Flourescent dye, a diagnostic tool commonly used in chemical engineering, has been put to a successful new purpose in unlocking the inner workings of packaged sewage plants. UK manufacturer WPL used a fluorescent dye to map the hydrodynamics of its own Submerged Aerated Filter (SAF) systems.

This research, which began in September 2014, has shed light on the dynamics of media loading and revealed a way that a new type of media can be used to increase biological oxygen demand (BOD) removal by 20 percent.

Engineer and researcher Tim Holloway of WPL says, “SAF systems have been around for a long time, and we know they work, but very little was understood about the hydrodynamics within the system and what effect it has on process performance.”

Surprising results

Holloway believed that there was a reason why some plants seemed to be more efficient than others. As part of his MSc research supervised by Dr. Ana Soares at Cranfield University in Bedfordshire, UK, Holloway proposed using a fluorometer to examine the internal hydrodynamics of SAFs.

Holloway explains, “Utility companies commonly have their own prescriptive loading rates that govern the way media is loaded. The usual approach is to fill the SAF tank with as much polypropylene biofilm media as possible.” While Holloway was aware of such a convention, his research aimed to uncover the ways that this accepted method of loading media within the system might not have been the most efficient. He proceeded to see what would happen if some of the media was taken out.

WPL installed its purpose-built SAF system on-site at Cranfield University, and Holloway began experimenting with different levels of media and aeration. Using internally mounted fluorometers, he was able to create data both showing how the biofilm media was reacting with contaminants and also revealing the influence of hydrodynamic conditions inside the tank.

The fluorometer measures the changes of small quantities of the fluorescent dye within a system, showing how it interacts with the internal hydrodynamics.

“Using the fluorometer, you can assess the flow, both vertically and horizontally, seeing where the peaks are and how the contents dilute according to how the fluorescent dye fades,” says Holloway. “It is a technique commonly used in chemical engineering to test what is happening in chemical reactors, but it has not been widely used in biological processes, such as sewage treatment.”

Holloway’s findings suggest that while nitrifying bacteria are best removed by densely packed media, carbon-consuming heterotopic bacteria are more efficiently removed by a tank that contains less media and has more space for circulation and movement.

“No one has looked at the process in this amount of detail before,” Holloway says. “It is really quite unique for someone to look at the internal hydrodynamics of SAFs.”

Additionally, Holloway’s research suggests that the removal of organic contaminants can be improved by as much as 20 percent when the ratio of media to wastewater is adjusted inside the tank. Holloway believes that this new understanding of the hydrodynamics of wastewater processing could be used to improve the way facilities are designed.

Further insights

In addition to its applications for increasing the efficiency of organic contaminant removal, Holloway’s method of using a fluorometer to assess what is happening within facilities could shed new light on other aspects of the wastewater treatment process. WPL is currently collaborating with Nigel Palmer, principal process consultant in Southern Water’s engineering and technical solutions division, to explore the way hydrodynamic analysis using a fluorometer could improve performance and be used to design more efficient systems.

Holloway presented his findings to delegates at the European Waste Water Management conference in Manchester, England, in October 2015.

Media design

WPL Technical Director Andrew Baird, who worked closely with Holloway on the research, believes that this new insight into the internal hydrodynamics of SAF systems could improve the way they are designed and built. The Hampshire-based company, which has over 25 years of experience designing packaged sewage plants, is carrying out further experimental research in order to build on what has been learned about the way the SAF systems work.

“I think we are among the first people to start looking at the way media is loaded and designed,” Baird says. “Our feeling is that by doing clever things with the media, we may eventually be able to eliminate dead zones, reduce energy use, and reduce the carbon footprint of our plants.”

Baird explains that a reduction in the need for scouring has already been shown and that design changes have already made it possible for WPL to create a self-scouring plant. Such innovations could be significant in terms of reducing energy costs when the whole-life cost of a unit is taken into consideration. When evaluated from the perspective of totex, or total expenditure, the whole-life cost and the whole-life benefits of the investment become apparent.

WPL specialises in customised wastewater treatment systems, which can be installed even in hard-to-reach locations. The company offers full design services and manufactures most SAF units off-site, thereby reducing installation time as well as reducing health and safety risks.

In a forward-thinking final remark, Andrew Baird says, “Our engineers already know a great deal about making packaged plants as effective and reliable as possible. But we believe as the water industry starts to look seriously at whole-life costs, it will become valuable to see how even tried and tested systems can be adjusted to become more efficient.”