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Irrigation

How irrigation systems work

Figure 6. Irrigation system schematic.

Irrigation systems are used to distribute milk house wastewater on pasture or cropland. These systems are designed to distribute the effluent at agronomic rates on large areas with minimum erosion potential defined as flat areas (less than 3% slope) with minimal ground cover or well vegetated areas with up to 15% slopes. Components of the irrigation system include a primary septic tank, a dosing tank, pump, piping, and irrigation heads. Critical design inputs for the system are the daily effluent volume, the effluent nitrogen and phosphorus concentrations, pump and piping requirements to meet the pressure and flow needs of the irrigation heads, and all site elevations. Figure 6 is a schematic of a typical irrigation system. The irrigation system can handle all wastewater from the milk house and unlike the other systems can handle some waste milk.

Pretreatment

Effluent from the milk house must be pretreated in a primary septic tank with a minimum three-day Hydraulic Retention Time (HRT) to remove large solids and some of the fats and oils. The primary tank must also be larger than the bulk tank and have a capacity of at least 1,000 gallons. Effluent from the primary septic tank flows into a second septic tank, sized to provide an additional three-day HRT, that serves as the dosing tank for the system. Additional storage capacity in the dosing tank provides a margin of safety if a system failure occurs or if irrigation needs to be suspended for crop harvesting. Both the primary septic tank and the dosing tank must be constructed and installed according to MN Rules Chapter 7080. Additional tanks for pretreatment can be used for additional solids settling. However, the longer the HRT, the greater potential there is for odor emissions during irrigation.

Sizing the irrigation area

The primary design consideration for sizing the irrigation system application area is the amount of pasture or cropland required for using the nutrients in the effluent at agronomic rates. The nutrient distribution requirements can be based on either phosphorus (P) or nitrogen (N). Wastewater volumes and effluent characteristics suggest that a milk house will produce 0.98 lbs/c/yr of N per year and 0.84 lbs/c/yr of P. Approximately 60% of the total N in the effluent is in the ammonia form (NH3) and will likely be lost through volatilization during irrigation. This means approximately 0.39 lbs of N are available in milk house wastewater per cow per year.

Where:

AP and AN are the area in ft2 required for wastewater application based on either P or N rates.
FP and FN are the crop nutrient uptake in lb/acre for P₂O₅ or N respectively.

For systems where the effluent will be spread to permanently vegetated areas or flat areas where there is little risk of soil erosion, the irrigation area should be based on the N removed (i.e. in the harvested crop) from the area. These values are given in University of Minnesota Extension Publication #BU-06240 Fertilizer Recommendations for Agronomic Crops in Minnesota which are summarized in Table 4. For irrigation sites where the effluent will be spread on cropland that has a high potential for soil erosion the irrigation area should be based on the P removal of the crop. Irrigation areas based on P removal will be significantly larger than those based on N removal.

Using these recommended values the following equations can be used to determine approximate areas needed for nutrient distribution from milk house wastewater only (no parlor water).

Irrigation zones and heads

Milk house wastewater irrigation systems typically are designed with a minimum of two separate irrigation areas or zones. One or more zones are used for winter application and one or more zones for summer application. When effluent is applied to pasture or cropland the zones can be managed to allow one zone to dry prior to harvest or grazing. Additional zones (more than two) may be required based on pumping and pressure requirements but may also be desirable because of the increased flexibility in nutrient distribution and hydraulic loading of the zones to better manage crop harvest or pasture use (e.g. more flexibility drying of zones for harvest or grazing). Multiple zones also offers some backup should one zone fail due to freezing or clogging of the sprinkler heads. The total area required for nutrient distribution is the sum of all the zone areas used.

Wobbler™ head used for winter or summer application.


Impact irrigation head used for summer irrigation.

Irrigation zones are often designated by the type of irrigation head used. During winter, a Wobbler™ head with a 9/32 inch orifice diameter is used. This is a frost-resistant head that emits effluent in a 50 foot diameter circular pattern which covers an area of approximately 2000 ft2. These heads are designed in such a way that they do not freeze during winter application. Wobbler™ heads can also be used in the summer but typically a traditional impact head is used because of their larger spread pattern.

