Reducing the Risk of Groundwater Contamination by

Improving Milking Center
Wash Water Treatment

Fact Sheet 10

Wash water from the dairy milking center includes wastes from the milking parlor (manure, feed solids, hoof dirt) and milkhouse (bulk tank rinse water and detergents used in cleaning).

From an environmental perspective, delivery of milking center wash water to a liquid manure storage facility, if available, makes the most sense. Dewatering options are overland flow and slow surface infiltration. Overland flow is the more effective option.

Your drinking water is least likely to be contaminated if you follow appropriate management procedures. If you choose to dispose of excess wash water off the farm, use proper offsite disposal practices to avoid contamination that could affect the water supplies and health of your family and others.

Combining milking center wash water with manure has the advantage of allowing a common disposal system for both types of waste. A liquid manure storage facility, properly constructed and sized, provides the additional flexibility of storing wastes until they can be applied at the right time to the right sites (Figure 1).

Applying milking center wash water with manure to fields at rates that do not exceed crop needs for nitrogen reduces the risk of groundwater contamination from either waste. Care must be taken, however, to keep soil phosphorus levels from accumulating to levels that will harm crops. On steep land, application rates must be closely monitored to prevent wash water and manure, along with eroding soil, from contaminating nearby streams and lakes.

Milking center wash water combined with runoff from solid manure storage or livestock lots can be stored in a detention pond. The contents of the pond can be applied to fields when conditions are appropriate. Site conditions that need to be considered before land application include weather, soil moisture, nutrient requirements of present and future crops, and the farm management plan.

Figure 1: Wash water and manure handling options for dairy farms. Source: Agricultural Waste Management Field Handbook, USDA-Soil Conservation Service, 1992.

While soil and plant systems have a large capacity to absorb and use wastes, treating wash water to remove some wastes before it gets into the soil can extend the effective life of a soil application area. Such pretreatment usually consists of a basin that holds the wash water long enough for heavier particles to settle and lighter solids to float.

A settling basin also provides a place for bacteria to decompose some wastes before disposal (Figure 2). This process causes a scum to form on top of the water in the tank. Removing the scum layer every few weeks can keep the system operating more efficiently and allow for annual removal of solids.

If a mechanical separator is used initially to remove the larger particles, a smaller settling basin may suffice. Removing waste products before washing into a settling basin requires extra effort, but it reduces the rate of solids accumulation, which can extend the period between basin cleanouts. Manure and excess feed, for example, can be treated like, and thus combined with, other animal wastes.

Passing wash water through a shallow treatment pond (also called an aerobic lagoon) results in a more thorough pretreatment. Algae growing in the pond generate oxygen, which can help decompose organic compounds without creating obnoxious odors. Mechanical aerators also may enhance aerobic action.

Solids that settle to the bottom of the pond usually decompose in the absence of oxygen. To prevent groundwater contamination, such ponds must be built of an impervious material such as packed clay or concrete, or synthetic liner. In some cases, wash water can be discharged to a lagoon without first going through a settling tank. After settling, the waste is best applied at low rates to croplands. Be aware that decomposition processes in this arrangement may generate odors.

Solids from settling basins are an excellent source of organic matter. Gardeners without access to manure can use the solids to improve topsoil.

Figure 2: Settling basins. Source: Livestock Waste Facilities Handbook, Midwest Plan Service, 1985.

Treating wash water for direct discharge to a stream or lake is too expensive for most dairy farms. The soil provides the most cost-effective treatment and use of wastewater. Direct, daily discharge is not recommended because it will result in application to frozen, saturated, or snow-covered soil. Several methods are available, however, for dewatering and using wash water from retention and treatment structures. Two options are approved for use in Washington. They are:

• Direct cropland application
• Slow surface infiltration

Applying wash water to cropland at low application rates poses the least danger to groundwater or surface water. The soil can assimilate the dispersed manure and crops can use some of the nutrients, thus preventing nutrients from entering groundwater or surface water.

Any methods that involve application of wash water or manure to the soil surface should be preceded by a soil analysis and a plan for use of these nutrients by crops. These applied nutrients should be credited in your fertilizer program.

Dairy wash water can be applied to croplands and pastures by portable irrigation equipment or a liquid manure spreader. Pipes with sprinklers can also be permanently installed to spray wash water over certain areas consistently. Field application timing and rates should be based on site-specific factors, such as soils, the crops to be grown, topography, flood hazard, and proximity to water bodies.

Milking center wash water applied to cropland at low rates poses little danger to groundwater because the soil filters or plants take up potential contaminants. Wash water and manure should be incorporated into the soil whenever possible. Avoid land application of wastes within 200 feet of water courses or water bodies, or in flood plains during flooding seasons unless incorporated. Do not saturate soils, since this can allow rapid percolation to groundwater or runoff to surface water. To maximize the efficiency of this system, harvest the crop or other vegetation. Windbreak or woodlot application may also be suitable, in which case harvest is not needed.

