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.