Exercises: Water contamination, eutrophication, BOD and treatment

Rivers in Ireland can be contaminated by agricultural run-off, sewage and industrial discharges.

1. a) Name two nutrients in fertiliser run-off that cause water pollution.
2. b) Name one chemical or biological contaminant associated with sewage.
3. c) Explain what is meant by eutrophication.

Biochemical oxygen demand (BOD) is used to assess the quality of water.

1. a) Define BOD.
2. b) State whether a high BOD indicates clean or polluted water, and explain.
3. c) The dissolved oxygen in a water sample is $9.0\,\text{mg}\,\text{L}^{-1}$ initially and $4.5\,\text{mg}\,\text{L}^{-1}$ after five days incubation in the dark at $20\,^{\circ}\text{C}$. Calculate the $\mathrm{BOD_5}$.

Drinking water is treated before it is supplied to homes.

1. a) List, in the correct order, the stages: filtration, flocculation, chlorination, screening, sedimentation.
2. b) State the purpose of chlorination.
3. c) Name a substance commonly used as a flocculant and state its role.

A sample of river water from the River Shannon is tested. The dissolved oxygen is $8.6\,\text{mg}\,\text{L}^{-1}$ on collection and $3.2\,\text{mg}\,\text{L}^{-1}$ after five days at $20\,^{\circ}\text{C}$ in the dark.

1. a) Calculate the $\mathrm{BOD_5}$ of the sample.
2. b) Unpolluted rivers typically have a $\mathrm{BOD_5}$ below $3\,\text{mg}\,\text{L}^{-1}$. Comment on the quality of this sample.
3. c) Explain how the BOD would change if untreated slurry were discharged into the river.

Describe the sequence of events in eutrophication of a lake in the midlands.

1. a) Explain why phosphate and nitrate from fertiliser cause rapid growth of algae.
2. b) Explain why the death of the algae leads to a fall in dissolved oxygen.
3. c) State two effects of this on aquatic life.
4. d) Suggest two measures that farmers could take to reduce nutrient run-off.

A water treatment plant doses water with chlorine to give a concentration of $0.50\,\text{mg}\,\text{L}^{-1}$ of $\mathrm{Cl_2}$. The plant treats $2.4\times10^{6}\,\text{L}$ of water per day.

1. a) Calculate the mass of chlorine used per day, in kg.
2. b) Chlorine reacts with water: $\mathrm{Cl_2 + H_2O \rightarrow HOCl + HCl}$. State which species is the disinfectant.
3. c) Calculate the amount, in mol, of $\mathrm{Cl_2}$ used per day ($A_r$ Cl = 35.5).
4. d) Explain why a small residual chlorine level is maintained in the distribution network.

Organic waste in water can be represented by glucose, which is oxidised by dissolved oxygen: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$. A discharge contains $150\,\text{mg}\,\text{L}^{-1}$ of glucose ($M_r = 180$).

1. a) Calculate the theoretical oxygen demand in $\text{mg}\,\text{L}^{-1}$.
2. b) River water saturated with oxygen at $10\,^{\circ}\text{C}$ contains about $11\,\text{mg}\,\text{L}^{-1}$. Calculate the dissolved oxygen remaining if the discharge is mixed 1:1 with saturated river water and the glucose is fully oxidised, and comment on the consequences for fish.
3. c) Explain why oxygen solubility is lower in warm water and why this makes summer discharges more harmful.

A lake monitoring programme gives the following data for phosphate concentration and dissolved oxygen at the bottom of the lake.

Month	Phosphate ($\mu\text{g}\,\text{L}^{-1}$)	Dissolved oxygen ($\text{mg}\,\text{L}^{-1}$)
April	20	9.5
June	45	7.0
August	90	3.0
October	60	5.5

1. a) Describe the trends shown by the data.
2. b) Explain the relationship between the two variables in terms of eutrophication.
3. c) Suggest why dissolved oxygen is lowest in August.
4. d) State two limitations of drawing conclusions from this data set alone.

Aluminium sulfate is used as a flocculant in water treatment.

1. a) Write an ionic equation for the formation of aluminium hydroxide when $\mathrm{Al^{3+}}$ is added to slightly alkaline water.
2. b) Explain how the aluminium hydroxide floc removes suspended colloidal particles.
3. c) Explain why the pH of the water must be controlled for this process.
4. d) Discuss one advantage and one disadvantage of chlorination as a disinfection method for a public water supply, with reference to by-products.