A Title 5 septic system is the minimum onsite sewage disposal system allowed by the Commonwealth of Massachusetts Environmental Code, and typically consists of a septic tank, distribution box, and subsurface soil absorption system. It provides basic mechanical and biological sewage treatment, but only minimal reduction of nitrogen, phosphorus, and other nutrients that are of concern in environmentally sensitive areas. An Innovative/ Alternative, or I/ A, sewage treatment system is an additional treatment unit that is installed between the septic tank and soil absorption system of a conventional Title 5 septic system. An I/ A system provides enhanced biological, and sometimes mechanical, sewage treatment in areas where it is critical to reduce nitrogen or other nutrients of concern.