Quantifying Fractional Atmospheric Nitrogen Losses in Municipal Wastewater Irrigation Systems.

Nitrogen (N) is often a critical design factor in municipal wastewater effluent (MWE) irrigation systems, thus a “source-sink” N mass-balance approach is used to determine the effluent hydraulic loading rate. The approach requires estimating the fraction of applied N lost to the atmosphere (f), however, few investigations have verified the f values suggested in literature. In this study, f values estimated from monthly N balances in tall fescue irrigated at ~50 mm wk^-1 with secondary-treated MWE (70 - 85% NO3-N) at the Penn State University Living Filter, were compared with f values suggested by the USEPA (0.1 for tertiary-treated effluents (C:N < 1) and 0.15 - 0.25 for secondary-treated effluents (C:N = 1-1.5)).  To enhance crop fertility, urea ammonium nitrate fertilizer was also applied to the grass field.  The effluent N was quantified from the effluent irrigation depth and the MWE total N concentration and the crop N uptake due to effluent alone was taken as the difference between the monthly measured N uptake and N uptake from the fertilizer. Fertilizer N uptake was estimated from the monthly fractional measured N uptake and annual N fertilizer applications. Leaching N was determined from NO3-N levels in soil water collected from 0.35 m-deep suction lysimeters and monthly water balances.   During the study, the mean monthly effluent C:N ratio was in the range of 0.04 to 0.2 and most of the f values were ≤ 0.1, and a few were within the 0.15 to 0.25 range. The f values were 0.14 (April and May 2011), 0.21 (July 2011), 0.09 (August 2011), 0.23 (April 2012), 0.05 (June 2012), 0.14 (July 2012), and 0.72 September (2012). The high N loss factor in September 2012 was due to low crop N uptake. An f value of 0.05 was assumed for June 2011 and May 2012, and 0.1 for September 2011 and August 2012, since total N removal due to crop uptake and leaching exceeded the applied effluent N in these months. For highly nitrified low BOD5 effluents, lower f values than those suggested by USEPA could be used for design purposes. The f estimates of 0.05 in May and June, 0.1 in August and September, and 0.2 in April and July were deemed appropriate for the study site. However, additional research (in the absence of commercial fertilizer addition) is needed to refine the values.