Presentation on theme: "Water Quality Control. Selection of parameters for assessment The first priority in assessing drinking-water quality must be to check microbiological."— Presentation transcript:
Selection of parameters for assessment The first priority in assessing drinking-water quality must be to check microbiological quality. This can be done by measuring, at a minimum, the “essential parameters” of drinking-water quality: faecal coliforms (or E. coli), and, when assessing treated water, chlorine residual, pH and turbidity. Other important priorities are the aesthetic quality of the water and contamination with chemicals of known health risk.
Measuring water quality The aesthetic quality of water, by definition, is determined subjectively by the user. Microbiological and chemical testing can be made either on-site, using field kits, or in laboratories. Where possible, field testing is preferred because it is logistically much easier, and in most cases significantly more cost effective. In addition, errors introduced from the preservation, transport and storage of samples for laboratory testing are eliminated. Properly trained field test kit operators can test a large number of water sources in a relatively short time, allowing the results to be obtained and shared with users within hours or days.
Microbiological analyses By far the most serious public health risk associated with drinking-water supplies is microbial contamination. Pathogens (bacteria, viruses and parasites) can cause a wide range of health problems when ingested in drinking water, but the primary concern is infectious diarrhoeal disease transmitted by the faecal-oral route. It is impractical to analyze water for every individual pathogen, some of which can cause disease at very low doses. Instead, since most diarrhoea-causing pathogens are faecal in origin, it is more practical to analyze water for indicator species that are also present in faecal matter.
When assessing faecal contamination, it is recommended to measure turbidity along with E. coli (or faecal coliforms), since pathogens can adsorb onto suspended particles, and to some extent be shielded from disinfection. When water has been disinfected, it is also important to measure chlorine residual and pH. These four parameters (E. coli/faecal coliforms, turbidity, disinfectant residual and pH) are considered the minimum set of “essential parameters” required to assess microbiological quality of drinking water.
Chemical analyses Chemical parameters can be analyzed much more rapidly than microbiological ones, because there is no need for incubation. Most parameters must be analyzed in a laboratory, at least for quantitative results. Some parameters, though, might change during storage and transport (e.g., pH) and should be measured at the sampling site. Inexpensive field kits are available for semi- quantitative determination of many parameters, and in some cases sophisticated equipment can be made portable for field analysis.
Precision and accuracy In all experimental measurements, there is a degree of uncertainty or error. When reporting data, the degree of uncertainty can be measured by considering the precision and accuracy of the analysis. The precision simply means the reproducibility of the analysis: if the same sample is analyzed multiple times, how much will the results vary? Accuracy, on the other hand, refers to how close the measurement is to the true value. Analytical precision can be assessed by making repeat measurements and calculating the ratio of the standard deviation to the average. This is called the coefficient of variation, and as a rule of thumb should be less than 10% for laboratory measurements. Precision will depend primarily upon the instrument and method, but also on the operator and quality control procedures.
Preventing Contamination Contamination prevention is a two-pronged process: 1- Reducing the amount of pollution entering the environment as a whole 2- Erecting barriers to prevent any contamination that is present in the environment from reaching water supplies.
Sources and pathways of contamination No natural water is absolutely pure – the chemical and physical characteristics of water are constantly changing through interaction with the environment. These changes can be positive: water is purified as its percolates down to aquifers and some adsorbed minerals can improve the taste and perceived value of water. Sometimes the changes can result in water that remains safe, but is unacceptable to consumers for aesthetic reasons (taste, smell or colour). And in some cases water can become unsafe for human consumption through contamination by naturally occurring chemicals (such as arsenic) or through pollution from human activities (such as pesticides).
Sources and pathways of chemical contamination There are two sources of chemical contamination: 1) Naturally occurring chemicals 2) Anthropogenic (caused by human activity) pollutants. There are several naturally occurring chemical compounds that pose a threat to human health, the most serious being arsenic and fluoride. Other natural chemicals affect the aesthetic quality of water and cause health problems indirectly by forcing people to use alternative sources that may be less safe. Iron is the most common such contaminant. Groundwater sources are typically the most affected by natural chemical contamination although there are cases of seriously contaminated surface water as well. In these cases, the chemical contaminant is present in the rocks and soils of the aquifer and is absorbed by the groundwater through a variety of chemical processes.
Natural chemical contaminants often affect large areas and multiple water sources, although there may be significant variation in contamination levels from source to source. Arsenic contamination levels, for example, are highly variable due to the complexity of the affected aquifers and the chemical processes involved. The level of contamination may also be influenced by the depth of the water source and whether or not it is capped. In general, natural contamination cannot be prevented. If an aquifer is affected, the only remedial measure is to tap another, unaffected aquifer (e.g., a deeper aquifer), use another source such as surface water or use filters or other treatment measures to remove the contaminant from the pumped water.
In addition, there are techniques that can improve the water quality in situ, in the aquifer itself. This involves reducing the contamination concentration levels through dilution by injecting uncontaminated water into the aquifer, or by inducing a chemical state in the aquifer that minimizes the adsorption of the contaminant in the water. These techniques are in general not fully developed, and used on a limited basis. Pollutants are harmful chemicals released into the environment from agricultural activities, industrial processes and household wastes. There are two types of pollution: 1) point source (such as effluents from factories) 2) non-point source (including run-off from fields and emissions of chemicals into the atmosphere.)
All types of water sources can be affected by pollution. Groundwater is contaminated though seepage from non-point source pollutants and from point sources such as leaking chemical storage tanks. Surface water is often contaminated through the release of industrial and domestic effluents directly into lakes and rivers, and from pesticide run-off from fields. Even harvested rainwater is sometimes at risk: rain can absorb and retain contaminates from air pollution, especially near certain types of industries. Pollution, especially point source pollution, can be prevented, as it is easier to identify and isolate.
Pathways for faecal contamination of water sources Faeces are the most serious water contaminant affecting people’s health and the interruption of the fecal-oral cycle is the key objective of most water and sanitation programmes in developing countries. Both animal and human faeces are health threats; however human faeces are generally the most dangerous. There are other sources of microbiological contamination besides faeces, but because faeces are by far the most common and the most dangerous, this section focuses on faecal contamination pathways.
Pathways for faecal contamination during transport and storage Protected water sources do not ensure that water used for drinking and cooking in the home is safe. Household water storage (a practice common in developing countries) contributes to drinking-water contamination. Water stored in homes is often faecally contaminated at levels far above the contamination level at the source. Studies show that water stored in homes routinely have faecal coliform levels hundreds of times higher than is present in the source – some studies have documented thousand-fold increases in faecal coliforms.
There are three reasons water quality deteriorates during the storage and transport of water: 1- Poor hygiene knowledge prevents people from taking basic steps to minimize contamination 2- Inadequate household latrines, hand-washing facilities and poor community environmental sanitation results in more faeces in and around households 3- commonly used transport and storage containers are easily contaminated.