Wash water can be applied at one end of a gently sloping grass filter strip or terrace. By spreading pretreated wash water over a vegetated soil surface, organic compounds and bacteria can be treated or filtered out as wastes flow in sheet form over the sloped, vegetated soil surface and percolate through the soil (Figures 3 and 4). High rainfall could overwhelm this system. This system works best on well-drained, loamy soils with at least 4 feet to bedrock or groundwater. The area should be designed to minimize runoff during heavy rain or snowmelt.

A controlled system of distribution and flow is required. Managing wash water safely requires that the retention pond and land application area be large enough to handle wastes during periods when land application is unacceptable. Harvesting the infiltration area is needed to keep vegetation from decomposing and releasing nutrients that could seep down to the groundwater.

Properly operated, a slow infiltration system poses a moderate risk of groundwater contamination by nitrate and other soluble compounds. There is a low risk of contamination by organic matter, pathogenic (disease-causing) microorganisms, phosphorus, or detergents.

Potential groundwater contamination from your milking center wash water
Your local health department, Conservation District or Soil Conservation Service office.

Sources of financial assistance
Your county Agricultural Stabilization and Conservation Service office.

Review of construction plans
To be sure that sanitation and water quality regulations are being met, contact a dairy sanitarian and your district Soil Conservation Service office.

Securing a permit
Before committing to a new facility, contact the Department of Ecology regional office for your area:

Northwest: (206) 649-7000
Southwest: (206) 753-2353
Central: (509) 575-2491
Eastern: (509) 456-2926

Designing wash water treatment systems
Your county Conservation District, Soil Conservation Service office, or Cooperative Extension Biological Systems Engineering, (509) 335-2910.

Publications are available from sources listed at the end of the reference section. Refer to number in parentheses after each publication.

Groundwater contamination, protection and testing

Managing Livestock Manure to Protect Groundwater EB1717 (1)

Keys to Dairy Manure Management for Water Quality EB1658 (1)

Livestock Manure Lagoons Protect Water Quality EB1642 (1)

Liquid Manure Injection EB1004 (1)

Washington Groundwater: A Vital Resource EB1622 (1)

Washington Agriculture: Sustaining Water, Land and People EB1634 (1)

How Fertilizers and Plant Nutrients Affect Groundwater Quality EB1722 (1)

Design Criteria and General Information

Agricultural Waste Management Field Handbook. 1992. (3) A recent,comprehensive guide addressing animal management and resource protection.

Dairy Housing and Equipment Handbook. Midwest Plan Service. MWPS-7. (2) Presents dairy facility and equipment planning and design. Discusses milking herd facilities, milking centers, manure management, housing needs, and basic farmstead planning principles.

Livestock Waste Facilities Handbook. Midwest Plan Service. MWPS-18. (2) Emphasizes planning and design of livestock waste facilities and equipment. Reports agricultural waste data for manure, bedding, feedlot runoff, and milking center wash water.

Land application of animal waste

How to Calculate Manure Application Rates in the Pacific Northwest PNW0239 (1)

Livestock Waste Facilities Handbook. 1985. Midwest Plan Service. (2) Includes information about animal waste characteristics, collection, and transport to storage, open lot waste handling, land application techniques, and waste use. Worksheet helps producers determine manure application rates for their system.

Publications available from...

1. 1. Your county Cooperative Extension office. There may be charges for publications, postage, and sales tax.
2. 2. Your county Extension office or the Midwest Plan Service, Iowa State University, Ames, Iowa, 50011, (515) 294-4337.
3. 3. Your district U.S. Department of Agriculture, Soil Conservation Service office.

The Homestead Assessment System is a cooperative project of Washington State University Cooperative Extension, Washington Department of Ecology, and the U.S. Environmental Protection Agency Region X

Home·A·Syst team members: Christopher F. Feise and Edward B. Adams, WSU Cooperative Extension Water Quality Coordinators; James D. LaSpina, Homestead Assessment System Project Associate.

Milking Center Wash Water Technical Reviewers: Ronald E. Hermanson, WSU Cooperative Extension; Andy Werkhoven, Dairy Farmer; Anne Schwartz, Washington Tilth; John A. Gillies, USDA-Soil Conservation Service; Philip A. KauzLoric, Washington Department of Ecology; John W. Bernard, WSU Cooperative Extension.

Adapted for Washington from material developed by the Wisconsin, Minnesota, and Kansas Extension Services and Farm·A·Syst Programs. Washington Home·A·Syst development was supported by the National Farmstead Assessment Program.

Information derived from Home·A·Syst worksheets is intended only to provide general information and recommendations to rural dwellers regarding their own homestead practices. It is not the intent of this educational program to keep records of individual results.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8, and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, religion, national origin, age, disability, and sexual orientation. Evidence of noncompliance may be reported to your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published September 1993. Subject Code 376. A. EB1746-F10