NC State Extension Publications

Introduction

In North Carolina, wastewater from a variety of sources is land applied to recover its beneficial components. Nitrogen (N), phosphorus (P), potassium (K), organic matter, calcium (Ca), magnesium (Mg), manganese (Mn), copper (Cu), and zinc (Zn), as well as the water itself, are all beneficially used for crop production. Wastewater can be land applied with a variety of techniques. Spray irrigation is a common method, and can result in release of odor-causing gases or in drift of fine droplets that transport odor-causing gases.

Another related issue is the loss of gaseous ammonia (NH3) during wastewater application. Ammonia in the soil readily converts to plant-available N forms, ammonium (NH4+) and nitrate (NO3-). One thousand gallons of anaerobic hog lagoon effluent contains about 4 to 6 pounds of total N. Irrigation of anaerobic hog lagoon effluent can result in loss of one-third to one-half of the applied N through ammonia volatilization (Balla, 2007). Although some organic gases in wastewater are likely the predominant cause of odor, ammonia is a pungent gas and can contribute to odor during land application. Loss of ammonia reduces the N value of the effluent, which may affect crop yield if not accounted for correctly. Ammonia losses can adversely affect public health and the environment. Methods for reducing ammonia loss during application can help minimize these issues.

This publication addresses application techniques that affect drift and odor problems associated with wastewater application, so that managers and designers of land application systems can make wise decisions on how to apply wastewater with minimal impact on neighbors and the environment.

Regulations

North Carolina requires permits for the land application of all types of wastewater (agricultural, municipal, and industrial). Typically these permits have conditions that state that wastewater shall not leave the intended target area or reach surface waters or wetlands by way of drift. Permit conditions that deal with odor are usually general in nature and state that the manager should control objectionable odors and not cause a public nuisance. These situations are difficult to define and are very subjective by nature, and thus are dealt with on a case-by-case basis.

When an odor complaint is made concerning wastewater application, an on-site assessment will determine whether the facility or the application of wastewater from the facility justifies regulatory action (North Carolina Administrative Code 15A NCAC 2D .1800). Usually, regulatory action starts with development of a management plan to reduce the likelihood of additional odor complaints. The plan can include management decisions or changes in equipment, or a combination of both. More detail on these options follows.

Factors that affect drift and odor

All types of wastewater, regardless of degree of treatment, contain some odor. This publication will explain the mechanics of odor loss from a wastewater application system and offer guidance to reduce odor losses. Odor is generated during wastewater application when dissolved odorous gases are released into the air or when fine effluent droplets evaporate, releasing the dissolved gases into the atmosphere.

Drift and odor during land application are affected by several factors. Physical or landscape features, such as windbreaks, can reduce drift and odor. Wastewater properties, such as total solids and pH, can also affect odor. Soil properties, such as soil texture, can affect odors as well. Application and weather factors that affect drift and odor include:

  • Droplet size from the land application equipment
  • Height from ground surface and angle of discharge of wastewater
  • Discharge pressure of the land application equipment
  • Nozzle size and type
  • Weather conditions

Droplet size: As the size of the droplet increases, the potential for drift and odor loss decreases. Some waste application systems apply wastewater such that individual droplets can­ not be recognized, the wastewater is applied in a “flooding” pattern. Application systems that create very fine droplets and aerosols create the highest potential for drift and odor movement.

Height of discharge: The height of the wastewater discharge relative to the ground surface has an impact on drift and odor loss for two reasons. First, when wastewater is released higher above the ground or the stream is angled upward, it encounters higher wind speed. In some application systems, the maximum height of the wastewater is not the height of discharge because the nozzle is angled up at some degree above the horizontal (up to 45 degrees). Although the higher angles provide greater wetted area coverage, they cause greater drift and odor. Finally, drift and odor (due to release of gases) will be greater because the wastewater stream stays in the air for a longer time due to greater height and angle of discharge.

Discharge pressure: The higher the pressure at discharge, the greater the potential to create smaller droplets.

Nozzle size and type: Nozzles are de­signed for a range of operating pressures. Droplet size will increase as the pressure is decreased. The nozzle type has a big influence on droplet size: the ring nozzle breaks up the droplets the most, whereas the taper bore nozzle gives the biggest droplets for the same orifice size and operating pressure. The taper ring nozzle is intermediate between the ring and taper bore nozzles in droplet size.

