Prepared by:
Ronald E. Sneed
Professor and Extension Specialist
Biological and
Agricultural Engineering
James C. Barker
Professor and Extension Specialist
Biological and
Agricultural Engineering
North Carolina State University, Raleigh, NC
Publication Number: EBAE 135-89
Last Electronic Revision: March 1996 (JWM)
Controlled Grazing An innovative method of utilizing wastewater currently being studied consists of grazing cattle on intensively managed bermuda grass or tall fescue pastures fertilized exclusively by land applying lagoon liquid. The grazing area is divided into subunits or paddocks and a group of animals confined to a paddock for one to three days graze the grass uniformly before being moved to another paddock. This rotation continues until the first paddock is ready for regrazing and the cycle repeats. Excess grass is harvested as hay. With livestock lagoon wastewater, pastures can be kept in good condition without additional fertilization, except in dry weather when supplemental water may be needed.
The pasture area to be irrigated will depend on the grass species and amount of available nitrogen in the wastewater. Systems are designed to apply nitrogen at optimum agronomic rates taking into account nitrogen from the cattle manure. Individual pasture paddock size is selected to allow rotation so that cattle are always grazing high quality forage. The number of paddocks can vary, but normally will be adequate to allow pasture regrowth in 2 to 3 weeks between grazing events. There should be adequate cattle numbers to graze the grass uniformly within three days and then move to the next paddock. There should be adequate recovery time for regrowth before cattle return to a paddock. Square paddocks represent the least fencing costs. Pasture length to width ratios should not be greater than about 4:1, especially if cattle have to move out of the pasture for shade and water. Excess traffic can cause compaction problems. A detailed discussion of paddock layout, fencing, cattle watering systems, and f orage management can be found in North Carolina State University publications by Mueller and Green entitled, "Getting Started with Controlled Grazing" and "Managing Pastures Receiving Swine Wastes".
Where possible, the irrigation system should allow watering of an individual paddock on the day after the cattle are moved to reduce direct consumption of waste adhering to the grass and to encourage forage regrowth. The amount of water applied at each irrigation will depend on the total amount of nitrogen and wastewater produced during the growing season divided by the number of applications and total pasture acreage. Where lagoon liquid nutrient concentrations have been highly diluted by rainfall or fresh water flushing, the amount of wastewater applied may need to be increased to supply adequate nutrients to the forage.
Permanent Irrigation Layout It is difficult to give a standard layout for permanent irrigation systems for land application, but some general guidelines can be suggested. Most permanent irrigation systems use Class 160 PVC plastic pipe for mains, sub-mains and laterals and either 1-inch galvanized steel or Schedule 40 or 80 PVC risers to near the ground surface where an aluminum quick coupling riser valve is installed. The pipe is usually buried 18 to 36 inches deep below the ground surface. A 1-inch diameter aluminum riser 12 to 18 inches tall is used to connect the sprinkler to a quick coupling riser valve. Class 200 and Schedule 40 PVC pipe would probably be needed for mains, sub-mains and laterals only in extremely rocky soil or extremely wet conditions where thicker wall, stronger pipe is required.
Sprinkler spacing should be based on nozzle flow rate and desired application rate. Suggested sprinkler spacing is 80 feet by 80 feet based on a minimum nozzle size for wastewater of 1/4 inch and an application rate no greater than 0.30 inch per hour. Normal spacing for irrigation of wastewater is 60 percent of sprinkler wetted diameter. For an 80-foot spacing, the sprinkler should have a wetted diameter of 133 feet (80 ft / 0.60). Recommended nozzle pressure is 50 to 60 psi.
Normally, enough sprinklers are purchased to irrigate an individual paddock or at least an acre at one time. Irrigation of wastewater is accomplished after a paddock is grazed. The amount of each irrigation event will depend on soil moisture levels and the lagoon liquid nutrient concentration to supply needed forage nutrients. With the sprinklers listed at the suggested 80-foot spacing, the 1/4-inch nozzle at 55 psi will apply approximately 0.20 inch per hour, or 5 hours pumping to apply one inch. The 9/32-inch nozzle at 55 psi has an application rate of 0.26 inch per hour and 4 hours will be needed to apply one inch of lagoon liquid.
_____________________________________________________________________________
Nozzle size, Pressure (psi)
inch 50 55 60
_____________ ____________________ ___________
FLOW DIA FLOW DIA FLOW DIA
gpm ft gpm ft gpm ft
_____________________________________________________________________________
Nelson F70APV
1/4 12.8 128 13.6 131 14.0 134
9/32 16.0 134 16.8 137 17.6 140
Rain Bird 70 CWH
1/4 12.9 124 13.6 126 14.2 128
9/32 16.3 131 17.2 133 18.0 135
Senninger 7025 RD-1-EFF
1/4 13.0 127 13.6 131 14.2 128
9/32 16.3 133 17.1 137 17.8 142
_____________________________________________________________________________
Several sprinklers meet these requirements and are available through most agricultural irrigation dealers. Characteristics of three brands taken from manufacturer's literature are given in Table 1. The Rain Bird* and Nelson* sprinklers are brass while the Senninger* sprinkler is plastic with stainless steel springs and fulcrum pin.
