A Polluted River: Explaining the Fish Kill and Choosing a Treatment
A 12 km river passes a housing estate of 300 households and a palm oil mill. Residents use the water for washing and children play at the bank. A monitoring team sampled over 24 hours at four points. Upstream (0 km): BOD 1.8 mg dm⁻³, dissolved oxygen 7.6 mg dm⁻³ at all times, no algal bloom. At 500 m below the mill's outfall: BOD 41 mg dm⁻³, dissolved oxygen 1.9 mg dm⁻³, milky brown water, dead fish along the bank. At 3 km downstream: BOD 7.5 mg dm⁻³, dissolved oxygen 11.2 mg dm⁻³ at 2 p.m. but only 2.8 mg dm⁻³ at 3 a.m., with thick green algae. At 8 km: BOD 3.1 mg dm⁻³, dissolved oxygen 6.9 mg dm⁻³, water clearer. The mill offers two remedies: a tertiary treatment plant costing RM 2.5 million, or a 2-hectare constructed wetland costing RM 300 000, on land currently used by four families for vegetable plots.
1. Explain the fall in dissolved oxygen 500 m below the outfall, and explain the day–night swing from 11.2 to 2.8 mg dm⁻³ at 3 km.
Read the worked answer
The oxygen sag at 500 m. Palm oil mill effluent is rich in organic matter - oil, fruit fibre and cell debris. Aerobic bacteria break this down and use dissolved oxygen for respiration. A BOD of 41 mg dm⁻³ means a very large oxygen demand, far beyond what the river can replace by diffusion from the air and by photosynthesis upstream. Oxygen is therefore used faster than it is replaced, and the concentration collapses from 7.6 to 1.9 mg dm⁻³.
The bloom at 3 km. By 3 km much of the organic matter has been mineralised, releasing phosphate and nitrate. With sunlight and these nutrients, algae grow densely. In daylight the algae photosynthesise, releasing oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2, so the water becomes supersaturated at 11.2 mg dm⁻³.
The night-time collapse. At night there is no photosynthesis, but the algae, the bacteria and other organisms continue to respire. Oxygen is consumed by all of them and by the decay of dead algae, so the concentration falls to 2.8 mg dm⁻³ by 3 a.m.
The judgement to make is that a single daytime reading of 11.2 mg dm⁻³ would make the river look healthy. It is only the 24-hour record that reveals the problem.
2. The effluent contains no toxic poison, yet fish died. Explain how a non-toxic discharge can kill fish, and why the upstream section was unaffected.
Read the worked answer
The mechanism is suffocation, not poisoning. Fish need dissolved oxygen of roughly 4–5 mg dm⁻³ or more. At 1.9 mg dm⁻³ they cannot extract enough oxygen across their gills and die of hypoxia. Larger, more active fish have the highest oxygen demand and die first, which fits the dead adult fish found along the bank.
Contributing factors. Decomposition of the effluent releases ammonia, which damages gill tissue, and suspended oil and solids can coat and clog the gills. These make the oxygen shortage worse, but the primary cause is oxygen depletion.
It is an indirect effect. The mill discharge does not poison the fish itself; the bacteria that feed on it remove the oxygen. That is why BOD, a measure of oxygen demand, is a better predictor of the kill than any toxicity test.
Why upstream is unaffected. At 0 km the organic load is low (BOD 1.8 mg dm⁻³) and the oxygen demand is matched by reoxygenation, so dissolved oxygen stays at 7.6 mg dm⁻³. The damage begins only where the effluent enters and the oxygen demand suddenly exceeds the supply.
3. Recommend which of the two remedies the mill should adopt, and justify your choice using evidence from the data. State one limitation of your recommendation and how it could be managed.
Read the worked answer
The wetland is the better first step. At RM 300 000 it costs about 12 % of the tertiary plant. A constructed wetland removes suspended solids and BOD by sedimentation, microbial breakdown and uptake by plants, and it also traps nutrients, which would limit the algal bloom at 3 km. It needs little power and can be operated by the mill's own staff.
But the wetland alone is not enough. The discharge BOD is very high at 41 mg dm⁻³, and a 2-hectare wetland may not reduce it sufficiently on its own, especially in heavy rain when the bed is flushed. It also takes months to establish and needs maintenance such as harvesting plants and preventing clogging.
Managing the limitation. Add a cheap oil trap and primary settling to remove most of the oil and solids before the effluent reaches the wetland, and monitor BOD and dissolved oxygen downstream at night as well as by day. If results still fail after 12 months, upgrade to tertiary treatment.
The social constraint must be settled. The land belongs to four families. They must be compensated and relocated with their agreement; a cheap scheme that takes farmland without consent is not a good solution.
Overall judgement. Start with pretreatment plus wetland, funded by the money saved compared with RM 2.5 million, and make the discharge licence conditional on measured improvement.