Bar screens are an important first-line defense when it comes to keeping debris and large solids out of a wastewater treatment system. For low-flow systems that need ultra-fine screening, a drum screen can be installed after the bar screen or at other critical points in the treatment process. They are particularly beneficial for certain industrial applications or in small municipal wastewater treatment plants (WWTPs) that utilize sensitive membrane filtration, such as membrane bioreactors (MBRs).

Source: WaterOnline.com

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Design Simplicity. Rear-return bar screen designs typically depend on brushes and spray nozzles to help clean debris from the bar scrapers before returning them down the “clean” side of the screen. Those components add cost, add maintenance requirements, and can compromise debris removal efficiency if they are not properly maintained on a continuous basis.
By contrast, front-clean/front-return link systems that rely on gravity to dump collected debris from scrapers avoid the need for add-on brushes and sprayers to assist in debris removal. Also, designs that eliminate jam-susceptible submerged sprockets avoid the need for shock absorbers or limit switches.

Cleaning Efficiency. In a rear-return design, any debris not removed from scrapers will unavoidably be flushed downstream. Worn brushes, worn seals, and compromised spray nozzle coverage become big factors in the declining efficiency of aging screening equipment. Unfortunately, it is all too easy to overlook scheduled maintenance of these items in a busy or understaffed WWTP facility.
Front-return screen designs route the drive chain and bar scrapers up the face of the screen, dump the debris, and return the scrapers back to the bottom of the channel all on the front side of the screen. Any debris that might stay on a scraper simply returns to the flow on the front side of the screen to be picked up by the next scraper. This eliminates the risk of captured debris carrying over into the flow on the “clean” side of the screen (Figure 1).

Figure 1.

In-Channel Maintenance

Beyond performance efficiency, design simplicity can also affect maintenance convenience, safety, and costs:

Ease Of Maintenance. Being able to do most maintenance from the top of the screen makes maintenance a quicker, easier process.
Maintenance Safety. Minimizing the need for WWTP operators to enter confined spaces below the deck of the bar screen channel — to clear jams, grease bearings, or perform emergency or routine maintenance — promotes operator safety by keeping them out of what can be a harsh environment (Figure 2). The time and effort of draining the channel and preparing for workers to enter there safely is also a big component in bar screen lifecycle operating costs.

Figure 2.

Life Cycle Considerations

With grit, debris, and corrosion-inducing flows, the headworks of a WWTP can be an unforgiving operating environment for mechanical equipment with rigid design parameters. Partner with a manufacturer who has a lot of experience applying their equipment in a variety of conditions and flexible, thoughtful design to customize to your sites unique challenges.  Mechanical reliability impacts the efficiency and lifecycle operating costs of the equipment as well, in the following areas:

Submerged Hazards. Eliminating chain-return sprockets and shafts at the bottom of the channel minimizes pinch points where larger debris can cause jams. Reducing such occurrences promotes maximum uptime that is especially critical in storm events, cuts labor-intensive maintenance and cleaning efforts in the channel, and minimizes the frequency of confined space entries for operators.
Equipment Maintenance. With the right bar screen design and reliable construction, annual mechanical maintenance can be quite simple. It comes down to occasional wash-downs and visual inspections, greasing a top-of-unit bearing quarterly to keep corrosive gases and liquids out of the housing, and replacing gearbox grease every  five years or 20,000 hours of service. That averages out to just a few hours per year per screen. By contrast, with screen designs that depend on regular replacement of brushes, seals, plastic teeth, and chain roller links, maintenance becomes cumulatively more expensive and time-consuming year after year. In one Western U.S. metropolis, an efficient bar screen design reduced the maintenance workload of 1,400 hours for five climber screens down to just 43 hours for the single bar screen that replaced it. It also trimmed the spare parts inventory required to support planned and emergency replacements.
Downstream Maintenance. How bar screens handle debris in the inflow channel also affects downstream maintenance. Efficient screening can reduce downstream pump damage and maintenance, plus reduce the frequency and cost of cleaning out clarifiers and digesters. For example, equipment with flexible link systems dynamically adjust to odd-sized, odd-shaped debris, and scrapers that fully penetrate all three sides of a bar effectively remove rag, rock, and other debris buildup.
Operating Economy. In addition to parts-and-labor maintenance costs, look for screen designs that can take advantage of low-horsepower motors to drive energy efficiency for lower lifecycle energy costs.
Handling Everything AND The Kitchen Sink. Given the wide range of debris that can impact the front of a bar screen — timber, tires, shopping carts, plastics, and more — its design needs to be simple, robust, and resilient. The more complex the screen design — in terms of the number of components, plastic parts, or close-fitting parts — the greater the maintenance challenges and potential for failure.

