Balancing Digester Diets

While energy-rich and an ideal substrate for digesters, organic waste can be loaded with contaminants.
By Katie Fletcher | September 02, 2014

Community digesters are built with the intention of creating sustainability within surrounding communities by diverting food scraps and other green waste from landfills and converting it into biogas to create useful commodities like heat, electricity, fertilizers and transportation fuel. Much like humans, community digesters require a balanced diet to maintain health and maximize biogas production.

AD is commonly used at wastewater treatment plants and dairy and swine farms across the nation. While these facilities use wastewater or manure as a feedstock, adding food and organic green waste to the substrate menu is trending. New digesters are taking in a whole new, eclectic assortment of feedstocks that necessitate a balance of nutrients for optimal biogas production.

“Essentially we are feeding a stomach, a very large one, but we are feeding a stomach nonetheless,” says Dean Foor, owner of JC-Biomethane, a biogas plant in Junction City, Oregon. “In very simple terms, we are just trying to feed it a balanced diet.”

A digester’s balanced diet is not so different than a human’s. Community digesters take source-separated organics. Like humans, digesters don’t want contaminants in their sustenance, which can and does happen in food waste streams due to human error in the source separation process. Common contaminants include plastic cutlery from cafeterias, glass, hard plastics and other nonbiodegradable components.

Certain practices, policies and regulations are in place to guide suppliers in making the source-separated organic material as devoid of contaminants as possible, but capacity for them to enter the feedstock stream remains. Hardware and other preprocessing techniques provide the solution, and these tactics are used at some facilities to depackage consumer products.

CleanWorld Partners, Harvest Power, Novi Energy and JC-Biomethane are just a few U.S. companies that are strategically located in areas that produce large quantities of organic waste.

On the Menu

“We can take almost 100 different types of feedstocks,” Bryan Heiss, plant manager of Novi Energy’s Fremont Community Digester in Fremont, Michigan.  FCD is a 100,000-ton-per-year, complete mix digester that sends 3 MW of electricity to the grid. A few of its suppliers include rejected batches of baby food from Gerber, high-strength effluent from Short’s Brewing Company, McDonalds’ apple dippers from Peterson Farms, and about 5 percent of the waste comes from manure. The digester can receive whole skids of packaged products.

CleanWorld’s Sacramento BioDigester can also accept fully packaged consumer products. The facility, which is scaling up from 25 to 100 tons per day by the end of the year, is located near the South Area Transfer Station, which Atlas Disposal, a Sacramento-based waste hauler, utilizes as a fueling station of renewable compressed natural gas (CNG) to deliver waste to CleanWorld’s facility. Campbell’s soup, Hyatt Hotel and HP Hood, the maker of products like Muscle Milk, have been assisted by the facility.
While the Sacramento digester may be getting its calcium from varying levels of muscle milk, JC-Biomethane, a 1.5 MW, 25,000-ton-per-year community digester feeds on fractions of ice cream from a local manufacturer. Sugar water from gummy bear production and waste from a biodiesel production facility are amongst other suppliers. The facility’s largest supplier, however, is the Portland City Metro, bringing in 450 tons per week of commercial food waste. 

About twice the size of JC-Biomethane is Harvest Power’s 50,000-ton food waste community digester near Orlando, Florida. Some waste from Disney World is brought to the facility, including the Ritz-Carlton and JW Marriott hotels at Grande Lakes Orlando.

Although it appears these digesters can consume anything, there are limits. “We take what we can get within guidelines,” says Steve Tourigny, senior vice president at CleanWorld. “We have a feedstock contract where we explicitly tell customers these are the types of things we absolutely want, these are the things we can tolerate and these are the things we prefer not to have in our feedstock—we work with them to improve the quality.”

State and federal policies can also impact what ends up at the facilities. One such example is the Metro Central Transfer Station, JC-Biomethane’s largest supplier, which is shifting to only accepting food waste. This policy eliminates loads containing regular and waxed cardboard by November. “We’re not designed to take those kind of materials and do anything with them,” Foor says. “Making amendments to their acceptability standard is changing things for the market over the next year.”

Bringing in the Trash

Wastes are transported through trucking companies, and many try to look at ecological ways to truck in the waste. JC-Biomethane shares trucks with Lane Forest Products, a compost facility located adjacent to the digester. Trucks transport Lane products north and are filled with food waste on the return trip south. 

CleanWorld’s Sacramento Biodigester is an example of a facility that converts biogas into RNG and CNG. This effort is part of CleanWorld’s closed-loop strategy fueling 10 trucks in Atlas’ fleet with CNG engines, about 500 diesel gallon equivalents per day. When the facility is 100 tons per day, the biogas produced can fuel up to 40 trucks.

