Onsite System Options Overview
In Minnesota, there are different methods to deal with on-site wastewater treatment. Standard systems are the simplest way to effectively treat wastewater, but occasionally site characteristics such as lot size, soil type, site disturbance, flow and wastewater characteristics often limit the options available. This document will define the different options.
According to Chapter 7080 a standard system is a trench, at-grade or mound system which is designed and constructed according to applicable requirements. The Minnesota Pollution Control Agency administrates these rules from a state wide level. Local governmental units such as a township or county administer the problem locally.
Standard systems set the acceptable treatment standards for system performance and management. These systems are designed with at least 3 feet of vertical separation. This assures that appropriate treatment levels can be met. An example would be a standard septic tank followed by gravity fed trench.
Alternatives to Standard Systems
There are two types of alternative to standard systems - 'other' or 'performance'. All of these systems need reviewal as a treatment system and different methods in achieving sewage treatment are what make them alternative. These systems have some similar characteristics. These systems do not have the same design criteria as standard systems. The permitting authority must approve the design and have reasonable assurance of performance. There must be a flow meter to monitor the flow. The estimated cost for construction, operation, monitoring, service, component replacement and management must be submitted to the local governmental unit. In addition, the anticipated system life and hydraulic and organic load must be determined. These systems are required to have a monitoring plan, which evaluates how is it working. The performance of these systems is measured. It includes sampling and testing to assure the waste is treated. The determination of what is to be sampled and how often is determined by the site characteristics, choice in system and local governmental unit (LGU). A mitigation plan, which lays out what will happen if the system fails to adequately treat needs to be submitted to the LGU. The differences between these systems are explained below.
These systems must be designed with a vertical separation of three feet or greater and have a soil texture of medium sand or finer. This soil treatment assures performance. At least 1-foot of the three feet must be original soil. They can not be loaded more than 1.2 gpd/ft2. An example would be a mound that was slightly downsized to accommodate a small lot.
Septic Tank ----> Downsized Mound
The permitting authority must approve performance system's design, monitoring plan and mitigation plan. Certain measurements must be taken to assure that the environment and public health are still protected. There must be at least 1 foot of vertical separation and an unsaturated zone between the seasonably saturated zone or bedrock. Twenty-five feet from the system the concentration of fecal coliform must not exceed the background concentration. If the system is close to a lake, fifty feet from the system the concentration of phosphorous must not be great than 1 mg/L above background levels. An example would be an aerobic tank followed by a trench system with less than 3-feet of vertical separation.
Septic Tank ----> Aerobic tank ----> Trench with less than 3 feet