TECH-TIP - chlorine residual measurement

Along with pressure and main breaks, chlorine residual is one of the key monitoring parameters for systems participating in the Partnership’s distribution system optimization program – and a critical distribution system parameter for all utilities.  For the distribution system optimization program, chlorine residual is used as an indicator of water quality integrity.  

The Partnership’s optimization goals for free and total chlorine concentrations are listed below.  These goals are for 95% of the routine readings each month, from samples collected from optimized sampling locations.  Additionally, there is a goal that individual sample sites should not have consecutive chlorine residual measurements outside of these goals.  

•    Free chlorine:  ≥0.20 mg/L and ≤4.0 mg/L
•    Total chlorine: ≥0.50 mg/L and ≤4.0 mg/L

In order for utilities to accurately assess their performance relative to the Partnership’s optimization goals, it is important that utilities have representative and accurate chlorine residual data that has been collected from the distribution system.  If sufficient data is not available at the current time, consider developing an action plan to take steps to improve the nature of the disinfectant residual data that is collected.

Chlorine may be monitored using grab samples or continuously monitoring analyzers, using a variety of analytical techniques.  Utility staff may select the measurement method based on a variety of factors, including data collection frequency requirements, access to power and communications at the monitoring site, waste considerations, maintenance considerations, and ease and frequency of staff access.  

Although many operators typically consider chlorine analysis to be a routine measurement, there are a number of variables that should be considered to avoid error and ensure the accuracy of analytical results for disinfectant residual.  Tips that can help operators obtain accurate, representative disinfectant residual results are summarized below:

•    “The analysis is only as good as the sample.”  Chlorine is volatile and reactive, so be sure that samples are collected in an appropriate manner.  Prior to collecting a sample, sampling taps may need to be opened and flushed until the sample is representative.  It is recommended that chlorine samples be collected in clean, glass containers after rinsing the container with a portion of the sample to be tested.  In many cases, the sample can be collected directly in the sample cell that is used for the analysis.  Grab samples for chlorine should be analyzed immediately after sample collection to minimize chlorine loss, and shaking of samples should be avoided.

•    Reagents – Using the DPD method for field measurement of chlorine can commonly mean that chlorine test equipment travels in the vehicles of distribution system field staff.  Since prepackaged reagents have a limited shelf life, which can be shortened by extremes of temperature or humidity, it is recommended to periodically inspect field testing equipment.  Check for damage to instruments and sample cells, and verify that reagents are not expired.  If the reagents appear questionable, verify their performance as described below.  When analyzing disinfectant residual samples, ensure that the correct reagents are being used for the parameter being measured (free total vs. total chlorine) and the sample volume used for the test.  It is important to be aware of any potential interferences in the sample that may impact the accuracy of the analysis.  For example, a positive interference may be experienced when using a DPD based method to test for free chlorine in the presence of total chlorine, and modifications to the testing method may be required to compensate for this.  

•    Verification – The performance of the analytical system, consisting of the instruments, reagents, and analyst, can be verified using standards of a known value.  Many manufacturers provide secondary standards that may be used to verify instrument accuracy.  These secondary standards consist of sample cells prefilled with a translucent pink-colored solid that is designed to simulate a specific chlorine reading.  They are read after zeroing the instrument with a clear standard designed to be used as a blank.  These secondary standards are typically provided with a certificate of analysis (COA), stating the chlorine concentration associated with each standard vial.  Readings are compared with those given on the COA to assess accuracy.  The analytical system may also be verified by using a primary standard.  A primary standard is a chlorine standard solution of a known value that is prepared and analyzed to assess accuracy.  Due to the complex nature of preparing chlorine standard solutions, they are typically prepared and measured in a lab setting.  Many operators also choose to verify the performance of online chlorine analyzers by comparison with a grab sample.  If the online and grab sample readings differ by a certain amount (typically between 10-20%), further troubleshooting may be required.

•    Online analyzers – Continuously monitoring analyzers are a useful tool that can provide utilities with information that helps them more efficiently control and operate their system.  However, analyzers require regular maintenance to keep them performing optimally.  Be sure to follow the manufacturer’s instructions for instrument maintenance, reagent replacement (if required), calibration (if required), and verification.  The performance of many online analyzers can be easily verified by comparison with grab sample results.  In addition to performance verification, the accuracy of communications between online analyzers and the SCADA system should be periodically verified.  

Consistency in measurement between all staff members analyzing chlorine is important.  To help build consistency, establish standard operating procedures (SOP) for chlorine analysis.  Detailed SOPs, for example those that specify how long to flush a sample tap or how often to verify performance, can help to improve consistency.  Training is recommended to ensure that staff is familiar with SOPs, and SOPs should be reviewed and updated on a periodic basis to maintain their relevancy.  Did you know that submittal of an SOP for routine chlorine residual sampling and analysis is a required component of the distribution system self-assessment report?

With attention to detail, consistent procedures, and periodic verification, utilities can take steps to ensure the accuracy of disinfectant residual measurements, allowing them to better understand their system and accurately assess performance relative to the Partnership’s optimization goals.