--- title: "Simplex method of aeration" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:simplex-aeration-method" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/simplex-aeration-method site: BCT Engineering --- # Simplex method of aeration Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2068 Magh (old course), 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/simplex-aeration-method - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question Describe the purpose, working and design considerations of the simplex method of aeration with neat sketch. ## Answer The **simplex aerator** is a mechanical surface aeration device used in the activated sludge process (a mechanical, non-diffused system). #### Purpose To supply oxygen to the mixed liquor in the aeration tank and to keep the activated sludge flocs in suspension and well mixed with the sewage, without compressed-air diffusers. #### Working ```text motor + gear | +--+--+ ~~~~~~~~~~~~~~ spray at surface | shaft| draft tube/cone ^ sewage lifted +--+--+ ___________|__|_______|_ ~~~~|~~~~~~| cone / \ /|~~~~~~ water level | | impeller draft tube |______|_____|______ aeration tank (liquor drawn up from the bottom) ``` - A vertical hollow **draft tube** (uptake tube) is fixed at the centre of the tank, with a cone-shaped impeller at its top, driven by a motor through a gear box. - The impeller draws the mixed liquor up through the tube from the bottom and throws it out over the surface as a thin sheet or spray, so that it picks up oxygen from the air. - This sets up a continuous circulation in the tank, so the whole of the mixed liquor is mixed and aerated and the flocs stay in suspension. - Sewage and return sludge flow into the tank, and the mixed liquor goes to the secondary settling tank. #### Design considerations - Oxygen transfer is about 1.2-2 kg O₂ per kWh; select the aerator for the oxygen requirement of the BOD load. - The tank is usually square (or circular), with a side water depth of 3-4.5 m, and is sized so that one aerator can keep its whole plan area mixed and aerated. - The power per unit volume must keep the MLSS in suspension (about 10-20 W/m³ for mixing). - The tank is designed from the F/M ratio (0.2-0.4), MLSS (2000-4000 mg/l) and detention time (6-8 h). - Provide standby units, access for maintenance and a splash guard against spray and noise. - Wind and temperature affect efficiency; a sludge return line and a scum removal arrangement are needed. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What two jobs does the simplex aerator do? A. Supplies oxygen and keeps flocs suspended and mixed B. Removes grit and grease C. Settles sludge and digests it D. Heats the sewage and filters it Answer: A. It oxygenates the mixed liquor and keeps activated sludge in suspension without compressed-air diffusers. (Key point: Purpose of aerator) 2. How does it transfer oxygen? A. It bubbles compressed air from the tank bottom B. The impeller draws mixed liquor up the draft tube and throws it over the surface as spray C. It injects pure oxygen D. It stirs sewage with no contact with air Answer: B. Liquor thrown in thin sheets or spray picks up oxygen from the air. (Key point: Working principle) 3. What is the draft tube (cone) for? A. Carrying return sludge B. Removing scum C. Guiding mixed liquor up to the impeller for circulation D. Housing the motor Answer: C. The vertical cone at the tank centre lets the impeller lift liquor from the tank and circulate it. (Key point: Components) 4. On what is the aeration tank design based? A. Grit size and weir loading B. Only on tank colour C. Surface overflow rate of 25-40 D. F/M ratio (0.2-0.4), MLSS (2000-4000 mg/l) and detention time 6-8 h; aerator chosen for oxygen requirement Answer: D. These parameters fix tank volume and the oxygen demand the aerator must meet. 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