March 2008, Vol. 20, No.3
Answering the Chlorine Question
Gaseous chlorine is a very effective water and wastewater disinfectant. It also is a very hazardous chemical. In recent years, many facilities have begun phasing it out in favor of less hazardous disinfection technologies, citing security concerns over the gas being accidentally or intentionally released.
Is a switch away from chlorine gas needed in all cases? Are there some instances where it is still the best candidate? And what are the factors that must be considered when switching to alternative disinfection practices?
Spartanburg (S.C.) Water has experience on all sides of this issue. The joint water and wastewater agency uses gaseous chlorine disinfection at its three drinking water and five wastewater plants. It uses chlorine tablets and ultraviolet (UV) disinfection at three wastewater treatment plants.
“As we make upgrades or changes at any of the facilities is see if we can select any type of safer technology,” said Rebecca West, director of technical services for Spartanburg Water and president-elect of the Water Environment Federation (WEF; Alexandria, Va.). That generally means trying to eliminate gaseous chlorine use and the hazards and extra safety measures that accompany it, she explained.
Eliminating a hazardous material is one of three reasons the City of Richmond (Va.) Wastewater Treatment Plant is switching from gaseous chlorine to UV disinfection, said Robert Steidel, deputy director of the Richmond Department of Public Utilities. The other two drivers are to facilitate a process change for nutrient removal and to treat wet weather flows from the city’s combined sewer system.
After the upgrade, the plant will remove nitrogen and phosphorus to meet Chesapeake Bay regulations using denitrification filters, Steidel said. Because the filters will remove most suspended solids, the effluent will be very clear, which is ideal for UV disinfection. If the filter should malfunction and nitrates begin to leak through, chlorine disinfection would be compromised. Nitrates consume chlorine and prevent it from killing pathogens.
“In case there is a breakthrough, we couldn’t feed enough chlorine, we felt, because of the nitrate demand on the chlorine, but we do feel we’ll have low enough suspended solids to allow disinfection to continue with ultraviolet light,” Steidel said.
The third reason for Richmond’s switch to UV is the large hydraulic swings associated with capturing and treating all flow from its combined sewer system. The city feels that UV disinfection will be a more sustainable technology than chlorine.
Steidel said the first phase of design will treat dry weather flows at 45 mgd (170,325 m³/d) and wet weather flows to 85 mgd (321,725 m³/d). The second phase will increase wet weather flow treatment capacity to 200 mgd (757,000 m³/d).
“It will be easier for us to apply more power to the [UV] disinfection than it will be for us to store the vast amounts of chlorine chemical we’d have to have onsite to treat those high flows,” Steidel said.
However, sometimes gaseous chlorine makes a better choice from a treatment point of view. Before the decision to move away from chlorine is made, “we look at what’s the best disinfection process for our facility,” West said.
For example, several of Spartanburg’s older facilities rely on lagoon treatment, and sometimes the lagoons allow incompletely treated organics out with the effluent, West said.
“By using gaseous chlorine, we get some additional oxidation of the organics,” West explained. “It does more than just disinfect the water,”
In late September 2007, WEF hosted a meeting with several utility managers, including West and Steidel, as well as representatives of the U.S. Environmental Protection Agency, to discuss disinfection methods. The meeting focused on the use of gaseous chlorine, when and why its use might be discontinued, and alternative disinfection practices.
The U.S. Department of Homeland Security (DHS) has released an interim final rule that imposes comprehensive federal security regulations for high-risk chemical facilities to prepare security vulnerability assessments. These assessments identify facility security vulnerabilities and develop and implement site security plans.
To date, water and wastewater treatment facilities have been exempt from DHS regulations; however, the George W. Bush Administration could reverse the exemption. This forces utilities to consider two questions, according to West. The first is, Can I find a safer technology? The second is, Will that process help to produce the best quality water at the end?
