GATE Civil Engineering (CE) 2020 Shift 1 Solved Paper

A river has a flow of 1000 million litres per day (MLD). BOD 5 of $5\mathit{m}\mathit{g}/$ litre and Dissolved Oxygen (DO) level of $8\mathit{m}\mathit{g}/$ litre before receiving the wastewater discharge at a location. For the existing environmental conditions, the saturation DO level is $10\mathit{m}\mathit{g}/$ litre in the river. Wastewater discharge of $100\mathit{M}\mathit{L}\mathit{D}$ with the BODs of $200\mathit{m}\mathit{g}/$ litre and DO level of $2\mathit{m}\mathit{g}/$ litre falls at that location. Assuming complete mixing of wastewater and river water, the immediate DO deficit (in mg/litre, round off to two decimal places), is