The Historical Evolution of Screenings Capture Efficiency

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The Historical Evolution of Screenings Capture Efficiency

Any individual who has operated a wastewater treatment plant will readily agree that a well-designed headworks system employing a reliable and effective screening technology is essential to a properly running facility. Treatment objectives have rapidly evolved since the beginning of centralized waste treatment. In North America, the practice is relatively young; the first recognized WWTP involving some level of treatment was established in the late 1800s. It was in 1887 that the first biological treatment using an intermittent sand filter was installed in Medford, Massachusetts. The first federal regulation of sewage quickly followed in 1899.

The rapid increase in the population (32 million in 1860 to 76 million in 1900), combined with large migrations moving to city centers due to the emergence of the American Industrial Age, acutely increased issues with health and safety. Stresses on living conditions, food supply and infrastructure, due to this surge gave entrance to government regulation and incentives to drive the changes needed to begin to solve these challenges.

Why should you care about the history of screening? 

You are probably asking, “What does this have to do with screening capture efficiency?” It is important to understand the underlying drivers of the ever-changing mission of the WWTP to understand the context. While the very first plants addressed serious health and safety issues, for the first 100 years or so, the primary drivers for the design of a WWTP were mostly due to government regulation shaping the treatment priorities. The treatment objective and technology used to meet those objectives have driven what form of screening would be used.

The mission of the screen was simple in the early days; it was designed to keep large objects out of simple processes through the use of a static screen panel using steel bars with spacing ranging from 1 inch to 3 inches or larger. As recently as the 1970s, a so-called “fine screen” in a WWTP process was typically in the range of 1-inch bar spacing.

The smaller a screen opening gets, the more debris it begins to capture. The increased rate of capture necessitated the ability to remove the debris captured on the screen mechanically. The reliability of this mechanism was the primary focus in the selection of a screening system. As the screen opening became smaller, the hydraulic effect of headloss emerged as a key design consideration. Facilities selecting screens initially focused on the screen’s hydraulic impact on the flow through the plant. Later on, hydraulics would become a critical factor for efficient screenings removal.  

Increasing treatment objectives

The turning point providing the impetus to move to a finer type of screen was the 1948 Federal Water Pollution Control Act Amendments, amended in 1972 and known as the Clean Water Act. With its passage, it gave rise to treatment process advancements such as nutrient removal, chemical conditioners (polymers), dissolved air flotation and thickening. New process configurations were put into place such as activated sludge, high purity oxygen, SBRs and MBRs. Improving these processes also gave entrance to improved sludge digesters and disinfection technologies.

Finer debris and small grit particles endangered the proper operation and lifespan of these more sophisticated processes. The mission of the headworks portion of the WWTP design became a critical point for optimization. An apex in the evolution of the WWTP, a focused understanding of wastewater screens as well as how to adequately select and design them became essential.

Get in line

Fabricating a machine such as a wastewater screen is not a complex operation, per se. The knowledge and experience of how screens are used and what environment they will be operated in will guide the desired outcome. With the new treatment objectives being implemented in WWTPs from 1972 onward, the market for a more sophisticated headworks screen emerged. Early innovators developed and patented screen designs still in use today. As the patents expired, many of these machines were copied. By the end of the 20th century, the plethora of screens was overwhelming. Many claims of performance were made, requiring some form of a universal standard by which to compare the machines.

In 1998 a method for third-party screen evaluation was developed. The U.K. water industry and the UKWIR funded a screen evaluation initiative that involved the creation of a National Screen Evaluation Facility (NSEF) at Chester Street WWTP in the U.K. The facility is used to conduct on-site testing of screens using the existing incoming wastewater at the plant. 

The testing protocol verifies the screen tested will meet minimum standards of removal of a specified size range of screenings during service. The testing provides a measure that came to be known as screenings capture ratio, or SCR — a value derived by sampling screen discharges and downstream gross solids loading. According to ThompsonRPM (the current third-party tester): “The ‘average SCR’ value for any particular screen has been accepted worldwide as the comparator for process performance when considering new screens.”

Several models of HUBER screens have undergone NSEF testing. While many of the results of the testing are not surprising, there were some interesting takeaways.

NSEF Certifications for select Huber Technology Screens
NSEF Certifications for select Huber Technology Screens

Mission critical

The bellwether regarding the critical nature of using a validated high SCR screen is membrane bioreactor technology. MBRs are highly sensitive to potential fouling environments. In particular, hairs and fibers can rapidly disable the membranes in the system. While the SCR rating on a given screen is an essential starting point in the design, several other factors are also at play in screen consideration. The design, operation and durability of a screen significantly affect short-term and long-term performance. As the capital cost of MBR technology is 10 times the cost of the related protective screen, great care should be taken in evaluating a screen against relevant factors.

While the SCR rating is helpful for initial consideration, something to consider is that the SCR testing at the NSEF site runs about two weeks of testing per machine. It provides a “snapshot” of the screen configuration capabilities. Varying types of designs for screens utilizing the same media produce radically different outcomes. A paper presented at WEFTEC 2010 titled “Saga of Two Screens” illustrated how a band screen configuration using a 3 mm perforated plate was not able to adequately protect a hollow fiber MBR from fouling. By adding a center feed drum screen using the exact same media downstream of the existing band screen, it solved the fouling problem. This empirical example illustrates how a screen’s hydraulic seal configuration can make the difference between success and failure.

In the article “MBR Screening Part 2: Selecting an MBR Screen,” the authors point out that a given screen type will have a changing SCR based on how the screen is operated. They summarize by saying, “Its value is dependent on both the rating and rotation speed of the screen, as well as on the velocity of the wastewater traveling through it.”

Each screen has its purpose

The SCR rating certification given to a particular screen by the NSEF is helpful for initial evaluation. However, it must be understood that the actual physical design of the screen, the robustness of construction (ability to deliver consistent performance over its entire design life), how the system is operated, and the hydraulics occurring during operation, all play an essential role in achieving and consistently maintaining the “real world” SCR.

No one screen design is the panacea that covers all attributes needed in the operation of a WWTP. Each design has a specific mission. A sturdily designed multi-rake bar screen is the perfect defense at the front end of the process to protect the plant by intercepting and removing heavy rocks, gravel, grit and coarse debris. An ultrafine screen with a high SCR rating would be taken out by a storm loading like that. Understanding where these screens are to be placed in the process is critical to success.

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