While these sprinklers will operate at pressures less than 50 psi and above 60 psi, lower pressures will reduce the discharge rate and diameter of coverage and give larger droplet sizes. Higher pressures will increase the discharge rate and diameter of coverage and produce smaller droplet sizes increasing the potential for drift.
Using quick coupling riser valves, with covers, it is possible to reduce initial cost several hundred dollars per acre by moving sprinklers from lateral to lateral. The quick coupling riser valve can be protected by placing a cement block around each valve, burying the block at field surface, then filling the core with sand or fine gravel around the riser valve (Figure 1).
Field size, shape, and proximity to the lagoon will determine the main line location. Only full circle sprinklers are recommended. The last sprinkler should be about 100 feet from buildings, roads, property lines, drainageways, water courses, etc. This leaves an area around the outside of the pasture that receives less fertilization, but it prevents spraying wastewater onto an area where it should not be applied.
Lateral pipe size is normally based on selecting a pipe where friction loss will not exceed 20 percent of recommended sprinkler operating pressure. For example, if a 50-psi sprinkler pressure is selected, then maximum allowable friction loss will be 10 psi (0.20 x 50). Inlet pressure to the first sprinkler on the lateral would be 55 psi while the last sprinkler would have a 45-psi pressure. This means that the discharge rate and diameter of coverage are reduced from the first sprinkler on the lateral to the last.
There is another consideration on PVC pipe. Flow velocity should not exceed 5 feet per second (fps). A detailed design accomplishes this objective by using several pipe sizes for the lateral line; however, this complicates equipment purchase and installation so most designers use only one pipe size and at most, two sizes. This means that the flow velocity near the main line exceeds 5 fps but is much lower toward the far end of the lateral.
Size of lateral 1/4-inch nozzle 9/32-inch nozzle PVC pipe, _________________________ ________________________ inches 50 psi 55 psi 60 psi 50 psi 55 psi 60 psi _____________________________________________________________________________ 1-1/4 3 3 3 3 3 3 1-1/2 4 4 4 4 4 4 2 7 7 7 6 6 6 2-1/2 10 10 10 8 8 8 3 13 13 13 11 11 11 4 23 23 23 20 20 20 _____________________________________________________________________________ * Based on using one lateral pipe size.
Table 2 lists the maximum number of sprinklers that can be used for different sizes of Class 160 PVC based on the 20% rule and for two nozzle sizes and three pressures. This design is for an 80 feet by 80 feet sprinkler spacing with the first sprinkler 40 feet from the main line. While this table gives the maximum allowable sprinklers per lateral, fewer sprinklers will give more uniform distribution. Beyond the last sprinkler on a lateral, there should be 5 to 10 feet of pipe used as a trash collector.
Lateral lines should be as short as possible. Individual laterals can be valved; however, each quick-coupling riser valve is closed when a riser is not installed. The main advantage of installing gate valves on individual laterals is that the entire system (main and all lateral lines) are not charged with water each time the pump is started.
The main or supply line is sized so that flow velocity does not exceed 5 fps. Table 3 gives the maximum flow rate for different size main lines.
Pumps Pumps used for land application of wastewater have generally been straight centrifugal pumps, normally powered by a direct drive electric motor. Pumps of this type can be used to pump swine and poultry lagoon wastewater that is relatively free of solids. It should be emphasized that neither this type of pump nor the sprinklers discussed are recommended for wastewaters with solids contents greater than approximately 1 percent without verification from an experienced designer.
_____________________________________________________________________
Pipe Size, inches Flow Rate, gpm
_____________________________________________________________________
2 55
2-1/2 85
3 125
4 210
6 450
_____________________________________________________________________
* If Class 200 or Schedule 40 PVC pipe is used, the designer should
consult the proper friction loss and velocity tables. Maximum flow
rate will be lower than that shown for Class 160 PVC.
