Although humans have been pondering the meaning of life since the beginning of time, municipal managers generally agree that the meaning of compost is a lot simpler: waste diversion. Numerous states have banned yardwaste from landfills, and many are looking to reduce greenhouse gases and meet air-quality goals. Approximately 3,500 US cities and counties operate greenwaste composting facilities, predominantly turning their materials into a soil amendment for the local community’s gardening needs.
But the opportunities for greenwaste compost to become something more than a backyard gardener’s tool are burgeoning. Ron Alexander, of R. Alexander Associates Inc. in Apex, NC, a consultant who specializes in developing markets and distribution avenues for compost programs, believes that now is the perfect time to expand compost’s reach.
“Because of global climate change,” he says, “there are more and more opportunities to use compost to bioengineer soils. We’re experiencing more and more drought stress in certain areas like the West and Southeast, and bigger storm events as well. Compost can be used to improve soils that currently hold too little water or that cannot absorb it fast enough—it can help soil hold and percolate more water, preventing erosion and helping to manage excess stormwater.”
Compost can be applied on farms, ballfields, and public parks, and as mulch around trees. It can be used in rain gardens to filter stormwater runoff, to control sediment at construction sites, to facilitate plant regrowth, prevent erosion, and to improve water quality. Greenwaste raw material can be used for harvesting fuel when combined with foodwaste, with the remaining digestate composted at the end. Studies have shown compost to be effective in remediating hydrocarbon pollution, and a new technology that collects steam vapor from compost to create heat and hot water is currently in agricultural use but could be used for municipal applications as well.
What’s the key to expanding the market and thus the range of end uses? Dan Noble, executive director of the Association of Compost Producers in California, says that municipal solid waste managers often still see compost as merely a disposal alternative. They need to see it as end users do: as a product.
Municipal greenwaste compost is often plagued with contamination issues, he explains, which is the result of residents throwing trash and other items in their greenwaste bins, setting up a hierarchy that sometimes prefers “clean green” compost from private landscaping companies. If more municipal managers begin valuing compost as the black gold commodity that it is—a first use rather than an end use, perhaps—the greater the success and range of markets and applications they will eventually find.
The Facts of Compost
What anyone already using compost knows is this: The black crumbly humus created from the controlled decomposition of organic matter by microorganisms has some amazing properties. Through its ability to improve the properties of soil physically, chemically, and biologically, its uses can be versatile:
- Compost can enhance the physical structure of soil, reducing bulk density and porosity and increase its gas and water permeability, thus making it ideal for plantings and for reducing erosion and preventing runoff.
- Compost can provide drought resistance.
- Compost can modify soil’s pH.
- Compost can help soil retain nutrients as well as provide additional nutrients and soil biota.
- Compost can suppress plant disease.
- Compost can bind contaminants such as heavy metals, hydrocarbons, and pesticides, thus reducing their leachability into waterways and absorption by plant matter.
The two basic methods of producing compost are aerobic (occurring in the presence of oxygen) and anaerobic (occurring in the absence of oxygen). According to Ron Alexander, aerobic techniques are by far the number-one method for making yardwaste compost in North America, either with outdoor windrows (rows of long piles) or aerated static piles. (A third aerobic technique, in-vessel composting, is generally too expensive to process yardwaste and is often used only for food or biosolids composting.)
Turned windrows are aerated naturally or mechanically. The ideal pile height, which is between 4 and 8 feet, allows for a pile large enough to generate sufficient heat and maintain temperatures, yet small enough to allow oxygen to flow to the windrow’s core. The ideal pile width is between 14 and 16 feet.
In aerated static pile composting, waste is mixed together in one large pile instead of rows and is not turned. To aerate the pile, layers of loosely piled bulking agents (e.g., wood chips) are added so that air can pass from the bottom to the top of the pile. The piles also can be placed over a network of pipes that deliver air into or draw air out of the pile.
Anaerobic digestion technologies, on the other hand, are still developing in the United States, and yardwaste digestion in this manner is not currently operational here. Low-solids anaerobic digestion is still generally used for manure, biosolids, and/or wastewater solids. The only large-scale high-solids digester on the horizon in North America is in Canada and will process yard- and foodwaste, collecting biofuel as renewable energy and then composting the remains.
