--- title: "Physical characteristics; decomposition of wastewater" subject: "Sanitary Engineering" chapter: 5 key: "semester_6/sanitary_engineering:physical-characteristics-decomposition" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/physical-characteristics-decomposition site: BCT Engineering --- # Physical characteristics; decomposition of wastewater Sanitary Engineering · Chapter 5: Characteristics and Examination of Wastewater - Asked in: 2071 Bhadra, 8 marks - Video: [5TH SEM CIVIL ENGINEEERING WS & WWE CH-7 Quantity and Quality of sewage PART-4(C)](https://www.youtube.com/watch?v=-HZSnFs2L4E) — Nilachal Polytechnic College Of Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/physical-characteristics-decomposition - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-5 ## Question Describe briefly the physical characteristics of wastewater. How does the decomposition of wastewater take place? Explain the processes. ## Short answer Physical characteristics include colour (brownish-grey fresh, black septic), musty or H₂S odour, temperature 20-25°C, turbidity, and solids 500–1000 mg/L. Decomposition: aerobic bacteria oxidise organic matter with oxygen to CO₂ and water; anaerobic bacteria produce methane and H₂S, causing foul smell. ## Answer #### Physical characteristics of wastewater 1. **Colour:** fresh sewage is light brownish-grey. As it becomes stale (septic), it turns dark grey and finally black because of sulphides. Colour tells the age and condition of the wastewater. 2. **Odour:** fresh sewage has a faint, musty smell. Septic sewage smells of hydrogen sulphide (rotten egg) and other gases like mercaptans. It is measured by the threshold odour number. 3. **Temperature:** usually 3-5°C higher than the water supply (about 20-25°C in Nepal). Temperature affects bacterial activity, DO solubility and the rate of decomposition. It is measured with a thermometer. 4. **Turbidity:** due to suspended and colloidal matter (clay, silt, organic matter); high in raw sewage. Measured in NTU with a nephelometer. 5. **Solids:** total solids are 500-1000 mg/L (suspended, dissolved, settleable). 6. **Specific gravity** is about 1.0 (nearly that of water). #### Decomposition of wastewater Organic matter in wastewater is broken down by bacteria. There are two ways. **1. Aerobic decomposition (in the presence of free oxygen)** Aerobic bacteria oxidise organic matter using dissolved oxygen: $$ \text{Organic matter}+O_2\xrightarrow{\text{aerobic bacteria}}CO_2+H_2O+NO_3^-+SO_4^{2-}+\text{new cells} $$ Nitrogen goes to nitrates, sulphur to sulphates, and carbon to CO₂. It is rapid, odourless and the products are stable. It occurs in streams, oxidation ponds, trickling filters and activated sludge. **2. Anaerobic decomposition (in the absence of oxygen)** When oxygen is exhausted, anaerobic bacteria work in two stages: - *Acid formation:* complex organic matter is changed to organic acids and alcohols. - *Methane formation:* these acids are converted to methane, carbon dioxide, ammonia and hydrogen sulphide: $$ \text{Organic matter}\xrightarrow{\text{anaerobic bacteria}}CH_4+CO_2+NH_3+H_2S+\text{new cells} $$ It is slow, and gives foul smell and a black colour, but methane is useful as biogas. It happens in septic tanks, sludge digesters and the bottom deposits of ponds. **3. Facultative decomposition:** facultative bacteria work either with or without oxygen (as in facultative ponds). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a black colour of sewage indicate? A. Stale (septic) sewage with sulphides B. Fresh sewage C. Clear water D. High oxygen Answer: A. Fresh sewage is light brownish-grey, turning black when septic. (Key point: Colour and age) 2. Which describes aerobic decomposition? A. Produces methane B. Bacteria use dissolved oxygen; rapid, odourless, stable products like nitrates and sulphates C. Foul smell D. Slow, black colour Answer: B. Aerobic breakdown gives CO2, H2O, NO3 and SO4. (Key point: Aerobic) 3. What are anaerobic products? A. Nitrates and sulphates B. Only CO2 and water C. CH4, CO2, NH3, H2S with foul smell D. Oxygen Answer: C. Acid formation then methane formation. (Key point: Anaerobic) 4. What are the two stages of anaerobic decomposition? A. Nitrification then denitrification B. Oxidation then reduction of oxygen C. Sedimentation then filtration D. Acid formation then methane formation Answer: D. Complex matter becomes acids and alcohols, then methane. (Key point: Anaerobic stages) ## Related in Sanitary Engineering - [Is treatment necessary? Town wastewater 300 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atreatment-needed-town-300-ls) — Sanitary Engineering, chapter 6 - [Degree of treatment, industrial wastewater](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adegree-treatment-industrial-waste) — Sanitary Engineering, chapter 6 - [Suspended and attached growth processes](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asuspended-attached-growth) — Sanitary Engineering, chapter 7, asked 3× ## Related in other subjects - [Physical and chemical characteristics of solid waste](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Asolid-waste-characteristics) — Environment and Pollution Control, chapter 5 - [Why hydration of cement takes place](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Ahydration-of-cement-why) — Civil Engineering Materials, chapter 5 - [Physical and chemical weathering](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aphysical-and-chemical-weathering) — Engineering Geology I, chapter 6, asked 5× - [Critical characteristics of the driver](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Adriver-characteristics) — Transportation Engineering II, chapter 1, asked 3×