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For centuries it has appeared that the cause of common diseases went unknown, meaning that epidemics could spread rapidly. Most recently, the discovery that micro-organisms are the prime cause of common infectious diseases has helped to eradicate many of theses diseases. From a medical point of view, the development of antibiotics on a global scale in the mid-twentieth century helped to control many pathogenic bacteria. A second approach to controlling diseases has been the increased ingenuity of public health systems, which act by adjusting the environment to prevent the spread of pathogens. One of the major transportation tunnels for pathogens is water, so water treatment is at the heart of the environmental engineering approach to disease control.
Although improved health and sanitation systems have been developed to adapt with most of the obvious pathogens, the increased populations will only stimulate the development of different pathogens, better suited to the new environment. All organisms compete with other for nutrients, habitats etc., so when one pathogenic micro-organism is eradicated, another can come to the fore. As such, environmental engineers must constantly adapt their approaches to face the new challenges.
In developing countries, where sanitation is poor or non-existent, water-related diseases flourish, accounting for millions of lives a year. The United Nations set out in its Millennium Development Goals the target to 'Halve, by 2015, the proportion of people without access to safe drinking water and basic sanitation' (UN, 2000). By implementing sustainable water treatment systems in developing countries, it is hoped that this target can be met and subsequently improved up.
2.1.1 Classification of Water-Related Illnesses
Many biologists classify water-related diseases by associating them with viruses, protozoa, bacteria, fungi, helminths, et, an environmental classification system was developed based on the method of transmission of diseases. This system was created by David Bradley (Lonholdt, 2005) and the four categories are:
Water-related insect vector
The infections themselves are grouped in a slightly different manner, in the following categories: Faecal-oral, water-washed, water-based and water-related insect-vector. All water-borne diseases and most water-washed diseases can be transmitted by the faecal-oral route, which involves faecal matter being transferred to the mouth.
18.104.22.168 Waterborne pathogens
Waterborne diseases are caused by contamination of water from human or animal waste and contracted by the ingestion of this water (Hassan et al., 2005). It is important to remember that all infections transmitted by the water supplies may also be directly transmitted from faeces to month, or via contaminated food (Bradley, 1980, McGranahan, 2001). The most effective preventative measures against water-borne pathogens are the implementation of adequate water treatment systems and supply networks which do not allow recontamination. By far the most common waterborne disease is diarrhoea, which can be caused by many different bacteria.
Water-washed diseases are diseases whose transmission is reduced by the use of greater quantities of water, almost regardless of the quality of the water. These diseases include diarrhoeal diseases, which are faecal-oral, and eye and skin infections, which are not (Howsam & Carter, 1996). The F-diagram in figure 2.6 illustrates the various measures that can be employed to control the spread of waterborne and water-washed diseases transmitted via the faecal-oral route. As can be seen, the primary barrier to spread of disease is the implementation of adequate sanitation.
Fig. 2.6 F-Diagram
Water-based diseases require an intermediate aquatic host to transport the pathogenic bacteria or worms. The main such disease is schistosomiasis, where the pathogenic worm spends part of its life cycle in an aquatic snail host. Another water-based disease is Guinea worm, the only infection exclusively transmitted in drinking water (Caincross & Feachem, 2000).
22.214.171.124 Water-related insect vector
These are diseases such as malaria and dengue fever, which are transmitted by insects, especially mosquitoes, which have a larvae stage in water. The main method for controlling these diseases is to ensure sufficient drainage, therefore preventing stagnant pools of water, which provide a good home for these insects (see Table 2.1).
Improve quality of drinking water
Prevent casual use of unprotected sources
Increase water quality used
Improve accessibility and reliability of domestic water supply
Reduce need for contact with infected water1
Control snail population1
Reduce contamination of surface waters2
Improve surface water management
Destroy breeding sites of insects
Reduce need to visit building sites
Use mosquito nets
1Applies to schistosomiasis only
2 The preventitive strategies appropriate to the water-based worms depend on the precise life-cycle of each (see Appendix C) and this only the general prescription that can be given.
Table 2.1 Transmission methods and main preventive strategies of water-related illnesses
(Source: Cairncross & Feacham, 2000)
Fewtrell et al. (2001) suggested adding a fifth category to the list, water- collection related diseases, covering the possible infection of water during the journey to and from the source. This cited an outbreak of meningococcal disease in a refugee camp as an example of this.
