Page 16: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium
In order to illustrate the use of QMRA for a municipal water treatment application, a number of test scenarios were analysed using fictional data and Health Canada's QMRA model (see Appendix D).
The municipal drinking water system in this case study is supplied by a surface water treatment plant that draws raw water from a large river. The watershed surrounding the river is largely wilderness, and generally has low turbidity (3-5 NTU), high colour (35 true colour units) and dissolved oxygen content (6.5 mg/L). There are only a few small communities upstream of the city, with minimal wastewater discharges. Large numbers of waterfowl (Canada geese, gulls and shorebirds) can be found on the river during migration, and some overwinter in areas that do not freeze completely. A few major tributaries drain agricultural areas, which may contribute both nutrients and pathogens from animal waste to the river. Thus, this particular source water is considered to be moderately impacted surface water. Monitoring data for various pathogens in raw water were collected over a number of years, and the resulting mean concentrations are shown in Table F1.
| Pathogen | Cryptosporidium (no./100L) | Giardia (no./100L) |
Rotavirus (no./100L) |
E.coli (Table 1 Footnote cfu/100mL) |
Campylobacter (Table 1 Footnote cfu/100mL) |
|---|---|---|---|---|---|
| Mean | 8.0 | 34.1 | 56.0 | 55.0 | 10.0 |
| Standard deviations | 12.0 | 72.2 | 62.0 | 55.0 | 10.0 |
Table 1 Footnotes
|
|||||
The water purification plant has a conventional treatment process that includes: coarse screening, coagulation, flocculation, sedimentation, dual-media filtration, chlorine disinfection, pH adjustment and chloramination. The population of the municipality is 100,000 with an average per capita consumption of 1.0 of unboiled tap water per day. It is assumed that there is no further treatment or disinfection of the tap water prior to consumption.
Physical removal performance for the treatment process is estimated to be 6.5 log for Cryptosporidium, 6.0 log for Giardia, 5.6 log for viruses and 2.7 log for bacteria, based on results from pilot plant challenge studies and full-scale surrogate testing (see Table F2). It is important to note that, although these log removal rates are higher than for most surface water treatment plants, they have been validated at pilot scale and full scale for this municipality's treatment process when operating under optimum conditions.
For primary disinfection, a typical free chlorine residual of 0.50 mg/L is used following a 60-minute contact time (pH=6.0, temperature=10 ºC). The contact time is based on mean detention time, rather than the T10 value, as this assessment is aimed at estimating the "actual" mean reduction through the treatment process.
Example 1: Performance of the existing treatment process
The overall log reductions for the existing treatment process are shown in Table F2.
| Process | Cryptosporidium | Giardia | Rotavirus | E. coli | Campylobacter |
|---|---|---|---|---|---|
| Coagulation/sedimentation (log10) | 1.3 | 1.3 | 1.9 | 1.5 | 1.5 |
| Filtration (log10) | 5.2 | 4.7 | 3.7 | 1.2 | 1.2 |
| Chlorine inactivation (log10) | 0.0 | 1.3 | > 8.0 | > 8.0 | > 8.0 |
| Total (log10) | 6.5 | 7.3 | > 13.6 | > 10.7 | > 10.7 |
Using the QMRA model and the log reductions shown in Table F2, the mean burden of illness from cryptosporidiosis is estimated to be 1.92 × 10-10 DALY/person per year, and that from giardiasis is estimated to be 4.51 × 10-11 DALY/person per year; the distribution of the estimates is shown in Figure F1. Levels of illness in this range would not reasonably be detected and are well below the reference level of risk of 10-6 DALY/person per year.