Traditional impact heads have a 100-150 foot diameter distribution pattern (areas of 7800 ft2 to 17,700 ft2). The impact heads should be brass (vs plastic for a longer life) and have a minimum orifice diameter of 1/2-inch to handle the solids that may be in the effluent. Some impact heads allow for part-circle irrigation giving greater flexibility in the layout and management of irrigation zones. Impact heads used can have either a low or standard trajectory (angle of water leaving the irrigation head), either is fine for this use. These heads can be installed on the edge of crop fields and the effluent can be spread over growing crops. Effluent has been spread on pasture, soybeans and corn with no negative effects. Application to alfalfa should be avoided due to its intolerance to high moisture conditions. Two to three days prior to harvest or grazing, the irrigation should be discontinued (switch irrigation zones) to allow for adequate drying of the area.

Effective application area

The effective area covered by a system with multiple risers and sprinkler heads is a function of the diameter of the spread pattern and the amount of overlap which is determined by the spacing of the risers. The overlap pattern is shown in figure 7 and calculations to determine the effective application area with systems that overlap is calculated below. Good overlap (spacing between risers less than 70% of the spread diameter) for good nutrient distribution in an area. However, it is typically more economical to not have any overlap between the sprinkler heads because more area will be covered with fewer heads. In calculating the application area with systems where the riser spacing is greater than 100% use the area covered per head (calculated by ΠD2/4) multiplied by the number of heads in the system. This area should be equal to or greater than the required application area based on nutrient loading.

Application area with heads with 360° spread patterns with overlap

The effective area for systems of head with 360° spread patterns can be calculated using the following equation. Note that this equation slightly overestimates the application area of the irrigation heads.

Where:

EAA = Effective Application Area (ft2)
NL = Number of heads along the length of distribution lines
NW = Number of rows of distribution lines
SL = Spacing of heads along distribution lines (ft)
SW = Spacing between distribution lines (ft)
D = Diameter of spread pattern (ft)

For example, a typical winter zone using Wobbler™ heads supplied wastewater at 10 psi will have a a spread diameter of 52 ft (D). Assuming a head spacing of 30 ft along the length (SL) and a 30 ft width spacing (SW), the effective area for 8 sprinklers in two lines (NW = 2) with four heads per line (NL = 4) as shown in the figure 7 below would be:

((4-1) x 30 +52) x ((2-1) x 30+52) = 142 x 82 = 11,600 ft²

Application area with head with 180° spread patterns with overlap

For a single row of impact heads set to apply only in a 180° spread pattern (such as on a fence line) the effective distribution area is calculated using the following formula.

For example, consider four impact heads (NL = 4) with 100 foot spread diameter (D) set along a fence line as per figure 8. Using a spacing of 80 feet (SL) between heads and setting the heads to only spray 180° the effective application area would be calculated as follows.

Effective Application Area (ft2) = ((4-1) x 80 +100) x 100/2 =17,000 ft²

Figure 8. Typical field layout for summer irrigation along a fenceline.

Irrigation head selection

Performance data for the Wobbler™ and a select impact head is given in Table 6. Avoid designs based on the minimum pressures listed in Table 6. Although it is possible to operate at those pressures the distribution pattern is not as even. The selection of the type and number of irrigation heads and number of zones is a function of several factors that will be discussed later. These decisions impact the pump selection.

Pumping and Piping

Effluent is pumped from the dosing tank through Schedule 40 PVC distribution pipe to the irrigation heads using a high-head effluent pump with high/low floats and a high-alarm. A simple 24-hour timer is recommended to allow for effluent pumping at certain times of the day. Timing of the irrigation is done to avoid odor problems during certain times of the day or to allow for irrigation during periods when the system can be observed. The timer could also be used to schedule winter irrigation during the warmer time of the day to minimize freezing potential.

Pump pressure requirements are a function of the type and elevation of the irrigation heads and the friction losses in the pipes. Table 6 provides information on the irrigation head pressure requirements. Friction losses in the distribution pipes are based on the values in Table 3. Besides the friction losses in the pipe, there are additional pressure losses due to joints in the pipe. These can be calculated on a per joint basis or can be estimated by adding an additional 25% of pipe length and respective friction loss. Note that the individual riser pipes have minimal flow and length and therefore minimal friction loss. Note also that the conversion of pressure in psi to feet of head is done by multiplying the psi by 2.3. For instance, the Wobbler™ head listed in Table 6 requires an operating pressure of 15 psi for a flow of 8.8 gpm. This pressure is equivalent to 34.5 feet of head loss.

