Authors as Published

*Overview of the Virginia Farm Assessment System

Table of Contents

I. Combined Waste Management Method
II. Pretreatment Methods
III. Land Application Methods
IV. Constructed Wetlands
Glossary No. 11
Contacts and References
 Worksheet 11



Wastewater from the dairy milking center includes wastes from the milking parlor (manure, feed solids, hoof dirt) and milk house (bulk tank rinse water and detergent used in cleaning). The amount of wastewater generated varies with milking preparation, equipment use, and the number of cows. A milking center for a 100-cow free-stall operation may use anywhere from 100 to 1000 gallons of water per day, and sometimes more.

Milking center wastewater contains organic matter, nutrients, chemicals, and microorganisms. Poorly designed or mismanaged waste disposal systems can contaminate surface and groundwater with ammonia, nitrate, phosphorus, detergents and disease-causing organisms.

It is illegal under Virginia Department of Environmental Quality (DEQ) regulations to allow milking center wastewater to be discharged off the property or into state waters. Water supplies are least likely to be contaminated if appropriate management procedures are followed.

Proper handling and disposal practices are essential to avoid risking water contamination and health problems. From an environmental perspective, delivery of milking center wastewater to a manure storage facility, if available for treatment and eventual land application, makes the most sense. Common methods for milking center wastewater treatment and management are described below.

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I. Combined Waste Management Method

Combining milking center wastewater with manure allows the use of a common disposal system for both type of wastes. A liquid manure storage facility, properly constructed and sized, provides the flexibility of storing milking center wastewater until it can be applied at the right time to an appropriate site. While this method may result in increased transportation and application costs, nutrients from milking center wastewater can be used to supply crop nutrient requirements and costs. Further, at certain timesof year, the extra dilution water may make the manure slurry more manageable. This option is limited, however, to operations where manure is handled in slurry form, or in treatment lagoons.

Risk to groundwater contamination from land application of milking center wastewater combined with manure is low if nitrogen application rates do not exceed crop nitrogen needs. Care must be taken, however, that phosphorus levels in soil do not reach excessive levels.

Milking center wastewater combined with feedlot runoff or leachate (from manure storage) can be stored in a detention pond. The contents of the pond can be applied to fields when conditions are appropriate.

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II. Pretreatment Methods

While soil has a high capacity to absorb and degrade wastes, pretreatment of wastewater to remove some pollutants before application to the soil can extend the effective life of a land application site. Pretreatment usually consists of a settling tank or basin that will hold the wastewater long enough for heavier particles to settle and lighter solids to float on the surface.

Milking center wastes should not be pretreated in an underground septic tank and disposed of in a soil absorption field. Soil absorption fields become plugged for one or more of the following reasons:

  • Increased volumes of water may not allow adequate detention time in the septic tank.
  • Milk solids and fats, or manure solids may plug the absorption field.
  • Sanitizers used in cleaning may reduce bacterial action in the septic tank.
  • Solids are not removed from the tank regularly and frequently.
When underground systems fail, wastewaterwill most likely surface elsewhere: in a ditch, farmyard, or a field. It is illegal under DEQ regulations to allow wastewater to be discharged off your property.

When a settling tank or basin is used, it should be cleaned every few months (or more frequently!). Otherwise the accumulated material may eventually move to the soil absorption area, clog the spaces between soil particles, and cause wastewater to collect on the surface. Manure and excess feed can be treated like (and combined with) other livestock wastes. Removing these and other waste products before washing into a settling tank requires additional effort, but it reduces the rate of solids accumulation, which can extend the period between tank cleanouts.

A settling tank also provides an opportunity for bacteria to decompose some wastes before the material is applied to the soil absorption area. This process causes a scum to form on the tank water surface. Removing the scum layer every few weeks can keep the system operating more efficiently.

Passing wastewater through a shallow treatment pond results in a more thorough pretreatment. Algae growing in the pond generate oxygen, which can help decompose organic compounds without producing obnoxious odors. Solids that settle to the bottom of the pond are decomposed by anaerobic bacteria in the absence of oxygen.

To prevent groundwater contamination, ponds and lagoons must be lined with an impervious material such as packed clay, concrete or a synthetic liner.

In some cases, wastewater can be discharged to a treatment lagoon without first going throughpretreatment in a settling tank. The material from these lagoons is best applied at low rates to croplands. However, waste decomposition processes may generate obnoxious odors, making them generally unattractive.

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III. Land Application Methods

Treatment of milking center wastewater by conventional methods for direct discharge to a stream or lake is generally too expensive for most dairy farms, and is not allowed by the DEQ. As an alternative, land application usually provides the most cost-effective wastewater treatment method. In a land application system, soil assimilates some pollutants and crops use some of the nutrients, thus preventing or reducing the amount which may enter groundwater or surface water bodies. A suitable land application area should be located at least 150 feet downslope from a well or spring. Suitable land application options may fall into one of three categories:
  • Direct cropland application with liquid manure
  • Direct cropland application through irrigation
  • Overland flow surface infiltration
Direct irrigation and overland flow infiltration must be carefully designed and installed, and carefully managed. Most often they are appropriate only for smaller dairies.

