Chlorine remains a widely used and highly effective disinfectant for wastewater. But how much chlorine is the right amount? What are the consequences of over- or under-dosing? And what are the benefits of dosing accurately and consistently?

Innovative technology from Hach is designed to give plant operators visibility to disinfection process data in real time, getting rid of guesswork and uncertainty. The RTC-C/DC (chlorination/dechlorination) software module can help operators optimize chemical usage and cost-effectively meet their effluent-quality targets across all shifts. This helps ensure continuous production, permit compliance and protection of public reputation.

The technology is part of the company’s Claros Process Management series of modules designed to help operators tackle the challenges unique to each facility. Bob Dabkowski and Dave Rutowski, application development managers with Hach, talked about the technology in an interview with Treatment Plant Operator.

What challenges in the marketplace was this technology designed to address?

Dabkowski: Operators sometimes can feel helpless when they don’t understand why the process is reacting in a certain way. Without the proper data, they don’t see what’s happening, and so they lack critical understanding of the disinfection process. At the same time, when you look at the EPA’s ECHO website (echo.epa.gov) you see very common violations such as too much chlorine in the effluent or too many fecal coliforms. So there is an issue around compliance.

In what basic way does the software help make operators more effective?

Rutowski: In our industry we talk about what a precious resource water is, but from a plant operations standpoint labor is also a precious resource. During storm events when something is going wrong at a plant, operators are tasked with many other things to do, so repetitive and redundant testing falls to a low priority. That is where hardware and software solutions can take over and keep everything running stable and consistent.

How have operators historically regulated chlorination and dechlorination?

Dabkowski: The typical way is to measure total residual chlorine at the end of the contact chamber with a grab sample maybe once a day or once per shift, and then evaluate whether the chlorine dose needs to increase or decrease. Obviously that misses an entire smorgasbord of data, including E. coli and fecal coliform counts.

What are the deficiencies of that method?

Dabkowski: That grab sample is an instantaneous snapshot of the process. It is that literal 10 mL in a sample cell, compared to the millions of gallons of water flowing through the plant in a day. And so, what’s happening the rest of the day, nobody knows. They can only try to guess based on that one sample.

Rutowski: Just getting that chlorine value as a static number, even if the number stays the same, doesn’t necessarily reflect the same process conditions, such as the flow of the plant or the organic loading it’s experiencing. Just because you have the same number doesn’t mean the chlorination process is running consistently.

In simple terms, how does the software module function real-time?

Dabkowski: Fundamentally, the chlorination/dechlorination management system lets users operate the disinfection process using the same science the engineers used when they designed the plant. It’s all about the concept of CT, which is well understood on the drinking water side as chlorine concentration multiplied by contact time. In wastewater we tend to just look at the chlorine concentration and neglect the contact time. But it’s the same disinfection process, so why don’t we use the same science and operate wastewater plants by way of the same CT concept?

In simple terms how would you describe the workings of the software?

Rutowski: The system uses feed forward and feedback loops. The feed forward chlorination calculation is based on continually maintaining an influent concentration of chlorine that correlates to the chosen CT by adjusting the dosage based on wastewater flow. The feedback calculation modifies the influent chlorine dosage based on the measurement of total residual chlorine at the end of the chlorine contact tank, to detect when chlorine demand changes.

How does this technology help optimize chemical usage?

Rutowski: It does so by enabling appropriate dosing at very short cycle times, instead of making major step feed changes once a day or once a shift. We can do multiple analyses as fast as the cycle time of the instrument, and augment that with measurement of other variables such as plant flow and hydraulic residence time in the contact chamber.

Is the system interfaced with chemical feed pumps so that dosage is adjusted automatically?

Rutowski: It can be, but it doesn’t have to be. The end user can make that decision. Most plants do decide to automate the process, but if there are certain conditions where they want manual control, the plant team has every right to use the values and the information as a suggestion engine, and then still make manual adjustments.

How does this technology potentially enhance operator safety?

Dabkowski: By dosing the proper amount of chlorine, we can reduce the quantity of chlorine gas used and the risk associated with changing out chlorine gas cylinders. If the disinfectant is sodium hypochlorite, then there are fewer incidences of trucks coming to the storage tanks. In addition, the elimination of grab samples reduces the general risk of operators being out in that part of the process.

What is involved in deploying and installing this system?

Rutowski: It depends on what the facility has in place. Many plants may have online instrumentation that provides data, or they read data and record it in a software system or by hand. We can integrate with existing instrumentation, or if the plant doesn’t have chlorine instrumentation in place, we can install it in the proper locations. Then we can pair that with a PLC or a compatible industrial computer that becomes the brain of the operation, integrating with SCADA or a stand-alone control system to send signals to the chemical feed pumps.

What experience provides confidence in the effectiveness of this software?

Dabkowski: The RTC-C/DC is another module developed out of our Claros Process Management platform. On that platform we have 29 modules with the same approach. We have deployed similar modules for diverse applications across North America and Europe, at some 1,700 sites across the globe. Fundamentally, we recognize that every wastewater plant has unique challenges. Our platform is very flexible, so we can work with operators to make it fit into their system.

Rutowski: The heart of the platform is the team that supports it. There are human beings behind the process. Claros Project Management is a specialized team of technical support, planned maintenance and field service within Hach. That team stands behind the successful implementation of each system.