Regarding the interference of chloride ions in COD determination
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We are a municipal wastewater treatment plant that operates in accordance with Class A standard. Due to the presence of industrial enterprises upstream, the chloride concentration in our effluent remains around 500–800 mg/L, which has a significant impact on the measurement of COD. By adding different amounts of mercury sulfate sequentially for testing, it was found that when 0.4 g of mercury sulfate is added to the same water sample, the COD level is around 55–65 mg/L, which exceeds the allowed limit; whereas when 0.8 g of mercury sulfate is used, the COD level falls within the 35–45 mg/L range, allowing the values to stay within the specified limits. Now we would like to know: what method can be used to determine whether it is actually our effluent that exceeds the standards, or whether it is the chloride ions causing the interference? Because according to the national standard, 0.4 g of mercuric sulfate per liter is required in theory to complex 2000 mg/L of chloride ions. Theoretically, adding 0.4g should be sufficient to complex all the chloride ions, but in practice, when I use 0.8g or even 1.0g, the results differ significantly.
There are a few questions: 1. Is the monitoring of chloride ions accurate?； 2. Are there any issues with the quality of the mercuric sulfate solution, such as insufficient purity of the solution? ； 3. Pure water should be used for experimental purposes, as tap water contains relatively high levels of chloride ions; 150 mg/L is a common value there ； 4. The water also contains other halogens and ions that form mercury compound precipitates.
You can create a gradient using one water sample to see within which range the COD remains stable