Describe the sources of water supply and the assessment of water quality.

Sources, Requirements and Water-related Disease

• Safe water is defined as water free from pathogenic organisms and harmful chemical substances, pleasant to taste and usable for domestic purposes
• The sources divide into three:
• Rain water — the purest natural water in origin, being distilled, but picks up dust, gases and micro-organisms on descent and from the collecting surface; it is soft and lacks minerals
• Surface water — rivers, tanks, ponds and lakes; readily available and adequate in quantity but always requires treatment, being exposed to contamination from every direction
• Ground water — wells, tube wells and springs; the commonest rural source in India
• Its advantages — likely to be free of pathogens because of natural filtration through soil; clear and colourless; requires no treatment in many cases; and available even in drought
• Its disadvantages — high mineral content and hardness; may contain fluoride, arsenic, iron or nitrate; may be contaminated where the water table is high or the well is badly sited; and requires pumping
• The chemical contaminants of Indian groundwater deserve naming because they are region-specific and cause real disease — fluoride in Rajasthan, Andhra Pradesh, Gujarat and Tamil Nadu causing fluorosis; arsenic in West Bengal, Bihar and Assam causing arsenicosis and malignancy; iron, salinity and nitrate
• The water requirement — a minimum of about 40 to 50 litres per capita per day for a rural household, of which only a few litres are for drinking; and 150 to 200 litres per capita per day for an urban household with sewerage connections
• The quantity of water frequently matters more than its quality, and this is a point worth arguing — a large volume of moderately clean water permits washing, bathing and domestic hygiene, which prevent trachoma, scabies, skin and eye infection and diarrhoea, whereas a small volume of perfectly pure water does not
• The water-related disease classification (Bradley) is the organising framework and should be given in full, because each class implies a different intervention:
• 1. Water-borne — the pathogen is IN the water and is swallowed: cholera, typhoid, hepatitis A and E, polio, amoebiasis, giardiasis, rotavirus and other diarrhoeas. Controlled by improving water quality
• 2. Water-washed (water-scarce) — caused by insufficient water for hygiene: trachoma, conjunctivitis, scabies, pediculosis, skin and eye infection and much diarrhoeal disease. Controlled by increasing water quantity and availability, and not by improving quality
• 3. Water-based — the pathogen spends part of its life cycle in an aquatic intermediate host: schistosomiasis, guinea worm. Controlled by reducing contact with water and by eliminating the intermediate host
• 4. Water-related vector-borne — transmitted by insects that breed in or near water: malaria, dengue, filariasis, Japanese encephalitis, onchocerciasis. Controlled by vector control and water management
• The practical value of this classification is that it prevents the common error of assuming that all water-related disease is solved by chlorination — chlorination does nothing whatever for trachoma or for dengue

