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Example: Bark Bed System
Assume a 60 cow dairy has an 800 gallon bulk tank. The producer has decided to install a bark bed on a silt loam soil with a maximum width of 25 feet. The elevation difference from the bottom of the septic tank to the top of the lateral lines is 8 feet, and the distance from the septic tank to the bark bed is 150 feet. Assume that no wells, sinkholes, lakes, streams or wetlands are located within 150 feet of the proposed bark bed site. Find the following.
- Required number and size of septic tanks
- Bark bed dimensions
- Length and spacing of the distribution pipes in the bark bed
- Amount of bark or wood shreds needed
- Pump pressure and flow required for the system.
1. Sizing of the septic tanks
- Estimated flow per day is 300 gallons (60 cows x 5 gallons/cow/day).
- The primary and secondary treatment septic tanks are each 1000 gallons. The two design parameters are the 3-day HRT or 900 gallons and the 800 gallon bulk tank. Therefore the smallest septic tank is the maximum of these two values. However, septic tanks are commonly found in 500 gallon increments hence the 1000 gallon septic tank would be specified in the design.
- The dosing tank size is sized with a minimum 1-day HRT so at a minimum a 300 gallon tank is required (larger is OK)
- An effluent filter (Zabel or similar) should be placed between the second tank and the dosing tank.
- The tanks selected could be a 2000 gallon tank with two 1000 gallon chambers followed by a second 500 gallon tank for dosing or a 1000 gallon tank followed by a 1500 gallon tank with a 1000 gallon chamber and a 500 gallon chamber used for the dosing tank.
- Consider future dairy herd expansion when sizing the septic tanks.
2. Calculate the sizing of the soil infiltration area
- The required size of the infiltration area (bark bed) is 2500 ft2 based on the daily flow (300 gpd) and a loading rate of 0.12 gpd/ft2 (from Table 2) for loam soil.
3. Calculate the piping requirements and layout of system
- The total length of the bed would be 250 feet using one distribution line (2500÷10), 125 feet using two distribution lines or 83 feet using three distribution lines. This is based on the 5 foot effective area on both sides of the distribution pipe.
- Using two lateral distribution lines with a center manifold (figure 5) gives a total bed length of 125 feet. The total length of the distribution line is 10 feet shorter than the bed length because of the requirement for 5 feet of cover on each end of the lines. With center distribution each of the two distribution pipes are 57 feet ((125 ft ÷ 2)– 5 ft).
4. Calculate the amount of bark needed for the system
- The total bark bed area is 2500 ft2. The minimum bark depth is 2 feet so the total bark required is approximately 5000 cubic feet or 185 cubic yards (cubic feet divided by 27 cubic feet per yard). An additional 10% of bark should be included for wastage. A total of 200 cubic yards (185 yd3 * 1.1).
5. Determine system flow rate
- The number of perforations for the system is 46 (115 ÷ 5 ft hole spacing = 23 holes per lateral x 2 laterals). With 46 holes, the flow required is 48 gpm (46 holes x 1.04 gallons per hole).
6. Determine the pipe sizing and determine pressure requirements
- The head required in the system is 5 ft.
- The elevation difference was stated as 8 feet.
- In general, 2 or 3 inch pipe is used for the mainline, depending on flow and distance. Distribution pipe are usually 1.5 inch diameter. Using Table 3, the friction loss at a flow rate of 50 gpm is 3.99 per 100 feet of pipe. For a distance of 150 feet this total loss is 6.0 feet (3.99 x 150 feet ÷ 100). An additional 25% of loss or 1.5 ft (0.25 x 6 ft) due to elbows and joints. This results in total pipe friction loss of 7.5 ft.
- No additional friction losses for the lateral pipes are added
- No additional friction loss for the manifold. Manifold sizing is the same as the mainline or slightly larger.
- Total head needed in the pump is (8 + 5 + 7.5) or 21 feet (always round up).
7. Determine the pump size
- From the pump sizing chart below and the calculated requirements of 50 gpm and 18 feet the pump sizing pump “c” would be selected for this site. Pumps “b” and “a” would also meet or exceed these requirements.
