Online monitoring of COD in high-chloride wastewater

This post was last edited by qugd on 2022-3-28 09:44. When testing COD and ammonia nitrogen in high-salt wastewater, there are some issues related to sample preparation and the accuracy of the tests. Most interferences can be eliminated for ammonia nitrogen by adding alkali to the sample and then distilling it. For online detection, the principle of Nessler’s reagent should not be used, as there are many interfering factors in wastewater; the salicylic acid method may yield better results. However, for COD testing, if the chloride content is too high, it is difficult to obtain accurate results even by using mercury salts; it is recommended to employ a chlorine correction method or the TOC analysis method. Online analysis can eliminate the interference caused by chlorine up to a level of 5000 ppm at most. A salt content of 10% is indeed quite high; it’s possible to reduce this interference to some extent, but not completely, as mercury sulfate cannot be added in unlimited quantities either. There shouldn’t be any difficulties in online pH monitoring; just make sure to choose electrodes that can withstand high salt levels and contamination. If you trust me, you can tell me the basic details of the wastewater as well as the on-site conditions and requirements, and I will prepare a rough plan for your reference. In the case of online monitoring, if it is not an emission point under environmental regulation, appropriate methods and sample processing techniques can be chosen. However, if it is an emission point subject to environmental regulations, things become more difficult. Nevertheless, you can discuss alternative solutions with the environmental authorities, as the actual conditions of the wastewater exceed the scope of detection covered by the standard environmental testing methods. In the case of monitoring emissions from pollution sources, it is also necessary to measure dissolved oxygen, turbidity, flow rate, and water temperature. These data must be transmitted to environmental supervision authorities, namely the factory’s safety and environment department, the local environmental protection agency (possibly via wireless transmission), as well as the company’s DCS or data recording system.

This is a company that deals with the comprehensive utilization of hazardous waste. Basically, it takes in solid waste (mostly residues from non-ferrous metal smelting and electroplating sludge), and since these wastes contain high levels of chlorine, it attempts to process them through washing before subjecting them to thermal treatment. Currently, they are still in the project environmental impact assessment phase. They are concerned that it may be difficult to accurately measure the CODcr of high-chloride wastewater using online sensors; if an online analyzer is chosen casually and problems arise with the online data once it is put into use, then the purchase of such online equipment will have been pointless. Their water samples after washing treatment were tested using their own so-called multi-functional water quality analyzer; the pre-digested tube method was employed for COD testing, and Nessler’s reagent was used for ammonia nitrogen testing, but neither method worked. Their original process was to have the rinsing water pass internal water treatment to bring the heavy metals to acceptable levels before it was sent to the wastewater treatment plant, but there were requirements regarding pipeline connection. It is necessary to meet the discharge standards for water pollutants specified in Table 1 of GB 31573-2015, \"Emission Standards for Pollutants in the Inorganic Chemical Industry\". In the environmental impact assessment, their monitoring measures include conducting self-tests, commissioning external tests, and using online monitoring for parameters such as COD, ammonia nitrogen, and pH. Based on your response, can it be assumed that: 1. The pH level should be fine; 2. A suitable online analyzer for ammonia nitrogen based on the salicylic acid principle should be found; 3. The local environmental protection authorities should be informed of the situation, and representative samples should be provided to discuss the use of TOC online devices for real-time monitoring of water quality.

The biggest confusion arises from the fact that although there is a certain range for the raw materials used in the rinsing process, the level of impurities in them can vary significantly. Relatively speaking, the likelihood of organic pollution in this company is low; however, there are large variations in the chlorine content in the wastewater. Thus, it increases the difficulty of detection and the feasibility of online monitoring.

The gist is clear. Choosing a pollution-resistant pH measurement system, selecting an appropriate measurement location, proper probe cleaning, and adequate maintenance should not pose any technical difficulties. There may not be many direct interfering factors in the methods for measuring ammonia nitrogen, but the reagents used for this measurement can sometimes be affected by certain hardness ions and transition metal ions present in the sample; this can cause the sample to become cloudy and change color, thereby affecting accurate testing. The Nessler’s reagent method has two drawbacks: first, the reagent contains mercury, and second, its stability is not very good, which may make it unsuitable for wastewater samples with complex compositions. Laboratories can use distillation for separation and testing; there are also online analyzers, and some manufacturers equip them with online distillation methods for separation followed by testing. The salicylic acid method is relatively less affected by the sample, which requires analyzing the sample composition before selecting an appropriate method for ammonia nitrogen detection. COD analysis is indeed a challenging task; high levels of chloride ions and other components can interfere with accurate measurements. If the COD value is already high, diluting the sample and removing the interference caused by chloride ions before testing may yield better results. However, for samples with low COD values but severe interference, dilution does not result in satisfactory test outcomes. If the TOC analysis method can be used, and by comparing it with the COD measurement values from the laboratory, this issue can roughly be resolved; however, such a solution does require approval through consultation with the users and the local environmental protection authorities.

Thank you, teacher, for your guidance! The company seems to have contacted those who manufacture online monitoring devices, intending to use Shimadzu-brand TOC and ammonia nitrogen (based on salicylic acid principle) online monitors; there aren’t many issues with pH monitoring online. Professional agencies conduct separate measurements of TOC and COD to create corresponding curves, and then work with the local environmental protection authorities to resolve the emission issues. For laboratory testing, the HJ/T 70 method \"Determination of chemical oxygen demand in high-chloride wastewater – Chlorine correction method\" is used to measure COD, in order to establish a correlation with TOC. For a company, being able to schedule tasks is the goal. The testing and water treatment processes involved are quite challenging.

I originally came here to learn about some maintenance tips for online pH meters, dissolved oxygen sensors, and online turbidity meters, but I accidentally came across these discussions here, which kept me reading until the end. I’ve learned from it; the question raised by the original poster is actually one we have also encountered, and the discussions among the teachers are very useful for reference.