Choosing Durability: WWTP Team Replaces Mixers’ Impeller Blades By Chris French | 03-2017 With more than 200 companies and some 50,000 people skilled in fields such as microelectronics, telecommunications, biotechnology, chemicals, pharmaceuticals and environmental services, Research Triangle Park in Durham County, North Carolina, is a place to find answers to all sorts of technical questions. At the nearby Triangle Wastewater Treatment Plant, operators’ experience made a case for mixers with stainless steel impellers. As a result, the plant team has been gradually replacing its fiberglass-bladed mixers with stainless steel. Variable flows Industrial wastewater accounts for 70 percent of the plant’s intake, but it also handles large flows from the weekday workforce at the research park, and from 6,000 local residents on weekends, when flows are appreciably slower. Stainless steel blades on mixer impellers help eliminate maintenance issues and enhance process performance at Research Triangle Park’s treatment plant. Joseph R. Pearce, deputy director of the Durham County Engineering and Environmental Services Department, says, “Although our percentage intake of industrial wastewater is significantly higher than the national average, we still have to deal with all the hair and rags that despite prescreening can cause clogging problems for any treatment plant.” Are fiberglass or stainless steel impeller blades better for a wastewater treatment plant’s mixers? At one of the world’s largest research parks, there is no shortage of expertise for weighing whether one material is better than another. During his near 10 years at the plant, Pearce and his team have labored long and hard to keep the original mixer blades going. “We’ve been doing everything we can, but the clogging, especially during storm events, became more and more frequent. The Durham County team chose mixers with stainless steel impeller blades for extended service life

"We had to pull up mixers from which heavy hair mixed with plastics and cotton swabs were hanging, taking them out of service. It became normal for this to be once per month, per mixer in some of the treatment facility mixing zones. Eventually, the blades' gel-coat front edge wears off, making them split, and at up to almost $10,000 per blade to replace, this was something we had to address." Processing solids Built in the early 1960s and expanded in the 1970s, the Triangle plant was upgraded in 2005 when the 6 mgd tertiary treatment facility was replaced with a 12 mgd five-stage enhanced biological...

In addition, the Triangle plant’s recycled water facility consists of four turbine pumps, a hydropneumatic tank, a 500,000-gallon storage tank, instrumentation and metering, a dual disinfection system and a distribution system. Recycling value Recycled water has become a community asset for landscape irrigation, cooling towers and construction activities. In Research Triangle Park, having a redundant water supply is important for water-critical facilities such as computer database services, pharmaceutical plants, LED manufacturing, and agricultural research and development in greenhouses. Triangle’s...