Mass balance analysis of reclaimed wastewater irrigation on paddy fields in Korea

An experimental field study was performed during the growing season to evaluate water and nutrient balance in a paddy field over a 3-year period. The plots were separated according to irrigation water: groundwater (TR#1), wastewater (TR#2), and filtered wastewater with ultraviolet (UV) treatment (TR#3). The hydrological and water quality of rainfall, irrigation, surface water, and infiltration were monitored throughout the crop stages. More than half of the total water inflow of about 1840 mm in each treatment was contributed by precipitation and the remainder by irrigation. The water balance analysis indicated that approximately 13% of the total outflow was lost through surface drainage, 30% was consumed, and 57% was lost by evapotranspiration and infiltration, respectively. Water quality data indicated that drainage from paddy fields can degrade the recipient water environment. Based on the total N (T-N) of irrigation water and total P (T-P) input, significant amounts of nutrients were lost through surface drainage and plant uptake. The nutrient balance for T-P and T-N in TR#1 indicated that most (56-67%) of the inputs were supplied by fertilization and most (57-95%) of the outputs were due to plant uptake. The T-N and T-P balance in TR#3 showed that most of the inputs were supplied by fertilization and irrigation, which were in the ratio of 1:2.2, and most (70-86%) of the outputs occurred as plant uptake. However, significant amounts (2-6%) of nutrients were lost through surface drainage because of high drainage outlet height. The results indicate that inadequate water management influences T-N and T-P losses during the irrigation period. Proper water management in wastewater reuse is required to reduce nutrient losses through surface drainage from paddy fields by high drainage outlet structures, irrigation scheduling, and minimized forced fertilization.