Table 2.2 lists the main diseases with each of the transmission routes described above
Faecal-oral (water-borne or water-washed)
Diarrhoeas and dysenteries
Shigellosis (bacillary dysentery)
skin and eye infections
Infectious skin diseases
Infectious eye diseases
Louse-borne relapsing fever
Water-related insect vector:
biting near water
breeding in water
B = Bacterium R = Rickettsia
H = Helminth S = Spirochaete
P = Protozoon V = Virus
M = Miscellaneous
Table 2.2 Common water-related diseases (Source: Cairncross & Feacham, 2000)
2.2 Properties and Contaminants of Water
In the United Kingdom the vast majority of waterborne diseases which occur are due to private water supplies. The most notable pathogens that occur are Campylobacter, Cryptosporidium and Giardia.
Fig. 2.1 Giardia
The presence of Campylobacter originates from high numbers found in faeces of infected pigs and poultry with contamination being relatively low. This is where the application of disinfection is applied in the form of chlorine, ozone, and UV to eradicate any Campylobacter organisms entering the water supply but firstly waters need to be cleaned properly to remove any dissolved organic material and particles especially when UV disinfection is needed.
Fig. 2.2 Cryptosporidium
The presence of Cryptosporidium originates from cattle, sheep and human sewage. Various catchment management control methods and appropriate disinfection use are needed to prevent the organism entering private water supplies. Unfortunately private water supplies which are associated with surface waters and springs will be at risk to contamination from agricultural sources.
Fig. 2.3 Giardia
The presence of Giardia originates from animal faeces contaminating a water source. If the correct coagulation, sedimentation and filtration processes are used it should be able to produce a 3-log (99.9%) removal of giardia cysts.
Fig. 2.4 E. Coli
E. coli is used as an indicator organism in many microbiological tests. It is a rod-shaped, gram-negative bacterium, part of the family Enterobacteriaceae. It is found naturally in the intestines of warm-blooded animals and unlike other members of its family, does not usually occur naturally in soil, water or plants. E. coli rarely grows outside the gut of humans or animals and therefore its inability to grow in water means that the detection of E. coli in water is an indicator of recent faecal contamination.
2.3 Disinfection methods
Disinfection is the destruction of pathogenic micro-organisms in water. A good disinfectant is characterised by the following traits:
Effectiveness to kill pathogenic micro-organisms
Non-toxic to humans or domestic animals
Non-toxic to aquatic species
Easy and safe to store, transport and dispense
Easy and reliable analysis in water
Provides residual protection in drinking water
Disinfection can be achieved through the use of
Chemical agents: These include chlorine and its compounds, bromine, iodine, ozone, phenol, dyes, soaps and acids
Physical agents: These involve the use of heat, light and sound waves, for example pasteurisation and UV treatment
Mechanical means: Micro-organisms may be removed using coarse and fine screens, grit chambers and sedimentation tanks
Radiation: Gamma rays have also been used for water disinfection, although there are no full-scale installations in operation using this technology
(Metcalf and Eddy, 2003)
The main disinfection methods are compared in Table 2.3 and a selection of the more common methods is described in more detail below.
Moderately low cost
Moderately low cost
Moderately low cost
Moderately high cost
Moderately high cost
Interaction with extraneous material
Oxidizes organic matter
Oxidizes organic matter
Absorbance of UV radiation
Non-corrosive and non-staining
Non-toxic to higher forms of life
Highly toxic to higher life forms
Unstable, must be generated as used
Unstable,must be generated as used
Toxicity to microorganisms
Toxicity at ambient temperatures
Table 2.3 Characteristics of the most widely used disinfection methods (Source: Metcalf and Eddy, 2003
Chlorination is by far the most commonly used method of disinfection. A number of chlorine compounds can be added to water to achieve disinfection, incluing chlorine gas (Cl2), sodium hypochlorite (NaOCl), chlorine dioxide and calcium hypochlorite [Ca(OCl)2].
It is not without its problems, however, as it is difficult to achieve the optimal dose of chlorine due to a complex series of reactions that take place in the generation of free chlorine (Andersen, 2005). The 'breakpoint' concentration must be achieved, whereby enough chlorine is added to react with all oxidisable substances in the water, such that any extra chlorine added remains as free chlorine.
Hypochlorous acid is the most effective of the chlorine compounds in the destruction of the pathogens. The hyochlorous acid reacts with unsaturated carbon-carbon bonds to form addition products and block further chemical reactions at this location. These unsaturated carbon-carbon bonds are essential to bacteria metabolism and so the addition reaction stops bacterial growth. This reaction does not kill the bacteria and is reversible, so a second step to the process is required. The addition of extra chlorine leads to the formation of the chloride ions, which break the carbon-carbon bond by oxidation and lead to cell death (McKinney, 2004).
