BOD-DO modeling and water quality analysis of a waste water outfall off Kochi, west coast of India

Water quality scenarios around an offshore outfall offKochi were simulated using MIKE21 water quality model, assuming a high Biochemical Oxygen Demand (BOD=50 mgl-1) effluent discharge. The discharge is introduced into the model through an outfall located at a distance of 6.8 km from the shore at a depth of 10 m. Three scenarios were simulated with different discharge rates such as 2, 5 and 10 m3 s-1, with BOD load of 8640, 21,600 and 43,200 kg day-1 respectively. Model simulations were carried out to estimate the assimilation capacity of the waters off Kochi for the three discharge rates. The results show that for 10 m3 s-1 effluent discharge, the initial BOD of 50 mg l-1 reduced to 3.33 mg l-1 at the outfall after 48 h. High BOD values were confined to an elliptical area of ∼ 8 km2around the outfall. Based on this, the assimilative capacity of the waters off Kochi in terms of BOD can be estimated as 38,000 kg day-1. It is suggested that offshore waters could be used as a feasible alternative to the Kochi backwaters for the disposal of treated effluent.