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Reducing diffuse pollution risks can bring a financial benefit to the business whilst also protecting the environment.  Runoff from around the farm can pollute local water quality and lead to wider impacts across a catchment.  The effects of diffuse pollution on water quality can often be seen miles away from the source, for example beaches designated as ‘bathing waters’ can be affected by runoff coming from further up the catchment.

Materials with a high Biochemical Oxygen Demand (BOD) and/or Chemical Oxygen Demand (COD) will be highly polluting if allowed to enter a watercourse by seepage or run-off. Anything with a value above that of the BOD of the receiving water could have a negative impact on water quality. Over-application of these substances to land can also result in a temporary soil oxygen depletion leading to poor plant growth. Table 1.1 outlines the pollution potential, measured in terms of BOD, from various substances common to a range of farming enterprises.

Table 1.1 Biological Oxygen Demand

Substance BOD (mg/l)*
Raw milk 140,000
Silage effluent 30,000 to 80,000
Pig slurry 20,000 to 30,000
Liquid sewage sludge 10,000 to 20,000
Draff run-off 10,000 to 20,000
Cattle slurry 10,000 to 20,000
Midden run-off 1,000 to 2,000
Dilute dairy parlour and yard washings 1,000 to 2,000
Lightly contaminated ’dirty water’ 1,000 to 2,000
Vegetable washings 500 to 3,000
Raw domestic sewage 300 to 400
Treated domestic sewage from septic tank <100
Treated domestic sewage from sewage works <20
River water (high quality) <3

*BOD is a measure of the amount of oxygen needed by microbes to break down organic matter present, therefore, used as a measure of polluting potential.