Weather conditions: Weather conditions that affect drift and odor include wind speed and direction, temperature, relative humidity, and atmospheric stability. These conditions interact in a complicated way with one another and may have opposite effects on drift and odor. As wind speed increases, drift is increased; however, odor is dissipated faster and does not linger. It is not advisable to spray wastewater when the wind direction can trans­ port drift toward neighbors’ houses and streams. Drift increases in warm and dry (low relative humidity) weather; because the droplets lose more moisture while floating downwards, become much smaller in size, and can be transported farther. Temperature can also increase drift by increasing turbulence in the lower atmosphere; however, its impact on odor is complicated. Although warm temperatures increase the activity of odor-causing bacteria, land application in warm weather will result in faster dissipation of odor due to greater turbulence. Increased relative humidity increases the perception of odor.

The atmosphere is stable when the air close to the ground is colder and heavier than the air above it. This usually happens during early morn­ ing hours when winds are low. Land application under stable conditions will result in odors being trapped close to the earth and spread­ing sideways, causing complaints from neighbors. However, drift is minimized under such conditions due to low wind, cool temperature, and high relative humidity. As the sun heats the ground, the warmed air rises upward, pulling down the relatively cooler air and creating unstable conditions. Unstable conditions may cause more drift, but less odor, because of better dissipation of gases.

It is clear that environmental conditions can affect drift and odor in different ways, making management difficult. It is generally recommended that spray application of wastewater should be done under low wind (less than 5 mph) conditions and after mid-morning, preferably in bright sunshine.

Selection Considerations

Several factors should be assessed when determining what type of wastewater distribution equipment is best for a given situation. Some of the options listed in Table 1 offer excellent control of drift and odor, but they may be impractical where large volumes of wastewater must be handled. The need for odor and drift control must be balanced with equipment costs and management requirements.

The need for odor and drift control may require a field-by-field deci­sion because it is related to the area where wastewater application occurs. Are there neighbors nearby, or is that potential likely? Is the site close to high-impact areas (historic sites, parks, hospitals)? Have there been complaints; and if so, what is the frequency of these complaints? If complaints or regulatory pressure has been low, then perhaps simple solutions such as irrigation timing decisions or a vegetative windbreak are all that is needed.

Any type of management strategy that affects the type of crop or tillage must complement the farm's exist­ing erosion-control plan. Further, soil incorporation may have limited value, as drift and odor can occur during application. Incorporation immediately after application can better control odor than allowing a waste product with persistent odors to remain on the surface. Most irrigated wastewater will soak into the soil and usually not result in persistent residual odor. In taller vegetation, odor may persist longer than on bare soil, because as the liquid intercepted by the vegetation evaporates, odorous gases are released. Liquids with higher solids content can present persistent odor problems. For those materials, soil incorporation in coordination with the farm management plan may be a good odor control measure.

There are many factors to assess when selecting wastewater application methods. Along with odor and drift, consider cost, ease of operation, applicability to the site, and maintenance requirements. Table 2 gives some general guidance and comments concerning various land application systems. The system types listed in Table 2 are not exactly parallel to those in Table 1. Table 2 defines the systems by category, without focus on the specific nozzle or sprinkler types that can be used with each system.


