Microbial quality is one of the primary indicators for the safety of a drinking water supply. Bacteriological examination of drinking water is made to determine whether the water consumed is contaminated, and microbial parameters can provide usefull information throughout the drinking water production process like catchment survey, source water characterisation, treatment efficiency and examination of distribution system.
Coliform organisms have been used to determine the biological characteristics of natural waters. Escherichia coli (E. coli) is generally used as an indicator organism. This organism is present in the intestine of warm-blooded animals, including humans. Therefore the presence of Escherichia coli in water samples indicates the presence of fecal matter and thus the possible presence of pathogenic organisms of human origin.
Faecal contamination is a common source of infectious microorganisms. These include bacteria, viruses and parasites that occur naturally in the gut of humans and other warm-blooded animals. The presence of waterborne disease causing microorganisms in drinking water may result in gastrointestinal illness or diarrhea and even lead to death. Microbiological contaminant parameters of EPA standards include Coliforms (total), Giardia lamblia, Heterotrophic Plate Count, Legionella, Pseudomonas sp., Pyrogens, Turbidity and Viruses).
Although treated, bacteria (Campylobacter jejuni/C. Coli, Esherichia coli, Vibrio cholerae, Salmonella typhi, Shigella, Legionella spp., Non-tuberculous Mycobacterium spp., Franciscella tularensis, etc.), viruses (Noroviruses, Rotaviruses, Enteroviruses, Adenoviruses, Hepatitis A, Hepatitis E, Sappoviruses, etc.), parasites (Cryptosporidium hominis/parvum, Entamoeba histolytica, Giardia intestinalis, Cyclosposa cayetanensis, Acanthamoeba, Naegleria fowleri, some invertebrates, including water mites, cladocerans and copepods, etc) and filamentous fungi and yeast (Aspergillus flavus, Stachybotrys chartarum, Psaudellescheria boydi, Mucor, Sporothrix, Cryptococcus, etc.) can be found in finished drinking-water, pipe biofilms and distribution systems. In a water distribution system faecal contamination may occur with an intrusion of faecal contaminant through broken mains and cross-connections or openings in storage tanks. In addition, construction, new pipe installation and repairs close to sewer lines can introduce contamination into the distribution system.
The presence of faecal pathogens is assessed by monitoring for indicator bacteria. The WHO Guidelines for Drinking-water Quality (WHO, 2011) recognize E. coli as the indicator of choice for faecal contamination, although thermotolerant coliforms (E. coli, Citrobacter, Klebsiella and Enterobacter) can be used as an alternative.
Although total coliforms are not a specific indicator group for contamination as for they can grow naturally in water and soils, they can be used to assess the cleanliness of distribution systems. Coliforms can arise from biofilm linings in pipes and fixtures or from contact with soil due to breaks or repair works. Testing for heterotrophic plate count bacteria is sometimes used for similar purposes. Total coliform numbers and heterotrophic plate count (also known as total or standart plate count) bacteria are used in operations as an indicator of system performance including a loss of disinfection efficiecy, intrusion of contaminants into drinking-water or the growth of biofilms that could support the presence of pathogens. The detection of any coliforms leads us a corrective action, such as increasing the chlorine dose at the water treatment plant, checking the operation of service reservoirs or pipe flushing and rechlorination of the affected area.
Standard plate count/HPC organisms are used for monitoring of efficiency of water treatment and disinfection processes or after growth in water distribution systems.
Total coliform bacteria (total coliforms) is used to evaluate quality of drinking water and related waters.
Fecal coliform bacteria (fecal coliforms) is used to evaluate the quality of wastewater effluents, river water, sea water at bathing beaches, raw water for drinking water supply and recreational waters.
Fecal streptococci (enterococci) are used in evaluation of treatment processes and recreational waters.
Clostridia (presumptive Clostridium perfringens) are used to indicate remote fecal pollution and to access efficacy of treatment and disinfection process.
Coliphages are used as indicators of the incidence and behaviour of human enteric viruses in the evaluation of drinking water. Also serves as an indicator of the presence of host bacteria.
Microbial pathogens including bacteria, viruses and protozoan parasites can be physically removed with other particles in treatment units such as coagulation / flocculation, clarification and filtration, or they can be chemically eliminated by disinfection. Since physical removal processes do not remove all microorganisms from the water, disinfection is important in maintaining the microbial quality of water. To control the microbial quality of water, the disinfectant residual that remains in the drinking water in the distribution system is important. It helps preventing bacterial re-growth after treatment and limiting the development of biofilms in the water pipes.