Diffused aeration: diffusers and blower

Description

Diffuser grid and blower for an aeration tank, from the standard oxygen requirement $SOR$ (clean water, 20 °C, zero dissolved oxygen). The field-to-standard correction (alpha, beta, temperature, dissolved oxygen) comes before this block, as in the activated sludge oxygen demand block.

Submergence and standard oxygen transfer efficiency, unless the SOTE is given:

$h = H - h_d \qquad SOTE = SOTE_m \cdot h$

Air for oxygen ($SOR$ in kg/h), with 0.2791 kg O₂ per standard m³ of air (20 °C, 1 atm), and air for mixing the floor:

$Q_{O_2} = \frac{SOR}{SOTE \cdot 0.2791} \qquad Q_{mix} = q_{mix} \cdot A$

The design air flow is the larger of the two. The default mixing flux, 2.2 m³/(m².h), is the minimum for a fine-pore grid (US EPA); coarse bubble layouts usually need more.

Diffusers, rounded up to a whole number per tank, and their spacing on a square grid:

$n = N \left\lceil \frac{Q_{ar}}{q_d \, N} \right\rceil \qquad s = \sqrt{\frac{A}{n}}$

The air per diffuser is checked at the minimum air flow (mixing, or the minimum oxygen demand when given), where the grid must still distribute the air evenly, and the spacing against the closest the layout allows.

Blower discharge pressure, inlet flow and adiabatic power:

$\Delta p = \rho g (h + h_{dif} + h_{tub})$

$P = \frac{w \cdot c_p \cdot T_1}{\eta} \left[ \left( \frac{p_1 + \Delta p}{p_1} \right)^{0.2857} - 1 \right]$

$p_1$ is the atmospheric pressure at the site altitude and $w$ the air mass flow.

Typical values by diffuser type, all replaceable by the manufacturer's:

References

METCALF & EDDY. Wastewater Engineering: Treatment and Resource Recovery. 5th ed. McGraw-Hill, 2014. Chapter 5 (aeration systems).

U.S. EPA. Design Manual: Fine Pore Aeration Systems. EPA/625/1-89/023, 1989.