Michigan Septic System Requirements

At the end of 2011, Michigan published a new version of their “Michigan Criteria for On-site Wastewater Treatment” (dated 12/21/11). This revision was an update to their 1994 document “Michigan Criteria for Subsurface Sewage Disposal”.

With a whole host of changes, this new document for Michigan is a dramatic shift from the old one.  Even though this is not an official rule (as of today’s date 12/12/2012), many Michigan County Health Departments require that this new document be followed.

One of the most significant additions to the new “Michigan Criteria for On-Site Wastewater Treatment” is it’s recognition and handling of varying strengths of BOD (Bio-chemical oxygen demand).  The new Michigan Criteria’s loading rate table (used to size absorption fields) lists separate loading rates for BOD 30-140 mg/L  and  BOD <30 mg/L.  Below is the loading rates tables as of today’s date.  I’ve listed it here for illustration purposes only.  If you are designing a system, please check Michigan’s Department of Environment Quality’s (DEQ) On Site Wastewater webpage for the latest version of this document.  Click HERE for a PDF copy( as of 12/18/12)

Utilizing an Aero-Tech ATU (aerobic treatment unit) that outputs BOD levels at 5mg/L easily qualifies for these favorable BOD loading rates (in the BOD<30 mg/L column).   Let’s now look at an example of how these new loading rates will effect the system size in a Fine Sand textured soil…

Michigan Soil Loading Rates


Example:  150 gallons per day x 4 bedrooms = 600 gallons per day.

.4 Loading Rate with BOD >30 and <140 = 1500 sq.ft.

1.0 Loading Rate with BOD <30 =  600 sq. ft.

As you can see utilizing an Aero-Tech ATU allows for a very significant 60% downsizing of the septic field!

Since the loading rate table only accommodates BOD up to 140 mg/L,  how are systems with BOD >140 mg/L sized?  This might be the case if you have a restaurant or other high strength facility.   In fact restaurants commonly produce waste with BOD >2000 mg/L.

I’ve been told that systems with BOD greater than 140 mg/L, that choose not to use a secondary treatment device to lower their BOD <30 mg/L, would have to proportionally increase their system size..

For example, someone with a known BOD of 280 mg/L (twice the limit of 140 mg/L) would have to double the size of their field system unless they use a device such as an Aero-Tech ATU that can drop the BOD below 30 mg/L.

Therefor, a restaurant with a BOD of 1400 mg/L would have a choice of  either loweing their BOD below 30 mg/L or installing an absorption field ten times larger than normal.  Obviously, the higher the BOD, the more financial incentive there is to install secondary treatment.

If you have questions about secondary treatment in Michigan, Indiana or Ohio, please don’t hesitate to Contact Me.