Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

Figure 8.10. 
Biological 
oxygen demand and 
water-dissolved oxygen 
content changes in the 
river water, in case of 
waste water discharge

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8. WATER POLLUTION
 177
In case of sewage inlet, the changes in water are characterized 
by the increasing demand for biological oxygen, decrease of the 
water-dissolved oxygen content, increase of the nitrogen content 
(and the nitrogen compounds usually are in the form of ammonium 
ions). In the event of water pollution with biologically labile 
substances, typical changes include alteration in water biocenosis 
(living organism communities) structure, which is primarily related 
to a drastic drop in biodiversity, and the increase of overall biomass 
of living organisms. 
Only at a certain distance from the point of discharge of 
pollutants, the water composition is restored to that which is typical 
to  the  environ ment  unaffected  by  anthropogenic  action.  River 
pollution with organic substances is caused by industrial facilities 
on river banks (food and wood processing complexes, paper mills 
and textile plants) discharging untreated or poorly treated waste 
water.  The  specific  character  of  river  ecosystem  response  to  this 
contamination is related to water exchange opportunities provided 
by the river currents. Depending on the speed of the current and 
the amount of discharged organic substances, the river ecosystems 
have certain self-cleaning abilities. The term «self-cleaning» denotes 
the capacity of microorganisms, plants and other aquatic organisms 
to break down, consume and mineralize the organic substances 
injected into the river, including those in their biomass or removing 
from the ecosystem in the form of minerals and carbon dioxide. The 
process takes place in several stages, each of which can be assigned 
to a certain zone downstream of the pollution intake site.
Usually there are four distinguishable self-cleaning zones:
1. Degradation zone. In this area of the river water mixes 
with pollutants. The water becomes cloudy, inadequate for 
development of water plants and other organisms. However, 
here species consuming the organic substance particles may 
reproduce. As the heaviest particles sink to the river bottom, the 
organic sediment layer is formed.
2. Active  decomposition  zone.  Here  the  concentration  of  organic 
pollutants causes rapid proliferation of water microorganisms, 
bacteria and fungi, and they commence the biological 
degradation of organic substances. A vital factor in this process 
is  the  oxygen  content  in  the  water.  All  aquatic  invertebrates 
and  fish  use  the  oxygen  for  respiration.  However,  in  the 
active zone it is also in large quantities consumed by aerobic 
microorganisms.  Therefore  the  oxygen  deficiency  is  observed 
here. It is characterized by the biological oxygen demand. While 
oxygen is still sufficient, the organic substances are degraded by 
the aerobic microorganisms. As the amount of oxygen decreases, 
they are replaced by anaerobic microorganisms. In this zone a