However, water treatment at cold temperatures improves dramatically when plants are a part of the system, says Stein. Either does a considerably better job than cattails. Stein and Hook think this is proof that there is a lot more to be learned about the cleansing abilities of other wetland plants, since they simply chose sedge and bulrush from among many possibilities because the two are widely available native plants. Understanding wetland plants' contribution to wastewater treatment has been complicated by the fact that early studies were most often done with cattails in warmer climates. In warm climates, the microbes seem to do most of the work, making the plants look inconsequential.
Organic carbon is the primary pollutant the two are trying to control with wetlands.
Controlling carbon is important because when it enters a stream, it depletes oxygen. If there is too much carbon, the free oxygen is eliminated from the system, harming fish and other life. You don't hear much about organic carbon pollution, says Stein, because municipal water-treatment systems can handle it rather well. But it is hard to get money to increase water treatment capacities as a town grows. Stein and Hook hope wetlands systems can be used to extend the life of those facilities or minimize the size and cost of new facilities, helping towns, reservations, even individuals treat wastewater economically.