Environmental Engineering Reference
In-Depth Information
32.2.2 t
ypES
of
w
atEr
p
ollution
There are two main types of water pollution from agriculture—point source pollution and diffuse
pollution.
32.2.2.1 Point source Pollution
Pollution of a waterway from a particular source such as a tank, building, or field is referred to
as point source pollution. This can result when slurry, silage effluent, or uncollected dirty water
reaches a waterway. It can also be caused by fuel oil, pesticides, or sheep dip, which can poison or
damage river life or make groundwater unfit for use.
When farm wastes get into a waterway, they are broken down by microorganisms. This process
uses oxygen needed by river life, including plants and fish, to survive. Biochemical oxygen demand
(BOD) is a measure of the amount of oxygen needed by these microorganisms to break down the
organic material. BOD is, therefore, a measure of the polluting strength of organic wastes. Farm
wastes have a high BOD, some values are shown in Table 32.2. In serious pollution cases, all river
life can be killed for considerable distances downstream.
32.2.2.2 diffuse Pollution
Diffuse pollution cannot be attributed to a single event or action and is generally caused by high
concentration of organic compounds, nitrates, phosphates, or pesticides.
taBle 32.1
Biogas Potential from Farm Wastes
dry matter
(%)
organic matter
(% dm)
Biogas yield
(m
3
/kg
-
om)
substrate
Liquid cattle manure
6-11
68-85
0.1-0.8
Liquid pig manure
3-10
77-85
0.3-0.8
Excreta from sheep
18-25
80-85
0.3-0.4
Excreta from horses
25-28
72-75
0.4-0.6
Excreta from poultry
10-29
67-77
0.3-0.8
Low fat milk
6-8
92-95
0.6-0.7
Whey
4-6
80-92
0.5-0.9
DM, dry matter; OM, organic matter
taBle 32.2
typical Bod values for Farm Wastes
Biochemical oxygen demand
(mg/l of oxygen)
Waste
Whole milk
100,000
Silage effluent
65,000
Pig slurry
25,000
Cattle slurry
17,000
Dirty yard water
1500
Raw vegetable washings
500-3000
Dilute dairy and parlor washings
1000-2000
Raw domestic sewage
300
