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Edmonton is a world-class composting city

State-of-the-art compost facility draws visitors from around the world

Apr-2009

A Canadian composting facility has become a hub for industry experts from around the world. Visitors from virtually every continent have visited the Edmonton Composting Facility to learn how this state-of-the art facility is combining sewage sludge (biosolids) with residential solid waste, to create a high-quality compost product.

In the 1990s, the City of Edmonton was in danger of reaching the maximum capacity of its landfill without any plans for a replacement facility. The city had relied on the landfill as the primary method of handling their disposal of municipal solid waste for years. That’s when city and community leaders began developing a comprehensive and cost-effective waste management plan for the city and what evolved was an industry leading recycling and compost program.

“The first step was a recycling program that was well-received by the community,” says Jim Lapp, supervisor of compost operations for the city of Edmonton. “Then in 2000 the compost facility was built and put into operation.”

Today, the city of Edmonton recycles about 17 percent of its waste, and the remaining residential solid waste is delivered to the Edmonton Composting Facility. These established programs allow the city to divert about 60 percent of its residential waste from the city landfill.

Located on 62 acres (25 hectares), the compost facility is the largest of its kind in North America. To put this in perspective, the compost facility buildings alone occupy an area the size of eight football fields or 416,500 square feet (38,690 square meters). The facility processes up to 198,000 tons (180,000 metric tonnes) of residential waste and 16,500 dry tons (15,000 dry tonnes) of biosolids each year.

State of the art process
The Edmonton Composting Facility is referred to as a co-composter because it composts residential solid waste together with dewatered sewage biosolids.

“We receive municipal solid waste from single-family and periodically from multi-family residences,” says Lapp. “There are two streams — recyclables and household waste. At the curb we have a blue-bag program, where the residents set out their recyclable materials, such as glass, tin, paper and plastics. That’s collected separately and goes to a recycling facility on our site. The other stream is basically the garbage stream or household waste that comes to the compost facility.”

Garbage trucks arrive at the compost facility and unload the household waste onto the tipping floor, a large indoor concrete pad. The tipping floor occupies 43,056 square feet (4000 square meters) of space or the equivalent of 18 sheets of hockey rink ice.

“Workers on the tipping floor conduct an initial or rough sort removing oversized items like furniture, large nonorganic items and household hazardous materials,” says Lapp. “Then wheel loaders push the waste into large concrete hoppers below the tipping floor.”

From there the waste is pushed by hydraulic rams into one of five large rotating mixing drums. Each drum measures 180 feet (55 meters) in length and 16 feet (5 meters) in diameter. The drums mix the household waste with biosolids that are sourced from the nearby wastewater treatment plant.

The biosolids arrive via an underground pipeline from the wastewater treatment plant or are transferred from nearby biosolids storage lagoons. When the biosolids materials arrive at the compost facility it’s between 2 and 5 percent solids. The biosolids are introduced to a dewatering system in the compost facility. Centrifuges use a spinning action, like a washing machine, to dewater the biosolids to about 23 percent solids.

“At this point the biosolids are almost like a cake mix mixture,” says Lapp. “The biosolids are injected into the mixing drum with the household waste and used to help control moisture and balance the nitrogen level inside the drum.”

The biosolids and household waste travel from one end of the mixing drum to the other over a period of one to two days. When the mixed product exits the drum, the organic material has been broken down. The product wouldn’t be described as compost at that stage, but it’s the first phase of the decomposition process. Onsite technicians test the product as it comes out of the drum to monitor the pH, carbon and nitrogen ratio.

Technicians can then recommend changes as needed throughout the process to ensure the correct ratio is attained, allowing the bacteria to do its job in further breaking down the product.

Once the biosolids and household waste mixture exits the mixing drum it is conveyed into two rotating trommel screens, which remove the larger non-biodegradable materials, like plastic and textiles over 3 inches (7.6 cm) in size. The oversize particles are taken to local landfills. Magnets placed above the conveyor belts remove ferrous materials, which are recycled.

“The compost material passes through the trommel screens and is conveyed to the aeration hall,” says Lapp. “The material is placed into three aeration bays where it will remain for 14 to 21 days. This is where the high rate composting process takes place.”

The aeration hall is the size of four football fields or 14 hockey rinks and is the largest stainless steel building in North America. The material inside the hall will reach temperatures of over 131 degrees F (55 degrees C) for at least three days, helping kill potentially harmful bacteria and pathogens. Mobile augers are used to turn and introduce oxygen into the material. The turning system can also add additional water to the material as needed to help maintain the decomposition process.

After the compost has cured in the aeration hall for 14 to 21 days it is conveyed to the finishing building, which is a final screening process designed to separate out small non-biodegradable materials. About 30 to 35 percent of the material that goes through the compost facility is non-biodegradable and is eventually sent to the landfill. Following this screening process, the compost is sent to a 17 acre (7 hectare) outdoor curing site for a period of four to six months. Seasonal temperatures in this region can vary from 30 degrees F below zero (34 degrees C below zero) in the winter to 90 degrees F (32 degrees C) in the summer and play a big part in how long the compost needs to cure.

