Recirculating Media Filters for Treating Milk House Wastewater

Kevin A. Janni, Professor and Extension Engineer

December 28, 2007

(Note: This is the fourth of a series of five articles on Milk House Wastewater. The previous articles were in publications dated October 13, November 10 and December 8.)

Recirculating media filters (RMF) 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.

Recirculating media filter systems include one or more septic tanks, a RMF unit 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 RMF aerobically treats the effluent coming out of the septic tanks. The purpose of the RMF is to reduce the organic matter concentration in the wastewater to levels similar to household wastewater, which the soil treatment system can treat over a long term. Milk house wastewater on the cooperating farms had biochemical oxygen demand (BOD) levels leaving the septic tanks between 500 and 2600 mg/L. Household wastewater typically has BOD levels around 200 mg/L. Milk house wastewater discharged to a soil drain field trench system without additional treatment to reduce the organic loading will overload the soil treatment. The organic matter and milk fat will plug up the soil and restrict infiltration.

The RMF units used in the study were proprietary products. They were sized according to the milk house wastewater's organic loading and flow rate. The RMF units had two tanks, a recirculating tank with a pump and a media filter vessel. The top of the media filter vessel was at the soil surface and open to the air. Septic tank effluent drains into the recirculating tank. The timer controlled pump doses the media in the filter vessel frequently and uniformly with wastewater in the recirculating tank. The frequent dosing keeps the media filter wet so that the microorganisms growing on the media can breakdown the organic matter in the wastewater. Wastewater typically is recirculated five to ten times through the filter media vessel before it is discharged to the underground soil treatment area. RMF units typically require around 400 square feet of space.

The infiltration area size is based on the expected organic loading coming out of the RMF unit, daily flow rate and soil characteristics. It is critical to infiltration area longevity that the RMF perform well to reduce the milk house wastewater organic load. 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. Drain fields can require between 4.2 and 21 square feet per cow depending on the soil characteristics.

Colostrum from fresh cows and waste milk from treated cows has a large amount of organic matter in it. RMF 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 septic tanks and the recirculation tank require pumping at least once per year to remove accumulated solids and scum. Septic tanks should be inspected quarterly for solids and scum build up to see if pumping is needed more often. The milk house wastewater septic tank and recirculating tank effluent can be land applied in accordance to the farm's manure management plan since it does not contain toilet wastes.

Recirculating media filter system costs, which were site specific, ranged from $12,000 to $20,000 for milk house wastewater flows less than 500 gallons per day. Annual operating costs were estimated to be $200 per year, which included the costs for emptying the septic tanks and running the pump.

The last article will describe another aerobic treatment unit. Fact sheets describing the project, milk house wastewater characteristics and each of the treatment systems are available on the web by going to http://www.manure.umn.edu/ and clicking on 'milkhouse waste'.

Producers interested in discussing milk house wastewater treatment options may contact Neil Broadwater, Regional Extension Educator, at 507-536-6300 or by e-mail. 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.