---
title: "Membrane filter technique for coliforms"
subject: "Water Supply Engineering"
chapter: 4
key: "semester_5/water_supply_engineering:membrane-filter-technique"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/membrane-filter-technique
site: BCT Engineering
---

# Membrane filter technique for coliforms

Water Supply Engineering · Chapter 4: Quality of Water

- Asked in: 2066 Jestha (old course); 2067 Asar (old course) (asked 2 times)
- Video: [Membrane Filtration Technique for Water Analysis (E. coli, Salmonella, Pseudomonas, Coliform etc.)](https://www.youtube.com/watch?v=K2UJzVxlIFM) — MicroChem's Experiments
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/membrane-filter-technique
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-4

## Question

Describe the membrane filter (filtration) technique for the determination of coliforms.

## Short answer

The membrane filter technique counts coliforms directly by filtering water through a 0.45 micron membrane and incubating it on selective medium. Colonies are counted and expressed as CFU per 100 ml. It is fast and good for clean water but unsuitable for turbid samples.

## Answer

The **membrane filter (MF) technique** is a direct way of counting coliform bacteria in a water sample. It gives the count of colonies (CFU) per 100 ml and is faster than the MPN method (results in about 24 hours), uses less media and gives a direct count.

#### Principle
A measured volume of water passes through a sterile cellulose membrane with 0.45 micron pores. Bacteria are retained on the surface. The membrane is incubated on a selective nutrient medium, where each bacterium grows into a visible colony.

#### Apparatus and media
Filtration unit (funnel and base), vacuum source, sterile membrane filters, forceps, petri dishes with absorbent pads, m-Endo broth (total coliform) or m-FC broth (faecal coliform), incubator (35 degrees C) or water bath (44.5 degrees C), sterile buffered dilution water, a counting lens.

#### Procedure
1. Sterilise the unit and place the membrane on the base with sterile forceps.
2. Filter 100 ml of sample (or a dilution) under vacuum, then rinse the funnel.
3. Place the membrane on the medium-soaked pad without air bubbles.
4. Incubate upside down: 35 degrees C for 24 hours for total coliform; 44.5 degrees C for 24 hours for faecal coliform.
5. Count the typical colonies (golden-green sheen with m-Endo; blue colonies with m-FC).

$$
\text{Coliform}/100\ \text{ml} = \frac{\text{colonies counted}}{\text{ml filtered}}\times100
$$

#### Merits and limits
It is good for large volumes with low counts (clean water). It is less suitable for turbid water that clogs the filter, and for water with many non-coliform bacteria.

## Check yourself

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

1. An advantage of MF over MPN is:
   A. faster (about 24 hours) and gives a direct count
   B. works best on very turbid water
   C. needs more media
   D. gives only a statistical estimate
   Answer: A. MF gives a direct CFU count with less media. (Key point: Advantage)

2. In the MF technique, bacteria:
   A. pass through into the filtrate
   B. are retained on the membrane and grow into visible colonies on selective medium
   C. are killed by vacuum
   D. are counted as gas in tubes
   Answer: B. Each retained bacterium forms a colony. (Key point: Principle)

3. With m-Endo, typical coliform colonies show:
   A. blue colour
   B. red halo
   C. a golden-green metallic sheen
   D. black pigment only
   Answer: C. Blue colonies are typical with m-FC. (Key point: Colonies)

4. MF is less suitable for:
   A. clean water with low counts
   B. large volumes of clean water
   C. checking absence in 100 ml
   D. turbid water that clogs the filter
   Answer: D. Turbidity clogs filters and many non-coliforms interfere. (Key point: Limits)

## Related in Water Supply Engineering

- [Indicator organism; membrane filter technique](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aindicator-organisms-membrane-filter) — Water Supply Engineering, chapter 4, asked 2×
- [Indicator organisms and membrane tube fermentation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aindicator-organisms-mtf) — Water Supply Engineering, chapter 4, asked 3×
- [Multiple tube fermentation (MPN) technique](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Amtf-mpn-procedure) — Water Supply Engineering, chapter 4, asked 5×