Who’s Doing What and How They’re Doing It
Featured here are the compost life stories of five municipal programs. Highlighting the end uses, from more traditional to high tech, these programs demonstrate that municipal operations throughout North America are bringing piles of leaves, brush, and grass to life as compost and the various ways they are giving it purpose.
Landscaping and Gardening Soil Amendment
“Composting greenwaste out West is a challenge,” says Mary Helen Giustizia, solid waste services manager of Tempe, AZ (pop. 167,000), “because of the variety of non-native vegetation. Not everything can be included.”
She’s referring to the city’s prolific eucalyptus plants and palm trees, which are toxic, as well as oleanders, one of the most poisonous plants in the world. These are not raw materials that can be made into soil amendment for vegetable gardeners.
Nevertheless, in 2010, Tempe decided to add separated greenwaste to the once-a-month collection of bulk and uncontained trash in a single neighborhood as a pilot project, after a massive education campaign about what can and cannot be included. Residents bag their grass trimmings and stack woody material on the grounds of their alleys for pickup. But instead of going to the landfill, the materials are taken to Singh Farms, near Scottsdale, where the city is learning how to make compost.
Singh Farms has been in business since the 1940s and has been at its present location, on the lands of the Salt River Pima-Maricopa Indian community, for about 30 years. Kenneth Singh took a 20-acre plot of dead land and brought it back to life—with compost—to grow gourmet vegetables. He also collects yard- and foodwaste from restaurants, hotels, colleges, and universities—and the City of Tempe’s curbside collection program—and returns compost to each generator.
In addition to promoting sustainability in an earth-groovy kind of way—“My definition of sustainability is to follow the path of nature; nature does not take things and run them into a landfill and bury them”—Singh is an advocate for microbes. “You must have the proper carbon-nitrogen ratio if you want to create the right heat,” he explains. “The heat is created from the microbes.”
He explains that three different types of bacteria—psychrophiles, mesophiles, and thermophiles—show up at different temperatures. “I don’t let my piles get any hotter than 158°, 159°, 160° max, because over 161° you are actually killing the microbes, which defeats your purpose.”
In the first phase of Tempe’s program, Singh was doing the initial grinding. As a second phase, he decided to ask Tempe to handle the grinding itself, which is now done onsite as collection rounds are made, so that any contaminants can be weeded out. The last phase, now that Singh has trained Tempe staff and given them the knowledge they need, will be for the city to find its own site and be independent.
“I’m an old man, and I did this to give things back,” he says. “I didn’t do this to create an industry.”
Singh uses front-end loaders for loading and turning his windrow piles, along with a Vermeer horizontal grinder, and a Powerscreen trommel screener, grinding to 4 inches to begin the process, to keep some air spaces in the piles, and after two or three months to 2 inches for finishing. Then he uses a respirometer to determine if the compost is horticultural quality or field quality, based on the carbon dioxide and oxygen concentrations.
Giustizia explains that the city is planning for the long term, assessing sites for its own compost operations. It’s gearing up for equipment purchases to serve the entire population but will be phasing in the program one community at a time. Overall, the city expects it will save $300,000 per year in landfill charges by composting about 12,600 tons of greenwaste.
“Composting is much more effective than a one-way street to nowhere,” Singh says.
Agricultural Soil Amendment
Located in the county ranking third in the United States for agricultural production, Bakersfield, CA, makes compost that feeds the Golden State’s vineyards, orange groves, and avocado trees. Farmers use compost to hold in water, increase soil’s porosity and permeability, getting more air to the roots of tender plants, and saving money on water needs.
Kevin Barnes, Bakersfield’s solid waste director, explains that 20 years ago, when farmers were more accustomed to spraying on liquid from a chemical tank, municipalities making compost had to convince them of compost’s value. “We gave them lots and lots of free samples, letting them try it out for two or three crop rotations to see if it made a difference for them. Slowly but surely, that built an awareness in the ag industry around here that compost is a soil-structure-building proposition.”
