Basic terms and concepts of biological treatment.
Prolonged aeration should be understood as the process of complete biological oxidation (complete biological purification) in the presence of dissolved oxygen. When carrying out extended aeration, one of the basic laws of engineering chemistry must be observed – the process must continue for as long as possible. Only in this case, the reaction products will contain a minimum amount of impurities. In our case, the reaction products are water, carbon dioxide and nitrogen.
The duration of extended aeration is determined by the formula:
ta = (Lin – Lout) / ρ ( 1 – S) a, hour where:
Lin – Lout – Difference of BODtotal values at the inlet and outlet of the aerotank, mgО2/l;
ρ – Specific rate of complete oxidation of organic substances with one gram of activated sludge (according to ash-free content), mgBODtotal / 1ghh (For a bioreactor designed for extended aeration and complete oxidation of organic substances, it is taken equal to 6.0 mgBODtotal / 1ghh or 6.0 gBODtotal / 1kghh);
a – Concentration of microorganisms, (dose of activated sludge), g/l or kg/m3. (For aerotanks with free-floating activated sludge, its dose is taken in the range of 1.5 – 2.5 g / l. For bioreactors with floating polymer loading, the recommended dose of sludge is 2.5 – 3.5 g / l.
S – Ash content of biomass (usually taken equal to 0.3 – 0.35 or 30 – 35%). From here, the required volume of the bioreactor is easily calculated:
W = Qh x ta , m3
So, for treatment facilities with a capacity of 24 m3 / day (Qh = 1 m3 / h), with the amount of pollution coming from one inhabitant in terms of BOD full 75 g / (day x person) (according to DBN V.2.5-75-2013 ) and a water disposal rate of 0.2 m3 / (day x person), with an allowable value of BODtotal at the outlet of the treatment plant of 20 mgO2 / l, the required aeration time will be:
ta \u003d ( 75 / 0.2 – 20 ) / ( 6 x 2.5 (1 – 0.35)) \u003d 36.4 hours.
Then, the required volume of the aerotank will be:
Wa \u003d 1 x 36.4 \u003d 36.4 m3
Note: Real data on the concentration of suspended solids, ammonium nitrogen and the value of BOD in wastewater entering such treatment facilities indicate a significant overestimation of the daily norms given in DBN V.2.5-75-2013. In other words, the actual amount of suspended solids and BOD intake is 1.5 – 2.0 times lower than the established norms. This indicates that the actual cleaning effect will be somewhat higher than the calculated one. The norms specified in the DBN are rather the maximum possible. Speaking of MOC, one should also take into account the slowing down effect on the rate of pollution oxidation of possible volley discharges of detergents (surfactants), disinfectants and other xenobiotics. At the same time, the smaller the inflow of wastewater, the more noticeable the effect of xenobiotics on the biological treatment process. Thus, the calculation of treatment facilities, in order to prevent the breakthrough of pollution, should be based on these data.
Growth and removal of excess activated sludge.
The growth of activated sludge is an important parameter of the operation of treatment facilities and implies the whole mass of waste products of bacteria and other contaminants introduced with wastewater (the mineral (insoluble) part of suspended solids and hardly oxidizable organic matter), which are no longer subject to biological oxidation, as well as a certain mass of bacteria, resulting from reproduction. The growth of activated sludge is determined by the formula:
Pr \u003d ((1- ∆) (Sin – Sout) + ∆pr (Lin – Lout) Qday \u003d g / day;
Lin – Lout – BODtotal value at the inlet and outlet, mgО2/l;
Svh – Sv – Concentration of suspended solids at the inlet and outlet, mg/l;
∆ – Share of hydrolyzed organic impurities of suspended solids;
∆pr – Mineralizable fraction of the growing biomass of bacteria;
The value of the sludge growth indicates that during the day the mass of activated sludge in the treatment plant will increase exactly by the found value. At the same time, it is this amount of sludge that must be removed from the bioreactor and is called excess activated sludge. Excess activated sludge must be regularly and correctly removed, since with a higher concentration of activated sludge (with insufficient removal), secondary pollution of the treated wastewater will occur, and with a lower concentration of sludge (when the sludge is removed more than its mass growth) – the system will not be in able to cope with the cleaning of incoming contaminants.
With prolonged aeration and the use of spatial succession of hydrobionts attached to the polymer load, when the hydrobionts of the next purification stage feed on the microorganisms of the previous one, it is possible to achieve very impressive results: In terms of 1 inhabitant, 2–4 g / day or 0.1– 0.2 l of gravity compacted excess activated sludge per day (at 98% humidity). In the presence of a sludge thickener, the moisture content of the sludge can be reduced to 96%. Then, accordingly, the volume of sludge will decrease to 0.05 – 0.1 l / (day x person).
The volume of excess sludge generated at the treatment plant