CHARLESTOWN, R.I. — A simpler, less-expensive, advanced on-site wastewater treatment system will soon be undergoing field tests in Rhode Island. In the coming weeks, the Rhode Island Department of Environmental Management (DEM) is expected to approve a pilot project to test an experimental nitrogen-removing septic system known as “layered soil treatment area” — or LSTA for short. Proposed by a consortium of the town of Charlestown and the University of Rhode Island’s Laboratory of Soil Ecology and Microbiology and New England On-Site Wastewater Training Program, the non-proprietary septic system would put nitrogen-removing technology within reach for more coastal homeowners. Since 2008, the state has required that failing septic systems in the coastal zone be replaced with advanced nitrogen-removing wastewater treatment systems. The requirement also applies to new construction and to major rebuilds.