--- title: "Wastewater examination and sampling" subject: "Sanitary Engineering" chapter: 5 key: "semester_6/sanitary_engineering:wastewater-sampling" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/wastewater-sampling site: BCT Engineering --- # Wastewater examination and sampling Sanitary Engineering · Chapter 5: Characteristics and Examination of Wastewater - Asked in: 2072 Asoj, 4 marks; 2068 Magh (old course), 4 marks (asked 2 times) - Video: [How Is Wastewater Tested?](https://www.youtube.com/watch?v=LpvpsesdmSg) — Water Movement - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/wastewater-sampling - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-5 ## Question Why is the examination of wastewater necessary? How is wastewater sampling done? ## Short answer Wastewater examination determines characteristics for design, operation control, and compliance. Sampling uses grab samples (instantaneous, for pH/DO) or flow-proportional 24-hour composites (for BOD/solids). Collect at mid-depth in well-mixed sections using clean bottles, rinse thrice, fill completely for BOD/DO, preserve at 4°C, analyze BOD within 6-24 hours. Label with location, time, temperature, collector. ## Answer #### Necessity of examination of wastewater 1. To know the **characteristics and strength** (BOD, COD, solids, pH, nitrogen, bacteria) of the wastewater. 2. To **design** the treatment plant and choose the process. 3. To **control and monitor** the operation and efficiency of the units. 4. To decide the **degree of treatment** necessary before disposal into streams, or reuse in irrigation. 5. To check compliance with **effluent standards** and laws. 6. To study **pollution** of the receiving water and the effect on the public health and the environment. 7. To trace infiltration and industrial wastes in sewers, and to detect toxic wastes. #### Sampling of wastewater A sample must truly represent the wastewater, because the flow and strength change hourly. **Types of samples** - **Grab (catch) sample:** a single sample collected at one place and one time; used for pH, DO, temperature, chlorine, and where the flow is uniform. - **Composite sample:** many small samples are collected at equal intervals (hourly) over 24 hours and mixed in proportion to the flow at that time. It gives the average quality and is used for BOD, solids, etc. - **Integrated sample** (several points at one time) is used for wide channels. **Procedure and precautions** 1. Select the sampling point: at an inlet works, a place with good mixing (flume, drop, manhole) and not at a deposit or a dead pool; sample at mid-depth and mid-stream. 2. Use clean polyethylene or glass bottles, rinse them 2-3 times with the wastewater, and fill completely (for BOD and DO, no air gap). 3. Take the sample quantity of 1-2 litres (more for composite). 4. Label with place, date, time, temperature and name of collector. 5. **Preserve** the sample at about 4°C (in ice), and analyse BOD within 6 hours (24 hours at the latest); add acid or other preservative for some tests as per Standard Methods. 6. Note site data (flow, weather, colour, odour) at the time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is wastewater examined? A. To increase BOD B. To make it drinkable directly C. To avoid sampling D. To design treatment, monitor efficiency and check effluent standards Answer: D. Examination guides design, control and compliance. (Key point: Necessity) 2. What is a grab sample used for? A. pH, DO, temperature, chlorine at one time and place B. Average 24-hour quality C. Flow measurement D. Sludge age Answer: A. Grab samples reflect a single moment. (Key point: Grab sample) 3. What is a composite sample? A. One sample at noon B. Samples taken at equal intervals over 24 h mixed in proportion to flow C. Sample from river bed D. Sample of sludge Answer: B. It gives average quality. (Key point: Composite) 4. How should a BOD sample be preserved? A. Store at 40°C B. Leave in sun C. Keep at about 4°C and analyse within 6 h (24 h at most) D. Keep 1 week Answer: C. Cooling slows biological change. (Key point: Preservation) ## Related in Sanitary Engineering - [Is treatment necessary? 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