A gate valve, discharge check valve, and totalizing propeller-type flow meter should be installed on the discharge side of the pump. The suction line and strainer should be floated in the lagoon such that the intake is about 18 inches below the water level to draw the most solids-free liquid. The pump should also be located as far from the inlet pipe to the lagoon as possible. If the lagoon is located in an area where a prevailing wind direction exists (particularly a long rectangular lagoon), the pump should be located on the upwind side of the lagoon since solids tend migrate to the downwind side by wind and wave action.
psi feet
sprinkler pressure = __________ x 2.31 = __________
1/2 of lateral line friction loss = __________ x 2.31 = __________
friction loss in main line = __________ x 2.31 = __________
riser height = __________
elevation difference = __________
Total (TDH) = __________
psi feet
sprinkler pressure = 55.0 x 2.31 = 127.0
1/2 of lateral line = (5.8 psi / 2) = 2.9 x 2.31 = 6.7
friction loss
friction loss = (0.96 psi/100 ft x 1060 ft) = 10.2 x 2.31 = 23.6
in main line
riser height = 1.5
elevation difference = 25.0
__________
Total (TDH) = 183.8
Electric motors up to 7.5 horsepower (hp) and in some locations to 10 hp can be installed on single-phase power lines without phase converters for three- phase service. This presents a limitation in many rural areas where three- phase power is not available. Growers may be limited to using the smaller single-phase motors or using internal combustion engines if they want to pump at rates exceeding the capacity of a 7.5- or 10-hp single-phase motor.
To compute motor or engine horsepower required, the flow capacity (gpm) and total dynamic head (TDH) has to be determined. Flow capacity is determined by multiplying the number of sprinklers operating at one time by the capacity of one sprinkler. The TDH is determined from the worksheet at the bottom of this page. Friction loss in the lateral line is determined from an irrigation slide rule and then divided by 2. Main line friction loss is determined from an irrigation slide rule or from friction loss tables. Each of these methods gives friction loss per 100 feet of pipe. This value is multiplied by the total pipe length divided by 100. The elevation difference is the vertical distance between the pump and the highest point in the field.
As an example of computing pump capacity and TDH (Page 5), assume that seven Nelson F70APV sprinklers with 9/32-inch nozzles are being operated at one time on a 2-1/2 inch lateral. Sprinkler pressure is 55 psi. Main line is 1060 feet of 3-inch pipe. Pump capacity is 117.6 gpm (7 sprinklers x 16.8 gpm). Riser height is 18 inches and total elevation difference is 25 feet. TDH is computed to be 183.8 feet.
The equation for computing motor or engine horsepower is:
pump capacity (gpm) x TDH (feet)
HP = __________________________________________
3960 x pump eff x motor or engine eff
Pump efficiency will vary from approximately 50% for a small self-priming
pump to 80% or more for a large straight centrifugal pump. Most wastewater
pumps will probably have an efficiency in the range of 60-70%. Electric
motor efficiency is normally taken to be 90%. Air-cooled gasoline engines
have an efficiency of approximately 65%. Water-cooled gasoline engines are
about 70% efficient while diesel engines have an efficiency of about 75%. In
our example, an electric motor is used. Pump efficiency is assumed to be
65%. The calculated motor horsepower is:
117.8 gpm x 183.8 feet
HP = __________________________ = 9.34 hp
3960 x 0.65 x 0.90
The available motor size closest to 9.34 is 10 hp. This provides little
capacity for wear on the pump, wear on sprinkler nozzles and friction loss in
fittings. Some designers will add 5 to 7.5% to the TDH to cover fittings
friction loss. If 7.5% were added for fittings loss in this example, the
required horsepower would be 10.05 hp. A 10-hp motor would still meet the
demand since an electric motor will operate at a small overload without
damage.
GPM pumped Electric Motor, hp
_______________________________
60 - 65 5
85 - 95 7.5
110 - 125 10
175 - 190 15
235 - 250 20
290 - 310 25
_______________________________
Some designers also add additional horsepower so that as the pump, motor and sprinkler nozzles wear, there will still be adequate capacity. While this is a good practice, often it is not followed to minimize equipment cost.
As a general rule in the Tidewater and Coastal Plain region of North Carolina, the following electric motor sizes are needed to pump the amounts of water shown in Table 4 at 80-85 psi pump pressures. Where higher pressures are required, the volume of water pumped will be reduced. Internal combustion engines and/or less efficient pumps will require higher horsepower.
Land Area Needed To minimize the amount of land and irrigation equipment needed, lagoon liquid is irrigated to supply optimum agronomic nitrogen rates to receiver crops. Table 5 provides typical dairy, swine and poultry layer lagoon liquid nutrient concentrations, irrigation rates, and minimum areas of fescue and bermuda grass pastures needed for controlled grazing. These application rates should supply ample nutrients for crop growth but should not be excessive causing soil or water quality problems. Timing of wastewater applications is important since some forages are cool season grasses while others thrive during warm weather. Wastewater should not be applied to these grasses during dormancy. Provisions such as extra lagoon storage, overseeding the summer forage with a cool season grass such as ryegrass, or having pastures with different forages should be considered.
