Peaking Factors And Hydraulics In Wastewater Screen Sizing

Dealing with fine particulate matter, nutrients, and pathogens is essential for wastewater treatment plants (WWTPs) striving to meet Clean Water Act (CWA) effluent standards. Before that stage, however, come important considerations about removing larger debris that can clog pumps, overload clarifiers, or disrupt other downstream processes — especially during periods of elevated flow and debris loading.
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Identifying The Challenges
Choosing an optimum screen design for a new or retrofit installation requires a good understanding of factors that can affect routine and peak flow conditions.

Inflow And Infiltration. Any older wastewater collection system — combined sewer or sanitary sewer only — can experience higher rates of groundwater infiltration (i.e., water entering the system through cracks and leaking joints). Even in dedicated sanitary sewers, illegal hookups from sump pumps or downspouts can contribute to elevated inflow volumes that can strain screening reliability during storm conditions.

Peak Flow Rates. Depending on variables of terrain, permeable vs. impermeable surfaces, seasonal weather patterns, climate change, etc., short-term peak flows can easily reach 10x to more than 20x that of typical daily flows. The increased volume typically impacts the flow velocity through the screen before impacting the depth of water in the channel leading to the screen.

“First-Flush” Loads. Weather-related surges in flow quickly scour the sewer systems, bringing high concentrations of debris in the first hours as the initial wall of water hits the bar screen.

Combined Sewer Systems. Increases in the volume of flow and the type/size of debris during storm events can vary more in these types of sewer systems than they do in sanitary sewers.

Cost Tradeoffs. The more debris that can be removed earlier in the process, the fewer the pump clogs, disruptions, and costly problems with clarifier cleanouts later in the treatment process.

Seasonal Considerations. Wastewater systems serving communities with highly variable seasonal populations — tourist areas, resorts, college towns, etc. — need to plan for efficiency based on populations varying from 5,000 people in the offseason to 20,000 people in peak season.

Conventional Methods for Screening Peak Flows

The most rudimentary screening solutions are fixed-position, manually cleaned screens with coarse openings and no moving parts.  High flows are often accommodated by bypassing the excess around the screen or allowing it to overflow the screen for the duration the rain event. This practice can result in frequent cleanouts at downstream clarifying tanks or digesters.

A second option is to use mechanical bar screens with slot spacing wide enough to accommodate extreme weather events without bypassing the screen or risking overflow during peak flows. Unfortunately, coarser screens will have sub-optimal hydraulics during normal flows and will allow a higher volume of debris to flow through the wider bar openings. This can lead to greater debris accumulation and additional maintenance requirements in downstream operations such as clarifiers or digesters.

Another approach is to add more mechanical bar screen channels to accommodate increased flows. The additional channel(s) can have the same narrow bar opening as the primary channel to remove the maximum amount of debris, or they can use coarser bar spacing to handle a greater volume of flow. Based on site preferences or space constraints, plant operators can choose to dedicate a primary channel exclusively to normal flow conditions and divert all storm flows to secondary channels, or they can simply use all channels during an increased-flow event.

Lastly, peak flows can be diverted to a holding tank or lagoon and then redirected back into the normal wastewater stream at an acceptable rate once the capacity becomes available.
Cost-Effective Design Considerations
A good approach to updating screen-system design is to monitor differences in performance between off-peak flows and peak flows. That includes comparing flow volumes and velocities at the screen, the nature of debris passing through existing screens, the frequency of cleanouts in downstream clarifiers, and other maintenance concerns. Armed with that insight, be sure to address the following considerations.

Play The Percentages. Instead of concentrating on peak flows exclusively, evaluate efficiency on combined year-round experience. Consider this example:

Peak Flow. If a WWTP handling a typical load of 1 MGD experiences a 20-MGD flow rate event for a duration of 12 hours once each month (a total of 6 days per year), it would be operating at that peak flow about 1.6 percent of the year. A 1” screen sized to accommodate the peak flow rate would process 120 million gallons of wastewater (approximately 25 percent of the total annual throughput) in just 144 hours of high-flow conditions.
Typical Flow. Over the remaining 98.4 percent of annual run time, that screen would be allowing 359 million gallons of wastewater (75 percent of annual throughput) to carry moderate-size debris through the oversized screen. That could lead to more frequent, expensive cleanouts of downstream clarifiers than necessary.

Follow These Rules Of Thumb. Industry guidelines for efficient screen operation underscore a series of important considerations for screen sizing and liquids/solids separation.

Channel Design. One good rule of thumb for designing channel capacity is to allocate one square foot of channel cross-section for each 1 MGD of wastewater flow. It also helps to have that submerged cross-section be closer to a square shape rather than being wide and shallow, which can lead to debris settling out if flow rates drop too low.

Minimum Channel Flow Velocity. Keep flow velocity at a minimum of 1.25 feet per second (fps) to keep solids and particles suspended in the flow.

Maximum Screen Flow Velocity. Maximum flow rates through the screen’s bars are typically designed to be between 3 fps and 4 fps (Figure 1). Flows above that velocity can force excessive debris through narrow openings — debris that would typically be screened out at normal flow velocities.

Figure 1. Hydraulic modeling generated with CFD software illustrates how flow velocity — represented here by a color-coded cross section — is affected by the size, spacing, and shape of the screen bars.

Head Loss. The height differential between the level of the feedwater approaching the screen and the water discharged beyond the screen is called head loss. Keeping routine head loss at an ideal range of 2” to 4”, and a maximum of 8” during peak flow events, keeps flow rates low enough to minimize the amount of debris getting sucked through the screens before it can be cleaned off the bars. These ideal head loss conditions, however, are not always achievable across all real-life scenarios.

Two methods used to predict head loss — the Bernoulli principle and Kirschmer method — have served the industry well across larger screen openings. Research of actual flows, however, shows deviations from theoretical performance as screen openings narrow. The answer is to modify the original calculations or apply computational fluid dynamics (CFD) to compensate for tighter screen spacing or bar profiles. This white paper outlines the differences between theoretical values and actual performance in use.

Blinding. Head-loss calculation as part of screen selection typically incorporates a blinding factor of about 25 percent as a safety factor. That percentage might need to be raised to 30, 40, even 50 percent to accommodate past experience — for example, where a utility experiences a high proportion of rags, wipes, or leaves.

Choosing a single ideal solution is difficult because no single screen can satisfy a range of real-world conditions. Unfortunately, current screen designs require compromising debris removal during routine operation or oversized bar openings for peak flows.  Thoughtful evaluation of the individual WWTP’s operating needs and due consideration of backup plans that incorporate added channels and screens can lead to improved performance.
Do you have an upcoming Wastewater Systems project? You’ll like working with us! Duperon provides solutions for coarse screening, fine screening, low flow screening, perforated screening, washing compacting and conveying.

We take pride in our ability to serve individuals and organizations with Wastewater technologies and services which exceed every expectation. Let’s Partner in Design