Figure 2. recommmended volumes for earthen septage storage basin. |
|---|
Ohio Septage Management in Ohio, Bulletin 854,s varied climate and cropping patterns make year-round land application of septage difficult. Storage facilities provide management flexibility for scheduling field application that reduces the risk of pollution. Adequate storage facilities allow pumping operations to continue until field conditions are suitable for land application. Septage storage is regulated by the County Health Department. The Ohio Environmental Protection Agency and the Ohio Department of Natural Resources have requirements for large storage facilities.
Facilities for septage storage are similar to the facilities used to store liquid manure. A storage facility should be designed by a professional engineer. A properly planned storage facility should include:
Septage may be stored properly in constructed earthen storage basins, below-ground concrete tanks, or above-ground concrete or steel, glass-lined tanks.
Earthen storage basins are earth-walled structures formed by excavation and berming of soil. They are usually partly above and partly below grade. Earthen storage basins provide long-term storage at a low to moderate cost. Proper site selection and soil conditions are necessary to prevent ground and surface water contamination. Evaluate the soil and geology for suitability before constructing an earthen basin. The soil must provide a stable foundation for the storage and not shift or settle after construction. The movement, settling, and shifting of an earthen structure, such as an embankment, could result in a total failure of the storage system.
Soil characteristics and geology of the proposed site should be studied by an engineer or soil scientist to determine soil sealing requirements, appropriate basin-wall sideslopes and dike construction, depth to bedrock, and water-table level. Keep the bottom of the storage at least 10 feet above bedrock and 4 feet above the water table.
In areas with high water tables, you may need to construct an embankment above ground to form the storage. Thoroughly investigate the construction area for existing or abandoned subsurface drainage systems. Remove or seal any existing subsurface drains in the construction area. An overlooked drainage tile can either fill the storage with excess drainage water or drain the storage into a nearby stream.
The soil used to construct an earthen storage basin must provide a seal and have low permeability. The permeability of the soil determines the ability of water to seep out of or into the basin. The coefficient of permeability of the soil lining the bottom and sidewalls of the basin should exceed 1 x 10-7 cm/sec (2.83 x 10-4 ft/day). A field or laboratory test will be needed to determine permeability. Ensure a good seal is provided before the storage is used. Locating and correcting leaks after the storage has been put into service is extremely difficult. If the naturally occurring soils cannot be compacted to the desired permeability, clay may be brought in to form a soil liner. Typically, compacted soil liners for waste ponds are 1-foot thick for impoundments up to 8-feet deep. If the impoundment is between 8- and 13-feet deep, a 2-foot thick soil liner is recommended. A 3-foot thick soil liner is recommended for impoundments from 13- to 18-feet deep. If enough suitable clay soil is not available locally to construct a seal, synthetic liners are available.
Keep clean surface water runoff and groundwater out of the storage to maximize the storage capacity of the basin. Provide surface water diversions around the structure to prevent runoff from entering the storage. Excess runoff reduces the working volume and may cause the storage to "top over" and flow into a ditch, stream, or pond.
The required capacity of a septage storage depends on its intended use. A small storage limits the amount of business a pumping operation can conduct when land application is not possible. The storage must account for the working volume, precipitation, clearance for pumping, freeboard, and the septage that will remain in the storage after pumping. Recommended storage volumes are illustrated in Figure 2.
|
Figure 2. recommmended volumes for earthen septage storage basin. |
|---|
To determine the working volume:
Plan for one to three months of septage storage in Ohio. Storage capacity for precipitation should include the expected average precipitation less evaporation for the storage period and two emergency storms (25-year, 24-hour storms). The average annual rainfall for Ohio varies from 44 inches in southwestern Ohio, to 40 inches in southeastern Ohio, to 32 inches in northwestern Ohio, to 36 inches in northeastern Ohio. Evaporation also varies throughout the state. The annual free water surface evaporation for Ohio ranges from 30 inches in southeastern Ohio; to 32 inches in northwestern, northeastern, and southwestern Ohio; to 34 inches in west central Ohio. Given these figures, the precipitation less evaporation in Ohio may be between 12 and 0 inches. Since most septage storages are used only a few months out of the year--when high precipitation and low evaporation is most likely--at least 6 to 12 inches of depth for rainfall storage should be included in the plan.
Additional storage depth must be provided in case of storms. Provide storage for two 25-year, 24-hour storms. In Ohio, a 25-year, 24-hour storm ranges from 3.7 to 4.8 inches. Incorporate at least 9 inches of storage depth to hold rain from storm events.
A minimum additional freeboard depth of 1 foot is needed for access while pumping and as an additional safety factor. Storage structures permitted by the Ohio Department of Natural Resources (ODNR) require a 3-foot freeboard.
Even a careful pumper has difficulty removing septage from the bottom of a storage facility. An additional 6 to 12 inches of permanent storage volume should be included to allow for a difficult-to-remove liquid and sludge layer at the bottom.
The final storage depth is determined after considering the site, storage volumes needed, groundwater conditions, and pumping equipment requirements. A deeper basin requires less area for the same capacity. A deeper basin also reduces the surface area exposed to precipitation and available for odor release.
The Ohio Department of Natural Resources (ODNR) regulates dam safety requirements for the state of Ohio. Any large impoundment, including earthen storage basins, requires a dam safety permit. A large impoundment is classified as any storage with an embankment height greater than 6 feet or a total volume greater than 15 acre-feet (4.9 million gallons).
