Ritu's Biology Hub

Continued from part 8

Water Potability

Potable water is defined as safe-drinking water which is free of pathogenic organisms, toxic chemicals and is tasteless, colourless and odourless.

Indicator organisms – It refers to the kind of microorganisms whose presence in water is evidence of the fact that the water is polluted with faecal matter from humans or other warm-blooded animals.

Significance – This kind of pollution means pathogens that occur in gastrointestinal tract (GIT) of animals may also be present. This is done because assay for detecting the presence of pathogens in water is difficult and time-consuming. High faecal contamination means water contains pathogen and is unsafe to drink. Thus, indicator organisms help to detect pathogens.

Characteristics –

1. It is present in polluted water and absent in unpolluted or potable water
2. It is present in large quantities as compared to pathogen population
3. It is present always when pathogens are present in water
4. The quantity of indicator organisms correlate with the level of contamination of water
5. It should be easily detectable
6. It does not multiply in water giving direct relationship between its amount and levels of contamination
7. It is generally harmless to humans
8. It should survive longer than the pathogens

Examples – E.coli, Enterobacter aerogenes, Klebsiella pneumonia, Clostridium perfringes, S.faecalis, Citrobacter

Coliforms – They are defined as facultatively anaerobic, non-sporing, gram negative, rod-shaped bacteria that ferment lactose producing gas within 48 hours at 37°C.

• coli – A faecal coliform found in faeces
• aerogenes, K.pneumoniae – Non-faecal coliforms found in soil and vegetation
• faecalis – Found in marine and brackish water
• perfringes – Long survival pathogen. Thus, an indicator of past pollution

Standards for potable water

There are 2 sets of standards defining potable water:

• EPA standards which is US based
• Indian standards

1. EPA standards

• Drinking water and swimming pool must be of highest purity
• There can be no more than 1 positive sample (>1 coliform/100 ml) in 40 and less than 40 samples tested in a month. Concentration of faecal coliforms must be zero. However, for more than 40 samples, 5% pathogens are allowed
• Natural bathing beaches and treated sewage are assayed for number of faecal coliforms. Total coliform counts are not used as a measure due to nearby ubiquitous presence of aerogenes and K.pneumonia in environment
• If counts are >400 faecal coliforms /100 ml, it indicates problem in treated sewage
• Maximum contaminant level goal (MCLG) – It indicates the exact number of organisms present. Should have zero coliforms, Cryptosporidium, Giardia intestanalis, Legionella and enteric viruses

2. Indian standards

• Throughout the year, 95% sample should not contain any coliforms
• There should be no coliforms/100 ml of water
• There should be no coli in water
• No sample should contain more than 10 coliforms/100 ml
• No 2 consecutive sample should contain coliforms

Precautions taken when sampling studies for potable water –

• The sample should be collected in a sterile bottle
• The sample must be a representative of the supply from where the water sample is taken
• Contamination of the sample should be avoided during and after supply
• The sample should be tested promptly as possible
• If there is to be a delay, then the sample must be stored at a temperature between 0-10°C

Nuisance bacteria

• They cause changes in colour, odour and taste of water
• Slime – forming bacteria – Produce gummy or slimy conditions
• S (sulfur) bacteria – Produce sulphuric acid and hydrogen sulphide which makes water very acidic and imparts foul odour. Precipitated S ions damage pipelines
• Fe bacteria – Transform soluble Fe compounds into insoluble form. Precipitate of Fe damage pipelines
• Algae – Turbidity, discolouration and unpleasant odour and taste

Standard Plate Count

• A rapid method for testing the total bacterial levels in water
• Usually 1.0 and 0.1 ml f water sample are plated on an agar medium for 24 hours, after which the colonies are counted
• Disadvantage –
  • There is no particular number of bacteria that is officially acceptable as water with a few pathogens is considered more dangerous than water containing high levels of non-pathogenic bacteria
  • Does not differentiate between coliforms and non-coliforms
• Advantage – Determines the efficiency of various water purification process

Membrane Filter Technique

• 100 ml of water is passed thorugh a membrane filter placed on a filtration unit
• Place the filter on an absorbent pad that is saturated with an appropriate culture medium
• Place the absorbent pad on a petri plate and incubate for 24 hours / 37°C. Look out for the colonies
• Advantages –
  • A large amount of water can be sampled
  • All microbes are retained on the disk
  • Results can be obtained more rapidly
  • By using selective and differential medium, certain bacterial types such as coliforms and non-coliforms can be identified by their colony characteristics
  • Good reproducibility
  • Single step results are possible
  • Filters can be transferred between media
  • Cheap and time saving
• Disadvantage –
  • High –turbid water or liquids containing particulate matter limit volumes sampled
  • High population of bacteria cause overgrowth
  • Metals and phenols can adsorb to filters and inhibit growth

Continued to Part 10