“That’s pretty much what we told EPA,” West noted. “Let that be a local decision, driven by [the questions], ‘Are you treating water to the best of your ability to protect public health?’ and ‘What’s the best process to do that by?’”
West added that plants still using gaseous chlorine maintain several safeguards, such as secured facilities, monitors to detect leaks, means of shutting down leaks remotely, and containment and air-scrubbing abilities.
“There are all kinds of provisions that are being put into facilities these days, if they choose to stick with gaseous chlorine,” West said.
A Bright Alternative
Steidel said he believes that chlorine is the most effective disinfectant but that UV is more sustainable than chlorine. In many cases where gaseous chlorine is being phased out, UV is the replacement. Of course, operating a UV system has its own set of challenges.
“The constraints are going to be a little bit different with UV,” Steidel said. “If we have summer rolling brownouts, we’re going to have problems getting enough power for our UV system. … I think the UV is more sustainable and a better solution than continuing to rely on chemicals.”
The main concerns with a UV system are the cost of power, ensuring a stable power supply, delivering that power, and maintenance and cleaning of the UV system, Steidel said.
“You have a higher O&M [operations and maintenance] cost, from our analysis, with UV than you do for gas chlorine systems,” Steidel said. “But again, that’s something we weight against many other factors to make it sustainable.”
Spartanburg’s plants that use UV have stepped up fats, oils, and grease removal, devised a means to recover from a power outage, and begun an aggressive maintenance program to keep the system operating in top form, West said.
“We knew that the grease would coat the bulbs and affect how effective it is,” West said.
Another major consideration was how to recover from a power outage and ensure that the disinfection chamber remained clear of pathogens. To do that, the plants keep sodium hypochlorite on hand to dose the chamber and kill the pathogens as the UV system gets back up to speed, West explained.
UV also requires quite a bit of maintenance, West said. “You can’t just turn it on and say, ‘I’ll replace the bulbs when they all burn out.’ You’ve got to keep these bulbs clean. You’ve got to have some continuous maintenance process where you’re making sure you’re getting the maximum efficiency out of those bulbs.”
In addition to the known changes, switching to a new process usually includes a few surprises — both good and bad. Steidel said that Richmond was pleasantly surprised when the findings of a joint research project with the U.S. Army Corps of Engineers showed that UV was effective at disinfection of combined flows that had only received primary treatment prior to disinfection.
On the other hand, one of Spartanburg’s UV plants was taken by surprise when a textile manufacturer discharged residual UV-protectant dye in its waste. The textile manufacturer had just received a government contract to make military uniforms with built-in UV protection for U.S. soldiers in Iraq, West said. When the UV-resistant materials were washed, excess protectant flowed to the treatment plant.
“We were fortunate that we didn’t wind up with a fecal violation,” West said. To correct the issue, Spartanburg worked with the industry to ensure that the textile manufacturer’s pretreatment process captured as much of the washwater as possible and also coordinated with them to turn up the power in the UV system each time they processed the dye.
“You have to be realistic about how UV disinfection works,” West said. “What I mean by that is if you have wastewater that has any color to it … it’s going to affect how effective the [UV] wastewater disinfection process is.”
In South Carolina, that becomes an issue because of textile and dye manufacturers discharging to the collection systems. Likewise, coastal and sunny communities have to take into account the amount of sunscreen that rinses down shower drains and flows into the wastewater treatment plant.
“That affects how effective the UV process is, because the sunscreen still has UV inhibitors in it, and you don’t always treat that completely through your treatment process,” West said. “You have to be sensitive that your wastewater may be able to absorb and affect the UV process, and so you’re going to have to basically ramp up your bulbs to ensure proper disinfection.”
Choosing which disinfection system to use, like most important questions, requires a balancing act. In this case, safety, costs, effectiveness, and sustainability all play a role. Also like most important questions, the true answer is different for each facility.
“First and foremost, look at your process, look at what you have in existence, and make the best decision for treating the water to the best of your abilities,” West said.
Steve Spicer, Operations Forum