Taking the smell out of compost
“When you’re working with compost there’s a definite odor,” says Lapp. But we developed a system to manage the odor.”

According to Lapp, the facility uses the method of negative variation, which draws the hot, humid and odorized air down through the compost material rather than forcing it up. This process also helps introduce oxygen into the compost material. The air is captured in large ductwork below the building floor and is drawn outside using six blowers each powered by a 400 hp (298 kW) motor. Once outside, the air passes through a 4 foot (1.2 meter) thick biofilter made of wood chips so the odors are removed from the air emitted to the atmosphere. Air pressure in all the buildings is kept slightly negative so that odorous air does not escape the building enclosures.

New approach to curing
In the past the compost has been placed in windrows measuring 25 feet (8 meters) wide by 246 feet (75 meters long and up to 10 feet (3 meters) high over the 17-acre (7-hectare) site. The city has utilized the services of a contractor to turn the windrows using a straddle-type compost turner. This method has worked well for the compost facility, but the city of Edmonton was looking for a better way to cure the compost and eliminate the wasted space between the windrows.

“Since we used the windrow stacking method we probably lost a lot of heat from the piles in the winter, which extended our desired curing time,” says Lapp. “With the contractor, the turning frequency was about every two to three weeks. Sometimes it depended on the contractor’s schedule, because they also work with other facilities. With our new method we can better manage the compost turning time depending on the condition of the pile.”

Following a great deal of research and discussion, the city decided the continuous windrow stacking method made more sense than the traditional windrow method. This process uses one continuous windrow covering the whole processing area, with the absence of open spaces between the windrows. This process typically allows for three to four times more material to be placed in the same space as conventional windrow composting.

“The continuous windrow stacking (or mass bed) method eliminated the wasted space between the windrows,” says Lapp. “Now we are able to cure a higher volume of compost in the same space, which ultimately leads to better utilization of our overall facility.”

Moving to a continuous stacking method means the city needed to find another way to turn the stack of compost, as a straddle-type windrow turner would not work with the new method. The city had heard about a compost-turning unit that was designed to work with either traditional windrow or continuous windrow stacking methods. In fact, the city of Edmonton originally owned one of these machines. With a little research, the City’s Waste Management Branch discovered Vermeer Corporation acquired the compost-turner technology.

Vermeer acquired the elevating-face technology and made significant enhancements to the machines. In fact, the company had just introduced the new Vermeer CT1010TX compost turner when the City of Edmonton contacted thier local dealer, Vermeer Canada, Inc.

“We had some conversations with the local dealer and a representative from Vermeer flew up from Pella, Iowa, to meet with us,” says Lapp. “We were sold on the technology and put out a tender offer to various suppliers for a unit. Vermeer presented the best deal and we purchased the first production unit.”

Lapp believes moving to a continuous windrow stacking method and purchasing the Vermeer compost turner will benefit the compost facility not only by utilizing the space more effectively, but it will shorten the curing time.

“By owning our own machine, we now have better control of the turning frequency and are working towards turning the piles based on various factors, such as moisture and temperature,” he says. “Because of the size of the continuous windrow stack we believe we won’t lose as much moisture in the summer and we will retain more heat in the pile during the winter months.”

Once the compost has cured and has met the Canadian guidelines for compost quality and standards, the compost is screened to 3/8 (9.5 mm) or ¼ (6.35 mm) inch (one last time and is marketed primarily to farmers, landscapers and nurseries. The compost is also being used as an absorbent to help clean up oil spills, which is a unique market because of the nature of composts and the fact Edmonton is situated in the heart of Canada’s oil country. The oversized particles are used at the landfill as cover material. So far, Lapp is happy with the new curing method.

“It’s quite interesting to see the Vermeer compost turner running through the bed and see how much material it actually moves,” he says. “Our Vermeer dealer is only 20 minutes away and their technicians have been great in training and support since we purchased the unit. We continue to find new ways to enhance our state-of-the-art facility and we can’t wait to share this innovation with other compost facilities around the world.”

Sidebar
The Edmonton Composting Facility isn’t resting on its laurels. The facility recently introduced the Gore Composting Facility at its curing site. Wood chips are mixed with dewatered biosolids and the mix is placed in windrows approximately 150 feet (46 meters) long and covered with a breathable fabric. Aeration lines run under each windrow and force air up through the material.

The facility sources brush and limbs from residents, tree care companies and the parks and recreation department, as well as clean pallets and used lumber from contractors. Chipping and grinding this wood waste used to be contracted out, but the city of Edmonton recently purchased a Vermeer TG7000 tub grinder for this purpose.

It’s just another way to keep green and wood waste out of our landfill and better utilize the biosolids from our waste water treatment plants,” says Jim Lapp. “The final product is used as top dressing for sports fields and erosion control.”

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