Milk House Wastewater Research Leads to Recommendations

Every dairy generates milk house wastewater. Dairy producers must decide how to handle their milk house wastewater in an environmentally appropriate and economical way that fits their management. This is the first article in a series that will describe two EPA 319 Grant funded projects administered through the Minnesota Pollution Control Agency (MPCA) that evaluated four types of milk house wastewater treatment systems. These projects were a team effort. Additional significant funding and collaboration were provided by other federal, state and local agencies and the cooperating dairy producers that participated in the two projects.

The purpose of these University of Minnesota led projects was to design, install and monitor sixteen milk house wastewater treatment systems in four Minnesota counties. The systems selected for evaluation were expected to be effective in cold climates. The results of the projects are being used to develop milk house wastewater design and management guidelines.

Milk house wastewater includes wash water from cleaning bulk tanks, milk pipelines, milking units and equipment and milk house floors. It includes residual milk that remains in the pipeline, receiver and bulk tank after emptying and is removed during cleaning. Milk house wastewater also includes cleaning chemicals in the detergents, sanitizers and acid rinses used to clean the milking equipment. And, it usually includes small amounts of manure, bedding, feed, grit and dirt. It may also include water softener recharge water.

Milk house wastewater is regulated under Minnesota Rules Chapter 7020, which restricts milk house wastewater from discharging into waters of the state, either through overland flow or subsurface tile lines. Milk house wastewater can be added to a manure storage system and then land applied at rates in accordance with a nutrient management plan.

Toilet wastes, if there was a toilet in the milking center, were not considered part of milk house wastewater for these projects. Toilet wastes must be handled with a system that meets the requirements to Minnesota Rules Chapter 7080, which regulates human wastewater and septic systems.

Colostrum from fresh cows and waste milk from treated cows has a large amount of organic matter in it, approximately 100 times that found in normal milk house wastewater. The milk house wastewater treatment systems studied for these projects were not designed to treat colostrum or waste milk. Colostrum and waste milk must be disposed of by other means such as feeding it to other farm animals or applying it to cropland.

The mixture of residual milk, chemicals, manure, feed and dirt gives milk house wastewater a high concentration of organic matter, fats and nutrients. This organically rich wastewater is more concentrated than residential wastewater. Milk house wastewater in these studies had average biochemical oxygen demand (BOD5) concentrations that ranged from 500 to 2,600 mg/L. Residential wastewater usually has a BOD5 concentration less than 200 mg/L. Standard septic systems used to treat residential wastewater cannot effectively treat milk house wastewater.

Fourteen of the dairy farms that participated in the projects had stall barns with pipeline milking systems. Two farms had parlors with parlor wastewater mixed in with the milk house wastewater. The milking herds ranged from 40 to 130 cows. Wastewater flow rates were estimated based on water meters strategically placed in the milk house. Wastewater flow rates varied from 2 to 7 gallons per cow per day.

Four types of treatment systems were installed and monitored. All of the systems started with one or more septic tanks sized to provide at least three days of retention in the tanks for primary treatment. The four treatment systems added after the septic tanks were bark beds, aerobic treatment units (ATUs) followed by a subsoil infiltration area, recirculating media filters (RMFs) followed by a subsoil infiltration area, and dairy surface irrigation to pasture or cropland. Bark beds have a large soil infiltration area covered by wood or barn shreds. System costs, which were site specific, ranged from $6,700 to $25,000.

All of the systems studied were effective when designed and managed properly. System design, size, layout and cost depended on water usage, farm layout and terrain, proximity to surface waters, soil types and their infiltration capabilities, and depth to ground water or bed rock. Producer preferences and management were also important factors considered when selecting a milk house wastewater treatment system.

Future articles will describe the four treatment systems. Fact sheets describing the project, milk house wastewater characteristics and each of the treatment systems are available on the University web.