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
|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.