The days of nursing those old coarse screens along will soon be behind you
That time has finally arrived – you are getting fine screens for your headworks. This will be a huge benefit for the operations of the entire plant, so the selection of the fine screen is not a decision that can be taken lightly. You know the choices you make today will be something that you and the plant will be living with for many years to come. Here are a few things to consider when selecting your fine screening solution.
Why a fine screen?
Fine screens help remove smaller pieces of trash, such as rags, wipes, cotton swabs, plastics and latex that will easily pass through traditional mechanical bar screens. Such trash quickly becomes problematic for operators downstream, as it collects on aerators, causes scum blanket accumulation in digesters, clogs pumps and floats in final clarifiers. Increased removal of inorganic solids can benefit treatment processes from activated sludge to oxidation ditches, as well as MBR facilities and lagoons. However, one challenge with fine screening is the removal of too many essential organics, such as fecal debris, from the waste stream. The biological process in the treatment facility is more effective in treating these solids than releasing at a landfill.
What is a fine screen?
Fine screens are traditionally defined as solids removal devices that have openings in minimally one dimension of 1/4 inch (6 mm) or less. It gets more complicated when you look at how that 1/4-inch or less opening is specified.
The one-dimensional fine screening captures debris in a similar method as coarse screens or step screens, utilizing closely spaced vertical bars. While these products will capture more than coarse screens, they are susceptible to passing items that can turn vertically and float on through. The most common item in today’s sewage that could slide past the vertical bars would be disposable wipes.
The alternative to this is two-dimensional screening. This method of screening meets the same 1/4-inch or less opening dimension but in two directions. Most screens of this type use perforated plate materials with round holes. This method of screening insures nothing larger than the minimum hole size can pass through the screen. All of JWC’s headworks fine screening solutions utilizes this two-dimensional perforated plate screening method.
Types of fine screens
Headworks fine screens can be further categorized into straight-through or in-to-out designs. Screens that employ the straight-through methodology typically sit directly in the channel and the flow intersects the screen perpendicular to the screening surface. Fluids pass through the screening surface and the screenings are carried up and over the top of the screen. Screens of this type, such as JWC’s Finescreen Monster, will capture significantly more materials than any bar screen. One of the important operational necessities of this type of screen is to fully remove the screenings from the panels as they pass over the top of the screen. If material is not removed it can easily be washed down the channel and into the plant as the panel re-enters the influent flow on the back side of the screen.
In-to-out screen configurations, often call band screens, have the flow enter the interior of the screen and then pass out through the screening panels on both sides. The debris is carried up the screening panels on the inside of the screen and washed into an interior trough to convey it out of the screen. In-to-out fine screen layouts, such as JWC’s Bandscreen Monster, offer the best capture of solids due to their non-carry-over design. These designs are better equipped to protect downstream high-tech treatment equipment, such as membrane bioreactors. MBR manufacturers frequently recommend a band screen or drum screen like JWC’s Monster Drumscreen with 1-2 mm perforations to prevent small trash and hair from fouling the membrane’s pores.
High-volume solids removal
Today’s fine screens possess narrower openings than previous generations of headworks screens to achieve increased capture rates. This also means they discharge more screenings than typical bar screens — sometimes twice as much. Screenings washer-compactors are now common handling this increased load of solids
Some manufacturers like JWC Environmental have developed innovative washer-compactor solutions that shred, clean, dry and reduce this excess debris so operators can dispose of the material at a landfill. The core element of these systems is a sewage shredder to precondition the screened solids prior to it entering the washer-compactor. JWC’s Muffin Monster grinder is capable of breaking up the rags and wastes to facilitate better washing. By doing this the plant can achieve maximum removal of fecal material from screenings and have both cleaner screenings as well as maintaining those essential organics within the waste stream. These systems also help to save time and money through fewer dumpster pickups while reducing odors at the facility.
The future of screening
Municipalities throughout the world are continually looking for ways to maintain or enhance the efficiency of their wastewater facility. The ongoing efforts within the industry to transition facilities from wastewater treatment plants to WRRFs will continue to drive the need for better process control and as a result fine screening. Operation of a WRRF, which can include utilizing FOG, along with biosolids as inputs, must be able to work with high-performance anaerobic digesters or other systems to achieve the desired output or resources. Removing more inorganic trash and rags out of the wastewater stream with fine screen and maintaining the organics loading with more powerful washer compactors are two tactics wastewater professionals use to improve the overall efficiency of the treatment process. When these technologies are incorporated at the headworks, downstream equipment stays protected and operators worry less about unscheduled maintenance and downtime.