Design and Specifications for Permanent
Wastewater Irrigation Systems for
Controlled Grazing
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
Published by: North Carolina Cooperative Extension Service
Publication Number: EBAE 135-89
Last Electronic Revision: March 1996 (JWM)
In the last several years the number of livestock and poultry on North Carolina farms has increased
significantly. These animal concentrations produce large amounts of manure and wastewater. The
increasing emphasis on protecting water quality has focused considerable attention on making
more effective and economical use of these organic resources.
Most swine production facilities and many poultry layer units use lagoons for manure treatment.
When these lagoons fill, the optimum type of system for land application of lagoon liquid depends
upon site constraints such as land availability, receiver crops and farm objectives. In most cases
sprinkler irrigation will be the most cost-effective land application method. The irrigation equipment
used must be matched with the characteristics of the wastewater being applied. Adequately sized
lagoons which provide sufficient storage capacity and pretreatment will influence the degree of
management required for land application of wastewater. Livestock lagoon design and management is
discussed in detail in North Carolina State University Manuscript EBAE 103-83, "Lagoon Design and
Management for Livestock Waste Treatment and Storage".
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 1inch 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 1/4inch 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.
Table 1. Irrigation Sprinkler Characteristics
_____________________________________________________________________________
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
1/4
9/32
12.9
16.3
Rain Bird 70 CWH
124
13.6
131
17.2
126
133
14.2
18.0
128
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.
Table 2. Maximum Allowable Number of Sprinklers per Lateral Line *
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 quickcoupling 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.
Table 3. Maximum Main Line Flow Rate for Class 160 PVC Pipe *
_____________________________________________________________________
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.
sprinkler pressure
1/2 of lateral line friction loss
friction loss in main line
riser height
elevation difference
Total (TDH)
=
=
=
=
=
=
psi
__________
__________
__________
sprinkler pressure
1/2 of lateral line = (5.8 psi / 2)
friction loss
friction loss = (0.96 psi/100 ft x 1060 ft)
in main line
riser height
elevation difference
Total (TDH)
x
x
x
2.31
2.31
2.31
feet
__________
__________
__________
__________
__________
__________
=
=
=
psi
= 55.0
= 2.9
x
x
2.31
2.31
=
=
feet
127.0
6.7
= 10.2
x
2.31
=
23.6
=
=
=
1.5
25.0
__________
183.8
Electric motors up to 7.5 horsepower (hp) and in some locations to 10 hp can be installed on singlephase 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:
HP
pump capacity (gpm) x TDH (feet)
__________________________________________
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:
HP
=
117.8 gpm x 183.8 feet
__________________________
3960 x 0.65 x 0.90
=
9.34 hp
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.
Table 4. Electric Motor Sizes Based on 65% Pump Efficiency and 80 psi Pump
Pressure
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.
Table 5. Typical Livestock Lagoon Liquid Nutrient Contents, Irrigated Application
Rates and Minimum Fescue and Bermudagrass Pasture Areas Needed for Controlled
Grazing
_____________________________________________________________
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-toper hd
30
.0070N
136
feeder
capacity
P2O5
53
K2O
133
feeder-toper hd
135
.034N
136
finish
capacity
P2O5
53
K2O
133
farrow-toper
433
.12N
91
weanling
active
P2O5
35
sow
K2O
89
farrow-toper
522
.14N
91
feeder
active
P2O5
35
sow
K2O
89
farrow-toper
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
Type of
Production
Unit
Plant Lagoon Liquid
Minimum Land
Available
ApplicationArea for Liquid
Nutrients
Ratec
Applicationc
----------#/animal Fescue Bermuda Fescue Bermuda
lbs/
unit
acre capacity
---acres/animal
inch /year---inches/year-unit capacity—
_____________________________________________________________
Table 5. (continued..)
________________________________________________________
________________________________________________________
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-to68
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-to45
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-to45
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-to68
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
P2O5
K2O
=
=
=
225 lbs/ac/yr
85 lbs/ac/yr
110 lbs/ac/yr
Bermuda: N =
P2O5 =
K2O =
325 lbs/ac/yr
85 lbs/ac/yr
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.
SUMMARY AND CONCLUSIONS
The permanent irrigation system for application of wastewater for controlled grazing is a feasible and
workable system for properly sized swine and poultry lagoons. This paper gives minimum design
criteria. Individual systems should be designed or verified by competent irrigation designers. Large
elevation changes in fields or between the lagoon and the field can create uneven distribution of
wastewater. The system will not handle wastewaters with high solids contents. Pumping from
inadequately sized or maintained lagoons will cause system failure.
Not Included: Figure 1. Swing Joint for Quick-Coupling Riser Valve
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and
program opportunities are offered to all people regardless of race, color, national origin, sex, age, or
disability. North Carolina State University, North Carolina A&T State University, U.S. Department of
Agriculture, and local governments cooperating.
EBAE 135-89
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