Total Residual Chlorine
Chlorine is a common wastewater disinfectant. Residual chlorine in effluent is toxic to fish in receiving streams and so must be removed with a dechlorinating chemical, such as sulfur di-oxide. Many NPDES permits place limitations on the amount of total residual chlorine (TRC) that can be discharged after dechlorination. Often, a permit will impose a limit of <0.019 mg/L or <0.011 mg/L TRC, although many permits require that “no-detectable” TRC be discharged. For most treatment plants, “no-detectable” TRC is equivalent to <0.099 mg/L.
There are three EPA approved test methods for measuring low-level TRC; (1) The Amperometric Titration method, (2) the Iodometric Electrode method and (3) The DPD Colorimetric method. This text focuses on the most commonly used method; the DPD Colorimetric, as described in Method 4500-Cl G., Standard Methods for the Examination of Water and Wastewater, 18th edition. The DPD method is a colorimetric method that takes advantage of the reaction that occurs between residual chlorine and N,N-Diethyl-p-phenylenediamine (DPD) indicator solution. When chlorine is present, a reaction with the DPD indicator results in the development of a pink coloration. The more chlorine present in a sample, the more intense the color development. Using a photometric device, the color intensity can be measured through absorption.
Samples to be analyzed for TRC should be collected in clean 100 – 500 mL polypropylene bottles. Ensure that representative samples are taken, i.e.; peak flow, well mixed flow stream. NPDES permits specify the exact TRC sampling location (generally, following the last treatment process). Residual chlorine samples are not stable, and there are no preservation methods. TRC samples are subject to change from excessive holding times, exposure to sunlight, increase in temperature and agitation. Much like pH samples, TRC samples must be analyzed within 15 minutes (preferably less). Benchsheets should record both the time that the sample was collected and the time that it was analyzed to demonstrate that the holding time was not exceeded.
The DPD method of measuring TRC can be performed on two different (but similar) instruments; (1) a spectrophotometer, and (2) a filter photometer. A spectrophotometer is a device that measures the absorption of light at various wavelengths. For TRC measurements using a spectrophotometer, a wavelength of 515 nm with a light path of a least 1cm is required. A filter photometer is a device that measures the absorption of light in a fixed range of wavelengths. For TRC measurements using a filter photometer, a wavelength range of 490 – 530 nm with a light path of a least 1cm is required.
Calibration to known standards is required for all photometric devices. The required frequency of calibration is dictated by individual circumstances. The recovery of known standards to check the instrument calibration is required as part of a quality control program. Because chlorine is unstable it cannot be used for making standards directly. Standards can be prepared either with Chlorine exposed Potassium Iodide (KI) or with Potassium permanganate (KMnO4). At least 5 calibration standards covering the Chlorine equivalent range of 0.05 – 4.0 mg/L should be used to prepare a calibration curve.
Assure that the instrument is set to the correct wavelength and that the optical sample cells are clean and not damaged.
Measure appropriate volumes of buffer and DPD reagents into a gradated cylinder and fill with appropriate sample volume (usually 50 mL).
Stopper the graduated cylinder.
Mix the buffer, DPD reagent and sample by inverting several times and allow 3 minutes for color development.
Using a matched optical sample cell filled with sample, zero the instrument.
When color of sample cell with buffer and DPD reagent is developed (3 min.), place cell in instrument and read absorption. (Most modern instruments display a reading directly in mg/L TRC).
Using distilled water, prepare a reagent blank that contains:
* A blanking agent
* Buffer and DPD
Read the absorption of the reagent blank with the instrument.
Subtract the reading obtained for the reagent blank from the reading obtained for the sample.
This yields the corrected sample TRC.
Samples containing significant turbidity should be filtered through a 3-micron membrane filter after reacting the sample with DPD reagent and buffer. When using this method, zero the instrument with a filtered sample blank.
Periodically prepare standards from Chlorine reacted with KI or from KMnO4. Determine recovery of these standards and establish lower detection limit as the lowest standard that can be detected with 95% recovery.
Externally supplied performance evaluation standards should be analyzed at least annually.