Pump flow specifications are based on the number and type of irrigation heads and operating pressures. For any single zone the flow is the sum of all the flow for all irrigation heads in that zone. Pump selection is based on the zone with the highest pressure and flow requirements. Use the calculated pressures and flows and manufacturers’ pump selection curves to select the correct pump. A typical pump curve is shown in Figure 9.

For more information on pump sizing and calculating of pressure losses in the system see Section 9 of the University of Minnesota OSTP Manual.

Distribution pipe

The diameter of the distribution pipe is based on flowrate and expected pressure losses but is typically two or three inches. Distribution pipe (schedule 40 PVC) is placed in trenches at least 18 inches into the soil to avoid freezing. Distribution lines must have a final minimum slope of 1% to insure drainback to the dosing tank. During construction, the distribution pipe trenches should be excavated on a 1.5% to 2% grade to insure the minimum 1% final grade requirements are met throughout the length of pipe. Standard installation practices for pressure pipes should be followed.

Wobbler™ heads

Summer irrigation with Wobbler™ heads.


Winter irrigation with Wobbler™ heads.

Wobbler™ heads are fed by one to two-inch riser pipes coming off the distribution lines. Wobbler™ heads, which are necessary in the winter but can also be used in the summer, must be mounted above the maximum snow depth, typically five to six feet above the soil surface. Risers are spaced every 25-30 feet to allow 50% overlap of the 50 to 60 feet diameter irrigation spray pattern. This overlap provides good distribution of the nutrients on the irrigation area. The elevation of all heads in a single zone should be within one-foot of each other to insure equivalent pressures at the heads and even effluent distribution between heads.

Impact heads

Figure 9. Typical pump curve for high head effluent pumps.


Summer application with impact heads.

Impact heads are fed with two-inch PVC riser pipes. Heads are typically mounted at five to six feet above the soil but can be mounted higher to spray above the crop canopy. This higher mounting height is critical when the effluent is to be irrigated on a corn crop where the heads need to be above the corn.

Generally, use spacing between heads of 70% of the spread diameter. For instance, with a spread diameter of 100 feet, the spacing between heads would be 70 feet.

Riser pipes

Riser pipes, the pipes between the distribution lines and the irrigation heads, have been constructed with 3/4-inch diameter PVC pipe wrapped in expanded foam and covered with 2” diameter PVC pipe. Successful systems have also been constructed with 2-inch diameter non-insulated PVC riser pipe. With rapid system drainback non-insulated riser pipes should be adequate.

Riser pipes must be well anchored to minimize vibration. Pipes are typically attached to 4-inch x 4-inch treated wooden posts or larger using galvanized pipe clamps. Posts should be set in the ground three to four feet in well compacted soil or set in concrete to insure maximum stability. Riser pipes in pastures need to be protected from animal damage using electric fencing. All riser pipes and posts need to be well marked for good visibility to minimize chances of mechanical damage by farm equipment.

Control valves

Valves for controlling irrigation zones located in manhole.

Valves for controlling the flow to different irrigation zones must be accessible throughout the year and insulated or protected from freezing. Control valves can be brass slide-gate or ball valves. Large diameter PVC pipe (8 to 24 in. in diameter) can be used to provide below ground (i.e. manhole) access to valve assemblies. Some type of valve handle extension must be constructed to reach the valves when a small diameter pipe is used for valve access rather than a manhole.

Valve access must be insulated to prevent freezing. Covering the access cover with a straw bale or filling the access pipe with insulation have both proven successful. Placing the control valves on top of the septic tanks also protects against frost.

The control valve manholes should be backfilled with six to ten inches of rock or gravel to prevent rodents from burrowing into the access pipe and covering the valves. The rock should extend 8-12 inches below the bottom of the valve assembly.

Design steps

1. Determine the number and size of primary septic tanks and the dosing tank based on the number of cows and flow estimate or water meter data.
2. Determine the amount of land needed based on crops and estimated nutrients (use phosphorus rates if area is in a cropping system where there is potential for soil erosion, use nitrogen rates when applying to permanent grasses).
3. Determine the number of zones and the number of irrigation heads per zone to meet the land application area requirements. This can be calculated using a trial and error method using equations 3 and 4 or by using Table 5.
4. Determine the flow and pressure requirements for each zone of the system using the irrigation head information for flow and pressure, the difference in elevation between the pump and the irrigation heads and the friction losses in the pipe.

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