Relatively little attention has been paid to integrating gaseous N(2)O generated by wastewater treatment into overall reactive nitrogen (Nr) pollution reduction. We propose that there is potential for substantial reductions in N(2)O emissions through the addition of denitrification processes to existing nitrifying wastewater treatment plants (WWTPs), which are designed to lower ammonia levels but currently do not reduce overall Nr. In addition to providing the benefit of reducing total nitrogen concentrations in the effluent, this kind of WWTP upgrade has been demonstrated to reduce energy consumption and fossil CO(2) emissions. We show that the creation of a greenhouse gas (GHG) crediting system for the wastewater sector could provide a potentially sizable economic incentive on the order of $10 million to $600 million per year in the U.S. for upgrading of nitrifying WWTPs that results in N(2)O reductions, with an ancillary benefit of another $30-100 million per year from electricity savings. Even if biological nitrogen removal (BNR) treatment were mandated by existing and future water quality regulations, a GHG crediting system could still be created to promote BNR design and operation that drive N(2)O emissions below a baseline to even lower levels. In this case GHG credits could offset around 0.5-70% of the operating and maintenance cost for the BNR.