Example Layout of System.
Example: Irrigation System
Assume a 60 cow dairy has an 800 gallon bulk tank. The producer has decided to install an irrigation system. Effluent will be applied on pasture land in the winter and cropland in the summer. The pasture land yields approximately 3.5 tons/acre while the cropland is in a corn soybean rotation with corn yields of 150 bu/acre and soybean yields of 45 bu/acre. The proposed application area is flat and is about 300 feet away from the milk house. The elevation difference from the bottom of the septic tank to the top of the lateral lines is 14 feet. There are no wells, sinkholes, lakes, streams or wetlands are located within 150 feet of the proposed application sites. Determine the following.
- Number and size of septic tanks.
- Size of the application areas.
- Number, spacing, and type of irrigation heads and zones.
- Flow and pressure requirements and specify pump.
- Other critical system elements.
1. Sizing of the septic tanks
- Estimated flow per day is 300 gallons (60 cows x 5 gallons/cow/day). The primary septic tank size must be 1000 gallons. This exceeds the minimum of 3-day HRT, bulk tank volume, or 1000 gallon minimum. The dosing tank size is also 1000 gallons to provide the minimum of 3-day HRT.
2. Sizing of the irrigation area
- Because the application area is flat the application area should be sized based on N requirements. Using 3.5 T per acre yield and a nitrogen utilization of 27 lbs/ton (Table 4) the amount of nitrogen required on the pasture is 95 lbs. Using Equations 2, the application area required using the N requirements on the pasture land is 10,900 ft2. For the cropland the average N use in the corn soybean rotation would be 138 lbs/year ((150 * 0.8 + 45 * 3.5)/2) requiring approximately 7400 ft2 of application area. Assuming that effluent is applied 5 months on the pastureland and 7 months on the cropland the area requirements for the winter and summer zones would be 10,900 * 5/12 and 7400 x 7/12 winter application area required is 4500 ft2 and the summer application area is approximately 4400 ft2.
3. Determining the number of zones and irrigation heads per zone
- Using Table 5, the minimum number of Wobbler® heads required is two. One impact head with half circle application will be enough to distribute the effluent to the cropland in the summer. Both the two winter heads and the single summer head will cover more area than is required as calculated in Step 2.
4. Determine the pump size for the system
- From Table 6, the lowest operating pressure recommended for the winter heads is 15 psi. The lowest operating pressure recommended for the impact heads is 25 psi. This is equivalent to 34.5 and 69 feet of head respectively. (The multiplier to convert from psi to feet of water pressure is 2.3).
- The flow rates for the Wobbler® and impact heads at these pressures is 8.8 gpm and 37 gpm respectively. With two Wobbler® heads the total flow would be 18 gpm in the winter.
- Using Table 3, the friction loss in 100 feet of 2-inch PVC pipe at a flow of 40 gpm (summer) or 18 gpm (winter) is 0.73 and 2.64 feet, respectively. With 300 feet of pipe used, the total friction loss in the pipe in winter is 3.33 and 7.92 feet. Since this friction loss is minimal, there is no need to use 3-in PVC pipe.
- If the irrigation heads are set at 6 feet, the elevation difference between the pump and the irrigation heads is 20 feet.
- The total pressure on the system is the sum of all the head pressure, friction loss and elevation difference for the zone.
- The winter zone total pressure is 57 feet (34.5 + (2.2 * 1.25) + 20). The summer zone total pressure is 97 feet (69 + (7.92 * 1.25) + 20).
- Using the pump chart (figure 9) and the winter requirements of 18 gpm and 57 feet of head, the correct pump would be pump “a”. For the summer zone, the requirements are 37 gpm and 97 feet of head. Irrigation systems require the use of high very head effluent so it is best to consult with a pump supplier when selecting the proper pump.
5. Additional Information
- Two valves are required, one for each zone in the system to allow switching between the summer and winter zones. These valves are typically installed in the system where the main supply line splits to the two zones.
Typical pump curve for high head effluent pumps.
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