A comparison of the requirements for large and small impoundments is listed in Table 9.
| Table 9. Impoundment requirements for small and large earthen septage storages | ||
|---|---|---|
| Small | Large | |
| Embankment height | less than 6 feet | 6 feet or greater |
| Volume | less than 15 acre/feet (less than 4.9 million gallons) | 15 acre/feet or greater (4.9 million gal or greater) |
| Free board | 1 foot minimum | 3 foot minimum |
| Storm volume | Two 25-year, 24-hour storms | 100 year flood |
| Regulatory authority | Local Department of Health | Ohio Environmental Protection Agency Ohio Department of Natural Resources |
| Sources: Ohio Administrative Code. 1981. Dam Permits and Inspection Laws. 1501:21.
USDA-NRCS. 1992. Agricultural Wastes Management Field Handbook. Part 651. | ||
With the primary emphasis on ensuring that adequate storage volumes are planned into an earthen basin, sometimes the area around the storage is overlooked. Using the storage as a point of reference and moving out, consider the important components of the berm and storage site (Figure 3).
Figure 3. Considerations for a septage storage site. |
|---|
Access by loading and unloading equipment and maintenance of the berm and embankment are important. Make the berm at least 12-feet wide to provide a minimum area to maneuver equipment. Slope the berm away from the storage to reduce the amount of runoff entering the storage. Because loading the storage is likely to occur during bad weather, a well-compacted gravel or paved loading area should be provided to reduce access problems and breakdown of the berm or embankment.
Mowing and maintenance of berms and embankments is important to the appearance of the site. Grade the outside berm of the storage to a sideslope no steeper than 3:1 (run:rise) for easier maintenance. Plant the sideslope in grass or other ground cover to minimize erosion.
The potential for drowning always exists when liquids are stored. Provide a safety station (Figure 4) with a pole, rope, and flotation device in a visible, well-marked location along the berm. Post the storage site with signs indicating that the area is a septage or waste storage pond and that dangers exist. Signs are available from safety equipment suppliers.
Figure 4. Safety station for liquid storage tanks and basins. |
|---|
Fence the storage area with at least a five-foot-high fence to keep people and animals away. Position the fence along the base of the embankment allowing enough room to maneuver maintenance equipment around the berm and fence. Set the fence back about 40 feet on the side of the storage used for loading and unloading to provide adequate room to maneuver equipment.
Septage has a foul odor and will create concerns for neighbors. Keeping the storage as far as possible from homes and businesses is the first step in being a good neighbor. Consider the prevailing wind direction and select a site where populated areas are not directly downwind. Wind breaks positioned around the storage help keep odors from being carried off site. Do not plant windbreak trees and shrubs in the storage embankment because the roots can form channels in the soil and weaken the embankment. Discharging septage into the storage below the surface will minimize surface agitation and the release of odors.
Finally, the appearance of the site will have an affect on neighborsSeptage Management in Ohio, Bulletin 854, perceptions of odor. Some people "smell with their eyes," thinking that if a site looks bad, it must smell bad. Attractive and well-kept landscaping goes a long way in calming the concerns of neighbors. Trees, shrubbery, and especially flowers should be planned for the storage site. Remember, do not plant trees and shrubs directly on berm.
The appearance of trucks entering and leaving the facility will also reflect on the operation. Provide a gravel or paved lane to the storage to keep mud from being carried by trucks onto the road. Also store a small supply of lime on site to spread on accidental spills to reduce odors.
Storage tanks are an option when soil characteristics, water table, depth to bedrock, and space limit the use of an earthen storage basin. Tanks can be constructed of reinforced concrete or coated metal. Engineered, premanufactured tanks may be purchased or custom-engineered tanks may be constructed in place by a qualified contractor. Details on the planning, design, and construction of concrete storage tanks is in the Concrete Manure Storages Handbook, MWPS-36, available through your county Extension office.
Tanks must be constructed on a stable foundation. Movement, settling, or shifting of the foundation can result in cracks and even complete tank failure. Below-ground tanks must be designed to withstand all anticipated earth, hydrostatic, and storage loads. Uplift forces from groundwater or a high water table can lift a tank out of the ground when empty. Anchor below-ground tanks or maintain a minimum depth of liquid to counteract any uplift forces.
Like earthen basins, below-ground tanks are potentially hazardous. Protect the storage area with a fence and signs. Include the same safety precautions described for earthen basins.
Below-ground tanks may be limited by depth to bedrock, water table elevation, and available space. An above-ground tank may provide the required storage capacity when a below-ground tank or earthen basin is not feasible. Above-ground storage tanks are usually 10- to 20-feet high and 30 to 120 feet in diameter. Carefully locate access ladders to reduce the risk of accidentally falling into the storage. Check valves and a manual safety valve on pump inlets and outlets must be included to prevent accidental emptying of the storage. A secondary containment berm made of earth around the tank storage site should be constructed to protect the area from an accidental tank or valve failure. The bermed containment area should be sized to hold at least 110 percent of the full tank storage capacity. Be sure to take into consideration the space occupied by the tank when sizing the containment.
Another option for septage storage is to investigate the availability of unused tanks or earthen storages at farms or industrial sites. Farm or industrial sites are often located away from residences and have roadways and other waste handling operations that compliment septage management enterprises. Wastewater treatment plants used seasonally (in summertime recreational areas, for example) may also have empty tanks and clarifiers during the year conducive to storing septage.
Safety is an important concern with existing facilities. Fencing may have to be added or upgraded. Signs should indicate that the tank or basin is used to store liquid. As with all liquid storage facilities, a safety station (Figure 4) should be installed in a convenient location and equipped with a pole, rope, and flotation device.
Septage, like other wastes, produces toxic gases as it decomposes. Entering any confined space that contains even a small amount of waste can be deadly. Eliminate confined, unventilated spaces if an existing facility is being used for septage storage.