Burning through waste, cooking up compost

Jeff Barber has worked at the Rapid City Solid Waste Division, in South Dakota, for 18 years and currently manages their yard waste and landfill operations. The division also provides residential collection of curbside recycling and operates an MRF. Barber's team produces three different types of compost, made from incoming MSW (municipal solid waste) or yard waste, including a 50/50 MSW/yard waste mix. The MSW and 50/50 composts are processed in a controlled environment, with the yard waste operation manually watering and turning the material to maintain an aerated, hygienically safe and finished product.

Annually, on average, the Rapid City landfill receives 18,000 tons of yard waste. As a regional facility, they accept and process waste from a large portion of southwestern South Dakota. Due to the limited number of composting facilities in the region, contractors in other counties, and even other states, also regularly rely on Rapid City to accept their material.

According to Barber, a significant amount of raw yard waste material is hauled in by landscape and mowing businesses in the surrounding region, and the facility sees a lot of pine trees and forest debris coming in from commercial fire remediation contractors who clean up around homes, trails and parkways. Once raw material is on the ground at Rapid City landfill, a meticulous process is used to convert it to a refined product that meets environmental standards and the end-use needs of consumers.

Essential elements
One of the key tasks at the Rapid City facility is grinding. Incoming carbon-rich, organic material is ground into something that can be processed and converted into saleable, high-quality compost. 

In his 18-year tenure at the landfill, Barber says four days of grinding in a workweek had been the routine. This was until they invested in a CBI 6800BT horizontal wood grinder for their application.

Now they grind one day a week, if that.

"What we grind now in a day used to take us three to four days," Barber says. "We used to grind four or five days a week with our other grinder. It's phenomenal how fast this thing runs material through it."

Logs, trees, branches and other materials are run through their 6800BT grinder and placed in windrows. Beyond watering and turning them, the facility heats the piles to a perfect temperature (140-150 degrees Fahrenheit) and lets it slow-cook for 14 days into a fresh-smelling, nutrient-rich product. 

"We try to get our compost to make the thirty to one carbon-nitrogen ratio," Barber explains. "If the carbon to nitrogen (C:N) ratio is too high in the compost pile, decomposition slows down. If the C:N ratio is too low, the pile takes on a foul odour. If you get it too hot you kill the good bacteria."

After the compost is held at ideal temperature for 14 days, a sample is brought to their lab to ensure it meets environmental standards.

Added benefits
The CBI 6800BT has been specifically designed for land clearing companies, mulch producers and yard waste processors, like the Rapid City Regional Landfill, who demand high-volume throughput and maximum reliability. 

According to CBI, the 6800BT processes more material at a lower cost per ton than other similar machines on the market, and is the first grinder that can grind whole trees as fast as a 30-inch chipper can chip. 

Barber confirms that he is impressed with the 6800BT's processing speed.

"Even when I help out our grinder operator, we can't keep the machine fed with two loaders and stay caught up with the machine," he says. "The efficiency frees up one man. When we're not grinding four or five days a week, it frees up a guy to run a windrow or a packer." 

Beyond cutting their grinding time down to a fourth of their previous benchmark, the 6800BT's reliability and maintenance-friendly design helps Barber's team get the most out of the day. Maintenance tasks that used to take an hour now take 15 to 20 minutes.

"It's a very simple machine and everybody was surprised with how open it is to work on it and where the hydraulics are located," Barber says. "Everything is easy to get at and easy to grease. You can open the throat up in it and change the teeth. You can change the whole set of teeth in about twenty minutes."

Barber emphasizes as well that they've had zero downtime with their new grinder. As a result, more green waste is recycled for its eco-friendly purpose as compost, and the pile at their facility stays at a manageable and safe size. He adds that due to the high production they've achieved with their 6800BT grinder, they've seen a secondary benefit.
"The grinder keeps our pile smaller so we don't have to worry about fire," Barber explains. "In years past we've had some major fires out here with our bulk pile getting too big. We're able to manage it well now."

He adds that they already now have a 50/50 compost product available to nearby residents for next spring's growing season. "We're providing a sustainable solution at a bargain price," he says, and is proud to add that Rapid City Solid Waste Division is now more focused than ever on safe, efficient and environmentally friendly waste services for the community. Plus, due in large part to the high productivity and efficiency of their 6800BT grinder, their increased diversion of yard waste is helping to significantly extend the useful life of the Rapid City landfill.

This article was the cover story and originally published in the November/December 2017 edition of Recycling Product News, Volume 25, Number 8.