BioCycle June 2004, Vol. 45, No. 6, p. 26
Facilities in Akron, Ohio and Calabasas, California -18 and 10 years old respectively – make process changes that have improved composting efficiency and economics, and built longevity into existing structures.
OVER 20 years ago, the city of Akron, Ohio began construction of a 73 dry ton/day in-vessel biosolids composting facility. Doors opened in 1986 and since that time, the plant has been processing 100 percent of the biosolids produced by the city’s wastewater treatment plant. While the original in-vessel equipment, supplied by Paygro, is still in use, some significant changes have been made in recent years that have resulted in a more efficient operation. Chief among those changes was the installation of new belt filter presses, and a shift to using sawdust instead of shredded bark as an amendment.
In California, the Las Virgenes Municipal Water District in Calabasas, built an 8-bay, 10 dry ton/day U.S. Filter/IPS biosolids composting plant, which began operating in 1993. Las Virgenes Municipal Water District and Triunfo Sanitation District operate the composting plant as joint venture partners. About two years ago, the plant installed a new biofilter system, an upgrade that has improved aeration efficiencies and reduced utility costs.
This article highlights changes at these two composting operations. In both cases, the insights of the operators shed light on how to solve challenges with cost-effective solutions.
NEW DYNAMICS WITH HIGHER SOLIDS CAKE
At the Akron composting facility, biosolids are pumped from the wastewater treatment plant into holding wells. Solids concentration ranges from four to eight percent. In 2002, the original belt filter presses were replaced with four Andritz S-14 model presses, resulting in an increased cake solids content from 22 percent (average) to over 30 percent. The changeover led to some interesting dynamics in compost system management, notes Annette Berger of KB Compost Services, Inc. based in Independence, Ohio, which has a contract with the city of Akron to operate the plant. “As a result of the higher cake solids, the volume of amendment required for the composting process was diminished, leading to a 25 percent reduction in volume to be composted,” she says. “In 2001, 9.97 cubic yards of amendment were used per dry ton processed. In 2002, only 3.33 cubic yards of amendment were used or a reduction of 64 percent in total amendment usage. Of course, with less amendment being used the volume going into the reactors dropped from 13.5 cubic yards per dry ton of processed solids to 6.79 cubic yards per dry ton. In turn, the amount of finished compost produced dropped by 45 percent. In the dead of summer, we use only two of the four composting reactors; in the toughest winter months, we may use three of the four.”
But the changes didn’t stop there, adds Berger. “The need for the volume of air to be scrubbed from the process decreased accordingly. In 2001, there was 895 cfm of air treated per dry ton of solids processed versus the 516 cfm in 2002, an overall reduction of 38 percent.” The facility uses two-stage Pepcon chemical scrubbers (a water phase to remove ammonia and a hypochlorite phase to remove organic compounds).
When it began operating, the facility had used shredded bark as amendment. Over time, a switch was made to ground yard trimmings to reduce costs. “That type of amendment is very fibrous, and it intertwines with each other,” notes Berger. “In turn, it takes a lot more for the equipment to go through that material. So we decided to switch to a mix of sawdust and yard waste. The trade off in labor alone was worth it, plus the sawdust was less expensive to procure. Our reactors are 700-feet long and with the sawdust amendment, the operator running the agitator could complete a reactor in eight hours versus before, it took 20 hours for one reactor alone. And we didn’t experience compaction due to using an amendment with less structure.”
An unanticipated consequence of the switch in belt filter presses was that the 30 percent cake had a higher nitrogen content per ton of cake processed. But with the switch, less amendment could be used. “Our goal is to make a fresh mix at 40 percent solids, so we could cut way back on the volume of amendment but in the process, we hadn’t taken care of the C/N ratio – we have a whole lot more N in that yard of cake,” she explains. “So we ended up having an ammonia issue, which negatively affected the smell of the finished compost.” The problem was solved by switching to 100 percent sawdust in the mix since it had more available surface area thus greater carbon available. That resulted in a C/N ratio of about 20:1. (It had dropped to about 16-17:1, which, says Berger, may not even have accurately reflected the amount of actual carbon available.)
