The additional microbial treatment step cuts down on the time energy-guzzling fans must be run to aerate the sewage, for instance. Denitrification also modulates the acidity of wastewater, making some chemical additives unnecessary, and reduces the amount of sludge that must be disposed of. Currently 43 treatment plants in the Chesapeake Bay watershed have been converted to denitrification, and plans are under way to outfit 58 more in the next five years. Yet reducing nitrogen in sewage alone will not do the entire trick. Much of the rest is runoff from farmlands.
Land management is the key to the latter goal.
This end can be achieved by proper grading of farmland and by tillage that minimizes soil disturbance, leaving a mat of plant debris to protect the surface. And forests or artificial wetlands can serve as a buffer between field and stream, providing a place for sediments to settle from runoff and for plants to take up dissolved nutrients. Fertilizer-reduction strategies, on the other hand, are geared at giving crops as much nutrient as they need but no more. Methods range from simple soil tests to computerized tractors that use satellite-based navigation equipment to direct the application of fertilizer. But some of the toughest problems are just now being recognized.