Equally challenging can be how extreme flow velocities from a storm “first-flush” event can affect the capture rate of different particle sizes. Coping with changing hydraulics and peaking factors requires up-front calculation of known debris characteristics and load volumes for normal and extreme wastewater flows. The more debris — large or small — that bypasses or gets forced through the screen, the higher the costs of pump maintenance and repair, filtration maintenance, and clarifier or digester cleanouts. Even if those costs are not directly attributed to bar screen maintenance or lifecycle costs, they are largely avoidable expenses that impact the overall annual operating costs of the WWTP.

Design Strategies For Handling The Unexpected
WWTPs looking to install their first mechanical screening system in place of a manually cleaned screen or upgrade an underperforming mechanical screen should take note of the historic impact of storm events on wastewater flows before choosing a new bar screen design.
Because even a few seconds of screening downtime can result in an overflow event, having a mechanical design that can accommodate a variety of object sizes, shapes, and weights is key. That includes large, heavy, and oddly shaped items such as timber, tires, shopping carts, sewer caps, and more. Equipment with mechanical components such as plastic teeth or submerged sprockets is more susceptible to damage/jamming with these types of debris conditions.
Be prepared to address the following factors based on the particular type of debris a sewer system tends to generate during storm events:
·     Channel Design Options. If space/design allows, it is not uncommon to have additional channels designed specifically to handle storm flows, separate from the normal wastewater screening flow. That way, the two screens can deliver optimized performance across both normal and extreme flow conditions. Take care that the velocity of wastewater approaching the screen fits within acceptable guidelines for both the channel flow and the slot velocity through the screen. If greater capacity is needed for existing or space-constrained sites, additional volume can be handled by increasing screening area — e.g., widening or deepening the screen field, adding more water depth — or by enlarging the bar openings.  Some sites can also evaluate diverting storm flows to some sort of storage basin and recycling it back from that holding area in controlled, manageable volumes once the flow has subsided.
·     Bar Spacing. Ideally, bar openings would handle both average and peak flows. Bar screen manufacturers have to size the bar openings to be able to handle peak flow effectively yet also work well in the typical conditions that are seen most days. To handle most flow ranges, bar openings are sized wider for the peak flow although it would benefit the average flows to have smaller openings. So choosing the bar opening is balancing multiple variables (a few primary: plant hydraulics, the ideal slot velocities for a given range, debris characteristics and what solids need to be removed to protect downstream equipment (example, sensitive systems like IFAS system or MBR or something more robust like a pump that can handle 3-inch solids).
·     Debris Removal Frequency. Having a wide range of debris removal speed capabilities will help accommodate a screen’s ability to keep up with varying flows and debris conditions experienced among both in dry and wet weather. While the speed of operation is important for removal capacity, so too is scraper spacing. Scrapers spaced closer together effectively clear more debris, as the cleaning frequency is greater. Scrapers spaced wider apart make it easier for the system to capture large, irregularly shaped items.
·     Adaptable, Continuous Cleaning. Designs incorporating multiple appropriately spaced scrapers can keep pace with first-flush loads better than single-arm articulating designs that need to travel down and up to complete one cleaning cycle. Keeping both the scraper cleaning pass and the return pass on the front side of the screen will eliminate the possibility of debris falling off the scrapers and into the downstream flow. Look for designs that can handle a wide range of debris encountered in first-flush flows — fully submerged, suspended, and floating.
·     Maintenance. With no margin of error for shutdowns during storm surges, choosing a design with low-maintenance requirements and no need for personnel to enter the screen-intake channel can prevent maintenance headaches. For example, avoiding designs with a sprocket at the bottom of the scraper chain eliminates the possibility for timber, rocks, or other hard debris to cause a dead jam condition. Another maintenance consideration is to avoid screens with lots of plastic “sacrificial” components that are susceptible to damage and requires replacement.
·     Design Experience. Perhaps the most important design strategy for challenging high-flow events is to team up with an experienced manufacturer who has proven installation experience across a wide range of wet-weather screen applications. The value of that experience will help in evaluating all appropriate options and in calculating the projected performance against the most extreme conditions experienced by the specific WWTP collection system involved.