Describe safe water criteria for psm
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Safe Water Criteria (PSM)
Based on Park's Textbook of Preventive and Social Medicine, WHO Guidelines for Drinking Water Quality (2011)
Definition of Safe and Wholesome Water
Water intended for human consumption must be:
- (a) Free from pathogenic agents
- (b) Free from harmful chemical substances
- (c) Pleasant to taste - free from colour and odour
- (d) Usable for domestic purposes
Water not fulfilling these criteria is said to be polluted or contaminated.
WHO Guidelines for Drinking Water Quality (2011)
The WHO recommends assessment of four broad categories:
I. Acceptability Aspects | II. Microbiological Aspects | III. Chemical Aspects | IV. Radiological Aspects
I. Acceptability Aspects
A. Physical Parameters
| Parameter | Guideline / Acceptable Level | Reason |
|---|---|---|
| Turbidity | ≤1 NTU (median); >4 NTU noticeable to naked eye | Interferes with disinfection; aesthetic |
| Colour | Up to 15 TCU (True Colour Units) | Levels >15 TCU detectable in a glass; aesthetics |
| Taste & Odour | Should be acceptable (no numeric health guideline) | Indicates pollution or treatment malfunction |
| Temperature | Cool water preferred (no specific guideline) | High temp encourages microbial growth |
| pH | 6.5 - 8.5 (optimal for distribution) | Extremes cause corrosion/scaling |
B. Inorganic Constituents (Aesthetic Thresholds - Table 4)
| Constituent | Level Causing Consumer Complaints | Reason |
|---|---|---|
| Colour | 15 TCU | Appearance |
| Turbidity | 1 NTU | Appearance; disinfection effectiveness |
| Aluminium | 0.2 mg/L | Deposits, discoloration |
| Ammonia | 1.5 mg/L | Odour and taste |
| Chloride | 250 mg/L | Taste, corrosion |
| Copper | 1 mg/L | Staining of laundry/sanitary ware |
| Hydrogen sulphide | 0.05 mg/L | Odour and taste |
| Iron | 0.3 mg/L | Staining of laundry/sanitary ware |
| Manganese | 0.1 mg/L | Staining; taste in beverages |
| Sodium | 200 mg/L | Taste |
| Sulphate | 250 mg/L | Taste |
| Total Dissolved Solids | <600 mg/L (good palatability); >1200 mg/L unpalatable | Taste, scaling |
| Zinc | >5 mg/L | Astringent taste, opalescent appearance |
II. Microbiological Aspects
(a) Bacteriological Indicators
The primary indicator is the coliform group of organisms.
- Coliforms = aerobic and facultative anaerobic, gram-negative, non-sporing, motile/non-motile rods fermenting lactose at 35-37°C within 48 hours
- E. coli is the key faecal indicator
- Supplementary indicators: faecal streptococci and sulphite-reducing clostridia
Bacteriological Quality Standards (Table 5 - WHO)
| Situation | Organisms | Guideline Value |
|---|---|---|
| All water intended for drinking | E. coli or thermotolerant coliform bacteria | Must NOT be detectable in any 100 mL sample |
| Treated water entering distribution system | E. coli or thermotolerant coliform bacteria | Must not be detectable in any 100 mL sample |
| Total coliform bacteria | Must not be detectable in any 100 mL sample | |
| Treated water in distribution system | E. coli or thermotolerant coliform bacteria | Must not be detectable in any 100 mL sample |
| Total coliform bacteria | Must not be detectable in any 100 mL sample; in large supplies - must not be present in 95% of samples in any 12-month period |
(b) Virological Aspects
- Drinking water should be free from any viruses infective for man
- Disinfection standard:
- 0.5 mg/L free chlorine residual after contact of ≥30 minutes at pH 8.0 (for areas endemic for hepatitis A)
- 0.2 mg/L free residual chlorine for 30 minutes in non-endemic areas
- Turbidity must be ≤1 NTU prior to disinfection for adequate treatment
- Ozone at 0.2-0.4 mg/L for 4 minutes is also effective but cannot be maintained in distribution systems
(c) Biological Aspects
- Protozoa: Water must not contain pathogenic intestinal protozoa (Entamoeba histolytica, Giardia spp., Balantidium coli). Sand filtration removes a high proportion.
- Helminths: Infective stages of roundworms and flatworms must be absent; filtration is effective.
III. Chemical Aspects (Health-Based)
Inorganic Chemicals of Health Significance (Table 6 - WHO)
| Constituent | Max. Limit (mg/L) | Max. Limit (µg/L) |
|---|---|---|
| Arsenic | 0.01 (P) | 10 |
| Antimony | 0.02 (P) | 20 |
| Barium | 0.7 | 700 |
| Boron | 2.4 | 2,400 |
| Cadmium | 0.003 | 0.3 |
| Chromium | 0.05 (P) | 50 |
| Copper | 2 | 2,000 |
| Fluoride | 1.5 | 1,500 |
| Lead | 0.01 | 10 |
| Manganese | 0.4 (P) | 400 |
| Mercury (total) | 0.006 | 6 |
| Molybdenum | 0.07 | 70 |
| Nickel | 0.07 | 70 |
| Nitrate (as NO₃) | 50 | 50,000 |
| Nitrite (as NO₂) | 3 (P) | 3,000 |
| Selenium | 0.04 | 40 |
(P) = Provisional guideline value
Key chemical notes:
- Fluoride >1.5 mg/L: causes dental/skeletal fluorosis
- Nitrate >50 mg/L: causes methaemoglobinaemia in infants ("blue baby syndrome")
- Lead: neurotoxic, especially in children; guideline 0.01 mg/L
- Arsenic: carcinogenic; provisional limit due to analytical constraints
IV. Radiological Aspects
- Radiological quality is assessed separately based on total indicative dose from alpha, beta, and gamma radiation
- WHO guidance value: total indicative dose < 0.1 mSv/year
Water Requirement
| Setting | Quantity |
|---|---|
| Basic physiological need (survival) | ~2 litres/head/day |
| Adequate for all urban domestic purposes | 150-200 litres/capita/day |
| Rural target (India) | 40 litres/capita/day |
Source: Park's Textbook of Preventive and Social Medicine, pp. 807-823, WHO Guidelines for Drinking Water Quality 4th ed. (2011)
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