WATER
treated water, and wou ld no t appear to be j ustifi ed in term s of cost to the commu n ity. T he inferred levels o f treatme nt for differen t source water qualities are listed in Table 5. T h is table indicates that C. jej1111i requires a hi gher leve l of treatmen t than C. p11n111111 1 reflecting the h igh er rate of infec tio n indicated in Fi gure 1B for C. jej11 11i w ith low n u mbe rs of organisms ingested. C. jej1111i is more easil y rem oved by c hlo rination than C. parvum , and this is taken into accou n t in the treatme nt require me nts infe rred in T able 5 . Rece nt studies (Malley, 2000) have shown that U V can redu ce th e in fec tivity of Cryptosporidi,1111 oocysts and th is may improve the e ffectiveness of disinfection .
Conclusions There is considerable uncertainty in i n terpreting monitorin g re sults for pathogens such as Cryptosporidi11111 and C11111pylo&acter. M o nitoring information often con sists of " non detect" res ults, with the occas iona l positive resul t . Ofte n the monitori ng information will pertain o nly to the source w ate r befo re treatment. K ey fac tors in the interpretation of m o nitoring informatio n include : • Th e reliability and recovery of the an al ytical methods. • T he species and st rains of the organisms present, and whether th ey are likely to give rise to infections in humans. [n th e absence of the ability to measure this, th e sou rce of the contam ination and w hethe r it is likely to be associated w ith human infec tion may be the best
indicator of viability and in fec tivity . • T he likelihood that low doses of organisms (eg o ne or two) w ill give rise to in fec tion . • The acceptable level of illness in the co mmu n ity and the objectives fo r mana ge m ent of pathogens in wate r supplies. These have not yet been agreed in Australia. T he analysis suggests that: • T he objectives for mana geme nt of pathogens in water supplies need to be defin ed. Adoption of target risk levels being discussed in th e USA and Europe wo uld in fe r chat the numbers of organisms present should be red uced to very low levels, well below our ability to m easure directly. Achieving such levels w ill requ ire a high level of treatment and substantial cost to the community.
Table 5: Inferred Levels of Treatment Required for Various Source Water Qualities (with respect to measured numbers of Cryptosporidium and Campy/obacter only) Source Water Quality (numbers of organisms . f erred f rom mon1"t on.ng 111 annual basis
Inferred Level of Treatment1 (Reduction In Numbers of Organisms) Required To Meet Target Target: Overall cost to the community is minimised
Target: No illness detected In the community
Target: Annual Risk of Infection less than 1 in 10,000
None req uired None requi red Disinfection only
None required S 1 log Disinfection on ly
s 2 log s 3 log Ret ention in reservoir, di sinfection, and reduction in distribution system may be adequate
None required S 1 log Disinfection only
S 1 log s 2 log Retention in reservoir, di sinfection, and reduction in dist ribution system may be adequate
s 3 log s 4 log Retention in reservoir, effective disinfect ion, and reduction in distri bution system may be adequate; if disinfection has low effectiveness then fi ltration wi ll be necessary.
;:: 1 log ;:: 2 log Retention in reservoir, disinfection, and reduction in distribution system may be adequate
;:: 2 log ;:: 3 log Filtration and disinfection
;:: 4 log ;:: 5 log High efficiency filtration and disinfection
Excellent Source Water
Infrequent detection at low nu mbers, organisms detected unlikely to be infective* C. parvum $ 1 per 10 0 L C. jej uni s 0 .1 per 2 L * infectivity < 0.1 that in Figure 1.
Good Source Water Infrequent detection at low numbers , organisms detected like ly t o be infective C. paNum s 1 per 100 L C. jejuni s 0.1 per 2 L
-
Poor Source Water Frequent detect ion, organisms detected likely to be infective C. paNum ;:: 10 per 100 L C. jejuni ;:: 1 per 2 L
Note: 1.Treat ment for Cryptosporidium and Campylobacter only. Treatment may be necessary for ot her water quality parameters. Treatment refers to reduction in numbers of viable organisms that will take place between the source and the point of use (includes loss in reservoir, treatment system, and distribution system). For cases where organisms are likely to be infectious it is assumed that Figure 1A and 1B dose-response curves apply.
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WATER MAY/ JUN E 2000