Table 1. Most commonly used types of wastewater application equipment with simple definitions and relative potential for drift and odor.
Equipment Type Description Relative Potential for Drift* Relative Potential for Odor*
Big gun A sprinkler with a large bore opening, ranging from 0.5 to 2.0 inch diameter. Typical operating pressure is 40-80 pounds per square inch (psi). High High
Stationary Typical height above ground surface is 4 to 6 feet
Traveling (Figure 1) Typical height above ground surface is 4 to 6 feet
Center pivot / linear move Typical height above ground surface is 10 to 12 feet
Impact Sprinker A sprinkler with a small bore opening, generally from 18 to 38 inch diameter. Typical operating pressure is 25 to 60 psi. Moderately high to high
(height dependent)
High
Stationary Typical height above ground surface is 1.5 to 5 feet
Center pivot / linear move Typical height above ground surface is 10 to 12 feet
Drop Nozzle A nozzle typically attached to a drop hose and pressure regulator to allow water discharge at a height just above or just below the crop canopy. Typical operating pressure is 25 to 60 psi. Low to moderate Low
Center pivot / linear move (Figure 2) Typical height above ground surface is 3 to 6 feet
Boom sprayer Typical height above ground surface is 2 to 4 feet
Low drift drop nozzle A specialized drop nozzle designed to create a stream of water and minimize the fine droplets that are prone to drift. Typical operating pressure is 5 to 20 psi. Very low Very low
Center pivot / linear move Typical height above ground surface is 3 to 6 feet
Boom sprayer Typical height above ground surface is 2 to 4 feet
Large diameter, low pressure discharge hose A device or opening designed for a large volume discharge at a very low pressure (less than 5 psi). These nozzles are typically 2 inches and larger, with a swath width usually less than 10 feet. Very low Very low
Boom sprayer Typical height above ground surface is 1 to 3 feet
Hosedrag sprayer
(Figure 3)
Typical height above ground surface is 1 to 3 feet
Tanker wagon These are liquid tankers that also use a large diameter, low pressure nozzle for water distribution. Typical operating pressure is less than 10 psi. Very low to low (height dependent) Low
Broadcast Typical height above ground surface is 3 to 10 feet
Injected Very low to none Very low
Drip emitter This is a specially designed tubing that discharges very low volumes of wastewater at low pressure. While internal operating pressures within the tubing may be high, discharge pressure is very low. Very low to none Very low
at ground surface
below ground None None
For wastewater application systems not shown here, information may be available from the manufacturer or equipment dealer.
* Source: Sheffield.

Table 2. Factors for selection and operation of various types of wastewater application equipment.
System Type Relative Cost Operational Demands Applicability to the Site Maintenance Requirements Comments
Stationary big gun Moderate Low Not suited to small fields or fields with slow infiltration rates Moderate
Small impact sprinkler Moderate Low Suited well to small or oddly-shaped fields Moderate Smaller nozzles clog readily with wastewater solids.
Traveling gun unit Moderate
(A tractor is assumed to be available)
Moderate Suited to medium to large fields. Not suited to moderately sloping or irregularly shaped land Moderate Mobile, easy to add additional acres.
Center pivot and linear move systems High Low Suited to large fields only. Not suited to moderately sloping or irregularly shaped land Moderate Per acre cost decreases as field size increases. Small nozzles are prone to clogging with wastewater constituents.
Boom sprayers Moderate High Not suited to moderately sloping or dissected land Moderate Typically used in buffer or problem areas. Not practical to move frequently to cover large areas. Small nozzles are prone to clogging with wastewater constituents.
Hosedrag systems Moderate High - requires full-time operator Applicable to a wide range of conditions. Possibly suitable for areas that cannot be covered by irrigation systems. High - unit plus tractor Relative cost does not include tractor, assumes one of adequate size available.
Tanker wagon system High High - requires full-time operator Applicable to a wide range of conditions. Possibly suitable for areas that cannot be covered by irrigation systems. High - unit plus tractor Cost figure does not include tractor, assumes one of adequate size available.
Drip irrigation Very high Very high Suited for small fields and small flows High Could be used in buffers/sensitive areas in conjunction with other systems. Wastewater must have very low suspended solids.

Figure 1. Traveling gun irrigation system.

Figure 1. Traveling gun irrigation system. Gun pressure typically ranges from 40 to 80 psi.

Figure 2. Center pivot or linear move.

Figure 2. Center pivot or linear move with high-pressure (20-30 psi) drop nozzles.

Figure 3. Hosedrag sprayer.

Figure 3. Hosedrag sprayer with five nozzles operating at less than 5 psi.

Summary

Many factors affect the potential for drift and odor from a wastewater application system, as well as the potential for such drift to become a problem for the community or environment. Drift and odor will increase with application equipment factors such as operating pressure and discharge angle (and height). Weather factors such as wind speed affect both drift and odor; however, some weather factors that can in­ crease drift (such as high temperature and low relative humidity) can reduce odor. An operator should evaluate both management and equipment factors to help with odor and drift control.

Acknowledgements

This publication is a revision of an earlier version. The authors would like to thank K. Shaffer for his earlier contributions.

Authors:

Undergraduate Teaching Coordinator
Crop and Soil Sciences
Extension Specialist and Professor
Biological & Agricultural Engineering

Publication date: May 11, 2015
AG-439-69

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.