Protecting MBR systems with ultra-fine screens
In 2000, the hot topic in the wastewater industry was prescreening for membrane bioreactors (MBRs). In order for this advanced technology to perform well in wastewater treatment, it was essential to stop hair and fibrous material from binding the membrane media within the cassettes. Having to remove such fine material was quite new in wastewater treatment. It made every screen manufacturer in the country lick its proverbial lips in anticipation of potential big business. Looking back, many pioneers may wish they had been a little more cautious in their approach.
In the 1980s and early ’90s, a 6-mm bar screen was considered “fine” and suitable to protect every treatment process in the marketplace. Little did the industry know that by the turn of the century, the debate would be focusing on “ultra-fine” screens for raw sewage. There was prodigious opportunity for large projects, especially in the southern U.S., considering the benefits of reusable water for irrigation.
It was a lot of work for MBR manufacturers to establish a best practice for the supply of fine screens. Various onsite testing helped establish screening protocols and performance specifications. One early experiment simply involved collecting bags of hair from local hairdressers, dyeing it all bright red and passing it through various screen bands. The MBR suppliers demanded a more precise evaluation, however, as well as measurable performance testing from potential screen manufacturers.
Leading companies, therefore, paid to have their screen products evaluated by United Kingdom Industry Research (UKWIR), based at the National Screen Evaluation Facility, which resulted in a screenings capture ratio for each model tested. This allowed direct comparison between different styles of screens. It became clear that some type of drum screen was necessary to ensure total capture of solids, because once debris is caught inside a cylinder, it truly is entrapped and cannot escape downstream. The problem with traditional drum screens, however, is that they are difficult to install in traditional concrete channels and generally need to sit in steel tanks and receive a pumped flow. The popularity of center-flow screens as an alternative to drum screens grew rapidly. These screens are basically elongated drum screens with a band of hinged panels that articulate around the top and bottom of the frame and elevate the screenings on a vertical plane. This style of screen can be installed easily in a regular channel and receive a gravity flow.
The debate always was energetic; however, a majority of the industry settled on a center-flow screen with 2-mm perforated panels. Most MBR manufacturers agreed that this would act as an effective barrier, preventing hair, fiber and stringy material from ragging the membrane cassettes. Imaginative methods of operation were introduced, allowing the screen band to mat, creating a blanket of material on the screening media. A slow band rotation was controlled by a variable-frequency drive, ensuring minimal disturbance of the blanket and more efficient screening.
Despite significant research and testing, many early 2-mm screen installations were unsuccessful. Some were even disastrous. The actuality of real-life applications was illuminating. It was clear that many screen systems were dramatically undersized and could not pass the rated peak flow. Formulae used to estimate flow on previous generations of screens were inaccurate on these new ultra-fine models. Selecting the appropriate screen model involves estimating the percentage of screen band open area that can be matted with screening while still allowing flow. Manufacturers underestimated the solids loading and applied insufficient blinding factors to the face of the screen band. The screens were overwhelmed, overworked and inevitably liable to break down.
Grit problems also had not been anticipated. The low approach velocities inherent to this type of screening encouraged these heavy abrasive particles to sink in front of and inside the screen band. In one particular case, two fine screens were installed in a 10-ft-wide-by-20-ft-long stainless steel tank that had a permanent 12-in. bed of grit in the bottom. The grit penetrated seals, getting between moving parts of the screens, wearing out chains, guides and drive sprockets, and tearing holes in the 2-mm perforated panels. It sounds like a horror story—and for the screen and MBR manufacturers, it was.
The screen market has learned from its mistakes and matured. Hydro-Dyne remains confident that center-flow technology has an important role in protecting MBR plants. The independent testing at UKWIR proved that its 93% screenings capture rate is the highest of any generic screen design. The goal is to successfully educate potential customers and consulting engineers considering MBR technology that the problems of the past have evolved into a strong solution for the future. A center-flow screen with 2-mm perforated panels, installed as a two-stage screening system and effective grit removal should be the standard, not just the preferred layout. The concept is gaining wider acceptance, although cynics remain.
The Shepherdstown Project
In 2010, Hydro-Dyne had the opportunity to put the theory to the test in Shepherdstown, W.V., when mandatory compliance with its Chesapeake Watershed Nutrient Removal Permit drove a wastewater system improvement project. A major upgrade was necessary, and the town decided to replace the existing conventional activated sludge plant with MBR. Fortunately, the customer and consulting engineer were aware that robust headworks were vital to the success of the project. In 2003, they replaced an aging macerator and emergency manual bar rack system with a fine screen, quickly realizing the benefits to the downstream treatment process. There were minimal rags in the aeration tanks and no blockages of the return activated sludge pumps.
The Shepherdstown project provided Hydro-Dyne with the opportunity to work with the Chapman Technical Group, the consulting engineering firm, to showcase a complete integrated headworks package custom designed to protect the MBR system. Shepherdstown’s average flow is 0.8 million gal per day (mgd), with a peak flow of 2.2 mgd. Space was at a premium, and the new building was just 23 ft wide by 35 ft long. The challenge was to deliver a traditional screen, a complete grit system including pump and classifier, two center-flow fine screens and conveyance to a screening washing and compaction system within these four walls.
The Triden, with a 3-mm laced link bar grid, was selected for the coarse screen. Its compact design only needed a 21-in.-wide channel. It has an estimated SCR of 65% with hooked elements designed to remove the bulk rags and plastics. Immediately downstream, a small, 7-ft-diameter Hydro-Dyne vortex grit trap was proposed to catch 95% of all grit particles greater than 200 μ at peak flow.
A self-priming grit pump transfers the grit to a stand-alone Hydro-Dyne Grit Screw Classifier. The flow exits the grit chamber and splits into two 42-in.-wide fine screen channels, each containing a Hydro-Flo screen with 2-mm ultra-high molecular weight polyethylene perforated panels. The upstream security allows them to truly be polishing screens, trapping the fibrous, stringy material still in the flow stream. The screenings from all three screens are sluiced to a single washing screw compactor, where they are thoroughly dewatered.
The Hydro-Flo screens needed to sit in deeper water than traditional screens. Any ultra-fine screen band naturally has a small open area to pass flow and is expected to see heavy matting. This means that the screens must be relatively large, with greater panel submergence to maintain acceptable velocities and head loss. Having input on the entire headworks design from the start allowed Hydro-Dyne to recommend the channel dimensions and influence the hydraulic profile. Without a 12-in. step down in the concrete after the grit chamber, it would have been difficult to install an effective 2-mm screen system.
Even a well-designed vortex grit trap will let some fine grit pass, which could potentially grind away at mechanical equipment. A center-flow screen is most vulnerable where abrasive material gets between the rotating band and the static frame. Simple neoprene flaps or brush seals failed to block grit in early designs. Hydro-Dyne developed a positive seal system in which the stainless steel frame interlocks with the guide link on the perimeter of the screen band, thereby creating a positive three-sided closure.
Hydro-Dyne will continue to monitor this installation closely. To date, the screen system is working well, and, crucially, so is the MBR. With new advanced treatment technologies entering the market regularly, ultra-fine screens are here to stay.