Getting customers to seamlessly separate the waste picked up from these trucks is a challenge. “With source-separated organics, the intent is for them to be all organics, maybe a little bit of paper, cardboard, napkins, some disposable knives and forks, however, the reality is it’s only as good as the sorting that’s done,” says Tourigny. “Whenever we have a load, we actually dump it onto a pad and do a visual inspection, because it is not unusual to have a car battery or a bicycle that’s been thrown in the wrong bin.”

Preparing the Meal

The inevitability of imperfect waste streams has been addressed by development of preprocessing hardware and mechanisms. Although facilities can use a different piece of hardware to clean the waste streams, they all work in relatively the same way.

JC-Biomethane was the first to employ the Swiss-made Hybag Bioseperator in the U.S. The bioseparator is a horizontally orientated cylinder that consists of 26 blades attached down  a shaft in the middle, spinning at around 1,500 revolutions per minute. “Essentially what they do is pulverize the food waste, so anything that can lose its shape or reduce its size will do so,” Foor says. 

The pulverized material then gets pushed through a series of 5-milimeter screens. “Anything that won’t reduce its shape, like a stick or a fork or a piece of plastic, gets rejected,” Foor says. “It’s blown out by high-volume air that is blowing through the unit, pushing the light out and the heavy like a fork or knife that don’t get chopped up settle to the bottom and get carried out by a conveyor.”

Doda USA Inc., a subsidiary of Doda Snc in Italy, manufacturers another kind of bioseparator employed at CleanWorld Sacramento that depackages and removes contaminants. Trucks carrying the raw or packaged food waste and organic material dump it into a pit, and the fully automated system augurs it right into the bioseparator. “Anything that isn’t going into our tanks is taken to the local municipal urban recycling facility,” Saville says.

Scotts Equipment Turbo Separator at Harvest Power’s facility in Florida works in a similar way as the Doda. The waste is dumped into a receiving pit, where a piece of equipment called a clamshell scoops up batches of the food waste and deposits them into the Turbo Separator, which smashes the material with rotating paddles. The organics fall through an underlying screen, and the contaminants are pushed to a chute with a screw auger that transfers them to a receiving bin to be cleaned.

The Secret Recipe

Once the feedstock is contaminant free, the ingredients are ready to create the digester’s meal. Often, the material is pumped into a separate mix tank where material is mixed and held before being fed to the digester. Sometimes, material enters a hydrolysis tank where the waste slurry is not only mixed, but brought up to the required temperature, either mesophilic (95 to 104 degrees Fahrenheit) or thermophilic (131 to 140 degrees F), to begin the digestion process.

Each facility has a mix of feedstocks based on the ingredients at its disposal, and, in a sense, develops its own secret recipe to keep the AD process running smoothly. Monitoring of the process over multiple weeks can spur biogas producers to change the recipe of the blends, increasing fractions of various high-strength materials or lowering where needed.

After contaminant removal in Harvest Power Florida’s low solids AD process, “the mix tank is used to mix some of the fat, oils and grease, food waste and the other types of materials and blend them together so you don’t send slugs of one heavy material over another into the digesters,” says Brandon Moffatt, senior vice president of energy for Harvest Power. “It’s trying to have the right mix and always trying to keep that in balance, so we’re continuing to optimize our recipe and make sure we have balanced feeding to make sure the system is stable.”

JC-Biomethane’s waste is pumped into a homogenization tank. Foor says changes to the digester’s diet need to be made carefully. “When we are adding high-strength waste water, such as fats, oils and greases, we do it in a very calculated manner,” Foor says. “We are keeping a very small percent of our overall feed coming from those very energy-dense fractions, because they have lots of energy they can produce lots of gas, but they can also change up our biology quite simply.”

Foor compares the balance to a human’s diet. “It’s a little bit like eating too many potato chips. Eventually, your stomach starts telling you it’s a bad idea.”

CleanWorld collects and analyzes feedstock data to understand the nutrients it contains to give feedback to the monitoring software. “Every waste stream is analyzed and given an average profile of what it contains,” Tourigny says.

Novi Energy has a lab in-house. “We test everything daily as far as the digester, the health of the digester, the product we have coming in,” Heiss says. “There is some type of recipe that we try to keep it at—we have a mix of 30 different types of feed that we are given on a daily basis to try to get the maximum amount of gas out of the digester.”

Author: Katie Fletcher
Staff writer, Biomass Magazine