Managing a headworks in the new world of membrane bioreactors
Membrane bioreactor (MBR) systems are taking treatment plant performance to a whole new level, yet they present a unique set of demands on managing the headworks process—the first step as flow enters a wastewater treatment facility.
Most membrane manufacturers and consultants select fine screens for their MBR projects with either 2 or 3 mm circular openings as the preferred method of screening. Moreover, a unique style of perforated plate screen, called center flow or bandscreen, appears to be the most effective technology at protecting the membranes.
A bandscreen positions the rotating screening loop parallel to the wastewater channel and flow. Wastewater enters the inner loop of the screen and travels 90 degrees left, right or down through the perforated screening curtain. The solids are captured and conveyed exclusively on the inner loop, and there is little to no chance of screenings carryover, an important protection feature for MBR systems.
Because the small perforation size will capture more organic and inorganic solids, plant operators can expect a larger volume of discharged screenings. Controlling this flow of material and keeping it from becoming an odor, containment and landfill problem is key to successfully managing bandscreen headworks.
As a solution, JWC installs a Screenings Washer Monster (SWM) in partnership with Bandscreen Monsters, a complete set-up called a Monster Separation System. The compact SWM uses a unique six-stage process to separate and wash organic fecal material off of discharged screenings and filter it back into the treatment flow upstream of the screen so it can eventually be processed by settling and the MBR process and turned into beneficial biosolids.
The SWM is an incredibly powerful washer compactor, and it has been popular with operators and consultants since its introduction five years ago.
At many SWM installations, operators have cut the screenings discharge rate in half or more. One site even did away with a screenings bin, instead allowing finely shredded discharge to fall to the floor, scooping it up with a shovel and into the grit bin once or twice a day. This is an unimaginable practice for plants using old-fashioned compactors.
Also important to operators and plant managers is the incredible reduction in odor the SWM achieves. The discharge is visibly cleaner and drier, making a remarkable difference in odor when compared to previous compactors, which produced big, brown, sludgy piles of screenings.
Installation of an SWM, which grinds, washes, compacts and dewaters screenings, at the Meriden (Conn.) Water Pollution Control Facility dramatically reduced organic content and overall volume, helping to eliminate expensive remote landfill costs and reduce handling needs and odor, according to the plant’s manager.
“Before the landfill adjacent to our site was closed, all we had to do was haul the screenings over there in our own 8 cu. yd. truck,” recalled Robert T. Mercaldi, assistant director, Water Pollution Control Division and manager of the 12 mgd plant, a two-time winner of EPA Region 1’s Operation and Maintenance Excellence Award.
“After the closure, we had to find a company licensed to haul it, certify testing, rent a specially-lined dumpster for $1,200 a month and pay landfill drop-off charges ranging from $40-50 a ton,” he said. “Meanwhile, we had a complex new task in the plant, moving around solids heavily loaded with organics and heavy odor, requiring double and triple handling.”
To combat the overload of screenings, Meriden selected JWC’s SWM. Mercaldi said discharge was virtually free of organic matter, with the exit plug typically 40-50% dewatered and reduced in volume by about 75-80%.
“We’ve been able to reduce the weight of our screenings from 16,000 to 660 lb per month, and the volume was reduced from 15 to 1 cu. yd. each month,” he said. “In addition to eliminating the cost for the out-of-state dumping and all that extra work we were doing with a backhoe, front-end loader, wheelbarrows and the dumpster, we no longer need the bar screen at our pump station.”
A controller accepts the input signal as synchronized with a screen or conveyor, and the auger operates automatically in an on/off cycle. Captured solids are diverted to the SWM’s hopper, flushed through a grinder and then processed by an auger, which washes and separates soft organics from plastics, paper, cans and other undesirable elements.
Grinding helps expose more surface area and break up fecal clumps for the spray water to further clean out unwanted solids. Plant backflow water is used to wash screenings down the chute, and potable water is used for the spray in the washbox. Plant managers and consultants have their choice of conveying methods to move material from the screens to the SWM, including a sluice, auger conveyor or direct discharge.
Organics and wash water pass through the auger’s perforated trough and are returned to the plant’s waste stream. The remaining material goes through compression and dewatering, with more water squeezed out in the tapered exit chute.
Mercaldi said the final product has the appearance of shredded newspaper, although considerably compacted.
“I was a little concerned when nothing had come out the end for the first five days after startup, but I also noticed there wasn’t any odor,” Mercaldi said. “Finally, a very dry solid, resembling shredded newspaper, extended 6 or 8 in. above the exit chute, like ashes at the end of a cigarette. That falls off into a bin, and we bag it up for disposal as special waste and very viable use in trash-to-energy plants. The Connecticut Department of Environmental Protection is working with the trash-to-energy plants to have the material re-classified as municipal solid waste.”
Simpler & cleaner
JWC reported similar success with an SWM unit at a California treatment plant, where an independent lab study found moisture content of rags and screenings coming off a screen was reduced from 86.2% to a 47.2% cake, and there was a large reduction in organics.
In addition, staff there previously dumped rags and screenings on a daily basis and now only do it once every two weeks, with a coincidental 30% reduction in hauling costs.
MBR treatment plants are more compact and produce higher quality effluent; on the other hand, they place more demands on the headworks area of the process.
Specifying the highest quality, most advanced screens and washer-compactors for the headworks will help ensure the efficient, reliable operation of the membranes and provide relief from an overload of discharged screenings.