Explanation of 47-Year BOD Alternation in a Japanese River Basin by BOD Generation and Discharge

Abstract

Average and 75 % biochemical oxygen demand (BOD) in the Yamato-gawa River, Japan, in 1963 were smaller than the criteria determined in 1970 (5 mg l−1); however, they were deteriorated up to 22–32 mg l−1 in 1970. The deterioration was caused by the population increase and economic and urban development. The national-level regulations on ambient water quality and pollutant discharge have been established in 1970. Municipal wastewater is the major contributor in pollutant discharges in the river basin. Estimated BOD discharge (PD(BOD)) in the river basin reduced to the level of 1963 in 2000; however, average and 75 % BOD were more than the criteria. Analysis on the relationship between BOD and PD(BOD) resulted in the five phases in 1963–2009. The equilibrium conditions changed from phase I to II because of the perturbation caused by the excess BOD discharge, self-correcting mechanisms were found in phase III, and equilibrium conditions changed from phase III to IV and V. The comparison of estimated BOD based on pollutant generation (PG) without measures and monitored BOD represented the following situations in 47 years: (1) It took several years before the effects of centralised WWTPs were observed, (2) PD(BOD) was less than 35 t-BOD day−1 for the linear relationship of PD(BOD) and BOD, and (3) combined effects of wastewater treatment, river water purification facilities and soft measures were estimated to be about 10 mg-BOD l−1 in 2010.