---
title: "Unit operation, process; need of trickling filter"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:unit-operation-process-trickling-need"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/unit-operation-process-trickling-need
site: BCT Engineering
---

# Unit operation, process; need of trickling filter

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2081 Chaitra, 3+5 marks
- Video: [Unit operations & unit processes of wastewater](https://www.youtube.com/watch?v=jmeIqdZmRB8) — Environmental engg
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/unit-operation-process-trickling-need
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

Define unit operation and process in wastewater treatment system with examples. Explain the need of trickling filter along with its pollutant removal mechanism compared to intermittent sand and contact bed filter.

## Short answer

Unit operation uses physical forces (screening, sedimentation); unit process uses chemical/biological reactions (activated sludge, chlorination). Trickling filter gives 65-90% BOD removal by biofilm oxidation on coarse media; intermittent sand filter strains through sand with intermittent dosing, contact bed adsorbs on stones then rests.

## Answer

#### Unit operation and unit process
- A **unit operation** is a treatment step in which pollutants are removed by **physical forces** only. Examples: screening, grit removal, sedimentation, flotation, filtration, flow equalisation and mixing.
- A **unit process** is a treatment step in which pollutants are removed or changed by **chemical or biological reactions**. Examples: activated sludge (biological oxidation), trickling filter, anaerobic digestion, coagulation, chlorination (disinfection), nitrification.

In a plant the operations and processes are combined (preliminary, primary, secondary, tertiary stages) into a treatment train.

#### Need of a trickling filter
Primary treatment removes only about 30-35% of the BOD, and sewage still holds dissolved and colloidal organic matter. A trickling filter is a secondary treatment unit that:
- gives 80-90% BOD removal (high rate 65-85%) with simple operation and no continuous power for aeration (only for dosing);
- needs less skill and cost than activated sludge, and handles shock loads better;
- is suitable for small and medium towns that have a suitable gravity head.

#### Pollutant removal mechanism
Sewage is sprayed over a bed of stones (40-80 mm) or plastic media. A slimy biological film (zoogloeal film) of bacteria, fungi, algae and protozoa grows on the media. As the sewage trickles down in a thin layer, air moves up through the voids. Organic matter is **adsorbed** on the film and **oxidised aerobically** by bacteria in the outer layer, which converts it to CO₂, water and new cells; the inner layer is anaerobic. Old film sloughs off and is removed in the secondary clarifier.

#### Comparison with intermittent sand filter and contact bed
| Point | Trickling filter | Intermittent sand filter | Contact bed |
|---|---|---|---|
| Medium | Coarse stones/plastic (40-80 mm) | Fine sand | Broken stones (25-50 mm) |
| Hydraulic loading | High (much higher than the other two) | Very low | Low |
| Operation | Continuous | Intermittent dosing and resting | Fill, contact, drain, rest cycle |
| Removal mechanism | Biofilm oxidation, continuous aeration | Straining plus oxidation in sand grains | Adsorption on stones, then oxidation during rest |
| Land | Small | Very large | Medium |
| Clogging | Little | Frequent | Frequent, with low efficiency |
| Efficiency | 65-90% BOD removal | High, but only for small flows | Lower than a trickling filter |

Because of higher loading rate, continuous operation, less area and less clogging, the trickling filter has replaced sand filters and contact beds.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which pair is correctly classified?
   A. Screening - unit process; chlorination - unit operation
   B. Sedimentation - unit process; activated sludge - unit operation
   C. Screening and sedimentation - unit operations; activated sludge and chlorination - unit processes
   D. Both are the same
   Answer: C. Unit operations use physical forces; unit processes use chemical or biological reactions. (Key point: Unit operation versus unit process)

2. Why is a trickling filter needed after primary treatment?
   A. Primary treatment removes all BOD
   B. Primary removes only about 30-35% BOD, leaving dissolved and colloidal organics
   C. Primary produces too little sludge
   D. Primary adds BOD
   Answer: B. Settling cannot remove dissolved and colloidal matter, so a secondary biological unit is needed. (Key point: Need for trickling filter)

3. How does a trickling filter remove organics?
   A. By straining through fine sand
   B. By anaerobic digestion in the whole bed
   C. By chemical coagulation
   D. By adsorption on the slime film and aerobic oxidation by its bacteria, using air through the voids
   Answer: D. A biological film grows on the media; organics are adsorbed and oxidised to CO2, water and new cells. (Key point: Removal mechanism)

4. Why has the trickling filter replaced intermittent sand filters and contact beds?
   A. Higher loading, continuous operation, less area and less clogging
   B. It uses fine sand
   C. It needs a larger land area
   D. It works only intermittently
   Answer: A. Its coarse media and continuous operation allow high hydraulic loading on a small area. (Key point: Advantage over sand filter and contact bed)

## Related in Sanitary Engineering

- [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
- [HR trickling filter, 5 MLD, RR 1.5](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-hr-5mld-rr1p5) — Sanitary Engineering, chapter 7, asked 2×
- [Two-stage trickling filter, 7665 m³/day](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atwo-stage-trickling-7665) — Sanitary Engineering, chapter 7
- [HR trickling filter, 2.5 MLD; change if standard](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atrickling-hr-2p5mld-std-compare) — Sanitary Engineering, chapter 7
- [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

## Related in other subjects

- [Municipal wastewater treatment plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Amunicipal-wastewater-treatment-plant) — Environment and Pollution Control, chapter 4
- [Removal mechanism of permanent hardness](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apermanent-hardness-removal) — Water Supply Engineering, chapter 6, asked 2×
- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6
- [Treatment processes, impurities removal, settlement](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-sedimentation-factors) — Water Supply Engineering, chapter 6, asked 2×