# 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](/files/wCAMNmvYxuNK3zf0f9wK)


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