The overall operating costs of the Akron Composting Facility were reduced by the improved dewatering operation. The largest savings came as a result of a 44 percent reduction in plant staffing followed by a 50 percent savings in electrical usage. Savings also were realized from a reduction in overall maintenance and operational costs (less amendment, chemicals and inventory).
BIGGER BANG FOR BIOFILTRATION BUCK
The Las Virgenes Municipal Water District made the switch to composting biosolids after 15 years of land applying on a district-owned farm. Wet weather conditions limited operations at the farm. In addition, the joint board’s commitment to full beneficial reuse led to the district’s decision to construct the agitated bay facility, which actually is located on the farm where land application took place (and is still the back- up management option during emergencies or routine plant maintenance). About 80,000 gallons of sludge at three percent solids are pumped daily from the district’s wastewater treatment plant (about four miles away) to the Rancho Las Virgenes Composting Facility. Before composting, the material is mesophilically digested and dewatered to about 22 percent solids in high speed centrifuges. Fuel cells were installed in 1999, using 80 percent of available digester gas to provide 60 percent of the plant’s power needs. Dewatered biosolids are mixed with sawdust and wood chips made from ground up dead trees and loaded into the bays. After a 30 day retention time, compost is conveyed to a curing building.
The facility originally was built with a 16,000 sq. ft. biofilter designed to treat process air from the compost reactor building. The base was comprised of perforated pipes in a gravel layer. The media is comprised of ground wood chips (including waste wood and mixed pine). “Years ago, we received some odor complaints from the site neighbors and began looking at ways to reconstruct the biofilter to improve its performance,” says Carlos Reyes of the Las Virgenes Municipal Water District, who oversees operation of the composting plant. “We opted to install a concrete slab with some sort of aeration channels underneath to pump air through. Construction was underway, when we were approached by BacTee, which was selling biofilters as a package unit. We told them that the only way we could use the technology was if they could sell the air plenum base plates separately.”
The plates, made from polyethylene, are corrosion resistant and designed to provide more uniform air distribution through the biofilter, as well as improve the drainage of leachate. “The company agreed to sell us the plates, so we made a change in design and poured an 8,000 sq. ft. slab – right next to the existing 16,000 sq. ft. biofilter – with trenches to customize air flow for the base plates,” explains Reyes. “We upgraded the original biofilter, installing new underdrains and new gravel and pipes. We also made a decision to use both biofilters at the same time.”
Construction was completed in January 2002. A smoke test was done on the smaller biofilter to ensure the air distribution was working properly. There was one section where the air wasn’t coming out, and it quickly was recognized that this was where the tractor laying the media had exited, compacting the media in that section. The problem was fixed by fluffing the media. Reyes notes that while smoke tests are valuable to assess air flow, the best way is to go out early on a cold morning and watch the vapors coming out of the biofilter. “You can stand there and watch till the cows come home, whereas a smoke bomb lasts only 35 to 45 seconds. It’s a much better way to assess the performance of the biofilter.”
The one significant difference between the original biofilter and the one with the base plates is that the latter does not create a lot of back pressure, he adds. “I think the best indicator of that is we are able to run the dual speed fans pulling air from the composting building into the biofilter at a low speed, whereas before, we had to run them at high speed just to evacuate the air out of the building. In addition, we only need to use three of the six fans that are installed. As a result, energy consumption has been cut by quite a bit.”
Having the extra biofiltration capacity has been good insurance in terms of controlling any negative impacts on site neighbors. In addition, more media can be put on top of the base plate biofilter if the original one declines in its performance. “Currently, we only have 3-feet of media on top of the base plates,” says Reyes. “But we can always add more media on top of that and likely achieve the same results as we get from running both biofilters simultaneously.”
