Question

a) 1 ton of methanol are accidentally discharged into a lake. By how much is the biological oxygen demand of this water increased? Assume that all ethanol is transformed into CO₂ and assume the lake contains 108 liters of water. Explain your answer with the chemical reaction involved and any assumptions you make. (5) b) A lake with a cross sectional-area of 1km² and a depth of 70 m has aneuphotic zone that extends 15 m below the surface, what is the maximum weight of the biomass (in grams of carbon) that can be decomposed by aerobic bacteria in the water column of the lake below the euphotic zone during the summer when there is no circulation with the upper layer? The bacterial decomposition reaction is \left(\mathrm{CH}_{2} \mathrm{O}\right)_{n+n \mathrm{n}_{2}} \quad \rightarrow \quad \mathrm{nCO}_{2}+\mathrm{nH}_{2} \mathrm{O} The solubility of oxygen in pure water saturated with air at 20°C is 8.9mg/L. (5)

Fig: 1

Fig: 2

Fig: 3

Fig: 4