Cleaning processes include: mechanical sedimentation of suspended matter, biological or chemical oxidation of organic and other pollutants by their mineralization and precipitation; chemical processes involving oxygen, neutralization of heavy metals and similar pollutants; absorption of various pollutants by bottom sediments and aquatic vegetation and other similar processes.
The process of self-purification from non-conservative pollutants is accompanied by the consumption of oxygen for the mineralization of organic substances and the dissolution of oxygen coming from the surface of the water surface, the so-called reaeration.
The process of oxygen consumption is characterized by the equation
Lg(VA,) = ~*it, (1.9)
WhereL-a- BOD total at the initial moment of the oxygen consumption process, mg/l;L,-BODtotal over time{, mg/l;To\- oxygen consumption constant (BOD) at a given water temperature;t-time during which the processes of oxygen consumption and reaeration take place, days.
The solubility of oxygen in water is relatively limited, therefore, due to its low content in water, the intensity of oxidative processes decreases. Also, the intensity of oxidative processes is influenced by the initial oxygen content in water and the intensity of its replenishment from the air through the water surface as it is spent on oxidation.
The process of oxygen dissolution is characterized by the equation Lg(D t /DJ = -k 2 t, (1.10)
WhereD. a- deficit of dissolved oxygen at the initial moment of observation, mg/l;D t -the same after time /, mg/l; /с 2 - oxygen reaeration constant at a given water temperature.
Considering the simultaneous occurrence of both processes in mutually opposite direction, the final rate of change in oxygen deficiency over time t can be expressed by the equation
4=AA(South‘"-102- a)/(* 2 -TO )+ A- 1<¥ й. (1.11)
Equating to zero the first derivative of equation (1.11) with respect to tCan get expression for tKp, corresponding to the minimum oxygen content in water:
"cr = log((*2/*i))