Nowadays, Bakersfield’s compost comes from the household yardwaste of 550,000 people within the city limits and throughout Kern County. The Bakersfield compost facility takes in 100,000 tons of yardwaste each year—60,000 tons from curbside collection and 40,000 from self-haul from landscapers—and produces about 40,000 tons of finished compost on 60 acres.
The process starts with false-bottom household bins—a grill on a hinge—to prevent grass from sticking to the container and to allow about 18% of the moisture to seep out before collection, preventing the materials from being delivered wet and sticky. Once dumped out, the materials are scooped up with a front-end loader and dumped into a McCloskey 737 electric trommel screener fitted with 2-inch screens. Rosebushes, limbs, and other larger materials are ground to 4 inches and then join the rest of the materials. An eight-man crew from a local homeless shelter—part of a contract to provide low-cost labor and high-value job training—work at the Morbark picking station to sort such rejected items as hoses and bricks.
Then the greenwaste is loaded into live-floor trucks and deposited into windrows that are 20 feet wide at the base and 8 feet wide in the middle. Bakersfield uses a Scarab turner to turn the material five times in the first 15 days, per state regulation, to keep the temperature at 131°F. Then, the compost is turned once a week for eight to 10 weeks, with regular watering keeping the moisture at 45% to 50%.
When the compost is done, it’s screened to one half-inch using another McCloskey 737 trommel screener with five electric conveyors instead of a diesel loader. The overs go through an Airlift Separator to suck off the bits of plastic that often cling to them, then are trucked over to the self-haul composting windrows where private contractors deposit grass for compost.
“Bakersfield makes two kinds of compost,” Barnes explains. “The very clean self-haul compost, which gets sold for landscaping, and the curbside collection compost, which generally has more debris,” such as pieces of glass. Luckily, since the city is surrounded by endless rolling fields of agriculture, and there are plenty of crops that can tolerate the foreign material, such as trees and vines, Bakersfield maintains a steady market. The compost is sold to farms in the Central Valley through a broker.
Roadway Vegetation and Erosion Control
The Blackland Prairie, like many areas of Texas, is made up of nearly impenetrable black clay soils and rock. The soil’s not lacking in nutrient value, but it’s packed so tight that no oxygen can get through, so plants can’t thrive.
The Texas Department of Transportation (TxDOT) used to be one of many entities that struggled with roadside erosion. It needed a way to encourage vegetation. It needed something that would help plants take root to keep the soil from washing away.
At the same time, in what was once an unrelated matter, the Lone Star State generates about 4 million tons of yardwaste per year that once filled up landfills.
To bring these concerns together, TxDOT worked with the Texas Commission on Environmental Quality, the Texas Transportation Institute, and the compost industry to develop a specification for compost, back in 1995. This convergence created a market for yardwaste throughout the state, gave TxDOT the materials it needs, and made the state a national leader in compost marketing and end use.
The city of Plano is one of many municipalities selling yardwaste compost to contractors working for TxDOT. Sherrian Jones, the compost sales and marketing manager for Texas Pure Products, which is owned and operated by the city of Plano, explains that she sells anywhere from 50 cubic yards to upwards of 10,000 cubic yards for roadway improvements. She’s sold compost for road projects in dozens of other cities and towns throughout the state.
Since 1992, the city of Plano has partnered with four other member cities (Richardson, Allen, McKinney, and Frisco, with a total population of roughly 700,000) in an inter-local agreement to create compost and divert materials from the landfill. All yardwaste materials are taken to the North Texas Municipal Water District facility, the “grind site,” for first-stage processing. From there it is trucked to the regional composting facility at the landfill, where it is windrowed using a Backhus windrow turner and a McCluskey 733 screen. After six months in windrows, the compost is screened to one-half-inch for garden and landscape beds and one-quarter-inch for topdressing for lawns.
From 100,000 tons per year collected from the member cities and local landscapers, Plano sells about 75,000 cubic yards of finished compost. Each member city earns back credits, based on the amount of yardwaste they contributed, for the amount of compost they can obtain. Jones sells it not only to TxDOT contractors but to local landscapers, garden designers, and homeowners.
