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Aerobic treatment units for treating milk house wastewater

Kevin A. Janni

Published in Dairy Star February 1, 2008

(Note: This is the fifth and final article on Milk House Wastewater. Previous articles: Milk house wastewater research leads to recommendations, Bark bed systems for treating milkhouse wastewater, Surface irrigation systems for treating milk house wastewater and Recirculating media filters for treating milk house wastewater.)


Air mixing in an aerobic treatment unit tank.

Aerobic Treatment Units (ATU) 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.

Aerobic treatment units are one part of a wastewater treatment system that includes septic tanks, an ATU and a soil treatment area. The septic tanks provide primary treatment by removing grit, dirt and other solids, some organic matter and milk fat. The septic tanks are sized to hold at least three days worth of milk house wastewater.

The ATU aerobically treats the effluent coming out of the septic tanks. The ATU reduces the organic matter concentration in the wastewater to levels similar to household wastewater, which the soil treatment system can treat over a long term. The milk house wastewater leaving the septic tanks had biochemical oxygen demand (BOD) levels between 500 and 2600 mg/L. Household wastewater typically has BOD levels around 200 mg/L. Milk house wastewater discharged to a soil drainfield trench system without sufficient treatment will plug up the soil and restrict infiltration. ATU systems are not designed to treat the large amount of organic matter and milk fat from colostrum or waste milk from treated cows. 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 ATUs used in the study were proprietary products. The systems were sized according to the milk house wastewater’s organic loading and flow rate. Septic tank effluent drains into an ATU tank. Air is blown to the bottom of the ATU tank and mixed with the wastewater where aerobic bacteria breakdown the organic matter in the wastewater. The wastewater must remain in the ATU tank and be given sufficient air to reduce the BOD level to below 200 mg/L before it is distributed to the infiltration area. An ATU typically requires around 200 square feet of space.

The infiltration area size is based on the expected organic loading coming out of the ATU, daily flow rate and soil characteristics. Milk house wastewater infiltration trenches must have at least 2 feet of separation between the bottom of the trenches and bedrock or the seasonally high water table to handle the organic load and water flow. A mound may be needed in a wet area with a high seasonal water table. Drainfields can require between 4.2 and 21 square feet per cow depending on the soil characteristics.

The septic tanks and the ATU tank require pumping at least once per year to remove accumulated solids and scum. Septic tanks should be inspected quarterly to see if pumping is needed more often. The milk house wastewater septic tank and ATU tank effluent can be land applied in accordance to the farm’s manure management plan since it does not contain toilet wastes.

ATU system costs, which were site specific, ranged from $10,000 to $20,000 for milk house wastewater flows less than 500 gallons per day. Annual operating costs were estimated to be $300 per year, which included the costs for emptying the septic tanks and running the aeration pump.

Fact sheets describing the project and each of the treatment systems are available on the University of Extension Manure website.

Producers interested in discussing milk house wastewater treatment options may contact Neil Broadwater, Regional Extension Educator, at 507-536-6300 or by e-mail at Producers interested in upgrading their milk house wastewater treatment system may want to contact their local USDA NRCS office to investigate opportunities for cost sharing.

Milk house wastewater economics

Many dairy operations add milk house wastewater to a manure storage unit and land apply the mixture onto cropland at agronomic rates. Manure application costs about $10 per 1000 gallons of manure slurry. Using an average value (5 gallons per cow per day), a cow generates about 1,825 gallons of milk house wastewater per year. The application cost alone is $18.25 per cow per year. Storage costs would be additional. Over a 15-year period, the typical life of a wastewater treatment system, total application costs would be $275 per cow, which becomes $27,500 for a 100-cow herd. This cost should be compared to the initial and operating cost of the milk house wastewater treatment systems.

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