Using the solids flux theory from the previous sections, equations will now be derived to calculate the maximum allowable hydraulic overflow rate as a function of the sludge concentration in the aeration tank, for the two limiting cases of clarification and thickening. It will be shown that is advantageous to have clarification as the limiting process. The sludge recycle ratio at the transition point between thickening and clarification as the limiting process is called the critical recirculation rate. An optimisation procedure is presented to size the combined system aeration tank - final settler. The solids flux design method is compared with other (empirical) design methods currently in use.
- Optimised design procedure for the final settler
- Determination of the critical sludge recirculation factor
- Optimised design procedure for the system aeration tank - final settler
- Applicability of the solids flux design method
(1) Optimised design procedure for the final settler
The fist step is to calculate the maximum allowable hydraulic loading rate. This maximum rate can be calculated irrespective whether clarification or thickening is the limiting process in the final settler. Due to non-ideality of the final settler (because of wind, gravity effects and non-uniform distribution of the mixed liquor flow over the settler area) a safety factor has to applied. Based on the values of maximum allowable hydraulic overflow rate, safety factor and influent flow rate, the required settler surface area can now be calculated. The application of this procedure is demonstrated in Examples 6.4 and 6.5. To download this section, click here.
(2) Determination of the critical recirculation factor
The superficial loading rate decreases with increasing sludge concentration. This increase is exponential in the case of clarification and even more accentuated in the case of thickening. As the increase of the required settler volume with increasing sludge concentration is so rapid, it is advantageous to increase the sludge recirculation factor until clarification becomes the limiting function of the settler: this value is called the critical recirculation rate. The relationship between the sludge concentration in the aeration tank, the return sludge concentration and the critical recirculation factor is shown in Figure 6.9. To download this Section, click here.
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| Figure 6.9 | Relationship between k·X t and k·Xr and the critical recirculation factor sc (k = Vesilind constant) |
(3) Optimised design procedure for the reactor - settler system
In the previous sections it was shown that it is possible to rationally design a final sludge settler for specified values of the inlet and return sludge concentration, if the values of the Vesilind settling constants are known. The objective of this section is to attribute values to the reactor- and return sludge concentration, such that the activated sludge process is operationally stable and that the efficiency of liquid-solid separation in the final settler is high, at total minimum construction costs. To download this section, click here.
(4) Applicability of the solids flux design method
In this section the maximum hydraulic loading rate calculated according to the solids flux theory presented earlier is compared with empiric guidelines from the EPA (US), STOWA (Netherlands), WRC (England) and ATV (Germany). It can be observed in Figure 6.12 that there is a close correlation between the theoretical values of the maximum hydraulic loading rate derived in this chapter and the empiric values observed in full scale settlers. This correlation is observed over the complete range of practical interest, i.e. where the empiric curves are valid (200 ml/l < sludge volume < 600 ml/l, and applies to poor, fair and well settling sludges. From Fig. 6.12 it is also confirmed that a safety factor of 2 leads to a good correlation between empirical and theoretical results. To download this section, click here.
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| Figure 6.12 | Theoretical values of maximum hydraulic overflow rate as a function of the sludge concentration for good, medium and poor settleability (safety factor = 2) as compared to empiric values from the ATV (1976), STORA (1981) and the experimentally determined value ranges indicated by the STOWA 2002 results (250 < sludge volume loading rate < 500 l/(m2.h)) |