Despite it being so widely used, there are some significant disadvantages to the chlorination process. Also, chlorine is a highly toxic substance in its natural form and is transported mainly by road and rail. Some disinfection by-products (DBPs) of the chlorination process, especially the trihalomenthanes (THMs), are known to be carcinogenic, while residual chlorine in treated water is toxic to aquatic life.
Ozone is an unstable gas formed when molecules of oxygen dissociate into single atoms. It can be produced by electrolysis, radiochemical reactions and photo chemical reactions (Asano et al., 2007). Ozonation systems generally involve passing air through a high voltage electrical discharge. The ozone is then pulled into the water to be treated via a vacuum created by a Venturi throat. Post treatment is required to remove insoluble mental oxides formed during the process. It is believed that disinfection by ozonation occurs via direct cell lysis (Asano et al., 2007).
A major problem associated with ozonation is that it is highly energy-intensive. According to McKinney (2004), only about 5% of the total energy used to produce ozone is actually used in the process. If an adequate energy supply is available, however, ozonation can be an effective alternative to chlorination. As an extremely reactive oxidant, ozone is believed to kill bacteria through direct attack on the cell wall. Ozone is also generally considered to be more effective against viruses compared to chlorine (Metcalf and Eddy, 2003). Ozone must be generated on-site, as it breaks down to oxygen readily. As with chlorination, ozonation has harmful associated by-products, especially if bromide is also present. Although THMs are not produced by ozonation, other by-products such as various acids and aldehydes can be created.
2.3.3 UV Radiation
UV radiation has long been recognised as an effective disinfectant, targeting the DNA and RNA within mico-organisms. Light in the germicidal range (~254mm) is known to cause adjacent thymine bases on A DNA strand within a micro-organism to form a thymine dimmer (figure 2.5). This blocks protein synthesis and prevents proper replication and transcription during the cell division cycle (Blake et al., 1999, McDonnell, 2007). Viruses tend to be more resistant than bacteria to UV radiation, although most viruses are treatable using the method (Field et al., 2003).
Figure 2.5 Formation of double-bonds in UV-treated microorganisms (Metcalf & Eddy, 2003)
For UV radiation to be used effectively as a disinfectant, the water needs to be clear enough to allow the light to pass through. It should therefore follow some form of physical pre-treatment, removing the majority of the turbidity in the water (Field et al., 2003).
126.96.36.199 UV Lamps
In the disinfection of water using UV, three main types of lamp are used: low-pressure low intensity, low-pressure high intensity and medium-pressure high intensity.
These lamps have a broad radiation spectrum, but peak dramatically at 254 nm, with approximately 85-88% of the output at 254 nm, making such lamps efficient disinfectants (Asano et al., 2007). The low operating temperature, in the region of 40Â°C, means that these lamps have a longer lifespan to higher intensity designs.
These lamps are similar to low-pressure low-intensity lamps, but the use of a mercury-indium amalgam as opposed to mercury gives them a far greater output.
These lamps operate at temperatures of 600 - 800Â°C, and generate roughly 50 to 100 times the UV-C output of low-pressure lamps (Asano et al., 2007). These are the most effective lamps, but are more expensive and require more safety measures than the previous two types discussed.
Figure 2.6 Aquada UV Filter System
188.8.131.52 UV System configurations
UV disinfection can take place in both open and closed channel systems. Low-pressure lamps tend to be employed in open channel flow regimes, while medium-pressure lamps are more usually employed in closed channel flow.
2.4 Private Water Supplies
What is a Private Water Supply?
A private water supply can be defined as a water supply which is not provided by a water company and where the responsibility for its maintenance and repair lies with the owner or person who uses it. It normally serves a single dwelling which can provide less than 1 mÂ³/day or even serve many properties such as commercial and industrial premises which provide 1000 mÂ³/day or even more. These sources can consist of either a borehole, well, spring, lake, stream or river.
The requirements for monitoring private water supplies are governed by the Drinking Water Directive legislation which varies accordingly due to the extent of the supply. Also private water supplies need to be classified by the size of the supply taking into consideration to which the supply serves:
a single dwelling for domestic usage;
for domestic usage for persons normally residing on the premises; or
for supplying premises used for commercial food production or with changing populations
Figure 2.7 Spring source