Both the Akron and Las Virgenes plants use positive aeration in the composting bays, which means that operators have had to identify critical control measures to efficiently remove and treat that wet and odor-laden air and minimize corrosion of the composting building. The contract that KB Compost Services, Inc. negotiated with the city in 2002 (10 years with a five year extension) stipulates that the city pay a fixed price per month, and KB covers all capital improvements (bar a new roof on the compost building). One area being focused on is preservation of the interior of the prefabricated steel building housing the composting reactors. “The city had made modifications in 1998, which included installing fiber glass panels on the interior,” says Berger. “Then, in 2002 and 2003, we treated all the members in the prefabricated structure with a coating resin. In addition, the control room – fabricated with concrete – is located in the reactor area. There was water seepage, so we applied the coating over all the concrete and it is holding up well. In general, the resin seems to stop corrosion at whatever state it is.” The coating is supplied by Preferred Solutions, Inc.
The Las Virgenes plant installed a drop ceiling over the bays as a way to contain the air and protect the overall building structure. There also are plastic curtains at the front end of the bays.
“The ceiling is about 7-feet above the top of the bays – just enough to allow room for the agitators,” says Reyes. “The intake registers are in the ceiling, and the air is pulled to the biofilter. We also have eight fans that blow air into the space above the drop ceiling to keep the humid air out. The ceiling panels occasionally fall out of the supports, due to water condensation on the side exposed to the composting process. We are looking to replace the ceiling material, perhaps with a more rigid fiberglass coating.”
POSITIVE VS. NEGATIVE AERATION…THE DEBATE CONTINUES
THE original Beltsville aerated static pile composting method is based on negative aeration, pulling air down through the composting mass. That process was developed at the U.S. Department of Agriculture laboratory in Beltsville, Maryland in the early 1970s. A few years later, Rutgers University presented a different approach, that of using positive aeration. The debate between the two approaches is catalogued in the pages of BioCycle, but over the years, it seems that composting facilities settled into their own routines based on practicality or the technology being used.
For example, the agitated bay composting technology typically employs positive aeration, with the odorous air released out of the top of the bays. Typically, these systems are installed in buildings. Two downsides of positive aeration in an enclosed structure are corrosion and worker health (due to the ammonia levels in the emissions).
At the Akron, Ohio composting facility, KB Compost Services, Inc., the contract operator, experimented with negative aeration in its bays in an attempt to reduce building corrosion. “We evaluated the trade off between pulling the corrosive air through the gravel and grating and the possibility of having to replace the blowers – versus the price of having to replace the whole steel structure,” says Annette Berger of KB Compost Services. “In the end, we saw two things happening with our system when using negative aeration. First, it takes substantially more air movement through the pile to achieve the same process requirements as positive aeration and second, when the compost came out, it smelled horrible. Essentially, we were pulling all the leachate through the composting mass, which in turn acted more like a filter, so we couldn’t get a product on the back side. So when we put the material outside to cure, we blew the neighbors away with the smell. One aspect we didn’t think about in our discussions about negative versus positive aeration was how to deal with the volume of water that gets removed. The leachate filled up the air plenum and we didn’t have the size drains that were required to remove it. In the end, we also learned that it is always easier to push air than to pull it.”
KB does use negative aeration on its outdoor curing piles, with the exhaust air going through a minibio-filter. Originally, the aeration pipe was placed in an S-formation, but now the pipe is run parallel to the length of the piles, allowing the ends of the pipe to be exposed to enable operators to drain the pipes. Caps are put on the ends of the pipes, but just about every day, an operator pulls off the caps and turns off the blower to let the leachate drain out of the pipes. “The key is to be sure there is some way to remove the leachate,” says Berger. “The other choice is to have a much higher blower capacity. We’re able to use a 2,000 cfm blower on a pile with 2,000 cubic yards of curing compost.”
June 15, 2004 | General
COMPOSTING FACILITIES APPLY LESSONS LEARNED TO CAPITAL IMPROVEMENTS
BioCycle June 2004, Vol. 45, No. 6, p. 26