Sludge Stabilized in Septic Tanks, a Requirement That Validates Its Evacuation on Site and Enhances Its Reuse

The objective of this study was to evaluate the degree of sludge stabilization in septic tanks by assessing the mineralization of retained sludge at different retention times. Under normal operating conditions, sludge remains inside septic tanks for extended periods, typically ranging from 8 months to 2 years. From a process engineering perspective, septic tanks operate as semi-continuous systems, in which wastewater enters continuously while sludge is removed intermittently. To evaluate sludge stabilization, sludge samples were collected from three full-scale septic tanks operating under real field conditions. The quality of the sludge is determined by the volatile fraction; the lower this value, the higher the physicochemical quality of the sludge. The volatile fraction is equivalent to the percentage of volatile suspended solids, which is evaluated by determining the ratio between volatile suspended solids (VSSs) and total suspended solids (TSSs). A lower VSS/TSS ratio indicates greater sludge stabilization. The results obtained allow for the evaluation of sludge quality and stabilization under different operating conditions and the endogenous constant in various scenarios. Finally, it is confirmed that septic tanks operate as digesters, reducing volatile material, and that their endogenous constant is below that of traditional digesters; the observed overall value is 0.0078 (1/day), which explains the residence times of the biomass and the respective periods of discharge and evacuation of the sludge.