Greener Guidance: On-site Sewage Treatment

Note: This is the ninth edition of NACCHO’s Greener Guidance environmental health advice column. See past columns here. Submit a question here.

April 2019

Dear Greener Guidance,

In my area, there is a push toward increasing housing density without expanding central sewer systems, including in areas that have extremely coarse-textured soils with shallow water tables. What are some best practices to safeguard public health and environmental quality from the negative effects associated with higher concentrations of on-site sewage treatment in these areas?

– Soiled

Dear Soiled,

According to Professor Jose Amador, a soil science expert at the University of Rhode Island, this is a good opportunity to install advanced onsite wastewater treatment systems. Compared to a conventional septic system, said Amador, “the effluent from advanced systems has a much lower concentration of the pollutants they are designed to treat, and the effluent is dispersed closer to ground surface, which results in a greater separation distance from groundwater.”

Nony Howell, Chief of the Onsite Systems Division at the Maryland Department of the Environment, agrees. Properties in Maryland’s Chesapeake Bay, which often have high groundwater levels, said Howell, “are required to install best available technology (nitrogen reducing systems) to improve the effluent quality for onsite sewage disposal systems.”

Local or state regulations like these are important, said Daniel Wheeler, a research fellow in the Department of Soil, Water and Climate at the University of Minnesota. “An educated wastewater industry (through certification/licensing programs) in concert with regulations can further relieve concerns,” said Wheeler. “With these two pieces in place, all wastewater professionals will be aware of the proper site-specific choices to limit any groundwater or off-site impacts.” The options Wheeler recommends are pressure distribution, pre-treatment of effluent, clean washed sand-lined systems, and timed dosing of effluent.

Washington state is experiencing some of these same challenges, said Washington On-Site Sewage Association (WOSSA) Board Member Dave Lowe. Here is what WOSSA is proposing in Washington to address this issue:

First, a baseline needs to be established. If we can agree that septic tank effluent discharged into a gravity drainfield is the standard or baseline, then we can establish a maximum daily loading on a per acre per day basis. For instance, if we are planning on a three bedroom home as a standard design flow rate (in WA it equals 360 gpd) and a total nitrogen (TN) concentration of 60 mg/l or 0.178 pounds TN/day per lot. We calculated a base minimum number of lots per acre at 3.3. So 3.3 x 0.178 pounds of TN= 0.58 pounds on TN per acre per day. On lots with course sand we might increase the lot size to 22,000 sq. ft. In this case the max loading of nitrogen per acre would be 0.3511 pounds of TN.

If you wanted to create smaller lots with course sand you need to reduce the amount of nitrogen by treatment and/or disposal method. For instance, if a recirculating gravel filter (RGF) is used and we give a credit of 35% nitrogen reduction with the RGF we could decrease the lot size by 35%. The lot size would go to 14,300 sq. ft. If drip dispersal was a distribution method a credit of say 20% cold be given. This addition reduction is only on the remaining portion of the lot. So, 20% of the 14,300 or 11,440 sq. ft.

The actual reduction amounts are somewhat arbitrary in this example, but do represent how a mass loading calculation would effect lot size and still protect the drinking water source.


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