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Water treatment

3.2 Results and Specific

3.2 Results and Specific Discussion 3.2.1 Raw water Due to the intertropical convergence, there is a tendency to have two dry and two rainy periods yearly in the Andean Cauca Valley. The raw water quality is influenced by these periods. In fact, during 1990 (figure 3.11) the turbidity values were above the mean (93 NTU) during a significant part of the rainy periods. This result is part of the justification for the planned research test periods of around six months. 700 600 Turbidity (NTU) 500 400 300 200 100 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Turbidity Months Mean value Figure 3.11 Daily turbidity means in Cauca River raw water at Puerto Mallarino. 1990 100 80 Cumulative frequency (%) 60 40 20 0 Turbidity 1990 1991 - 1993 1990 - 1996 Mean 93 72 81 Stand. Dev. 117 79 88 Minimum 15 12 12 Maximum 1,800 1,400 3,665 N (Data) 7,194 25,031 57,322 10 100 1000 10000 Turbidity (N T U ) 1990 1991 -1993 1990 -1996 Figure 3.12 Statistics and cumulative frequency distributions for raw water turbidity values. Descriptive statistics and cumulative frequency distributions (figure 3.12) of turbidity data for three periods (1990, 1991-1993; 1990-1996) show a similar pattern for this parameter through seven years of intensive (hourly) monitoring at Cinara´s station. Close to 50% of the time the turbidity values were greater than 50 NTU, which is well beyond the treatment capacity of SSF technology without working in combination with other treatment stages. These high values, together with the abrupt water quality changes observed during the rainy periods (as shown in figures 2.24C and 3.11), also make it very difficult to apply compact or 89

direct rapid filtration water treatment plants because of their low hydraulic retention times. Only those more complex conventional plants having full time skilled operators, real-time monitoring and automatic dose control, could properly adjust the intake facilities or the treatment stages for such abrupt changes. A clearer picture of the raw water quality to be treated with the MSF pilot system is shown in table 3.6. Faecal coliform levels are beyond the most limits (usually 20,000 CFU per 100 ml) recommended for a surface water source used for human consumption, even with chemically based water treatment process (EEC, 1975). Based on the water quality limits established for water sources used for human consumption, the Brazilian Association for technical Standards (ABNT, 1989, quoted by Di Bernardo, 1993) would classify the Cauca River water in a special (D) category. This category requires conventional treatment (rapid filtration of chemically coagulated water and chemical disinfection) plus complementary treatment stages to be determined after specific studies. Table 3.6 Descriptive statistics for raw water quality parameters at Puerto Mallarino during the 1 st phase test periods. March 1991-February 1993 Parameters No. Standard Mean (Data) Deviation Min Max. 90 Percentile 95 Percentile Faecal Coliforms (CFU/100ml) 143 63,291 68,059 2,700 500,000 142,200 194,900 Turbidity (NTU) 1,584 64 71 12 900 120 180 Total Solids (mgl -1 ) 18 172 58 113 355 217 260 Suspended Solids (mgl -1 ) 145 111 143 11 978 200 403 Volatile Solids (mgl -1 ) 17 51 13 30 69 66 68 Colour (PCU) 145 57 41 16 250 100 137 Chemical Oxygen Demand (mgl -1 ) 18

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