The authors have applied mathematical basic and modified Streeter - Phelps equation to calculate the maximum allowable BOD load discharged into Dong Ba and Bach Yen branches, from which to assess their assimilative capacity (AC) according to three different scenarios. The results show that Bach Yen branch (BY) has the lowest AC followed by DB1 and DB2 of Dong Ba branch. Assimilative capacity of these branches in rainy season is much higher than that in the dry one in three scenarios. In addition, outputs of Streeter-Phelps model (represented by travel time and distance to critical sag, deficit at critical sag) indicate that it is necessary to consider nitrification and SOD when applying Streeter-Phelps equation combined with Fair model to assess AC. Moreover, the authors have conducted the calculation of actual BOD load from various sources of wastewater discharged into these branches and the actual BOD load is much lower than the maximum allowable. It indicates that Dong Ba and Bach Yen branches are still likeky to receive organic pollutants without violating their best-designated conditions.
Keywords: modified Streeter - Phelps, assimilative capacity (AC), BOD load, Dong Ba river, Bach Yen river.