Figure F1 - Text description
A graph showing the distribution of the annual burden of illness estimates from the consumption of drinking water produced using existing treatment processes. The distribution of the burden of illness estimates are shown for each of Cryptosporidium, Giardia, rotavirus, Campylobacter, and E. coli O157:H7. The x-axis of the graph is the annual DALY risk in DALYs per person per year. It is presented on a logarithmic scale and the axis values range from 10-15 to 1. The y-axis of the graph is the normalized probability distribution function. It is presented on a linear scale and the axis values range from 0 to 1. The reference level of risk of 10-6 DALYs per person per year is illustrated on the graph as a dotted vertical line. The annual illness estimates for each pathogen are normally distributed. The distribution curves extend approximately from: slightly less than 10-11 to 10-9 for Cryptosporidium; slightly less than 10-12 to 10-9 for Giardia; 10-15 to 10-13 for rotavirus; slightly greater than 10-11 to slightly less than 10-8 for Campylobacter, and slightly less than 10-11 to 10-9 for E. coli O157:H7. The peak of each probability distribution is normalized to 1.
Example 2: Effect of individual treatment barriers on pathogen risk
Using the same source and treatment data, the effect of each of the treatment barriers on health risks from the reference pathogens is investigated. Figures F2, F3 and F4 show the mean number of illnesses and DALYs for raw water with no treatment; physical removal by coagulation/sedimentation/filtration only; and physical removal by coagulation/sedimentation/filtration and inactivation by chlorine disinfection, respectively.
Figure F2 - Text description
A bar graph showing the burden of illness in DALYs per person per year from consumption of untreated river water. The burden of illness is shown for each of 5 pathogens (Cryptosporidium, Giardia, rotavirus, Campylobacter, E. coli O157:H7) and for the total burden of illness from these pathogens combined. The x-axis of the graph is labelled with the 5 pathogens and the pathogen total. The y-axis of the graph is the annual DALY risk in DALYs per person per year. It is presented on a logarithmic scale and the axis values range from 10-15 to 10. The reference level of risk of 10-6 DALYs per person per year is illustrated on the graph as a dotted horizontal line. The vertical bars extend approximately to: 10-3 for Cryptosporidium; 10-3 for Giardia; 10-1 for rotavirus; 10-1 for Campylobacter, 1 for E. coli O157:H7; and for the total of these pathogens combined, slightly higher than 1 DALY per person per year.
Figure F2 demonstrates that consumption of raw water (i.e., river water without treatment) would result in a high risk of gastrointestinal illness and a burden of illness well above the reference level of risk of 10-6 DALY/person per year, shown as the dotted line on the graph. In comparison, Figure F3 shows that adding a physical barrier for removal of pathogens (in this case, coagulation, flocculation, sedimentation and filtration) greatly reduces the burden of illness from Cryptosporidium and Giardia. However, the risk from viral and bacterial pathogens remains greater than the reference level of risk. Adding a disinfection barrier of free chlorine to the physical removal (coagulation/sedimentation/filtration) further reduces the risk from E. coli, Campylobacter and rotavirus to a negligible level, as shown in Figure F4.
Figure F3 - Text description
A bar graph showing the burden of illness in DALYs per person per year from consumption of river water treated using conventional filtration without disinfection. The burden of illness is shown for each of 5 pathogens (Cryptosporidium, Giardia, rotavirus, Campylobacter, E. coli O157:H7) and for the total burden of illness from these pathogens combined. The x-axis of the graph is labelled with the 5 pathogens and the pathogen total. The y-axis of the graph is the annual DALY risk in DALYs per person per year. It is presented on a logarithmic scale and the axis values range from 10-15 to 1. The reference level of risk of 10-6 DALYs per person per year is illustrated on the graph as a dotted horizontal line. The vertical bars extend approximately to: 10-10 for Cryptosporidium; 10-9 for Giardia; 10-6 for rotavirus; 10-2 for Campylobacter, 10-2 for E. coli O157:H7; and for the total of these pathogens combined, slightly less than 10-1 DALYs per person per year.
