Aerobic Treatment Units and Recirculating Media Filtration
How aerobic treatment units and recirculating media filtration systems work
These treatment systems consist of a primary septic tank, an Aerobic Treatment Unit (ATU) or Recirculating Media Filter (RMF), and a subsoil infiltration area. Both the ATUs and RMFs are designed to reduce the organic loading of the milk house wastewater to concentrations similar to household septic wastewater (i.e., 200 mg/L BOD5). This reduction in wastewater strength allows the effluent to be distributed into a standard sized septic soil infiltration system.
Both ATUs and RMFs are commercially available. These systems are sized based on the BOD5 concentrations and daily flow. Both systems use aerobic microorganisms to reduce the BOD5 prior to discharge to a standard septic infiltration area. To maintain these organisms, oxygen is added to the system with blowers, pumps or venturi systems. Many ATUs employ an inert support material to maintain microbial populations (fixed film.)
RMFs add oxygen by pumping the effluent over a porous media filter. The biological community, attached to the surface of this media, break down the organic matter. The media is contained in a watertight vessel either below the surface of the ground or wholly or partially elevated in a containment vessel. Proper function requires that influent to the RMF be distributed over the media in frequent, cycled, uniform doses. To achieve accurate dosing, these systems require a timer-controlled pump with associated pump chambers, electrical components and liquid distribution network. This frequent, cycled dosing keeps the media constantly wet. The effluent is collected in the bottom of the filter and returned to the recirculating/mixing tank where it either mixes with fresh septic tank effluent or a portion is discharged to the infiltration area. Flow splitting mechanisms are used to control recirculation and discharge to the subsoil infiltration area.
Infiltration area design
Infiltration areas are sized based on an assumed treated waste strength of 200 mg/l BOD5, the daily flow from the particular site (gallons per day), and the soil characteristics of the infiltration area. Soil Sizing Factors (SSF) are given in Table 7.
For milk house systems, the infiltration trenches must have at least 2 feet of separation from bedrock or the seasonally high water table to deal with the remaining BOD5 and hydraulic loading. Note that this is less restrictive than the household wastewater requirements of three-foot separation due to the lack of human pathogens in the waste. Infiltration areas should maintain a 100 foot setback from wells and 50 foot setback from water bodies (or follow applicable local and state requirements).
Installation and management
Installation of ATUs and RMFs and the related infiltration areas should be done by trained professionals. Manufactures’ recommendations should be followed for all operation and management of these systems. More information on ATU and RMF system installation and management can be found the University of Minnesota OSTP Manual Section 10.
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