76 ft or greater for a drilled well and 151 ft or greater for a bored/dug well. Minimum separation distances of 50 ft to a drilled well and 100 ft to a dug/bored well are stipulated under legislation. Locate storage sites for bagged, wrapped or tubed haylage (baylage) at least 30 ft. from surface water sources and field drainage tiles to reduce the risk of contamination.

Sizing of Small Seepage Tanks

A. For Horizontal Silos

• If crop is stored > 70% moisture, size seepage storage for 100 ft³/100 tons of crop stored.
• If crop is stored at/or below 70% moisture, use 50 ft³/100 tons of crop stored.
• The above design criteria will give, in most cases, a minimum of 2 days of storage for the seepage material. With very low moisture crops (< 70% moisture), up to one year of storage can be provided with this design, when no rainwater is collected.
• Rainfall Storage: Size for minimum of 1 in. or 0.083 ft. rain over entire silage storage area flowing to leachate storage in any one day. This material can be transferred to an outside liquid manure or runoff storage. If there is no liquid storage on farm, consider building the leachate storage to contain runoff and seepage for a minimum storage period of 240 days. Another option is to treat this dilute liquid on an approved vegetated filter strip.

Example 1:

Size a storage to contain seepage and runoff from a 40 x 100 x 12 ft. horizontal silo. Apron area is 40 x 20 ft. Crop moisture content is 75%. See Table 5.

Storage Capacity (T)

T70 = 1,080 tons (from Table 5)
         (storage capacity at 70% moisture)

T75 = 0.3 (T70)/(1-M) (storage capacity at 75% moisture)
      = 0.3 (1080)/(1.75)
      = 1,296 tons

Seepage Storage Volume

Seepage = 100 ft³/100 tons x1,296 tons
              = 1,296 ft.³ainfall Storage Volume

Rainfall Storage Volume

= (.083 ft.) x (area of silo + apron area)
= (.083 ft.) x 40 x 100 + 40 x 20) sq. ft.
= (.083 ft.) x (4,800 sq. ft.)
= 398 ft.³

Required Storage Size

= 1,296 + 398 ft.³
= 1,694 ft.³

Seepage and Precipitation Storage Size* (1,694 ft.³)

Use Table 7 to find the dimensions of the required storage capacity = width x length x height
1,764 = 14 x 14 x 9 ft.
2,156 ft.³ = 14 x 14 x 11 ft. (allows 2 ft. of depth for freeboard)

* Transfer leachate material from this storage to permanent outside liquid manure or runoff storage. During cropping season this material can be land spread on a regular basis.

B. For Tower Silos

• If crop is stored > 70% moisture, size seepage storage for 100 ft³/100 tons of crop stored.
• If crop is stored at/or below 70% moisture, use 50 ft³/100 tons of crop stored.
  
The above design criteria will give, in most cases, a minimum of 2 days of storage for the seepage material. Up to one year of storage can be provided with very low moisture crops, i.e. < 60% moisture.
• Tower silo is covered with roof to keep out rain.

Example 2:

Size seepage tank based on the following criteria:

1. 20 x 70 ft. tower concrete silo
   
2. Alfalfa silage at 70% moisture
   
3. For capacity see Table 6 in OMAFRA Factsheet, Tower Silo Capacities, Order No. 88-033.

Storage capacity = 703 tons

Storage capacity = 703 tons

Required seepage storage size

= 50 ft³/100 tons x 703 tons
= 0.5 x 703 = 352 ft³

Storage size (352 ft.³) = width x length x height

Use Table 7 to find the dimensions of the required storage capacity.

384 = 8 x 8 x 6 ft. > 352 okay
512 = 8 x 8 x 8 ft. (allows 2 ft. of depth for freeboard)

Table 5. Capacities for Common Horizontal Silo Sizes (Capacity is in tons for a grass or corn silage density of 45 pounds per cubic ft. at 70% moisture)

The following tables list the approximate wet tons capacity for a number of common silo sizes. The tables take into account a 1:2 sloping front face. Widths given are inside to inside and do not include space taken up by posts and planking. When using these tables, calculate the daily feed removal to ensure enough feed is removed to prevent spoilage. For capacity in Tonnes, multiply figures shown by 0.91.

The following table lists the approximate wet tons capacity for a number of common silo sizes.

Source: OMAFRA Factsheet Tower Silo Capacities, Order No. 88-033.

Source: OMAFRA Factsheet Tower Silo Capacities, Order No. 88-033.