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A. Direct cropland application

Where milking center wastewater is added tothe farm manure storage system, it can be field applied to crops and pastures using large irrigation equipment or liquid manure spreaders. Routine manure testing will provide guidance to permit farmers to include manure/wastewater nutrients in planning for nutrient needs and utilization by crops. Adequate storage can be provided to hold all wastes until they can be appropriately spread as a crop fertilizer.

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B. Low-rate sprinkler application

Pretreated milking center wastewater can be applied to cropland and pastures using small scale irrigation equipment. Where milking center wastewater will be applied without combining with other wastes, pipes with sprinklers can be permanently installed to consistently apply wastewater over dedicated grassed absorption areas. Wastewater application rates can be determined from the wastewater percolation rate in the soil; and the nutrient uptake by vegetation or crops. It is important that this system be designed by an expert to assure low-rate application, and to provide a sufficient number of application areas to allow "resting" between applications.

To properly manage the above system, the crop or other vegetation should be harvested and removed from the field. Harvested vegetation can be fed to livestock, if appropriate, or used as bedding. If harvested material is left on the ground, nutrients may leach into groundwater. Application areas should be rested for about a week prior to harvesting to help prevent compaction and damage to the absorption area. Similarly, avoid grazing cattle, if possible, on dedicated continuous application areas to prevent hoof compaction.

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C. Overland flow surface infiltration

An overland flow system is somewhat simpler than a sprinkler application system. Thismethod may be appropriate for small operations and suitable sites. Pretreated wastewater is applied uniformly across the top of a gently-sloping grassed absorption area in a thin sheet using a perforated pipe header or level application strip. The ideal soil for overland flow systems will have a good infiltration rate, high water-holding capacity, and good nutrient holding capacity. Soils should also be deep and not excessively permeable to the water table. Best results will be obtained on well-drained loamy soils with at least 3 feet of depth to bedrock or groundwater table. This will provide good filtering, and will be capable of supporting high forage yields for maximum nutrient removal. The application areas should be designed so that runoff is minimized during heavy rain or snow melt.

Alternating application areas may be desirable as described in the previous section. Management practices similar to those described for sprinkler application areas are necessary. It is necessary to harvest and remove vegetation from the site so that nitrogen and phosphorus are not released when the vegetation dies.

Uncontrolled gravity systems, such as an open drain pipe, should not be used because the application area remains wet, making mechanical harvesting of vegetation difficult. Also, because of the heavy concentration of flow from the drain pipe, wastewater will be channelized to nearby streams and ditches which is not allowed by the DEQ. By controlling the flow with a pump, wastewater can be uniformly applied and then the area can be allowed to dry out between applications. Alternating applications between several infiltration areas is another way to allow an area adequate time to dry out.

Applying wastewater intermittently in an overland flow surface infiltration system mayrequire a large retention tank or a holding pond and the land area should be large enough to handle large volumes of milking center wastes.

When operated improperly, overland flow surface infiltration systems pose a high risk of groundwater contamination by nitrate, ammonia and other soluble compounds, such as detergents.

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IV. Constructed Wetlands

There is considerable interest in development of constructed wetlands to make use of vegetation to remove nutrients from wastewater. This method may be appropriate for treating less concentrated and diffuse waste materials. However, constructed wetlands have not undergone sufficient longterm testing to warrant recommendation for treatment of milking center and other livestock wastes. Contact your Cooperative Extension or Natural Resources Conservation Service office for additional information.

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These terms may help you make more accurate assessments when completing Worksheet No. 11. They may also help clarify some of the terms used in Fact Sheet No.11.

Application - wastewater in droplet form to the soil.

Field application - Application of wastewater to croplands and pastures by irrigation ditches or equipment.

Overland flow surface irrigation - Application of wastewater to level or gently sloping fields to provide nutrients for actively growing crops.

Slab separator - A type of settling tank used for removing fine particles and sand from wastewater prior to pumping into a holding tank or lagoon.

Soil permeability - The quality that enables the soil to transmit water or air. Fine (heavy) soils such as clay are lowly permeable. Coarse (light) soils such as sand are highly permeable.

Solids separator - A screen apparatus over which milkhouse wastewater slurry is passed, allowing liquids and fine particles to pass through while retaining larger solids.

Sprinkler - Method of field application using pressurized sprinkler nozzles to broadcast.

Contacts and References

For review of construction plans and regulatory requirements, contact the Regional Office of the Virginia Department of Environmental Quality (DEQ).

To design a land application/wastewater treatment system, contact the Natural Resources Conservation Service (NRCS), private consultants, or the Biological Systems Engineering Department at Virginia Tech, Blacksburg, VA.


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 Worksheet 11, Milking Center Wastewater Treatment

View a list of the Virginia Farmstead Assessment System publications.


Reviewed by Bobby Grisso, Extension Specialist, Biological Systems Engineering

Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

May 1, 2009