During sanitary surveys, inspectors evaluate where chlorine residual sampling is being performed and will request to do side-by-side chlorine residual measurements with the operator. Chlorine residual monitoring is required both for entry point chlorine residual monitoring and also in the distribution system during bacteriological sampling. Public water systems that only use sodium or calcium hypochlorite or chlorine gas to form free chlorine for disinfectant residual should be monitoring and reporting free chlorine residual at all times. Conversely, any systems adding ammonia to form chloramines must monitor and report disinfectant residual as total chlorine residual. In this article, we will discuss handheld colorimeters and questions we’ve received from operators during sanitary surveys on properly measuring both high range and low range chlorine residuals. Improperly using an EPA accepted test method, using expired or incorrect DPD reagent or not verifying or operating disinfectant monitoring analytical equipment in accordance with manufacturer requirements can result in a violation of Regulation 11, Section 11.46. This was one of the Top 10 most cited violations cited during sanitary surveys in 2022.
The main issue inspectors often see in the field is measuring low range vs. high range free chlorine residual. For HACH pocket colorimeters (PCII) in the low range (LR) setting, a readout of 2.2 will flash repeatedly if the sample concentration is above 2.2 mg/L free chlorine. Recording the 2.2 mg/L as the chlorine residual is not correct and will result in a violation during a sanitary survey. The operator should be familiar with how to change their chlorine analyzer setting from low to high range. For a HACH DR300 or a HACH PCII, you must select the High Range (HR) menu option (please see your manual). For HACH units in High Range, the sample cell also changes to the plastic vial with a 5-mL sample and two 10‑mL DPD Free Chlorine Reagent Powder Pillows or two 10‑mL DPD Total Chlorine Reagent Powder Pillows must be used for each test. Please see your manual for your specific chlorine residual kit, and note that some manuals have to be downloaded online now.
Other issues that can result in field based violations for chlorine residual monitoring is using expired DPD reagent and glassware that is caked with DPD (see photo below).
Operators must be familiar with their colorimeter and should have written maintenance procedures and a maintenance log implemented for the analyzer, including routine verifications required by the manufacturer (HACH does not specify a frequency for pocket colorimeters but CDPHE does quarterly checks with GELEX standards on our pocket colorimeters). Please note that for online chlorine analyzers used for compliance with Regulation 11, online analyzers must be verified at least once a week by taking a parallel grab sample and analyzing it by another verified method (bench top) or by comparison with other parallel analyzers (see Policy 4 Guidance). All verification checks and results should be written down in a logbook.
Please avoid these field-based violations related to chlorine residual monitoring and ensure proper disinfection by being familiar with your analyzer and following proper sampling protocol. For any questions or concerns, please email our Field Services team at cdphe_wqcd_fss_questions@state.co.us.
➽ Connor Clarke, CWP, Field Services Section
➽ Heather Young, PE, CWP Field Services Section
