Buildup of Live Bacterial Biomass

The biozone layer is formed as a biologically active layer in the soil absorption system near the infiltrative surface by the growth of microorganisms feeding on the organic matter (BOD) of the septic tank effluent (See Figure 6:4-1). The amount of live bacteria biomass in the biozone is estimated using a mass balance equation assuming the biozone as a control volume. The mass balance equation of microorganisms (live bacteria biomass) in the control volume is then estimated by:

$\frac{d(Bio)}{dt}=\alpha*[\sum Q_{STE}*C_{BOD,in}-I_p*C_{BOD}]-R_{resp}-R_{mort}-R_{slough}$ (1)

where $Bio$ is the amount of live bacteria biomass in biozone (kg/ha), $C_{BOD}$,in is the $BOD$concentration in the $STE$ (mg/L), $C_{BOD}$ is the $BOD$ concentration in biozone (mg/L), $\alpha$ is gram of live bacteria growth to gram of $BOD$ in STE (conversion factor), $Q_{STE}$ is the flow rate of $STE$ (m$^3$/day), Ip is the amount of percolation out of the biozone (m$^3$/day), $R_{resp}$ is the amount of respiration of bacteria (kg/ha), $R_{mort}$ is the amount of mortality of bacteria (kg/ha), and $R_{slough}$ is the amount of sloughed off bacteria (kg/ha).

Figure 6:4-1 Vertical configuration of STE distribution chamber and absorption system