“Composting is part of our environmental sustainability program,” Jones says about why Plano composts. “We don’t want to put something in a landfill that would benefit our citizens and community. To waste this beautiful project by sending it to a landfill would be a crime.”
“Our clay is impenetrable,” she says, “but compost makes everything better. You can grow anything with it. You want a garden in Texas? You need to use compost. It’s almost too simple.”
In fact, she says, due to compost’s wide reach, “compost is building a garden throughout the state.”
Plano’s compost is not only certified by the US Composting Council, but it has earned a listing with the Organic Materials Review Institute, which certifies the product for use on organic farms.
Sediment Control, Wetland Construction, and More
The way Bill Malach, director of utilities for the city of St. Peters, MO, enumerates all the ways the city utilizes its own yardwaste compost makes compost sound like a municipal magic bullet: wetland construction, erosion and sediment control, top dressing, rain gardens (bioretention ponds), revegetation for levee and road construction projects, land reclamation, and more. “The city uses 40% to 50% of what we make for city property or city projects, and the rest is sold to contractors or individuals,” he says.
The city’s been making and using its own compost at its Earth Centre since 1991, when Missouri banned yardwaste from landfills. While it started with static windrows, the city found it had to time the turning of the windrows based on the wind direction to avoid wafting odors over to a nearby shopping area. In 2010, the city made the switch to aerated static piles, using a biofilter to control odors. The cost of the new system, which combines yardwaste with caked biosolids from the city’s wastewater treatment plant, is offset by not having to apply the liquids on agricultural lands any longer, which was also odorous, and expensive.
The city provides curbside pickup and allows residents and contractors to drop off, collectively amassing about 40,000 cubic yards of greenwaste per year, which is stockpiled on a 12-acre site. The ground yardwaste is combined with 10,000 cubic yards of 15% de-watered biosolids in a Rotomix compost mixer, the materials are measured by weight to a carbon: nitrogen ratio of 30:1, porosity of 2.9%, and a moisture content of 38%. The mixed materials are placed into eight Engineered Compost System (ECS) computerized aerated static pile composting bunkers.
Positive air or negative air is controlled by pressure sensors in the ductwork and four temperature sensors located in each bunker. These sensors send data back to a computer that controls the supply and exhaust air fan speed to ensure proper temperature and duration to meet EPA regulations, with all the data recorded electronically every hour into an Excel spreadsheet. The compost is processed at 131°F or greater for three days or longer and 120°F for 15 days or longer, then moved to an open bunker for an additional 10 to 15 days of curing. After a total of 25 to 30 days, the compost is removed and placed in a stockpile for six to nine months and then screened to one-half-inch with a Wildcat 516 trommel screener. Twenty-thousand cubic yards of grade-A finished compost results, which is certified by the US Composting Council. It makes its way to residents and about 10 or 15 local contractors.
One of those contractors is Soil-tek, of Lake St. Louis, MO, which uses it as part of its erosion and sediment control and stormwater pollution prevention services. The company uses compost-overs, which are large, irregular sized pieces of yard waste, for erosion control on construction sites, as well as finished screened compost as filtration material.
Soil-tek uses Filtrexx FilterSoxx in place of silt fence, as perimeter boundaries, as ditch checks, for inlet protection, around concrete washouts, and to de-water construction sites. These systems can also be used on slopes to de-power flowing water. The company fills Filtrexx’s proprietary mesh sacks with compost using a hydraulically powered auger, the Filtrexx FX machine. It then applies these compost-filled units to 50 or 60 sites a year at the request of municipalities doing sewer work, private companies working on construction, the Missouri Department of Transportation’s roadside projects, and other clients interested in sediment control—adding up to 200,000 or 250,000 feet of compost FilterSoxx per year.
“This is the stuff your grandma used,” Rob Caruthers, president of Soil-tek explains, reminding people that compost is not a high-tech, dangerous, or new material. “We as a society got away from using compost in the 1960s when we decided we could do everything we wanted with petrochemicals. Now we’re back to being green.”
