Subsurface Wastewater Program Policies

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Maine Subsurface Wastewater Disposal Rules

Policy Regarding Design Flow Reductions

Effective Date: April 5, 2001

Background: Advanced Treatment Units

Advanced treatment units (ATU’s) are now in wider use in Maine since the 1995 Subsurface Wastewater Disposal Rules (Rules) first recognized them. These ATU’s routinely produce very high quality effluent. Accordingly, the Rules allow reductions in disposal area sizing based upon effluent quality. In several instances, specific ATU models have been granted design flow reductions less than those allowed by the Rules.

Background: Proprietary Disposal Devices

Proprietary disposal devices (PDD’s) of various types have been used in Maine since the early 1980’s to dispose of wastewater effluent. These PDD’s have been granted reductions in square footage requirements based upon unobstructed void spaces, and in the case of some devices, unmasked soil interfaces. Disposal area size reductions are based upon a comparison with standard stone and pipe disposal beds. The extent of size reduction allowed varies with the type of device.


At issue is the use of ATU’s in combination with PDD’s, when the PDD size reduction is taken in addition to the adjustment factor (AF) reduction of the ATU. For example, an ATU with a 50 % reduction followed by a PDD with a 30 % square footage reduction would result in an AF 80 % reduction compared to a conventional stone and pipe bed. The hydraulic loading rate specified in the Rules could easily be exceeded in such an instance, leading to hydraulic overload of the disposal area and premature failure of the disposal area.

It was not the Division’s intent to allow extended reductions in disposal area sizing when PDD’s were granted size reductions and ATU’s were granted adjustment factors. The Division is not persuaded that such extreme reductions in disposal area sizing are warranted as a routine matter.


Therefore, it is the Division’s policy that multiple reductions in disposal area sizing are not appropriate. When used with an ATU, PDD’s must be sized at their respective ratings pursuant to the Rules, based upon the actual design flow, rather than the adjustment factor for the ATU.


W. Clough Toppan, Acting Manager
Wastewater and Plumbing Control Program