Illinois · small ground water systems
Illinois chlorine residual requirements
Short answer
Illinois requires continuous chlorination of community water supplies unless the supply is exempt, and keeps a higher distribution minimum than most states: 0.5 mg/L free chlorine (or 1.0 mg/L combined) in all active parts of the system at all times. Non-community systems such as campgrounds are regulated by the Illinois Department of Public Health, which requires at least 0.1 mg/L free chlorine where chlorination is required.
- Minimum residual
- Community: 0.5 mg/L free or 1.0 mg/L combined. Non-community (IDPH): 0.1 mg/L free where required
- How often
- Not set as a number in the community rule (forms have daily rows); with each coliform sample
- Report
- Monthly operating reports within 30 days after the month
Who has to chlorinate
- Continuous chlorination is required for all community water supplies unless they meet the residual requirement or qualify for an exemption. 35 Ill. Adm. Code 604.700
- Illinois EPA describes exemptions for systems that only chlorinate or fluoridate, use protected ground water sources and run an active cross-connection control program. Illinois EPA
- For non-community systems, IDPH requires disinfection of surface, spring and cistern supplies, and of ground water supplies of questionable sanitary quality or exposed to the atmosphere during treatment. 77 Ill. Adm. Code 900.40
Minimum residual
- Community supplies: at least 0.5 mg/L free chlorine or 1.0 mg/L combined chlorine in all active parts of the distribution system at all times. 35 Ill. Adm. Code 604.725
- Non-community supplies where chlorination is required: at least 0.1 mg/L free chlorine at distant points in the distribution system. 77 Ill. Adm. Code 900.40
Maximum
- 4.0 mg/L chlorine for community and non-transient systems; transient systems are covered only for chlorine dioxide. 35 Ill. Adm. Code 611.313
How often to test
- Community supplies must monitor the residual at different points in the distribution system; the rule itself doesn't set a frequency. 35 Ill. Adm. Code 604.725
- Community and non-transient systems also measure the residual at the same time and place as each total coliform sample. 35 Ill. Adm. Code 611.382
- Illinois EPA's monthly report forms have a row for chlorine tests on each day of the month. Illinois EPA
Test kits and methods
- IDPH says non-community test equipment should read to the nearest 0.1 mg/L below 0.5 mg/L, 0.3 mg/L between 0.5 and 1.0, and 0.5 mg/L between 1.0 and 2.0. 77 Ill. Adm. Code 900.40
Reporting
- Community supplies file monthly operating reports (Monthly Operation and Chemical Feeding, and Distribution System Operating), signed by the certified operator in responsible charge, within 30 days after the end of the month. Illinois EPA
Parks, campgrounds and seasonal systems
- Campgrounds, restaurants and highway rest areas are examples IDPH gives of transient non-community systems it regulates. IDPH
Keeping the log in Illinois
At 0.5 mg/L, Illinois community systems have less room below the target than most, so a residual that drifts down shows up as a problem sooner. Log every reading with its location, and set your working range comfortably above the minimum so a low reading prompts action before it becomes a violation.
Print a dated month with the chlorine residual log sheet, or build a sheet for your whole round with the daily water system log sheet.
Other states
For federal rules and every state's program, see chlorine residual requirements.
Sources
- 35 Ill. Adm. Code 604.700 — Chlorination
- Illinois EPA — Chlorination exemption
- 77 Ill. Adm. Code 900.40 — Non-community water system treatment (IDPH)
- 35 Ill. Adm. Code 604.725 — Chlorine residual
- 35 Ill. Adm. Code 611.313 — Maximum residual disinfectant levels
- 35 Ill. Adm. Code 611.382 — Disinfectant residual monitoring
- Illinois EPA — Daily operating reports
- IDPH — Non-community public water systems
Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.