Disinfection Dilemma

A simple lab test helps a plant team get to the bottom of a chlorination system problem and restore routine permit compliance
Disinfection Dilemma
Figure 1: One demonstrationwas very simple — a hydro-meter, a graduated cylinder and a reference chart.

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Another set of bad effluent fecal coliform results. This makes four times since the new chlorine system was installed. The plant superintendent is not going to be happy about this, especially after the last council meeting where he had to explain to the commissioners the reasons for the pending consent order from the state regulators.

The treatment plant had recorded no violations in the last five years, and with the new five-stage Bardenpho system, meeting nutrient criteria was a breeze. The plant had undergone major upgrades to meet the nutrient removal standards, and part of the modification was a new chlorine disinfection system: a switch to sodium hypochlorite from 1-ton 100 percent gas/liquid containers.

 

What is wrong?

The plant had used the gas from the 1-ton containers during its entire existence, going on 30 years, without a coliform violation or a chlorine-related accident or gas release. Joe, the chief operator, remembered when they used gas chlorine and life was less complicated. Now, they had all these pumps, tanks, a new open structure to house all this equipment — and coliform violations, as well. Why did they have to go with this new system, he wondered.

Joe brought up these issues at the monthly treatment plant staff meeting. His operations staff listened intently as he explained the situation between the treatment plant, the town council and the regulators.

Joe led a brainstorming session with his operators, gathering their thoughts and opinions on what might be a solution to the poor effluent disinfection. After all the more senior staff members voiced their thoughts, Joe noticed that his newest operator hadn’t said a word. He asked Mary what she thought and was intrigued by her response.

 

Valuable knowledge

A few months ago, Mary attended a workshop provided by a state Rural Water Association to get continuing education units toward her license. The course was about using chlorine as a disinfectant and applying the alternative forms of chlorine, like calcium and sodium hypochlorite. Joe asked Mary for more details, as it seemed Mary had come away from the training with more than just some CEUs.

During the workshop, the instructor explained how to measure the strength of the chlorine bleach solution and how bleach can lose its strength over time, especially weaker solutions in hot climates. He also noted that time played a big part in the solution losing its strength: The more it aged, the weaker it became.

The trainer performed a demonstration, describing how to measure the strength of the sodium hypochlorite solution. He had various samples of hypochlorite, and after donning eye protection and gloves, he showed the test methods. One demonstration was very simple — a graduated cylinder, a hydrometer and a reference chart (Figure 1).

The trainer poured sodium hypochlorite samples into several 1,000 ml graduated cylinders and used a special hydrometer to measure the specific gravity of each sample. He showed how to take the readings properly and to apply the readings to a small chart in an operator’s handbook provided by a water and wastewater equipment supplier.

 

Putting it to work

Mary explained that it was very easy to do, but seemed like it might not be accurate enough to meet the plant’s needs. In a second demonstration the trainer used a specially made bleach strength test kit (Figure 2). This method seemed much more accurate and measured the strength to the nearest tenth of a milligram.

Although close to the readings taken with the hydrometer method, the bleach strength test kit appeared more reliable. Through his lab supply vendors, Joe researched the different test methods, weighed the costs and availability, ordered a bleach strength test kit, and anxiously awaited its arrival.

A few days later, the bleach test kit had been delivered, and Joe and Mary went about testing their chlorine bleach supply. Mary had been trained in laboratory technique and was comfortable using the associated reagents and titration equipment. She set up a testing schedule and measured the bleach strength a few times per week for the next month. After that, she had some interesting results to share with the plant staff.

When a new bleach shipment arrived at the plant, its strength was exactly as advertised, 12.5 percent. However during the following few weeks, the strength dropped off dramatically, down to 8 percent in 14 days. After one month it was less than 5 percent strength as available chlorine.

 

Making changes

When the plant was modified, a flow-paced chlorine feed system had been installed to control the speed of the hypochlorite pumps, but it had to be overridden often to maintain an adequate chlorine residual. The longer the hypochlorite remained in the tank, the more the feed system had to be manually adjusted. This didn’t seem to be a problem during winter, but it became steadily worse as the heat of summer progressed.

Joe asked Mary to share the findings with the rest of the operations staff, and they offered ideas to prevent the sodium hypochlorite from losing strength. The team made several changes that made a major difference in the bleach feed system. They included:

• Ordering smaller quantities of bleach from the supplier, but more often

• Making a temporary shade to block the translucent hypochlorite tanks from the direct summer sun

• Installing a ventilation system to provide air movement through the hypochlorite tank and equipment structure, reducing heat

Once these modifications were made, the plant was once again in routine compliance with its operating permit. The course Mary attended was well worth the information she obtained, as well as the CEUs!

 

About the author

Ron Trygar is senior training specialist in water and waste-water at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.



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