We have an ORP probe hooked to our PLC which turns a metering pump off/on. Does anyone know what the best way is to configure these things to give a steady 6ppm of residual chlorine in the waste water stream?

 

We had requirement for lower ppm in 1980. The issue was that a very smart Chemical Engineer ended doing the chemistry part of the project as well as the Instrumentation. The multiloop Micon was busy closing the loop. An ORP probe was not used at all. Rather we had Orion analyser. I may be wrong depending upon other chemicals in the waste (for instance if there is lime). Fisher Porter used to be the specialist in chlorination.

 

There is no direct correlation of ORP to ppm chlorine, especially as you are using it in a combined chlorine chemical state. What is the goal of your process? coliform count, effluent chlorine residual? A good resource, www.stranco.com. Dennis Fairfield Automationdirect.com

 

Thanks for the reply, Dennis I've heard that the ORP probe may not be the right thing for the job, but apparently there are some articles around in process control journals, etc on it. The person who sold us this probe was supposed to fax them to us, but I haven't seen them(?). We are a recirculating aquaculture facility, and the 6 ppm residual chlorine is supposed to prevent any live pathogens from being released from our waste water. The water is diposed of in an underground disposal field for the same purpose. I'll check out the resource you mentioned. Thanks for your time Clint Demaray

 

The articles are in water/wastewater journals rather than process control journals. I used to design control systems in water and wastwater plants. The previous response with the Stranco information will lead you to the source of the ORP debate. ORP is, as stated, not directly related to Cl2 residual. Cl2 residual is also not directly related to kill rates. There are papers, however, that suggest that ORP is a more accurate measure of kill potential. In wastewater, amonias react with the Cl2 and require you to use a total Cl2 measurement rather than free chlorine. Total Cl2 requires a reagent and is a maintenance issue. You must provide a sample line (prone to clogging and freezing) and use an instrument that is much more complex and expensive than a simple "dunk" probe. Depending on the physical layout of your system, you may need to include a pump on your sample line. If you are going to use ORP, you need to work with your engineer to develop an ORP based control strategy. However, you may have a regulatory requirement for on-line Cl2 monitoring which requires you to have a Cl2 device. If you are using grab samples for permit only, then you can add ORP to control dosing, provided that you are comfortable that your ORP control strategy will keep you within Cl2 limits. One good part of using ORP is that you avoid online Cl2 monitoring and therefore may avoid reporting it. If you get into on-line Cl2 monitoring, you need to make sure that your control strategy is well designed and tested as even small excursions can become a permit issue.

 

Thanks very much for the information. I'll look up Stranco and see if there's a decent way to be pretty sure we're getting a good kill. The regulations really haven't been written for what we're doing, but since we have this equipment, we'd might as well make it work as well as we can.

 

Since Cl2 residual control can be a real bear, I just wanted to add a few more words of caution. Don't forget that with ORP control an ORP differential is measured. ORP into a plant varies widely so you have to control on the difference between it and your outlet point. With chlorine control, be sure to pay attention to the delay times between feed change and residual measurement. The presence of flow varriable retention times and, often, pH controls on the plant effluent can make it real tough to keep them within reasonable bounds. An approach that I used once, with some reasonable results, was to custom program a controller so that its tuning varied with of plant flow (retention time). Al Pawlowski, PE [email protected] dba ALMONT Engineering Baton Rouge, LA USA

 

Clint, We have used ORP to control CL2 and SO2 for our processes here in Manchester, CT with a lot of success. After you talk to Stranco, let me know if you need any other information. We were able to use it successfully to meet NPDES permit requirements that were on the lunatic fringe for reporting residual readings. It is a long story and will require some time to explain. Let me know if you need any more info. Bob Robert J. Dusza, Jr. Treatment Manager (V) 1-860-647-3219 (F) 1-860-647-3150 E-mail - [email protected] Manchester Water & Sewer Dept. 125 Spring St. P.O. Box 191 Manchester, CT 06045-0191