---
title: "Dorrco aerator in ASP"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:dorrco-aerator"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dorrco-aerator
site: BCT Engineering
---

# Dorrco aerator in ASP

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2074 Bhadra, 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/dorrco-aerator
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

What are the advantages in using the dorrco aerator in activated sludge process method; briefly describe its operation with neat sketch.

## Short answer

Dorrco aerator uses a rotating agitator with compressed air to produce fine bubbles and circulate mixed liquor. Advantages: good oxygen transfer, efficient mixing without solids deposition, independent aeration/mixing control, suitable for deep tanks and small plants, less air required than diffusers.

## Answer

The Dorrco aerator is a mechanical-cum-compressed-air aeration device (supplied by Dorr-Oliver) used in the activated sludge process.

#### Operation
```text
   motor
    |
  +-+-+  rotating agitator
  | | |   (hollow shaft)
~~|~~~|~~~~~~~~~~~~~~~ water level
  |   |  <- mixed liquor circulates
  | air supply pipe
  |___|_____ impeller/sparger plate ___
  tank bottom  (sludge kept in suspension)
```
- A vertical shaft carries a rotating agitator (impeller) in the aeration tank. Compressed air is supplied to the base of the agitator through a pipe or the hollow shaft.
- The rotating blades break the air into fine bubbles and throw the liquid outward and upward. This produces a circulation of the mixed liquor, so the bubbles are in contact with the sewage for a long time.
- The sludge is kept in suspension by the stirring action and the dissolved oxygen is supplied, so organic matter is oxidised by the micro-organisms.

#### Advantages
- Good oxygen transfer, since fine bubbles are produced by shearing and are retained in the liquid for a long time.
- Efficient mixing of sewage and return sludge; there is no deposit of solids in the tank, so clogging of diffusers does not occur.
- Aeration and mixing can be controlled independently, so the unit works for varying loads.
- Tanks can be deeper or of any shape, and the system suits small plants.
- Needs less compressed air than a plain diffuser system, and the pipe work is simple.

## Check yourself

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

1. The Dorrco aerator is best described as
   A. a purely diffused-air system with porous plates
   B. a trickling filter distributor
   C. a mechanical-cum-compressed-air aeration device with a rotating agitator
   D. a surface brush aerator using no compressed air
   Answer: C. It combines a rotating agitator with compressed air supplied at its base. (Key point: What the Dorrco aerator is)

2. Why is oxygen transfer good in the Dorrco aerator?
   A. The rotating blades shear the air into fine bubbles and circulate the liquid so bubbles stay in contact long
   B. Pure oxygen is used instead of air
   C. The tank is kept closed under pressure
   D. The air is blown through large pipes in one big bubble
   Answer: A. Fine bubbles retained for a long time in the circulating mixed liquor give good transfer. (Key point: Oxygen transfer)

3. Why does diffuser clogging not occur in the Dorrco aerator?
   A. It works only with filtered sewage
   B. Stirring keeps sludge in suspension, so no solids deposit and no diffusers are used
   C. Chlorine is added to the air
   D. The tank is cleaned daily by hand
   Answer: B. Efficient mixing prevents deposits, so there are no porous diffusers to choke. (Key point: Mixing and clogging)

4. Which is an advantage of the Dorrco aerator over a plain diffuser system?
   A. It needs more compressed air
   B. Aeration and mixing cannot be separated
   C. It needs less compressed air and aeration and mixing can be controlled independently
   D. It works only in shallow square tanks
   Answer: C. Independent control suits varying loads, and less air is needed. (Key point: Advantages)

## 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
- [ASP: removal of carbonaceous matter; recirculation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aasp-fundamentals-carbonaceous) — Sanitary Engineering, chapter 7