Before wastewater can be treated to minimize biochemical oxygen demand (BOD), nutrients, and pathogens, it is important to eliminate as much debris as practical, as early on in the treatment plant as possible, to avoid clogging pumps or compromising downstream performance. Careful consideration of design and operating factors can minimize total lifecycle costs and maintenance effort while protecting screening efficiency.
It All Starts With Design
When it comes to maintaining bar screens in a wastewater treatment plant (WWTP) environment, the cost saving with the greatest impact is the maintenance that does NOT need to be performed. Well-thought-out screen design can all but eliminate jamming, parts susceptible to wear, and other troublesome mechanical components. Here are several screen design considerations that deliver the benefits of reducing operating costs and saving hundreds of hours of maintenance over the service life of the equipment.
Keeping it Simple
It is better to choose a simpler screen design that streamlines efficiency via multi-purpose components rather than a design that relies on adding mechanical complexity to cope with each separate challenge.
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).
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.
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.