Bark Bed Systems for Treating Milk House Wastewater

Bark beds were one of four types of milk house wastewater treatment systems designed, installed and monitored on Minnesota dairy farms as part of two EPA 319 Grant funded projects administered through the Minnesota Pollution Control Agency (MPCA). The effort, which began in 2002, required significant funding and collaboration by other federal, state and local agencies, and the cooperating dairy producers.

A bark bed is a soil infiltration area covered with bark or wood shreds where treated milk house wastewater is distributed uniformly. Prior to distribution in the bark bed, the milk house wastewater flows through one or more septic tanks for primary treatment. Grit, dirt, other solids, some organic matter and milk fat are removed in the septic tanks. Some of the organic matter is broken down by anaerobic microorganisms. An effluent filter in the last septic tank prevents large particles from leaving the tank and being pumped into the bark bed.

Effluent from the last septic tank flows into a chamber where a pump with float controls pumps the wastewater to distribution lines that uniformly distribute it in the bark bed. A pressurized distribution system with a pump allows the bark bed to be located quite some distance from the milk house.

The bark bed distribution pipes, spaced at 10-foot intervals, can be up to 220 feet long. The pipes can be either laid on a gravel bed or hung in plastic chambers commonly used for septic drain fields. The infiltration area needs to be on relatively flat soil (slope of 6% or less) with at least two feet of separation to the seasonally high water table or bedrock. Effluent spreads out over the soil 5 feet on both sides of each distribution pipe. Once the distribution lines are laid, the entire infiltration area is covered with bark or wood shreds to a depth of 18 to 24 inches. The bark covering protects the infiltration area from freezing, enhances oxygen transfer, and aids wicking and evaporation of moisture from the system.

Bark bed size is both site and operation specific. In general, assuming five gallons of milk house wastewater per cow, a sandy-loam soil requires around 31 square feet per cow for the bark bed infiltration area. A site with loam soil typically requires 40 square feet per cow while a clay-loam requires 75 square feet per cow. Bark beds can be located in wooded areas.

Effective septic tank treatment and uniform distribution of the milk house wastewater effluent are critical for lengthening the bark bed’s useful life. Soil microorganisms break down organic matter in the effluent. Water either soaks into the soil or evaporates. Septic tank treatment removes some solids and fats, which reduces the organic content of the effluent. In these University of Minnesota studies, the milk house wastewater 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. Overloading the soil bed with organic matter leads to plugging of the soil surface, reduced effluent infiltration and premature bark bed failure.

The septic tanks, which provide primary treatment, are sized to hold at least three days worth of milk house wastewater. The septic tanks require emptying once per year to remove accumulating solids and fats; the effluent is land applied to cropland.

Colostrum from fresh cows and waste milk from treated cows has a large amount of organic matter in it. Bark bed systems are 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 bark beds built and monitored were observed to be very effective. Bark bed system costs, which were site specific, ranged from $6,700 to $25,000. Annual operating costs were estimated to be $150 per year, which included the costs for emptying the septic tanks and running the pump for the distribution system. Additional bark may need to be added every few years as it breaks down over time.

By Kevin A. Janni, Professor and Extension Engineer