What a log means
Each log is a tenfold reduction. Credits from separate barriers add together, so 3-log filtration plus 1-log disinfection gives 4-log total.
| Log | Removed | Left of 1,000,000 |
|---|---|---|
| 1 | 90% | 100,000 |
| 2 | 99% | 10,000 |
| 3 | 99.9% | 1,000 |
| 4 | 99.99% | 100 |
CT and UV dose
CT is the disinfectant residual (mg/L) at the end of the contact segment multiplied by T10, the time 10% of the water takes to pass through it (minutes). Baffling factor matters: a tank with poor baffling gives far less T10 than its volume suggests.
UV dose is in mJ/cm² and is barely affected by temperature, which is why UV holds up in cold northern water while chemical CT climbs steeply as the water gets colder.
Treatment targets
- Health Canada sets a minimum for surface water and GUDI supplies of 4-log viruses, 3-log Giardia and 3-log Cryptosporidium. Higher source contamination calls for more.
- British Columbia sums it up as 4-3-2-1-0: 4-log inactivation of viruses, 3-log removal or inactivation of Giardia and Cryptosporidium, 2 treatment processes, 1 NTU or less of turbidity (aiming for 0.1), and 0 total coliforms and E. coli. The Log Removal Chart can be set to these targets.
- There is no log target for bacteria. The guideline is no detectable E. coli per 100 mL, and treatment that meets the virus and protozoa targets covers enteric bacteria.
- Legionella and Mycobacterium avium are mostly a building plumbing problem: they grow in biofilm and warm stagnant water after treatment.
Where the numbers come from
- Filtration credits for protozoa and viruses: Health Canada Guidelines for Canadian Drinking Water Quality technical documents on enteric protozoa and enteric viruses.
- Chemical CT for Giardia and viruses: US EPA Surface Water Treatment Rule Guidance Manual tables, also used in Canadian CT calculations. Virus tables are based on hepatitis A.
- Cryptosporidium CT for ozone and chlorine dioxide, and all UV doses for protozoa and adenovirus: US EPA LT2ESWTR tables.
- Bacteria values, and UV doses for viruses other than adenovirus: representative published laboratory values. Treat them as order-of-magnitude guides, not credits.
- BC 4-3-2-1-0: BC Ministry of Health, Drinking Water Treatment Objectives (Microbiological) for Surface Water Supplies in British Columbia (2012).
- The practical, difficult and impractical bands are rules of thumb for a small plant, not regulatory limits.
This page is for training. For compliance, calculate CT with your approved tables or calculator for your actual pH, residual, temperature and T10.