Dissolved Oxygen (DO) is required to maintain a balanced community of organisms in lakes, rivers and ocean.
When oxygen-demanding waste (measured as BOD) is added to the water, the rate at which oxygen is consumed (deoxygenation) may exceed the rate at which oxygen is resupplied from the atmosphere (reaeration). This can lead to depletion of oxygen resources, with concentrations falling far below saturation levels.
When oxygen levels drop below 4 to 5 mg O2/L, reproduction by fish and macroinvertebrates is impaired. Oxygen depletion is often severe enough that anaerobic conditions develop, with an attendant loss of
biodiversity and poor aesthetics (turbidity and odor problems).
Consideration of the fate of BOD following discharge to a river is a useful starting point for examining the impact of oxygen-demanding wastes on water resources.
OXYGEN SATURATION
Oxygen Saturation or Equilibrium Concentration – is the amount of oxygen that can be dissolved in water at a given temperature.
In the warmer months of summer, stream flow is typically lower as well , offering less dilution of the waste.
ASSIMILATION (SELF-PURIFICATION) OF NATURAL STREAMS
When the wastewater or the effluent is discharged into a natural stream, the organic matter is converted into ammonia, nitrates, sulfates, carbon dioxide etc. by bacteria. In this process of oxidation, the dissolved oxygen content of natural water is utilized. Due to this, deficiency of dissolved oxygen is created. As the excess organic matter is stabilized, the normal cycle will be in a process known as Self-purification wherein the dissolved oxygen is replenished by re-aeration with atmospheric oxygen of wind.
Factors Involved in Self-purification of Natural Streams:
1.Dilution
When wastewater is discharged into the receiving water, dilution takes place due to which the concentration of organic matter is reduced and the potential nuisance of sewage is also reduced. When the dilution ratio is quite high (small quantity of oxygen-demanding waste and large DO in water), large quantities of DO are available which will accelerate the chances of purification and reduce pollution effects. Aerobic condition will always exist because of higher dilution. This will however, not be here if dilution ratio is small, i.e., when large quantities of oxygen demanding effluent is discharged into a small stream supplementing little oxygen or aeration.
3.Dispersion due to Currents
Self-purification of stream largely depends upon currents, (whirlpools, waterfalls and turbulent flow) which will readily disperse the wastewater in the stream, preventing local accumulation of pollutants. High velocity accelerates re-aeration and reduces the concentration of pollutants. High velocity improves re-aeration, reduces the time of recovery, though length of stream affected by the wastewater is increased.
Sedimentation
If the stream velocity is lesser than the scour velocity of particles, sedimentation will take place. This will have two effects.
(i) The suspended solids, which contribute largely the oxygen demand, will be removed by settling and hence water quality of the downstream is improved.
(ii) Due to settled solids, anaerobic decomposition may take place.
- Temperature
At low temperature, the activities of bacteria is low and hence rate of decomposition will also be slow, though DO will be more because of increased solubility of oxygen in water. At high temperatures, the self-purification takes lesser time, the quantity of DO will be less although the bacterial activities will be increased.
5.Sunlight
Sunlight helps photosynthesis of certain aquatic plants (as algae) to absorb carbon dioxide and give out oxygen, thus accelerating self-purification. Sunlight also acts as a disinfectant.
Ocean has the greatest capacity to assimilate the waste compared to rivers or lakes.
Rivers have greater capacity to assimilate the waste than lakes because of greater mixing.
Ideally, we have to treat the wastewater to a level such that its assimilative capacity of the wastes is not exceeded.
Sources: Lecture from
Introduction to Wastewater Treatment Lesson 7
