Water contamination, eutrophication, BOD and treatment
Every summer, some lakes in the midlands turn a murky green and fish float to the surface. Yet the water that comes out of your tap is clear and safe. What causes the first problem, how do we measure it, and what is done to the water before it reaches your kitchen? By the end of this page you will be able to answer all three.
Sources of water contamination
Rain falls, flows over land and through towns, and ends up in rivers and lakes. On the way it can pick up contaminants. Three sources matter most in Ireland.
| Source | Typical contaminants | Effect |
|---|---|---|
| Agriculture (fertiliser, slurry run-off) | Nitrates, phosphates, organic matter | Eutrophication, high BOD |
| Sewage and wastewater | Organic matter, phosphates, ammonium ions, pathogens (bacteria, viruses) | High BOD, disease |
| Industrial effluent | Heavy metals (lead, mercury, cadmium), solvents, acids | Toxic to living things, build up in the food chain |
Other contaminants include microplastics, which come from synthetic clothing fibres and breakdown of plastic waste, and are not removed easily by treatment.
Rivers in Ireland can be contaminated by agricultural run-off, sewage and industrial discharges. a) Name two nutrients in fertiliser run-off that cause water pollution.…
Rivers in Ireland can be contaminated by agricultural run-off, sewage and industrial discharges.
- a) Name two nutrients in fertiliser run-off that cause water pollution.
- b) Name one chemical or biological contaminant associated with sewage.
- c) Explain what is meant by eutrophication.
Eutrophication
Eutrophication is the over-enrichment of a water body with nutrients (mainly phosphates and nitrates), leading to excessive growth of algae and a fall in dissolved oxygen.
The sequence of events is:
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Fertiliser or sewage run-off adds phosphate and nitrate. These are the limiting nutrients for algae and aquatic plants, so the algae grow very rapidly (an algal bloom).
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The thick layer of algae blocks sunlight, so plants lower down cannot photosynthesise and die.
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The algae have short lives. Dead algae and plants sink and are decomposed by aerobic bacteria.
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The bacteria use up dissolved oxygen as they respire. The dissolved oxygen falls.
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Fish and other organisms that need oxygen die. Anaerobic bacteria take over and produce foul-smelling gases such as hydrogen sulfide.
Farmers can reduce nutrient run-off by applying fertiliser only in the amounts crops need and not before heavy rain, by leaving buffer strips of vegetation along rivers, by fencing livestock away from streams and by storing slurry properly. Better sewage treatment (removing phosphate) also helps.
Describe the sequence of events in eutrophication of a lake in the midlands. a) Explain why phosphate and nitrate from fertiliser cause rapid growth of algae. b) Explain…
Describe the sequence of events in eutrophication of a lake in the midlands.
- a) Explain why phosphate and nitrate from fertiliser cause rapid growth of algae.
- b) Explain why the death of the algae leads to a fall in dissolved oxygen.
- c) State two effects of this on aquatic life.
- d) Suggest two measures that farmers could take to reduce nutrient run-off.
A lake monitoring programme gives the following data for phosphate concentration and dissolved oxygen at the bottom of the lake. | Month | Phosphate…
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 |
- a) Describe the trends shown by the data.
- b) Explain the relationship between the two variables in terms of eutrophication.
- c) Suggest why dissolved oxygen is lowest in August.
- d) State two limitations of drawing conclusions from this data set alone.
Biochemical oxygen demand (BOD)
We cannot test a river for every possible pollutant. Instead we measure how much organic pollution it contains by measuring how much oxygen microorganisms would use to break it down.
The sample is kept in the dark so that algae cannot photosynthesise and add oxygen. The more organic matter present, the more oxygen the bacteria use, so:
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High BOD means heavily polluted water (a lot of biodegradable organic matter).
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Low BOD means clean water. Unpolluted rivers typically have a $\mathrm{BOD_5}$ below about $3\,\text{mg}\,\text{L}^{-1}$.
Theoretical oxygen demand
If we know what the pollutant is, we can calculate the oxygen needed from the balanced equation, using moles. For glucose:
$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$
Biochemical oxygen demand (BOD) is used to assess the quality of water. a) Define BOD. b) State whether a high BOD indicates clean or polluted water, and explain. c) The…
Biochemical oxygen demand (BOD) is used to assess the quality of water.
