The design of a completely mixed, activated-sludge process illustrates the general procedures involved (Metcalf and Eddy, Inc. 1991). When a BOD5 (5-day BOD) is used and the effluent produced is to have <20 mg/l BOD5, the following equation applies:
where aXe = the biodegradable portion of effluent biological solids. An a = 0.63 is generally applicable (Metcalf and Eddy, Inc. 1991). Furthermore, the values of the following parameters in the design exercise that follows are Si (influent BOD5), Q (influent flow rate), X (MLVSS), 6c (SRT), and Xr (recycled sludge concentration). A wastewater temperature of 20°C is assumed. The following exercise summarizes the design of a completely mixed activated-sludge process:
1. Compute the aeration basin volume (V) using Equation 7.25(7). In this case, the values of X and 6c are assumed, and the values of kinetic coefficients are taken from Table 7.25.4. Note that 6 (LRT) is defined as V/Q, where Q is the influent flow rate.
2. Compute the amount of sludge to be wasted per day. The amount of MLVSS produced due to the removal of BOD (Px) is as follows:
Yo is defined in Equation 7.25(9). The amount of sludge lost in the effluent is QXe10-6 (kg/day). Therefore, the amount of sludge to be wasted per day is Px — QXe10-6 (kg/day).
3. Compute the sludge waste rate (Qw) using Equation 7.25(4).
4. Compute the LRT (9) of the aeration basin.
5. Check the F/M ratio and BOD removal efficiency (E) using Equations 7.25(10) and 7.25(11), respectively.
6. Compute the oxygen requirements. The amount of BODl (ultimate BOD) removal is Q(Si — Se)10—6/f, where f is the conversion factor for converting BOD5 to BODl. (Note that f = 0.65-0.68 for municipal wastewater.) Therefore, the amount of oxygen required (kg/day) is (Q/f)(Si — SJ10-66 — 1.42Px.
7. Compute actual air requirements (diffused aeration). The design air requirements (m3/day) are as follows:
[(SF)(Q/f)(Si — Se)10-6 — 1.42PJ/(%)(0.232)G3d) 7.25(30)
SF = safety factor (usually 2)
7a = specific weight of the air, kg/m3
¡3d = oxygen transfer efficiency of the diffused aeration equipment 7a = p/RT
p = sum of atmospheric pressure and air-diffuser discharge pressure R = universal gas constant T = absolute temperature
For mechanical aeration equipment, the power requirement (kW) is calculated as follows:
[(SF)(Q/f)(Si — Se)10—6 — 1.42PJ/24N 7.25(31)
where N = field oxygen transfer capacity of the mechanical aeration equipment (kg O2/kW-hr). Table 7.25.5 summarizes the typical ranges of field oxygen transfer capacities of various mechanical aerators.
8. Calculate the required recycling ratio (r) as r = X/(Xr — X).
The secondary clarifier design is an integral part of the overall activated sludge process design. Details on the secondary clarifier design are presented in Section 7.32.
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