New approach to oxygen requirement for low-load activated sludge

Low-load activated sludge treatment plants are by far the most numerous and are continuing to develop given their enhanced capacity for nitrogen removal. In general, the electrical consumption of aerators accounts for around 80% of total electrical consumption. It can therefore easily be understood that it is important to be able to evaluate the oxygen requirement for biological treatment as accurately as possible. This evaluation of oxygen requirement is made by the designer during the project phase, using the conventional formula [O2 = 1/α(a′Le + b′Xv + c′N)]. While this approach is entirely satisfactory for the designer (who essentially needs to know the adequate oxygenation capacity), it meets the needs of the operator only imperfectly; the latter needs to know the specific electrical consumption of the aerators as accurately as possible (SEC—kWh.kgBOD−15). It is for this reason that an alternative method has been developed allowing calculation of the oxygen requirement for biological treatment (ODCW—kgO2.kgBOD−15) and specific aerator consumption (SEC—kWh.kgBOD−15) for low-load activated sludge type plants (Charpentier, 1988). Initially, an ODCW-standard value is obtained, by calculation or from a graph, from the desired concentration in TKN or NH+4.N for the effluent. ODCWstandard = mass of oxygen which the aerators must be able to dissolve in clean water for 1 kg of BOD5 to be eliminated in “standard” operating conditions. For “non-standard” operating conditions (volumetric load <0.18 kg BOD5.m−3.d−1; [VSS]aeration ≠ 3 g.l−1; [TKN/BOD5]influent ≠ 0.25; [BOD5]influent > 1000 mg O2.l−1), the ODCWstandard value must be combined with correction factors k1, k2, k3 and k4 of which the values and conditions of application have been defined:ODCWplant=ODCWstandardXk1k2k3k4 SEC=ODCWplant/SOTR