October 25, 2005 | General


BioCycle October 2005, Vol. 46, No. 10, p. 30
Canadian facility develops ways to prepare compost for markets while processing biosolids and MSW as feedstocks. Part II
Scott Gamble

THE HISTORY as well as current operations of the Edmonton Composting Facility (ECF) was described in Part I of this report in last month’s issue of BioCycle. Included was an account of how the Edmonton Waste Management Centre developed when the City’s existing landfill became close to reaching capacity in the mid-90s. Originally, the ECF was expected to produce about 125,000 metric tons per year of finished product following a three to four month curing stage. While the quality of the compost has met or exceeded initial expectations, the quantity produced has not come close to the expectations. In 2002, approximately 70,000 metric tons were produced. In 2003 and 2004, production dropped because of the maintenance shut down.
One reason for the reduced production was a higher than anticipated residual rate for the ECF, which has resulted in less material being converted to finished compost. Modeled after experience at other facilities – most notably Cobb County, Georgia and Sevierville, Tennessee – it was anticipated that the overall residual rate in Edmonton would be in the order of 35 percent. However, experience over the past five years indicates that 45 percent is a more realistic expectation.
A compost finishing system is located downstream of the aeration bays. The finishing circuit has been the biggest single contributor to this difference in anticipated versus actual residual rate. The finishing system, consisting of a series of trommels, density separators, and magnets, was built with an expectation that it would have a 20 percent reject rate. In reality, the amount of rejects is over 50 percent, of which a significant portion is organic material. The primary cause is that the density separation equipment was not capable of differentiating between clumps of wet compost and inerts such as rocks or glass. As a result, the compost would end up on the reject belt.
In order to achieve an acceptable balance of product quality and rejects while still maintaining acceptable production, the process in the plant was changed during the first year so that material fed into the finishing circuit had a moisture content in the 30 to 35 percent range. Obviously running at this moisture content had implications on the composting process, both inside the plant, and in the curing operation. In the latter, curing times increased from an expectation of three to four months, to six to 12 months. The resulting levels of dust created by screening compost at this moisture level also created health and safety concerns for staff in that part of the facility, which needed to be addressed and managed.
The finishing equipment has been problematic in other respects as well. In part as a result of its complexity, it was found to be a very maintenance intensive system. “Between routine servicing, replacement parts, and repairs, we were spending between $300,000 and $400,000 per year to keep this system functional,” says Bruce Blackett, plant manager. “That was way out of line with maintenance costs in the rest of the plant.”
The City is about to commence with a project to change out the finishing system. Material will be taken from the aeration bays and moved to a temporary curing/storage area near the landfill. Once the new finishing system is installed (the city currently is evaluating proposals it received this summer), stored compost will be processed through it.
While the changeover is being done to respond to the performance issues just noted, it also is driven by changing markets for the finished product. Originally, the compost produced was to be cured and then used to reclaim strip mines west of the Edmonton facility that were operated by the facility’s original developer, TransAlta. However, this market is not as viable as it once was, and the city has had to refocus on other applications including agricultural, sports fields, and soil blends. These markets require a higher level of finishing so portable screens were being used at the curing pad. However, as market demand has increased over the past few years, most notably in the past 12 months, this is becoming inefficient for the volumes of material being handled.
In addition to handling residential MSW, the Edmonton facility also processes biosolids from the City’s wastewater treatment plant. Biosolids are used primarily to adjust the moisture content of the incoming MSW during the nonsummer months, and to a smaller extent, improve the C:N ratio. Biosolids are retrieved from storage lagoons adjacent to the plant using a custom-designed dredge mounted on a barge. The biosolids are pumped to the plant and dewatered using three large centrifuges. Dewatered cake is injected into the mixing drums using a Schwing positive-displacement pump.
As it was originally conceived, the ECF would process 22,500 dry metric tons of biosolids annually. This target was based on several assumptions, most notably the moisture content of the MSW. Since opening, the MSW has consistently proven to be wetter than anticipated therefore less moisture needs to be added to the drums in the form of biosolids. This translates into a direct reduction in the amount of dry tons that can be processed by the ECF. This has been compensated for somewhat by increasing the moisture target in the drums upwards from the 48 percent recommended by the designers. But experience has shown that once the moisture level exceeds about 55 percent, the 80 mm trommels immediately downstream of the drums blind off more frequently and screening rejects rates become unacceptable.
Another key assumption behind the biosolids processing target was that the dewatering operation would be able to consistently produce cake with a solids content of 28 percent. A significant effort has been made over the past five years trying to achieve this target. Various polymers have been tried, as have modifications to the dewatering equipment. Additional staff positions also were added so that dedicated personnel could be assigned to the dewatering operation. The best success has been achieved in the past 12 months with the assistance of the centrifuge manufacturer. “When everything comes together inside, and the dredge operator finds a good pocket of biosolids in the storage lagoon, we’ve been able to achieve 28 percent and even 30 percent for a couple of hours during the day,” says Noah Ganes, Operations Manager. “But more importantly, we’re consistently getting a solids content of 25 percent, compared to the 21 percent we were getting three years ago.”
