Abstract: A wastewater treatment system includes a tidal, sequential vertical flow marsh cell system having at least three marsh cells in serial forward fluid communication. Incoming wastewater to be treated is channeled to at least the first and a second marsh cell, and a portion of water exiting a final marsh cell, to the first marsh cell surface. Each marsh cell is alternately flooded and drained for enhancing aeration of incoming water and marsh cell contents, which include media and plant roots having biofilms growing thereon, the biofilms including a population of nitrifying and denitrifying bacteria, for achieving substantially simultaneous nitrification and denitrification of the wastewater.