These values also should be used when planning a new system. Existing livestock operations or new units already in operation should begin a program of wastewater sampling and nutrient analyses and use the results to determine application rates thereafter. A wide variation of nutrient concentrations will exist in different seasons. The NCDA Plant Analysis Lab analyzes wastewater for primary and micronutrients for $4 per sample. Lagoon liquid samples can be collected at a flush tank or from about 6 inches underneath the lagoon surface 10-15 feet away from the bank edge. Representative samples from several locations should be combined with about 3/4 pint placed into a pint nonmetallic container, iced or cooled, and transferred to the lab as soon as possible.
_____________________________________________________________
Type of Animal Animal TotalPlant Total
Production Unit Unit LagoonNutrientNutrients
Unit Equivalent Liquid
Live to be
WeightIrrigated,a
acre-inch/ lbs/
animal unit acre
lbs /year inch
_____________________________________________________________
DAIRY
heifer per hd 1000 .25N 137
capacity P2O5 77
K2O 195
milk cow per hd 1400 .34N 137
P2O5 77
K2O 195
SWINEb
weanling-to- per hd 30 .0070N 136
feeder capacity P2O5 53
K2O 133
feeder-to- per hd 135 .034N 136
finish capacity P2O5 53
K2O 133
farrow-to- per 433 .12N 91
weanling active P2O5 35
sow K2O 89
farrow-to- per 522 .14N 91
feeder active P2O5 35
sow K2O 89
farrow-to- per 1417 .39N 136
finish active P2O5 53
sow K2O 133
POULTRY
pullet per 1500 .34N 179
1000 bird P2O5 46
capacity K2O 266
layer per 4000 .93N 179
1000 bird P2O5 46
capacity K2O 266
_____________________________________________________________
Table 5. (continued..)
________________________________________________________
Type of Plant Lagoon Liquid Minimum Land
Production Available ApplicationArea for Liquid
Unit Nutrients Ratec Applicationc
----- ---- ----
#/animal Fescue Bermuda Fescue Bermuda
lbs/ unit
acre capacity ---acres/animal
inch /year---inches/year-unit capacity--
________________________________________________________
DAIRY
heifer 68 17 3.3 5.8 .075 .042
57 14 1.5 1.7 .17 .14
146 36 .75 2.0 .33 .12
milk cow 68 24 3.3 5.8 .10 .059
57 20 1.5 1.7 .23 .20
146 50 .75 2.0 .46 .17
SWINEb
weanling-to 68 .48 3.3 4.8 .0021 .0015
feeder 40 .28 2.1 2.1 .0033 .0033
100 .70 1.1 2.6 .0064 .0027
feeder-to- 68 2.3 3.3 4.8 .010 .0072
finish 40 1.4 2.1 2.1 .016 .016
100 3.4 1.1 2.6 .031 .013
farrow-to- 45 5.4 5.0 7.2 .024 .016
weanling 26 3.1 3.2 3.2 .037 .037
67 7.9 1.6 3.9 .072 .030
farrow-to- 45 6.5 5.0 7.2 .029 .020
feeder 26 3.8 3.2 3.2 .044 .044
67 9.5 1.6 3.9 .086 .036
farrow-to- 68 26 3.3 4.8 .12 .081
finish 40 15 2.1 2.1 .18 .18
100 39 1.1 2.6 .35 .15
POULTRY
pullet 90 30 2.5 3.6 .13 .093
34 11 2.5 2.5 .14 .14
199 67 .55 1.3 .61 .26
layer 90 84 2.5 3.6 .37 .26
34 32 2.5 2.5 .38 .38
199 186 .55 1.3 1.7 .72
________________________________________________________
a Total liquid manure plus average annual lagoon surface rainfall surplus;
does not account for seepage.
b 400-# sow and boar on limited feed, 3-wk old weanling, 50-lb feeder pig,
220-lb market hog, 20 pigs/sow/yr.
c N leaching and denitrification and P2O5 soil immobilization unaccounted
for.
Fertilization rates: Fescue: N = 225 lbs/ac/yr Bermuda: N = 325 lbs/ac/yr
P2O5 = 85 lbs/ac/yr P2O5 = 85 lbs/ac/yr
K2O = 110 lbs/ac/yr K2O = 260 lbs/ac/yr
As an example, suppose a producer with a 1,000-head capacity swine feeder- to-finish unit wishes to irrigate lagoon liquid onto bermuda grass pastures. From Table 5, the total annual volume to be irrigated would be 34 acre-inches (1,000 head x 0.034 ac-in/hd/yr). The total lagoon liquid nitrogen concentration would be 136 lbs/ac-in. The total annual plant available nitrogen would amount to 2,300 lbs N (1,000 head x 2.3 lbs N/hd/yr). The minimum pasture area needed would be 7.2 acres (1,000 head x 0.0072 acres/ hd) and the typical application rate would be 4.8 inches/year.
Not Included: Figure 1. Swing Joint for Quick-Coupling Riser Valve
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