Evaluation of the treatment efficiency of septic tanks – Second experimental campaign

This study aims at improving the characterisation of the treatment efficiency of septic tanks (particularly precast concrete ones) to anticipate possible evolutions of standards and regulations.

The present report completes the Cerib report “Evaluation of the treatment efficiency of septic tanks” (287.E) which presents the test results of treatment efficiency of four prefabricated concrete septic tanks based on the protocol of the NF EN 12566-3. It presents the result of the second phase of the experimental programm, including more important overloading and underloading based on the protocole from the “Arrêté du 7 septembre 2009 modifié”.

This study confirms the treatment efficiency of the prefabricated concrete septic tanks expressed in terms of treatment efficiency ratios. It highlights the difficulty to define simultaneously in terms of concentrations the three characteristic parameters BOD5, COD and TSS for an effluent released by septic tanks or primary treatment device because of the variability of the influent entered septic tank.

This confirms the approach adopted in the NF EN 12566-3+A1 which expresses the results in terms of treatment efficiency ratios and not in concentrations of the effluent released by septic tanks. If the typical values of the effluent released by septic tanks defined by the NF EN 12566-6 and by the “Arrêté du 7 septembre 2009 modifié” can be used as references for waters intended to enter in the septic tanks for testing them, they could not be likened to the thresholds values to characterize the treatment efficiency of a septic tank.

Keywords: Concrete, Septic tank, Treatment efficiency