Seasonal variations in MSW composition means that the late fall and winter months are when biosolids at the ECF are required to amend moisture and, to a lesser degree, C:N ratio. However, those months are also the most expensive ones in which to extract and dewater biosolids from the City’s storage lagoons. Conversely, extraction and dewatering of biosolids are easiest and cheapest during the warm months of the year.
As a result, the City sought an additional composting technology to complement the existing operation. It was felt that two processes would have the capacity to handle all of the biosolids generated. A smaller separate process would also provide the ECF with the flexibility to produce higher quality compost from source separated feedstocks should the opportunity to do so arise in the future.
Of the technologies reviewed, the Gore Cover system was felt to be the most suitable for the application. Earth Tech was commissioned to help the City with a detailed assessment of technical viability of the Gore system, including a detailed review of the claims made by Gore, and inspections of facilities operating in Europe. Based on this technical assessment as well as the City’s financial evaluation, an agreement was entered into with W.L. Gore. Construction started in July 2002 and the first compost pile was constructed at the end of September 2002.
The separate biosolids composting operation occupies approximately 3 hectares, and consists of a composting pad, a curing area, bulking agent storage areas, and a surface water management pond. The active composting pad is sized for sixteen 50 m long composting piles or “heaps”. This pile size and configuration, in combination with Gore’s recommended eight-week operating cycle, provides an input design capacity of almost 95,000 m3 per 12 month period. The facility uses biosolids with a dry solids content of about 24 percent. A ratio of 2.5 parts bulking agent (wood chips and fines) to 1 part biosolids (by volume) has been found to be effective. A Supreme Enviro-Processor mixes the biosolids and ground yard trimmings and unloads directly onto the concrete compost pad.
The pad is surrounded on two sides (north and south) by 3.35 m (11 ft) high concrete back walls. In addition to providing containment, the back walls serve as the support structure for the winder assemblies used to place and remove the covers. For each of the 16 piles, a single 2 hp blower provides air via two embedded concrete aeration channels. Blowers are controlled by oxygen and temperature feedback.
Composting of municipal solid waste on a large-scale basis is complex, so it comes as no surprise that in a one-of-a-kind facility like Edmonton’s, challenges have been encountered. In most cases, these have been addressed by modifying operational practices. However, redesign or reengineering of equipment has been needed to address others. Despite the challenges, the City of Edmonton and Earth Tech continue to remain committed to the long-term viability and success of the facility. Perhaps more than anything else, this commitment is important to ensure success.
Scott Gamble is with Earth Tech Canada Inc. in Edmonton, Alberta.
Sidebar p. 33
ALL COMPOST SITES have unique hazards (e.g. dusts, bioaerosols, process off-gases) which need to be accounted for through design and operational practices. At Edmonton, the nature of the feedstocks, as well as the volumes handled, introduce additional risks.
The Edmonton Composting Facility is an industrial site, not unlike the generating plants, petroleum refineries and chemical plants that are its neighbors. Heavy equipment, conveyors, rotating equipment, and automated control systems which are situated within 9.5 acres of building can lead to many safety challenges. Like any work industrial environment, a coordinated program for protecting the health and safety of employees, subcontractors, and visitors needs to be provided.
One of the very first things that Earth Tech did when they took over operations was to review the existing health and safety program, and begin a program of improvement. Earth Tech safety professionals and industrial hygienists reviewed site conditions and programs, and a part-time Health and Safety Coordinator position was established. These positions worked together to identify areas of greatest risk, and developed a plan to address them through a combination of capital projects, changes in operating practices, training and protective equipment. Six months into this program, it became apparent that staffing the resident H&S Coordinator on a part-time basis was insufficient, so the position was converted to a full time job.
Through the work of the coordinator, supervisors, and most especially the facility’s front-line staff, the health and safety program has basically been rebuilt since 2001. A significant effort was put into reviewing and rewriting all of the facility’s standard operating procedures to include hazard assessments and safety requirements. Formal safety “tailboard” meetings are held at the start of each of the four shifts every day to make staff aware of changing conditions throughout the plant. Short training “vignettes” are also regularly incorporated into the tailboards to supplement the formal training program which involves both on and off site training.
Statistics have shown that new workers are the most prone to injury and incidents, so particular emphasis is placed on new employees at the plant. Prior to starting work, all new staff members spend up to two days going through various safety training programs and site orientations before they are allowed to work on their own. Then, depending upon the position, they job shadow with an experienced staff member to further reinforce the training.
In 2003, a medical monitoring program was introduced for all plant staff. This involves an initial medical screening upon being hired, followed by annual reviews. In addition to benefiting employees, the program helps to ensure that new employees are fit for work in the plant environment.
While incidents do still occur at the plant, we strive for lower frequency and severity. All incidents are formally investigated to identify the root cause, and practices and procedures are amended as necessary to prevent the incident from reoccurring. Near misses are also reported and investigated.
In an industry which historically hasn’t had a great safety track record, the Edmonton Facility has managed to achieve an exemplary level of safety performance. In fact, they have achieved one of the highest industry rankings ever in a voluntary provincial safety certification program. “The City places a high priority on safety in all of its operations,” says Allan Yee of the City of Edmonton, “and through their ongoing efforts, staff at the composting facility demonstrate their commitment to this principle. We are very pleased with the success and gains they have made.”

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