Igcse Model Questions Vol1 2025 Chemistry Question 6

An industrial wastewater treatment plant processes water that initially contains (300\ \text{mg\ L}^{-1}\) biochemical oxygen demand (BOD) and (200\ \text{mg\ L}^{-1}\) suspended solids. The plant operates two stages: (1) Primary sedimentation removes 30 % of the suspended solids; (2) A biological treatment stage reduces the BOD by 80 % of the amount that remains after the first stage. The discharge limits are BOD \(\le\) (30\ \text{mg\ L}^{-1}\) and suspended solids \(\le\) (50\ \text{mg\ L}^{-1}\). (a) Identify which pollutant, if any, exceeds its limit after the two stages. (b) For each pollutant that exceeds its limit, calculate the minimum additional percentage removal that a tertiary filter must achieve to meet the limits.