Pollutant transport model and the carrying capacity of the Asahan River, North Sumatra

The only estuary of Lake Toba is the Asahan River, which extends for a distance of 147 kilometers. The river has a depth ranging from 1.6 to 7.2 meters. Current conditions indicate a decrease in discharge and an increase in pollutants. Monthly discharge data for 9 years is the result of measurements from Perum Jasa Tirta I in 2009 - 2018 and measurements in 2022 show a minimum discharge of 62 m3 / second. The amount of discharge fluctuation is very Asahan depending on the season and the use of water by hydropower managed by PT INALUM. This study will look at the pattern of pollutant transport in the Asahan River originating from the Miils Outfall of PT Toba Pulp Lestari Tbk with a maximum discharge of 0.425 m 3 /sec and coordinate position 522680.79/272684.34. The model employed is DHI Mike 21 Flow FM, which was operational for a period of one year, during which time it was subjected to a series of alterations in discharge. Model input parameters are TSS, BOD and COD. Furthermore, these parameters are employed in order to ascertain the Asahan River’s carrying capacity. The water quality hue for BOD, COD, and TSS in the Asahan River was 0.4 mg/L, 1.6 mg/L, and 5 mg/L, respectively. In contrast, the BOD, COD, and TSS in the outlet mills were 14.8 mg/L, 155 mg/L, and 35.3 mg/L, respectively (PT Toba Pulp Lestari Tbk, July 2022). The model results show that the maximum concentration of BOD at the mill outfall is 0.84 mg/L with a Zone of Initial of Dillution (ZID) of 0.683841 Ha, the maximum concentration of COD at the mill outfall is 6.23 mg/L with a ZID of 4.195784 Ha and TSS at the mill outfall is 4 mg/L with a ZID of 0.220491 Ha. which occurred in May, November and December. In accordance with the quality rules for river water and similar bodies of water set forth in Appendix VI of Government Regulation No. 22 of 2021, the concentration of BOD, COD, and TSS discharged into the Asahan River through the mill outfall does not exceed the Class 1 quality standard of 2 mg/L. The Maximum Pollutant Load Allocation for the 3 parameters TSS, BOD and COD (kg/day) are 268,286; 16,097 and 134,143 respectively, while the Actual Pollutant Load for the 3 parameters are (kg/day) 16,097; 2,146; and 8,585. The Asahan River has a carrying capacity of 252,189, 13,950, and 125,557 kg/day for total suspended solids (TSS), biochemical oxygen demand (BOD), and chemical oxygen demand (COD) parameters, respectively. The Asahan River continues to demonstrate excellent capacity and car