Dairy Effluent Discharged into a Farm Pond
A 50,000 L/day dairy processing plant near Erode discharges untreated effluent into a farm pond used by nearby farmers to irrigate fodder grass. Final-year microbiology students sample the pond: pH 8.5, BOD5 480 mg/L, COD 900 mg/L, oil and grease 40 mg/L, and a multiple-tube fermentation test gives an MPN of >1,600 coliforms per 100 mL. The pond has visible foam, a foul smell and algal blooms. Farmers say the fodder grass grows well but leaves have a slimy coating; children sometimes swim in the pond. The plant manager argues that dairy waste is “natural” and therefore harmless, and that dilution in the pond makes treatment unnecessary. The students must present a reasoned report to the panchayat.
1. Interpret the BOD5, COD and MPN results. What do they indicate about organic load, oxygen status and faecal contamination?
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BOD5 of 480 mg/L is very high; typical domestic sewage is around 200–300 mg/L BOD, so this effluent has a large biodegradable organic load from milk solids, lactose, proteins and fats. COD of 900 mg/L measures total oxidisable matter and is consistent with a high organic load. The BOD/COD ratio of about 0.53 suggests that a substantial part of the waste is biodegradable. When this waste enters the pond, bacteria consume dissolved oxygen, so the pond becomes hypoxic or anaerobic, causing foul smells and harming fish and other aquatic life.
MPN of >1,600 coliforms per 100 mL is a strong indicator of faecal contamination. Coliforms are indicator organisms; they do not prove that pathogens are present, but they show that the water is contaminated with material of intestinal origin and is unsafe for drinking, swimming or irrigation of crops eaten raw. The high pH and algal blooms are consistent with nutrient enrichment, especially nitrogen and phosphorus. Foam and oil and grease add to the pollution load.
2. Explain why dairy effluent can still harm the pond, soil and crops despite being ‘natural’, and outline the risks to the farming community.
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“Natural” does not mean harmless at high concentration. Milk solids and fats are rapidly broken down by microbes, which consume oxygen and create anaerobic conditions. Anaerobic decay produces hydrogen sulphide and other foul-smelling compounds, and the pond can become hostile to aquatic life. Nutrients such as nitrogen and phosphorus cause eutrophication and algal blooms, which further reduce oxygen when they die and decompose.
On land, repeated irrigation with untreated effluent can clog soil pores with fats and solids, reduce infiltration and create anaerobic soil. Ammonia and salts can damage fodder grass, and the slimy coating on leaves can block stomata and reduce photosynthesis. Pathogens from cattle and human waste can contaminate fodder, soil and groundwater. Children swimming are at risk of gastroenteritis, skin infections and ear infections. The community may also be exposed through contaminated vegetables or groundwater. Dilution in the pond does not remove the pollution load; it only spreads it.
3. Propose a low-cost treatment and monitoring plan for the plant and the pond, including at least three specific control measures.
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The plant should first reduce the load at source: recover milk solids from rinses, use dry cleaning before wet cleaning, and screen out solids. Effluent should pass through a screening and grit chamber, an equalisation tank and a sedimentation tank to remove fats and settleable solids. A low-cost anaerobic lagoon or covered anaerobic digester followed by an aerated lagoon, stabilisation pond or constructed wetland can reduce BOD and nutrients. Treated water should be reused for irrigation or landscaping only after testing, and should not be discharged into the farm pond without control.
Monitoring should include pH, BOD, COD, total suspended solids, oil and grease and coliforms at least monthly, with records shared with the panchayat. The pond should be fenced and signposted, and children and farmers should be educated not to swim in it or use raw effluent on fodder. A buffer strip of vegetation can filter runoff. The panchayat should set discharge limits, inspect the plant and require corrective action. These measures are affordable and can be implemented in stages.