*3.3.5.1 Testing for coliforms: membrane filter method*

A measured volume of sample is filtered through a special membrane filter by applying a partial vacuum [10, 39].

The filter, a flat paper-like disk, has uniform microscopic pores small enough to retain the bacteria on its surface while allowing the water to pass through. The filter paper is then placed in a sterile container called a petri dish, which contains a special culture medium that the bacteria use as a food source [39].

Then, the petri dish is usually placed in an incubator, which keeps the temperature at 35°C, for 24 h. After incubation, colonies of coliform bacteria each containing millions of organisms will be visible [10]. The coliform concentration is obtained by counting the number of colonies on the filter; each colony counted represents only one coliform in the original sample [10, 39].

Coliform concentrations are expressed in terms of the number of organisms per 100 mL of water as follows:

coliforms per 100 mL = number of colonies × 100/mL of sample (11)

### **4. Water quality requirements**

Water quality requirements differ depending on the proposed used of water [19]. As reported by Tchobanoglous et al. [19], "water unsuitable for one use may be quite satisfactory for another and water may be considered acceptable for a particular use if water of better quality is not available."

Water quality requirements should be agreed with the water quality standards, which are put down by the governmental agency and represent the legislation requirements. In general, there are three types of standards: in-stream, potable water, and wastewater effluent [19], each type has its own criteria by using the same methods of measurement. The World Health Organization (WHO) has established minimum standards for drinking water that all countries are recommended to meet [25].

### **5. Conclusion**

The physical, chemical, and biological parameters of water quality are reviewed in terms of definition, sources, impacts, effects, and measuring methods. The classification of water according to its quality is also covered with a specific definition for each type.

*Water Quality - Science, Assessments and Policy*
