Design a pretreatment lime stabilization system for Omega Three (Example 27-2). Determine the volume of the reactor tank, mixer size (kW, rpm, turbine diameter), lime feed rate, and lime storage volume for two-week delivery. Assume the specific gravity of the sludge is 1.012, that lime has a bulk

Question

Design a pretreatment lime stabilization system for Omega Three (Example 27-2). Determine the volume of the reactor tank, mixer size (kW, rpm, turbine diameter), lime feed rate, and lime storage volume for two-week delivery. Assume the specific gravity of the sludge is 1.012, that lime has a bulk density of 640 kg/m³ , and that a 14-day supply will be stored.

Accepted Answer

a. From Example 27-2, the volume of sludge delivered per shift is 90 m³ . From Table 27-10, a standard tank that will hold this volume is 115 m³.

b. From Table 27-10, select a mixer with the following characteristics: 2.1 m turbine, 14.5 kW motor, and shaft speed of 37 rpm.

c. The mass of dry solids is determined using Equation 15-9. From Example 27-2, the percent solids is 3.8 and the volume of the sludge per pumping cycle is 90 m³.
[latex] \sout{V}_{sl}\frac{M_{s}}{(\rho )(S_{sl}(P_{s}))} [/latex] (15-9)
[latex]V_{sl} = \frac{M_{s}}{(ρ)(S_{sl})(P_{s})}[/latex]
[latex]M_{s} = [/latex] (90 m³)(1, 000 kg/m³)(1.012)(0.038)
= 3, 461 or 3, 500 kg per pumping cycle

d. The mass of lime (CaO) for each batch is the ratio of the gram molecular weights for the slaking of lime. From the reaction shown below, one mole of CaO reacts with water to produce one mole of hydrated lime (Ca(OH)[latex]_{2}[/latex]).
[latex]CaO + H_{2}O \rightleftharpoons Ca(OH)_{2}[/latex]

e. From Table 27-9, assume a dose of 300 g Ca(OH) /kg dry solids for WAS. The mass of lime (CaO) for each batch is then
[latex]\frac{GMW CaO}{ GMW Ca(OH)_{2}} (300 g/kg)(3,461 kg)(10^{−3} kg/g)[/latex]
[latex] = \frac{56}{74}(300 g/kg)(3,461 kg)(10^{−3} kg/g)[/latex]
= (0.76)(1,038.3) = 785.7 or 790 kg per batch

f. The lime feed rate is set to match the flow rate of the sludge. The lime feed rate is then
[latex]\frac{785.7 kg per batch}{60 min delivery per batch} =[/latex] 13.1 kg/min

g. The volume of the lime storage silo is
[latex]\sout{V} =[/latex] (3 batches per day)(785.7 kg per batch)(14 d supply)(1/640 kg/m³)
= 51.56 or 52 m³

Comment. For practical, operational use, the specific gravity of sludge is often taken to be 1.0.

Question 27.3: Design a pretreatment lime stabilization system for Omega Th...

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