So Many Choices: Putting the Right Screen in the Right Place
When applying screening technologies in a Wastewater Treatment Plant (WWTP), there are a large selection of possible configurations. WWTPs vary significantly around the world due to differing site conditions and treatment objectives. This diversity of environments has produced many screen variants and related peripheral equipment, some of which have worked well, and some not so well. By understanding some of the originating situations that drove the design of a particular screen technology, stakeholders can provide direction for effective application, while also helping to avoid potential pitfalls.
Different Circumstances Requires Different Solutions
It is important to understand that most screening equipment in a waste treatment process has the potential to be an effective solution when used in the conditions they were designed for. For instance, the article "The Historical Evolution of Screenings Capture Efficiency", focused on the importance of the Screening Capture Ratio (SCR) to properly select screening technologies able to protect delicate processes such as MBRs that are highly susceptible to fouling from fibers. The chosen screen for MBR technology is typically placed farther into the treatment process schematic. In many cases, MBR screening is after a primary headworks, grit removal, and clarification treatment step.
The substances which cause the most hazard for the MBR plants would be best described as fibrous sludge. To fully capture hairs and fibers, the screening media and seals used to prevent bypass must be carefully examined. The mechanisms used for the transference of the captured fine material from the MBR screens are also potential pitfalls; mechanical conveyance systems such as augers would have difficulty transporting the screening material obtained in those screens. In many cases pumping the screenings would be more effective. Because the screenings are, for the most part, a fluid and fibrous sludge, care must be taken not to have the fine particles escape during the handling. Wash presses considered for this operation need to be able to wash, dewater, and compact this material without inadvertently returning hairs and fibers to process.
It Has To Be Tough
Conversely, the job of a headworks screen assembly has an entirely different role. The headworks is often described as the first line of defense in a WWTP. As a result of the equipment’s position in the beginning of the treatment scheme, the screen employed needs to have a structural integrity that can handle an environment that could range from large rocks/heavy gravel, logs/sticks, ragging, grease slugs, etc. Depending on the type of collection system, the screen must be designed to handle significant variations in incoming flow and screenings/grit concentrations. The robustness of the design and efficiency of material capture is a priority when considering the proper technology to be used in a headworks application. The capacity for screenings the wash press needs to handle during peak loading events should also be considered. A large portion of the average daily screenings load coming into plant headworks may occur during a peaking event. In many cases, a peak flow event could bring 4 to 5 times the average screenings load in the span of one hour.
In a headworks environment with severe peak loads, it is more realistic to employ screens that can handle the excessive debris loading and extract material higher than ¼” or without binding or blinding. The most common screen used in these conditions is a bar screen with multiple rakes for rapid material removal from the incoming flow to the WWTP. These types of screens have lower screenings capture efficiency (SCR) compared to other screening media, but they have the structural strength to protect the plant from varying loadings with minimized hydraulic headloss.
Protecting Process Downstream
Finer screenings capture becomes more critical to downstream processes such as mechanical aerators or clarifiers and digesters. When excess debris causes problems, perforated plate media offers significantly higher SCR for the same nominal spacing as compared to a bar screen. The second dimension of the perforated media’s screening performance results in a more reliable capture and removal of thready or fibrous material.
The tradeoff for better capture, however, is headloss. The percentage of open area of perforated plates is significantly smaller as compared to a bar screen of similar nominal openings (i.e., ¼”). The result is a comparatively higher amount of headloss that needs to be taken into account within the headworks design. When considering similar footprint comparisons, the perforated plate screen occupies more space than a bar screen rated for the same flow. The size increase is due to a diminished flow per unit area of the wetted screen surface.
Increased SCR also translates to a much higher screenings load captured. Higher capture increases the required size of the screenings wash presses and related transport mechanisms. Also, the screenings themselves will have a higher retained organics (fecal material, etc.) and need to be thoroughly cleaned before disposal. Wash presses that also launder the screenings should correct the issue of excess retained organics. The organics themselves are useful in the process and returned downstream of the screens.
While less common in North America, many larger WWTPs in Europe employ a two-step screening arrangement in their headworks design. Two-step headworks provides the best of both scenarios of severe duty (bar screens) and high SCR (perforated plate) performance.
Retrofits of existing plants pose a unique challenge, especially if they are looking to increase the flow rating of the headworks or increase screenings removal rates. In many cases, the goal is to use existing structures while minimizing modifications and/or disruption of existing operations. StepScreen® is a unique bar screen design for this purpose. While the StepScreen is still considered a bar screen, the thin lamina used as the “bar” medium creates a large percentage of the open area; comparatively, this allows for nominal spacing down to 1/8” while simultaneously allowing for a similar flow rating of 1/4” spaced conventional bar screens. In essence, a finer removal can occur without changing the existing channel arrangement. Because the lamina used in the design is thin, the screen works best in environments that do not typically receive heavy sediment loading (such as a gravity feed collection system that is subject to infiltration issues).
Specialty screens are employed in plant designs to accomplish protection and screenings removal at critical points in the WWTP process. Specialty screens are a helpful solution whenever the finer screening requirement represents only a small fraction of the entire plant flow, and it is not practical to revise the existing headworks. Some examples may be: Inline screens provide the ability to screen more viscous material (thin sludge, septic waste, etc.) while it is being pumped without breaking line pressure: Confined space screens are used to protect pumps from being disabled by rags and debris at lift stations: Septage receiving stations create a controlled environment to accept batch loads of varying loads of waste from septic system cleanouts and portable toilets: Grit Classification plants are designed to take street sweepings and vac truck loadings for separation and classification of the grit, gravel, and organics allowing for material reuse and minimizing landfill.
Most screens that are in use today in WWTPs across the globe can be an effective treatment solution when understood and placed in the appropriate location within the design. The paper “Screening Considerations – A Guide to Selection” explores the specific design considerations of many of the screen types discussed here in this discussion.