--- title: "ASP: removal of carbonaceous matter; recirculation" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:asp-fundamentals-carbonaceous" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-fundamentals-carbonaceous site: BCT Engineering --- # ASP: removal of carbonaceous matter; recirculation Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2078 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/asp-fundamentals-carbonaceous - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question Describe the fundamentals on removing dissolved carbonaceous substance in the Activated Sludge Process (ASP). Also, justify the need of sludge re-circulation in the ASP with suitable examples. ## Short answer Dissolved organics are adsorbed on bacterial flocs, oxidized for energy, and synthesized into new cells; endogenous respiration follows. Sludge recirculation maintains MLSS, controls F/M ratio and sludge age, seeds incoming sewage, ensures good flocculation, and allows smaller tanks; e.g., recycle ratio 0.43 for 3000 mg/L MLSS and 10,000 mg/L return sludge. ## Answer #### Fundamentals of removing dissolved carbonaceous matter in ASP In the activated sludge process the dissolved organic (carbonaceous) matter measured as BOD is food for aerobic heterotrophic bacteria kept in suspension in the aeration tank as flocs (activated sludge, MLSS 2000-4000 mg/l). 1. **Mixing and contact:** settled sewage is mixed with return sludge and aerated. The dissolved and colloidal organics are quickly adsorbed on the sticky bacterial flocs (biosorption). 2. **Oxidation (energy):** a part of the organics is oxidised by the bacteria with the dissolved oxygen supplied by diffused or mechanical aeration: $$ C_xH_yO_z + O_2 \xrightarrow{\text{bacteria}} CO_2 + H_2O + \text{energy} $$ 3. **Synthesis (growth):** another part, with nitrogen and phosphorus, builds new cells: $$ C_xH_yO_z + O_2 + NH_3 \rightarrow C_5H_7NO_2\ (\text{new cells}) + CO_2 + H_2O $$ 4. **Endogenous respiration:** when food is short, the cells oxidise their own mass: $C_5H_7NO_2 + 5O_2 \rightarrow 5CO_2 + 2H_2O + NH_3 + \text{energy}$. 5. **Flocculation and settling:** the grown cells clump into settleable flocs. In the secondary clarifier they settle, leaving clear effluent with 85-95% less BOD. 6. **Controlling factors:** $F/M=\dfrac{QS_0}{VX}$ (0.2-0.4 for conventional), sludge age $\theta_c$ (3-15 days), dissolved oxygen above 2 mg/l, pH 6.5-8.5, nutrient ratio BOD : N : P = 100 : 5 : 1, temperature and absence of toxins. Oxygen needed is about 0.5-1 kg O₂ per kg BOD removed. #### Need for sludge recirculation Return activated sludge (RAS) is the settled sludge pumped from the secondary clarifier back to the inlet of the aeration tank. It is necessary because: 1. It **maintains the required concentration of active micro-organisms (MLSS)** in the aeration tank; otherwise the biomass would be washed out with the effluent, since the growth rate of bacteria is slow compared with the detention time of a few hours. 2. It **keeps the F/M ratio and sludge age at the desired value**, so a stable process is obtained. 3. It **seeds** the incoming sewage with adapted, flocculent organisms that are already in the endogenous or declining-growth phase, which speeds up adsorption and oxidation. 4. It **gives good flocculation and settleability**; without return sludge, the dispersed growth would be in the log phase and would not settle. 5. It lets the plant treat the same load in a **smaller tank**. *Example:* if 3000 mg/l MLSS is needed and the return sludge has 10000 mg/l, the return ratio is $\dfrac{3000}{10000-3000}=0.43$, i.e. 43% of the inflow is recycled continuously. Without this return, the tank would hold only the few hundred mg/l of bacteria that grow in one pass, and BOD removal would fall sharply. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What removes dissolved carbonaceous matter in the ASP? A. Aerobic heterotrophic bacteria in flocs, which adsorb, oxidise and use the organics for growth B. Anaerobic bacteria in the sludge blanket C. Chemical coagulation D. Algae producing oxygen Answer: A. Bacteria suspended as flocs in the aerated mixed liquor take up the organics as food. (Key point: Who removes the BOD) 2. What happens to the organics taken up by the bacteria? A. All of it is converted to new cells B. Part is oxidised to CO2 and H2O for energy, and part (with N and P) is built into new cells C. All of it is oxidised to CO2 with no cell growth D. It is converted to methane Answer: B. Oxidation gives energy and synthesis makes new cells; both lower the BOD. (Key point: Oxidation versus synthesis) 3. What is the main reason for recirculating the settled sludge? A. To dilute the incoming sewage B. To remove nitrogen in the clarifier C. To keep a high concentration of active micro-organisms (MLSS) so they are not washed out D. To cool the aeration tank Answer: C. Bacteria grow slowly relative to the few-hour detention time, so the biomass must be returned. (Key point: Why return sludge) 4. With MLSS 3000 mg/l and return sludge 10 000 mg/l, what fraction of the inflow is recycled? A. 3.33, from 10000/3000 B. 0.30, from 3000/10000 C. 0.70, from 7000/10000 D. 0.43, from 3000/(10000 - 3000) Answer: D. Return ratio = X/(Xr - X) = 0.43, i.e. about 43%. (Key point: Return ratio example) ## Related in Sanitary Engineering - [Activated sludge and ASP treatment process](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-sketch-treatment-process) — Sanitary Engineering, chapter 7 - [Activated sludge: properties, mechanism](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-properties-biochemical) — Sanitary Engineering, chapter 7 - [Short note: Need of sludge treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-need-examples) — Sanitary Engineering, chapter 8 - [Unit operation, process; need of trickling filter](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aunit-operation-process-trickling-need) — Sanitary Engineering, chapter 7 - [Sludge treatment need; sand drying bed](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-treatment-necessity-drying-bed) — Sanitary Engineering, chapter 8 - [Sludge treatment need; digester sketch](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-necessity-digester-sketch) — Sanitary Engineering, chapter 8