9.4.3 Chemical parameters
Natural water has different elements and compounds in it due to the ability of water to dissolve many substances. Unlike microbial contaminants, most chemical constituents in drinking water only cause adverse health effects after a prolonged period of exposure. A huge concern is if a massive accidental discharge of chemicals occurs in the drinking water system. Experience has shown, however, that in many such incidents, the water will exhibit changes in appearance, taste and odour that prompt consumers to cease using it. For example, if the residual chlorine level exceeds a certain level, consumers notice the taste and immediately reject the water. Ethiopian chemical quality standards for the usual parameters in drinking water are presented in Table 9.3. So, taking aluminium for example, its quantity in 1 litre of water must not exceed 0.2 mg. There are other components that also have limits, such as pesticides, but these will only be analysed for if a case of contamination by these types of substances is suspected.
| Substance | Maximum permissible level (mg l–1) | Health and other risks associated with high intake |
| Aluminium | 0.2 | Deposition of aluminium hydroxide flocs in water pipes and exacerbation of discoloration of water by iron |
| Ammonia | 1.5 | Objectionable (pungent) odour |
| Arsenic | 0.01 | High incidence of skin and possibly other cancers |
| Barium | 0.7 | |
| Boron | 0.3 | |
| Cadmium | 0.003 | The kidney is the main target organ of toxicity |
| Calcium | 75 | |
| Chloride | 250 | Undesirable (salty) taste |
| Chromium | 0.05 | Carcinogenicity suspected of chromium (VI) compounds |
| Copper | 2 | Acute toxicity is high; effects on thyroid and particularly the nervous system if long-term exposure occurs |
| Cyanide | 0.07 | Effects on thyroid and particularly the nervous system if long-term exposure occurs |
| Fluoride | 1.5 | At low concentration, prevents dental caries; at high (7 mg l–1) concentration, increased risk of dental fluorosis; much higher concentration leads to skeletal fluorosis. |
| Iron | 0.3 | Causes reddish-brown colour, promotes iron-bacteria growth and stains laundry and plumbing fixtures |
| Lead | 0.01 | Toxic to both the central and peripheral nervous systems, causing neurological effects |
| Magnesium | 50 | |
| Manganese | 0.5 | Neurotoxicity and other toxic effects |
| Mercury (total) | 0.001 | The kidney is the main target for inorganic mercury, whereas methyl-mercury mainly affects the central nervous system |
| Nitrate (as NO3) | 50 | Causes methaemoglobinaemia in infants and suspected risk of certain forms of cancer |
| Nitrite (as NO2) | 3 | Causes methaemoglobinaemia in infants and suspected risk of certain forms of cancer |
| Potassium | 1.5 | |
| Residual chlorine | 0.5 | |
| Sodium | 200 | Undesirable taste |
| Sulphate | 250 | Causes noticeable taste and corrosion of pipes |
| Total hardness | 300 | |
| Total dissolved solids | 1000 | Undesirable taste |
| Zinc | 5 | Imparts astringent taste and opalescence; develops a greasy film on boiling |
9.4.2 Physical parameters