Waste Not, Want Not: Tyler, Texas Reduces Costs with Lean Six Sigma

When residents depend on you to provide essential services at the lowest costs possible, municipal officials can feel like they’re between a rock and a hard place. The city of Tyler, Texas, solves this dilemma by applying Lean Six Sigma to everything from garbage collection to its hiring process.

The city has completed 103 projects in the past six years, projects that have enhanced the quality of life for its residents while yielding some $5.4 million in savings.

For example, officials noticed the Westside Wastewater Treatment Plant used varying amounts of magnesium hydroxide, which prompted an investigation by employees trained in Lean Six Sigma. Magnesium hydroxide stabilizes pH levels, controls the odor of raw wastewater, reduces sludge, and settles solids.

The team set out to identify the optimum magnesium hydroxide level that would meet all of the necessary functions and comply with the Texas Commission of Environmental Quality’s pH level requirements.

The Challenge

The Westside Wastewater Treatment Plant handles about 60 percent of the city’s wastewater, processing an average of 7.5-8 million gallons per day. Standardizing the amount of magnesium hydroxide added to the wastewater would reduce costs. The project team first needed to determine what caused the fluctuations, and see how the varying amounts of the chemical affected the treated wastewater.

Tyler, Texas, has saved more than $5.4 million to date through Lean Six Sigma projects, including one that standardized the amount of magnesium hydroxide added to wastewater at the city’s Westside Wastewater Treatment Plant.

At the treatment plant, employees mix magnesium hydroxide with the wastewater at the headworks to achieve the desired pH level throughout the treatment process. Samples are taken every two hours to ensure that state permit requirements are maintained.

Magnesium hydroxide is costly, so employees sought the optimum amount that ensured effective wastewater treatment. They evaluated the system and identified several process variables, which included the magnesium hydroxide dosage and the alkalinity of the incoming wastewater.

The team needed to measure each variable to determine how they affected the pH level, and also to investigate possible correlations between the variables. Did the level of alkalinity influence the amount of magnesium hydroxide needed? Was magnesium hydroxide the only factor that raised and lowered the pH level? To answer these questions, the project team used Minitab Statistical Software.

How Minitab Helped

Because it’s designed to make statistics accessible to anyone, Minitab takes away the fear many people have about data analysis. “Minitab made it easy for employees to analyze the data,” says Guillermo Garcia, Master Black Belt and member of the City of Tyler’s management staff. “For example, the Assistant feature guides team members through an analysis, even if they have never used the software before.”

The project team used Minitab Statistical Software to generate a regression equation—which describes the relationship between a variable and response—and displayed the data graphically. The results helped them see the relationships between magnesium hydroxide and both alkalinity and pH level. “The images offered clarity on what the data had been telling us,” Garcia says.

After tracking the range of magnesium hydroxide dosages over time, the team identified periods of low alkalinity in the incoming wastewater as the cause of the increased chemical dosages. Employees originally believed that adding more magnesium hydroxide would correct the low alkalinity. Now that they knew why treatments were fluctuating, the team could focus on testing levels of alkalinity and pH to determine if additional magnesium hydroxide was really necessary.

The graphical summary above displays the variability in magnesium hydroxide levels added to the wastewater.

The fitted line plot above reflects the insignificant change in the pH level with fluctuating magnesium hydroxide dosages from as little as 12 gallons to as much as 55 gallons per two hour intervals.

This I chart shows that the pH sample results changed very little after the team identified the ideal magnesium hydroxide usage.

The team’s analysis showed that adding magnesium hydroxide changes the alkalinity and the pH level of the wastewater very little. Based on the results, the team identified the ideal amount of magnesium hydroxide to add to the wastewater, and they used Minitab’s control charts to validate and illustrate their success.

Results

Data analysis showed that magnesium hydroxide dosage does not substantially affect the pH level or correct low alkalinity. The facility can stabilize the dosage—and cut costs—with complete confidence that the treated wastewater will still comply with state regulations. The magnesium hydroxide variability was reduced from 106.17 to 15.57 gallons per day. The savings for this project exceeded the initial estimated dollar amount, and has totaled more than $79,000 to date. The Westside Wastewater Treatment Plant also established standard operating procedures, which ensured the continued efficiency of their process and savings for the city and taxpayers.

As the city’s residents know, this is just one of many improvement projects—and the benefits are really adding up. “All of our Green Belts and Black Belts have completed projects that are focused on improving a city process, saving either time or money,” says Mayor Barbara Bass.

“I believe our success can be attributed to the synergy of our Lean Six Sigma program combined with other employee engagement strategies,” says City Manager Mark Daniel. “So far, the accumulated savings for the city’s projects is over $5.4 million.”