7 months ago


Water treatment

systems. The differences

systems. The differences between MSF alternatives are not considered significant in terms of management, and environmental impact. Demographic data is not a decisive factor either, as the economies of scale of the MSF options are similar. As a consequence, the selection guide focuses on three key criteria (Galvis, 1993) • The sanitary significance of the raw water quality, • The treatment efficiency required to meeting water quality guidelines or standards, • The sustainability of the MSF option as indicated by the socio-economic acceptance at local level. 6.1 Basic Parameters of Sanitary Significance in Surface Water Sources Based on the approach followed by Lloyd and Helmer (1991) for monitoring small WS systems and adopted later by WHO (1993 and 1997), the sanitary risk of surface water sources will be characterised with two basic parameters: faecal coliform counts and turbidity. Besides, considering that communities may reject water through aesthetic considerations, colour is included. Considerations about iron and manganese are also presented. Faecal coliform levels. As indicated in section 2.2, health risks associated with microbial contamination are so important that their control is the top priority. Besides, If a large number of water sources need to be compared, the classification of risks makes it possible to establish priorities for remedial actions to reduce the acute sanitary risks in a given region. Lloyd and Helmer (1991) have published a grading scheme (A to E) for faecal coliform levels per 100 ml to classify small water supplies in which A = 0; B = 1 - 10; C = 11 –100; D = 101 – 1000; and E > 1000. The results of sanitary inspections and water quality analyses carried out for different water sources in a region can be brought together in one model (see figure 6.1). If this model is applied to several watershed areas in a region, a surface water source in a well-protected area, with very little human activity, should have low risk level and low treatment cost. The application of this model will help to define where improvements need to be made to reduce sanitary risks to acceptable levels. In harmony with the multiple barriers strategy (section 1.2) it is possible to improve the position of a small WS system in the model by, for example, changing the water source, modifying patterns of open field defecation and cattle breeding, or introducing on or off site waste water treatment. The application of the multiple barrier strategy should increase public health impact and reduce the capital and running costs of water treatment. The short to medium term target should be to bring all surface water supplies at least to B (low hazard) grade. Besides, where institutional and community development make chemical disinfection possible the microbiological target should be consistently grade A (Lloyd et al, 1991). Under the Surface Water Treatment Rule in the USA, among other criteria summarised in section 2.3, the level of faecal contamination must be < 20 CFU/100 ml in 90% of the samples before chemical disinfection (Pontius, 1990). Lloyd et al (1991) and later WHO (1993, 1997) suggested ≤ 3 and 25 CFU/100 ml for the mean and maximum faecal coliform levels before terminal disinfection, as summarised in table 2.15. To obtain water grade B before terminal disinfection the treatment objectives proposed for faecal coliform 191

acteria in the effluents of MSF alternatives are ≤ 3 and 10 CFU/100 ml for the mean and maximum faecal coliform levels respectively. Figure 6.1 Zoning of combined risk analysis of sanitary inspections and faecal coliform contamination levels for prioritising remedial action strategies to reduce microbiological contamination (Lloyd and Helmer, 1991). Turbidity levels. Turbidity affects both the treatment efficiency and acceptance of the water by the consumers as summarised in section 2.3. Results presented and discussed in chapter 3 (section, table 3.21) indicate that SSF can achieve desired levels of faecal coliform removal with turbidities ≤ 5 NTU in 98% of samples taken from SSF effluents. This seems to be in harmony with previous findings in the literature (Bryant et al 1992) indicating that pathogen removal effectiveness of SSF should not be considered on the basis of turbidity. This parameter is particularly relevant to the effectiveness of disinfection by chlorine. High turbidity increases the chlorine demand and reduces the impact of disinfection. Under the Surface Water Treatment Rule in the USA, among other criteria summarised in section 2.3, the turbidity has to be < 5 NTU before chemical disinfection but sometimes higher values are accepted provided they occur no more than twice a year (Pontius, 1990). The water quality guidelines of WHO (1993 and 1997) indicate that disinfection with chlorine can be effective if the previous treatment steps provide water with a maximum turbidity of 5 NTU and an average turbidity < 1 NTU. Based on previous considerations, the treatment objective proposed for turbidity is ≤ 5 NTU in the effluents of MSF alternatives, before terminal disinfection. Colour levels. Colour affects both the treatment efficiency and acceptance of the water by the consumers as summarised in section 2.3. Colour is an indirect indicator for the presence of fulvic and humic acids, which often constitute an important portion of organic material in surface water sources (Spencer and Collins, 1991; WHO, 1993). Chlorine reacting with organic material can cause oxidation by-products (OBPs) that may present chronic health risks. These types of chronic risks should not be ignored but they are much less significant to pubic health than acute risks associated with microbial contamination and should not compromise terminal disinfection of polluted surface water sources (Craun et al, 1994; Otterstetter and Zepeda, 1996; Galal-Gorchev, 1996). Based on aesthetic considerations the 192

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