What’s coming to the Metro Vancouver, BC, area is, some would say, revolutionary: an anaerobic yard- and foodwaste digester, known as a high-solids anaerobic digestion (HSAD) facility, that will produce up to 2 MW of power and heat per year.
Twenty-four municipalities that are part of Metro Vancouver, a regional planning commission, deliver their yard- and foodwaste to the Richmond, BC–based Fraser Richmond Soil & Fibre plant, a subsidiary of Harvest Power of Waltham, MA. When the digester opens in 2012, the plant will collect biogas to generate electricity and heat.
Fraser Richmond Soil & Fibre, selected from 23 bidders to operate the digester, currently operates a covered aerated static pile (CASP) facility on the site and sells its compost and compost-based soil blends to local landscapers, farmers, municipal parks and maintenance departments, golf courses, and homeowners. Digestate remaining after energy extraction in the new digester will be composted in the piles as well.
Many of the cities, corporations, townships, and districts have been collecting yard trimmings curbside from single-family homes since at least 1989, and the vast majority of them have been bringing materials to this site for composting since it opened in 1993. Some operate additional facilities as well, such as the city of Vancouver, which offers static-pile windrow composting of drop-off yard waste.
In the last few years, seven of these communities began co-collecting food scraps and delivering to the facility as part of their yardwaste collection routes, and others have plans to do so as well. Currently, Metro Vancouver contracts with Fraser Richmond to process up to 50,000 metric tons (55,000 tons) of residential organic waste per year.
According to Andrew Marr, a senior engineer with Metro Vancouver, municipalities that are close enough to haul directly will pay less than half the cost per metric ton than disposing of it. Suzanne Bycraft, manager of fleet and environmental programs in the city of Richmond, which collects 10,000 tons per day curbside, is a case in point—she says the city is saving $550,000 per year on tipping fees. Those that have to transfer and transport will pay roughly 60% of the cost per metric ton overall compared to disposal at a landfill.
This type of anaerobic digestor can process scoopable feedstocks, rather than slurries, that do not have to be chopped or ground and do not require water. The first step of the process is to load collected materials—from pizza crusts to twigs—into hydrolysis percolators so that the organic matter can be biochemically degraded into organic acids. During a two-week period, most of the acids drain into tanks. A portion of the acids are then pumped into the methane digester, which contains bacterial cultures that consume the organic content of the acids and produce biogas, which is pumped to an energy production center. Whatever’s left in the percolators is transferred to the covered aerated static piles to provide moisture for composting.
At the compost pile, curbside-collected materials are piled as high as 25 feet. When about 2,000 cubic yards have been collected, a 6- to 12-inch organic cover layer is applied to prevent odors. A network of perforated pipes and a blower pulling air through the pile maintain aerobic conditions. The compost generally takes about eight weeks to cook and is then screened for contaminants and cured in windrow-like piles.
“Fraser Richmond’s compost is the number-one most popular thing at special events,” Bycraft says of the end product, which occasionally takes the form of a pile of free compost donated to communities.
The whole operation comes out of Metro Vancouver’s Zero Waste Challenge and a goal to recycle 70% of the region’s waste by 2015 and 80% by 2020—up from 55% now. Some of the individual communities also have their own goals, such as the city of Vancouver’s plans to become the “greenest” city in the world by 2020. “This initiative is definitely pushing our programs in that direction,” says Bob McLennan, an engineer in the solid waste branch.
Wolf Material Handling Systems at Elk River, MN, offers complete engineered material handling systems and equipment for the biomass industry, supplying systems for chipping, screening, storing, reclaiming, re-chipping, and feeding digesters. Wolf has expertise in handling a wide variety of feedstocks, including woodwaste, cotton stalks, almond hulls, orchard pruning, grape vines, and construction debris. Wolf understands the unique challenges required to handle biomass and takes complete design and supply responsibility of the material handling system. The company’s custom systems are tailored to meet the customer’s unique feedstock and production requirements.
Wolf provided the material handling systems for three of the largest solid-waste-fueled power plants in the US, developing a proprietary system for processing these materials for use in this demanding environment.