- a) Define BOD.
- b) State whether a high BOD indicates clean or polluted water, and explain.
- 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 $\mathrm{BOD_5}$.
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…
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.
- a) Calculate the $\mathrm{BOD_5}$ of the sample.
- b) Unpolluted rivers typically have a $\mathrm{BOD_5}$ below $3\,\text{mg}\,\text{L}^{-1}$. Comment on the quality of this sample.
- c) Explain how the BOD would change if untreated slurry were discharged into the river.
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…
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 ($M_r = 180$).
- a) Calculate the theoretical oxygen demand in $\text{mg}\,\text{L}^{-1}$.
- 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.
- c) Explain why oxygen solubility is lower in warm water and why this makes summer discharges more harmful.
Treating drinking water
Raw water from a lake or river is not safe to drink. A treatment plant makes it clear, free from pathogens and suitable to supply to homes. The stages are in this order:
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Screening: bars and screens remove large objects such as sticks and litter.
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Flocculation: a flocculant, usually aluminium sulfate, is added. It makes tiny suspended (colloidal) particles clump together into larger flocs.
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Sedimentation: the heavy flocs settle to the bottom of a tank.
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Filtration: water passes through beds of sand and gravel, which remove the remaining fine particles.
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Chlorination: chlorine is added to kill bacteria and viruses.
Other steps in many plants are pH adjustment (so that pipes are not corroded and flocculation works), fluoridation (to help prevent tooth decay, as is practised in Ireland) and in some plants UV treatment to disinfect.
Flocculation in detail
Colloidal particles in water carry a negative charge, so they repel each other and stay suspended. Aluminium ions neutralise this charge, and also form a gelatinous precipitate of aluminium hydroxide in slightly alkaline water:
$$\mathrm{Al^{3+}(aq) + 3OH^-(aq) \rightarrow Al(OH)_3(s)}$$The sticky, jelly-like $\mathrm{Al(OH)_3}$ traps the particles as it forms and they settle together. The pH must be controlled: if the water is too acidic the hydroxide dissolves, and if it is too alkaline it can dissolve as aluminate ions, so in both cases less floc forms.
Chlorination in detail
Chlorine reacts with water:
$$\mathrm{Cl_2 + H_2O \rightarrow HOCl + HCl}$$Chloric(I) acid, HOCl, is the disinfectant. It kills micro-organisms. A small residual level of chlorine is left in the water so that it keeps protecting it against contamination as it travels through the pipes to your home.
Advantage: cheap, effective, and the residual protects water in the network. Disadvantage: chlorine can react with organic matter in the water to form harmful by-products such as trihalomethanes (chlorinated organic compounds), and it can give the water a taste and smell.
Drinking water is treated before it is supplied to homes. a) List, in the correct order, the stages: filtration, flocculation, chlorination, screening, sedimentation. b)…
Drinking water is treated before it is supplied to homes.
- a) List, in the correct order, the stages: filtration, flocculation, chlorination, screening, sedimentation.
- b) State the purpose of chlorination.
- c) Name a substance commonly used as a flocculant and state its role.
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…
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.
- a) Calculate the mass of chlorine used per day, in kg.
- b) Chlorine reacts with water: $\mathrm{Cl_2 + H_2O \rightarrow HOCl + HCl}$. State which species is the disinfectant.
- c) Calculate the amount, in mol, of $\mathrm{Cl_2}$ used per day ($A_r$ Cl = 35.5).
- d) Explain why a small residual chlorine level is maintained in the distribution network.
Aluminium sulfate is used as a flocculant in water treatment. a) Write an ionic equation for the formation of aluminium hydroxide when $\mathrm{Al^{3+}}$ is added to…
Aluminium sulfate is used as a flocculant in water treatment.
- a) Write an ionic equation for the formation of aluminium hydroxide when $\mathrm{Al^{3+}}$ is added to slightly alkaline water.
- b) Explain how the aluminium hydroxide floc removes suspended colloidal particles.
- c) Explain why the pH of the water must be controlled for this process.
- d) Discuss one advantage and one disadvantage of chlorination as a disinfection method for a public water supply, with reference to by-products.
Summary
Vuelve al principio: the green lake is eutrophic, caused by nutrient run-off that starves the water of oxygen, which we measure as BOD, while the tap water is safe because of the five treatment stages.