Abstract:
Rivers and streams are the important components of the biosphere as they serve dual purpose as a major resource for utilization and waste assimilation, River supports multitude of uses such as drinking, bathing, irrigation, recreation etc., and therefore, it is expected that river water quality be maintained as per the criteria developed by Central Pollution Control Boards (CPCB) for various uses of water. However, it is seen that due to discharge of industrial and municipal wasteloads, waste assimilation capacity of a river is exploited to such an extent that the water becomes unfit for various uses.
The Assimilative Capacity (AC) of a perennial river at a given location for a given pollutant is defined as the maximum amount of the pollutant load that can be discharged into the river at that location and still maintain the quality of water to the desired criteria with respect to BOD, DO and other parameters. Waste assimilative capacity for a river is usually computed for minimum or low flow conditions. The methodology is based on flow data analysis; identification of wastewater discharges; identification of critical water quality parameters and behaviour of river stretch for modelling; model setting and choice of model coefficients; model calibration; and number of iterative simulations for estimation of assimilative capacity of the river.
The paper presents assimilative capacity estimation study for Kanhan River in Nagpur district. Kanhan River enters in Nagpur District near village Temurdoh, and after flowing for 120 km joins river Wainganga near village Chichghat. The river network is defined by tributaries Kolar, Pench and Nag. The Irrigation Department Govt. of Maharashtra has installed four gauging stations at Temburdoh, Kamthi, Mathani and Chichghat in this 120 km stretch. The major wastewater carrying stream to river Kanhan is Nag river meeting at 96 km from Temburdoh. Monthly flow data at various locations in the river indicated that river flow in summer and winter ranges from 5-50 m3/s. Accordingly, 5, 23 and 50 m3/s were considered as critical seasonal flows
for estimation of assimilative capacity of river Kanhan.
MIKE 11, a software tool for simulation of hydrology, hydraulics, water quality in estuaries, rivers, irrigation systems and other inland waters was set up for simulating 1-D open channel flow and water quality in the river. With tributaries Pench and Kolar, the river receives four wastewater drains, of which wasteload from Nag river was considered for AC estimation. The river morphology was defined at 17 stations by river survey. Model results for DO-BOD simulation indicated that the assimilative capacity of Kanhan River near confluence with Nag river for minimum river flow (Q) 5.85 m3/s is 1033 kg/day of BCD; and for winter flow of 50 m3/s is 6220 kg/day of BOD, in order to satisfy class C water quality criteria in the river according to CPCB Classification.