Water Quality, Purification and Standards

• Water quality is assessed by bacteriological, chemical and physical examination
• Bacteriological examination uses indicator organisms rather than seeking pathogens directly, and the reasoning behind that choice is examinable:
• Pathogens are few, intermittent, difficult and slow to isolate, and dangerous to handle; whereas an indicator is present whenever faecal contamination has occurred, in large numbers, is easy and cheap to detect, and survives at least as long as the pathogens
• The chief indicator is escherichia coli and the coliform group, with faecal streptococci and Clostridium perfringens as supplementary indicators; Clostridium spores indicate remote or intermittent contamination, since they survive long after coliforms die
• The standard to be quoted — drinking water must contain NO E. Coli in any 100 mL sample, and NO coliform organism in any 100 mL sample
• The methods — multiple tube (presumptive coliform count, giving the most probable number); membrane filtration; and the H2S strip test as a simple field screen
• Chemical and physical parameters — turbidity, colour, taste and odour; pH (6.5 to 8.5); total dissolved solids and hardness; chlorides, which suggest sewage contamination; nitrates, whose excess causes methaemoglobinaemia (blue baby syndrome); fluoride, with an optimum of about 0.5 to 1.0 mg/L in India, below which dental caries increases and above which fluorosis occurs; and arsenic, lead and other heavy metals
• The fluoride relationship is worth stating as a curve rather than a threshold — too little causes caries, the optimum protects teeth, and excess causes dental then skeletal fluorosis; the optimum is lower in a hot climate because people drink more, which is a point specific to India
• Purification on a large scale proceeds in three stages:
• 1. Storage — a natural purification, in which sedimentation removes suspended matter, sunlight and predation reduce bacteria, and aeration oxidises organic matter; about 10 to 14 days is optimal, since longer storage allows algal growth
• 2. Filtration — the most important single stage bacteriologically
• Slow sand (biological) filters work chiefly through the vital layer (schmutzdecke or zoogleal layer), a slimy biological film on the sand surface that does the actual purification
• Its consequences are practical — the filter must ripen for a few days before use, and cleaning by scraping the top layer destroys the vital layer, so the filter must ripen again afterwards
• Rapid sand (mechanical) filters require prior coagulation with alum and sedimentation, are faster and need less land, and are cleaned by backwashing in minutes, but require more skill, more chemicals and more reliable chlorination afterwards
• Slow sand filters remove a greater proportion of bacteria (about 99.9%) than rapid filters (about 98 to 99%), which is why the rapid filter depends more heavily on disinfection
• 3. Disinfection — chlorination, considered separately
• The principles of chlorination — the water must be clear before chlorination, since chlorine is consumed by organic matter and turbidity shields organisms
• Chlorine demand is the amount consumed by organic matter and reducing substances; the chlorine requirement is the demand plus the desired free residual, which should be at least 0.5 mg/L after one hour of contact
• The free residual is the safeguard, since it indicates that demand was met and provides protection against recontamination in the distribution system
• The break-point is the point at which the demand has been fully satisfied and free residual chlorine begins to appear
• Chlorine acts through hypochlorous acid, is more effective at lower pH and higher temperature, and is reliable against bacteria and viruses but not against the cysts of Giardia, Entamoeba and Cryptosporidium
• That last limitation is important and is regularly examined, since it means chlorinated water may still transmit giardiasis and amoebiasis unless filtration has removed the cysts
• Testing is by the orthotolidine (OT) test for residual chlorine
• Quantity often matters more than quality, since washing and bathing prevent trachoma, scabies and much diarrhoea
• Chlorination does nothing for trachoma or dengue, which is why the Bradley classification is worth learning properly
• Chlorine is unreliable against cysts of Giardia, Entamoeba and Cryptosporidium, which filtration must remove
• Water must be clear before chlorination, since turbidity shields organisms and consumes the chlorine
• Free residual chlorine is the safeguard, showing demand was met and protecting against recontamination downstream
• Aim for half a milligram per litre residual after one hour of contact, which is the figure to quote
• The vital layer does the work in a slow sand filter, which is why it must ripen and why scraping resets it
• Rapid filters depend more on chlorination, removing a smaller proportion of bacteria than slow sand filters
• Indicator organisms are used, not pathogens, because pathogens are few, intermittent, slow to isolate and dangerous
• No E. Coli in any hundred millilitre sample is the drinking water standard to state exactly
• Clostridium spores indicate remote contamination, surviving long after coliforms have died
• Fluoride is a curve, not a threshold; too little causes caries and excess causes fluorosis
• The optimum fluoride is lower in a hot climate, since people drink more water, which matters in India
• Nitrate excess causes methaemoglobinaemia, the blue baby syndrome, which is a groundwater problem
• Arsenic affects Bengal, Bihar and Assam, and fluoride the drier western and southern states, which is worth locating
• Chlorides suggest sewage contamination, and are a useful chemical indicator alongside the bacteriological ones
• Store water ten to fourteen days, since longer allows algal growth to undo the benefit
• Groundwater is filtered by soil, which is why wells are often safe without treatment where properly sited
• Rain water is purest in origin but collects contamination on descent and from the collecting surface
• Surface water always needs treatment, being exposed to contamination from every direction
• Chlorine works better at lower pH, acting through hypochlorous acid, which is why pH is monitored
• Water-based disease needs contact reduction, not water treatment, since the pathogen is acquired through the skin