■What did the modified methods change? Tracing the lineage of the activated sludge method, the modified methods that emerged from the standard method can be organized as follows: 1. Load conditions: Long-time aeration method, etc. 2. Method of aeration: Tapered aeration method, etc. 3. Flow type: Oxidation ditch method, etc. 4. Method of inflow: Step aeration method, batch-type activated sludge method 5. Method of solid-liquid separation: Membrane bioreactor (MBR) ■Characteristics of plug flow and complete mixing In terms of flow type, plug flow has the characteristic of suppressing bulking, while complete mixing has the characteristic of alleviating sudden load fluctuations. ■How do they differ when compared numerically? The representative values for the standard method and long-time aeration method are as follows: 1. BOD-MLSS load: 0.2–0.4 and 0.03–0.05 kg/(kg·day) 2. Retention time in aeration tank: Approximately 8 hours and approximately 24 hours 3. Design DO: Typically 1–2 and 3–5 mg/L As the load is significantly reduced, the tank capacity increases, and the retention time of the water also becomes longer.

■What is the strength of the long-term aeration method? It can reduce the amount of excess sludge by decomposing it through endogenous respiration at low loads. It is suitable for small-scale wastewater treatment, is resilient to load fluctuations, and is relatively easy to operate. ■What are the weaknesses of the long-term aeration method? On the other hand, the sludge has poor cohesiveness, and depending on the conditions, the quality of the treated water may be inferior to standard methods. It also requires high construction costs and a large installation area. There are constraints that make it difficult to adopt unless the regulations for discharge points are not strict. ■How to use standard methods in factories with large fluctuations? In wastewater with significant fluctuations, such as dairy wastewater, it is desirable to equalize the quality and quantity of the wastewater in a balancing tank before sending it to the aeration tank. There are also examples where measures have been taken to reduce excess sludge through aerobic digestion. ■Efforts to equalize DO at each stage! In aeration tanks with a push flow, there are examples of preventing bulking by using tapered aeration to maintain a constant DO at each stage. ■Consultation on DO meters and aeration devices At Torii, we offer consultations on the maintenance of DO meters and the renewal of aeration devices. Please feel free to reach out to us.