Figure F4 - Text description
A bar graph showing the burden of illness in DALYs per person per year from consumption of river water treated using conventional filtration with disinfection. The burden of illness is shown for each of 5 pathogens (Cryptosporidium, Giardia, rotavirus, Campylobacter, E. coli O157:H7) and for the total burden of illness from these pathogens combined. The x-axis of the graph is labelled with the 5 pathogens and the pathogen total. The y-axis of the graph is the annual DALY risk in DALYs per person per year. It is presented on a logarithmic scale and the axis values range from 10-15 to 1. The reference level of risk of 10-6 DALYs per person per year is illustrated on the graph as a dotted horizontal line. The vertical bars extend approximately to: 10-10 for Cryptosporidium; slightly less than 10-10 for Giardia; 10-14 for rotavirus; 10-10 for Campylobacter, slightly less than 10-10 for E. coli O157:H7; and for the total of these pathogens combined, almost 10-9 DALYs per person per year.
Example 3: Effect of chlorine CT and addition of UV on risk from Giardia
Using Giardia for illustration, a comparison of microbiological risk was made for various values of CT for disinfection achieved within the treatment plant. Figure F5 depicts the burden of illness (in DALYs per year for the entire population) as a function of CT in mg•min/L.
The minimum target (shown as the first vertical dashed line) corresponds to a 0.5 log Giardia disinfection target required to meet regulatory compliance in many jurisdictions. The "current CT" value indicated is the disinfection level currently being achieved in the treatment plant, although this level varies somewhat with operating conditions and seasonal temperatures. The graph does, however, indicate reduction in Giardia health risk for increasing CT.
Figure F5 - Text description
A graph showing the Giardia-related burden of illness in DALYs per person per year as a function of the CT level in milligram minutes per litre. The x-axis of the graph is the CT values in milligram minutes per litre. It is presented on a linear scale and the axis values range from 0 to 70. The y-axis of the graph is the DALYs per person per year. It is presented on a linear scale and the axis values range from 1 x 10-10 to 8 x 10-10 with the x-intercept at zero. The graph includes two vertical dotted lines. The first line indicates the minimum CT requirement for a 0.5 log inactivation for Giardia. This line is at 13 milligram minutes per litre. The second vertical dotted line indicates the actual CT applied in the example. This line is at 30 milligram minutes per litre. There are two lines plotted on the graph. The first plotted line shows the burden of illness from Giardia as the CT level increases for water treated using filtration and chlorine. This line is shaped like an exponential decay curve. The curve begins at a burden of illness from Giardia of 8 x 10-10 DALYs per person to year at 1 milligram minute per litre and decays to approximately 0 DALYs per person per year at 60 milligram minutes per litre. The curve intercepts the two vertical dotted lines at approximately 2.5 x 10-10 (at 13 milligram minutes per litre) and at approximately 0.5 x 10-10 (at 30 milligram minutes per litre). The second line shows the burden of illness from Giardia as the CT level increases for water treated using filtration, chlorine and ultraviolet light. This line is horizontal, intercepting both vertical dotted lines at approximately 0.
It is important to note that while microbial risk may decrease as the CT value is increased, DBPs may also be increasing to levels above those recommended in the Guidelines for Canadian Drinking Water Quality. Adding UV disinfection at a fluence dose of 40 mJ/cm2 reduces pathogen risk to a level too small to appear on the graph and reflects the added protection of a multi-disinfectant strategy such as free chlorine + UV disinfection.
Conclusion
It can be seen in this case study that the microbiological risk of drinking water from this water treatment facility is negligible. The case study overall indicates that the protozoan pathogens Cryptosporidium and Giardia are key pathogen risks, and filtration is the most significant barrier in reducing those risks. Without the primary disinfection barrier of free chlorine, however, bacterial pathogens (as represented by the reference pathogens Campylobacter and E. coli O157:H7) would be a significant pathogen risk in this scenario, because of their high occurrence levels in the source water and the severe health consequences associated with infection. Thus, it is evident that both the removal and inactivation barriers are critical to controlling overall microbiological risks in this drinking water system.
Page details
- Date modified: