Water Pollution 161
water and thus to a severe degradation of an aquatic ecosystem; oxygen depletion may bring about the death of fish or other aquatic organisms and create
disagreeable septic conditions through the release of methane and hydrogen
sulphide. The oxygen dissolved in the water is not a limiting factor for primary productivity but it may limit populations of aquatic consumers, e.g. fish.
Organic materials are a common pollutant, since wastewater from cities
and stock-raising establishments, as well as liquid waste from agricultural
and other industries, contain significant concentrations of organic compounds which usually wind up in receiving water. Organic matter serves as
sumption of the dissolved oxygen in the receiving water due to aerobic respiration, according to the chemical reaction:
Organic matter O
CO H O heat.
+
→
+
+
2
2
2
In the technology of water pollution, organic matter is frequently measured by its deoxidising effect, biochemical oxygen demand (BOD). BOD of a
quantity of organic matter is the mass of oxygen that is consumed during its
biological oxidation; BOD 5 is the mass of oxygen consumed during the first
5 days. In urban wastewater, BOD 5 is usually 65–75% of BOD. For exclusively domestic wastewater, BOD 5 is 55–80 grams per inhabitant per day.
The receiving water is capable of maintaining a maximum concentration
of dissolved oxygen, which is known as the saturation concentration. The
saturation value depends for the most part on temperature and falls when
the temperature rises. Another factor affecting the saturation concentration
is the salinity of the receiving water. Due to its greater salinity, seawater has
lower saturation concentrations for the same temperature, around 75–80%
of the comparable values of freshwater.
When the concentration C of dissolved oxygen in the receiving water is
lower than the saturation value, we observe the phenomenon of oxidation
during which oxygen from the atmosphere enters and is dissolved in the
water, thus enriching it (Figure 8.1).
According to European Union Directives, an absolute minimum dissolved
oxygen concentration of 5 mg/L (100% of the time) must be ensured, whereas
the average concentrations required are significantly higher depending on
the use of the recipient of the wastewater. In the case of marine recipients,
significant ecological disturbance may be noted even for values of dissolved
oxygen concentration of around 90–95% of the saturation value.
8.3 PhoSPhoruS and nitrogEn—EutroPhication
Pollution of water bodies with phosphorus or nitrogen salts is caused by
overfertilisation of fields or by the disposal of urban or other wastewater.
water and thus to a severe degradation of an aquatic ecosystem; oxygen depletion may bring about the death of fish or other aquatic organisms and create
disagreeable septic conditions through the release of methane and hydrogen
sulphide. The oxygen dissolved in the water is not a limiting factor for primary productivity but it may limit populations of aquatic consumers, e.g. fish.
Organic materials are a common pollutant, since wastewater from cities
and stock-raising establishments, as well as liquid waste from agricultural
and other industries, contain significant concentrations of organic compounds which usually wind up in receiving water. Organic matter serves as
sumption of the dissolved oxygen in the receiving water due to aerobic respiration, according to the chemical reaction:
Organic matter O
CO H O heat.
+
→
+
+
2
2
2
In the technology of water pollution, organic matter is frequently measured by its deoxidising effect, biochemical oxygen demand (BOD). BOD of a
quantity of organic matter is the mass of oxygen that is consumed during its
biological oxidation; BOD 5 is the mass of oxygen consumed during the first
5 days. In urban wastewater, BOD 5 is usually 65–75% of BOD. For exclusively domestic wastewater, BOD 5 is 55–80 grams per inhabitant per day.
The receiving water is capable of maintaining a maximum concentration
of dissolved oxygen, which is known as the saturation concentration. The
saturation value depends for the most part on temperature and falls when
the temperature rises. Another factor affecting the saturation concentration
is the salinity of the receiving water. Due to its greater salinity, seawater has
lower saturation concentrations for the same temperature, around 75–80%
of the comparable values of freshwater.
When the concentration C of dissolved oxygen in the receiving water is
lower than the saturation value, we observe the phenomenon of oxidation
during which oxygen from the atmosphere enters and is dissolved in the
water, thus enriching it (Figure 8.1).
According to European Union Directives, an absolute minimum dissolved
oxygen concentration of 5 mg/L (100% of the time) must be ensured, whereas
the average concentrations required are significantly higher depending on
the use of the recipient of the wastewater. In the case of marine recipients,
significant ecological disturbance may be noted even for values of dissolved
oxygen concentration of around 90–95% of the saturation value.
8.3 PhoSPhoruS and nitrogEn—EutroPhication
Pollution of water bodies with phosphorus or nitrogen salts is caused by
overfertilisation of fields or by the disposal of urban or other wastewater.