A Burning Answer for Manure

New system turns hog manure into dry fuel.

Transforming manure into energy is hardly a new idea. People have been burning cow chips and chicken dung for centuries.

Until recently, the best energy option for hog manure was capturing methane gas emissions from an anaerobic digester. But thanks to a project spearheaded by Envirogain, that's about to change. Dried hog manure can also serve as a good energy source. In fact, hog manure is as good a fuel as chicken manure mixed with wood chips, company officials proclaim.

Interestingly, producing dry fuel came almost as an afterthought for this company. Unlike the projects and research in the United States geared towards developing alternative energy sources, the Canadian-based company has taken a decidedly different course. With hydro-energy priced at $0.065/kilowatt/hour in Quebec, it is easy to understand why.

A Joint Effort

Envirogain, based in the outskirts of Quebec City, Canada partnered with F. Ménard Inc. hog farms, Agriculture and Agri-Food Canada, Processys Inc., the University of Sherbrooke and École polytechnique de Montreal, to produce a dry organic fertilizer from mixed sludge that had a real commercial value. It would also solve the province's ever-growing problem with manure disposal in areas with limited land base.

FEOS (Fabrication d'Engrais Organique Seche, translated, means “dried organic fertilizer manufacturing”) rises above other drying processes because it can be dried on site, requires minimum handling and relies mostly on recovered energy.

“This process solves many environmental problems,” says Rock Chabot, vice president of Envirogain, including manure management, waterway contamination (caused by the spread of excess manure or non-composted manure), unpleasant odors and soil nutrient depletion from intensive farming.

“It is also right on target for countries where the energy market is the driver, and where people are looking to make energy with biomass, at the lowest cost possible but with very high efficiency,” he says.

The Fertilizer Process

To make fertilizer, the FEOS system starts with mixed sludge obtained from yet another process developed by Envirogain, called Biofertile, which purifies the organic load of the effluent (see Figure 1).

This aerobic system first separates the manure by removing large, suspended matter from the liquid and produces solids with approximately 35% dry matter content. The remaining liquid is then treated in a bioreactor, in which bacteria feeding on nutrients create biomass. Further filtration of this biomass results in solids with 16-18% dry matter (DM). The filtered liquid is then fed through a biological filter sitting on plastic media.

The bacteria, thriving on the nutrient-rich water and air, virtually eliminate all odors. Once the liquid is decanted, it is ready for dispersal. Nitrogen and phosphorus levels are equivalent to 50-60 ppm, so the treated water can be used for irrigation purposes.

How FEOS Works

To ensure the dry organic fertilizer has a good commercial value, Envirogain targets maximum 13% water content, which is the threshold for safe storage. They also made sure its specifications met the requirements of the organic fertilizer markets.

Cost efficiency of the process was insured by capitalizing on the energy produced by the activated sludge processes and from the FEOS process to dry the mixture.

The three-part process starts by combining the solids with 35% DM with the sludges, yielding 16-18% DM together. The mixture is then fed through a bio-dryer at 158°F to produce a bio-stabilized product free of pathogens and odor.

In the final drying process, warm, dry air is recovered from the bio-drying process, then circulated between layers of product on an overlap conveyor to reduce the moisture by another 20%.

The end product is a dry, organic fertilizer (85-87% dry matter), ready for commercial use that can easily be shipped or stored. It can also be used as an efficient combustible with a calorific value in the range of 6,000-9,000 Btu/lb.

The FEOS process insures that the product retains all the nutrients, carbon and biological properties while odors and pathogens are destroyed. But, if the goal is to simply produce an efficient combustible, the FEOS bio-drying system can be adapted to work with an existing centrifuge process.

Transforming sludge into a dry, stable, organic fertilizer using FEOS instead of fossil energy is not only good for the environment, it can also provide a sizeable return on investment, says Chabot.

“This return might even increase if the facility would also process sludge or biosolids from other sources, like local municipal waste, with over 20% DM,” he says.

A recent study made by École polytechnique of Montreal, using Envirogain's FEOS, sized to treat 14,440 lb. of biomass extracted from 13,000 gal. (US, liquid)/day of fresh pig manure at an approximate cost of $400,000, states that the rate of return for the process was 14%, but it could reach 25% if the price of fertilizer increases as expected.

The same logic applies if the FEOS system is used to produce dry fuel. “The payback for producing methane with a biodigester is a minimum of 10 years,” explains Chabot. “You can cut that by half when producing dry fuel with hog manure using FEOS. And, the same plants now burning chicken or cow manure, can now burn hog manure just as efficiently.” Envirogain plans on making a foray into the United States this fall.

On-site drying reduces greenhouse gas (GHG) emissions since there is little or no transportation involved for the treatment of the sludge, and most of the energy source required to dry the fertilizer can be redirected from the Biofertile treatment station. There are no methane (CH₄) emissions since the sludge is treated shortly after it is recovered, and there is no need for storage tanks.

Nitrogen oxide (N₂O) and CH₄ emissions, which are usually released when disposing sludge, become minimal. Since the dry organic fertilizer is reintroduced to soils within the same region, the need to haul nutrients from greater distances is avoided.

Further reductions of GHG's can be achieved if each FEOS plant also processes waste from local industrial, agri-food and municipal wastewater treatment stations, which have a minimum organic load concentration of 41.8 lb./gal. SS (suspended solids).

The GHG assessment of the Biofertile process, done by Natural Resources Canada, shows that GHG emissions are reduced by 1,779 tons of carbon dioxide equivalent (CO_2eq)/year for a treatment capacity of 21,662 gal. of hog manure/day. The corresponding reduced emission with the basic 13,000 gal./day Biofertile station was then evaluated at 1,085 tons CO_2eq/ year.

The FEOS for this corresponding Biofertile unit was also evaluated to reduce the GHG emissions at 1,192 tons of CO_2eq /year, for a total of over 2,205 tons of CO_2eq/year when Biofertile plus FEOS are installed.

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