Many factors that attribute to the deficiency of dissolved oxygen are as follows:
One of the most important criteria for a river’s health is dissolved oxygen (DO). With decrease in DO, survival of life becomes difficult.
In extreme cases under anaerobic conditions, the right forms of life can get extinguished. Under such conditions, fungal growth and floating sludge can create noxious conditions.
There are many factors which attribute to the deficiency of DO:
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1. Temperature with the increase in temperature, especially in summer, the solubility of oxygen decreases. With decrease in temperature, especially in winter, the floating ice prevents penetration of atmospheric oxygen.
2. Presence of oxygen demanding wastes reduces DO and with increasing temperature rate of aerobic biodegradation increases and DO drops drastically.
3. Respiration of organisms living in water as well as in sediments removes oxygen resulting in DO drop.
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4. The green plants in the water body although add DO during day through photosynthesis, they reduce DO at night causing harm to the living organisms.
In a simplified model, the two most important parameters considered to assess DO are:
(i) Deoxygenating:
The removal of DO by microorganisms due to biodegradation re-aeration replenishment of oxygen at the interface between the river and the earth’s atmosphere.
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In order to explain the effect of the above two processes, it is assumed that the water and the wastes flowing down the river are uniformly mixed at any given cross section of the river (point source) and there is no dispersion of wastes in the direction of the flow (plug flow).
(ii) Re-aeration
The rate of replenishment of oxygen at any given location is proportional to the difference (D) between the saturated dissolved oxygen concentration and actual dissolved oxygen at any point, at a given time.
The aeration rate constant kr varies with the condition of the river at a time. A fast moving white water river will have higher re-aeration constant compared to a sluggish river.
Saturated value of dissolved oxygen depends on key parameters like temperature, atmospheric pressure and salinity.
At the point of disposal of wastes, decomposition of the higher amount of organic material leads to a greater drop in oxygen level and the rate of removal of oxygen is greater than the rate of re-aeration to bring other oxygen to the same level.
With distance, the oxygen drop decreases but continues and a point arrives, the critical point, where the rate of removal of oxygen equals the rate of addition of oxygen by aeration.
And beyond this critical point, the re-aeration rate supersedes deoxygenating caused by microorganisms and dissolved oxygen reaches to the previous saturation value and the river recovers.
When the rate of deoxygenating exceeds rate of re-aeration, DO drops. At critical points, both are equal. Beyond the critical point, re- aeration exceeds decomposition, DO curve goes upward and DO reaches saturation value.
The lowest value of DO is found to be 6.0 mg/1, an amount sufficient for the most aquatic life. If BOD is added in excess, DO drops below this lowest value creating unhealthy conditions, killing fishes and other animals might also diminish.
From water quality view point, there are thick mats of fungi, sludge, blood worms and many kinds of bacteria growing, leading to further drop in DO and an anaerobic condition might occur resulting in the formation of toxic gases like ammonia, hydrogen sulphide, etc.
Due to anaerobic decomposition by microbes, which in long run creates completely lifeless condition, the DO level falls to zero.
Even if the pollutant load is constant the drop in DO might also vary with seasons, temperature and time of the day.
With increase in temperature, solubility of oxygen decreases, aerobic decomposition of wastes becomes faster and re-aeration slower. Thus, a particular river which has sufficient DO in winter will have unacceptable deficit in summer.
Photosynthesis also affects DO. Algae and other aquatic plants which add DO in day time due to photosynthesis, use oxygen for respiration at night due to which DO drops.
Thus, for a river which is already loaded with excess BOD and choked with algae, at night it becomes unfavourable for sustaining life.
The idea regarding deoxygenating, re-aeration and the effect of BOD on the condition of the health of a river will be helpful in taking decisions on the extent of on-site-waste treatment and the maximum amount of BOD which can be allowed to a river or generally to water resources.