--- title: "Oxidation pond; flushing device role" subject: "Sanitary Engineering" chapter: 7 key: "semester_6/sanitary_engineering:oxidation-pond-mechanism-flushing" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/oxidation-pond-mechanism-flushing site: BCT Engineering --- # Oxidation pond; flushing device role Sanitary Engineering · Chapter 7: Wastewater Treatment - Asked in: 2081 Chaitra, 4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/oxidation-pond-mechanism-flushing - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7 ## Question Define oxidation pond. And, describe pollutant removal mechanism of the oxidation pond. Also, discuss about the role of flushing device to keep the sewerage system functioning well. ## Short answer Oxidation pond is a shallow earthen basin (1–1.5 m) treating sewage by bacteria-algae symbiosis; aerobic bacteria oxidize organics using algal oxygen, algae use released CO2/nutrients, bottom sludge digests anaerobically, and sunlight kills pathogens. A flushing device suddenly discharges 1–2 m³ of water to scour deposits from flat/dead-end sewers, preventing blockages, H2S, odour and corrosion. ## Answer #### Oxidation pond An **oxidation pond** (waste stabilisation pond) is a shallow, man-made earthen basin, 1-1.5 m deep, in which sewage is treated naturally by bacteria and algae using sunlight. Detention is typically 5-30 days and the BOD removal is 70-90%. #### Pollutant removal mechanism 1. **Bacterial oxidation:** aerobic bacteria in the upper layer oxidise the organic matter with oxygen: $\text{organics}+O_2\rightarrow CO_2+H_2O+NH_3+PO_4+\text{new cells}$. 2. **Photosynthesis by algae:** algae use sunlight and the CO₂, ammonia and phosphates released by the bacteria and produce oxygen: $CO_2+H_2O+\text{light}\rightarrow\text{algae}+O_2$. This oxygen, along with wind aeration at the surface, feeds the bacteria. The two groups live together in **symbiosis**. 3. **Sedimentation and anaerobic digestion:** settleable solids go to the bottom, where anaerobic bacteria convert them to CH₄, CO₂ and H₂S. 4. **Pathogen removal:** sunlight (UV), high pH during the day, long detention and predation kill most pathogens. ```text sunlight \ | / ~~~~~~~~~~~~~~~~~~~~~~~~ surface (wind) Algae --O2--> Bacteria (aerobic) Algae <--CO2,NH3,PO4-- Bacteria - - - - - - - - - - - - - - - - Anaerobic sludge layer -> CH4, CO2 ================ bottom ``` #### Role of the flushing device in sewerage A **flushing device** (flushing tank or flushing manhole/hydrant) sends a large quantity of water into a sewer from time to time to clean it. - Sewers laid at flat gradient, dead ends and house sewers at the start of a line do not get the self-cleansing velocity (0.6-0.9 m/s) because the flow is small. Solids then settle and decompose, producing blockage, hydrogen sulphide, corrosion and odour. - The sudden discharge of 1-2 m³ of water (automatic siphonic flush tank, or hand-operated flush from a hydrant or tanker) raises the velocity and depth, scours the deposits and carries them to the larger sewers. - Types: automatic flush tank (siphon type, filled from a water-supply connection and discharging at set intervals), non-automatic (hand-operated), and flushing by hydrants and fire-engine pumps. - Result: sewers remain free from silting and blockage, the sewerage system functions well, and odour and corrosion are reduced. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An oxidation pond is A. a deep covered concrete tank treated with chlorine B. a shallow earthen basin 1-1.5 m deep where bacteria and algae treat sewage using sunlight C. a closed digester producing methane D. a trickling filter with rock media Answer: B. It is a shallow man-made earthen basin with natural treatment by bacteria and algae. (Key point: Definition of oxidation pond) 2. What do algae contribute in the pond? A. They digest sludge into methane B. They produce oxygen by photosynthesis using CO2 and nutrients released by bacteria C. They remove oxygen to kill pathogens D. They settle solids at the bottom Answer: B. Algae use sunlight, CO2, ammonia and phosphates and release O2, which the bacteria use (symbiosis). (Key point: Algae role) 3. What happens to settled solids at the bottom of the pond? A. They are oxidised by algae B. They are digested anaerobically to CH4, CO2 and H2S C. They are carried out with the effluent D. They are converted to oxygen Answer: B. Anaerobic bacteria in the sludge layer produce methane, CO2 and H2S. (Key point: Bottom of pond) 4. Why is a flushing device needed in a sewerage system? A. To increase the sewage BOD B. To add oxygen to the sewage C. To supply drinking water to houses D. To supply a sudden large flow that raises velocity and scours deposits in flat or dead-end sewers lacking self-cleansing velocity Answer: D. Small flows in flat sewers and dead ends do not reach self-cleansing velocity; flushing scours the deposits and prevents blockage and odour. 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