 Harvest Power Energy Garden located near Vancouver, British Columbia. |
Where is the value in today’s anaerobic digestion (AD) market with low natural gas prices and no national alternative energy policy? Some good news is on the horizon: more than one driver may make AD feasible. Some of the most common reasons to build an urban digester are:
- Renewable energy revenue;
- Energy independence and smart grid energy generation;
- Compressed natural gas (CNG) vehicle fueling;
- Building heat and district heating;
- Odor control (capacity expansion for high-odor potential feedstocks);
- Fertilizer production and connection to urban agriculture;
- Mass/moisture reduction; and
- Clean-tech job creation.
The AD industry is growing in spite of the absence of a federal renewable energy or zero-waste policy. While AD is an expensive undertaking from a capital investment perspective, at three to six times more than composting on a per-ton of capacity basis for non- lagoon systems, the combination of high disposal costs and high energy costs is driving some projects off the drawing board. AD is gaining traction in locations where renewable energy markets are generous and reliable and tipping fees are relatively high. Currently, projects are either operating or under construction in these locations:
- Vancouver, Chilliwack and Surrey, British Columbia;
- London and Toronto, Ontario;
- San Jose, Sacramento and Monterey, Calif.; and
- Orlando, Fla.
A number of digester types are available. What type makes sense? It depends. If contamination rates are high, then high solids batch digestion or vertical plug flow digestion makes sense. If glass is a problem, continuously stirred tank reactors have an advantage. Several hybrid systems with a list of acronyms also are available: SEAD (shear-enhanced anaerobic digestion), UASB (upflow anaerobic sludge blanket), BIMA (biologically induced mixing arrangement) and AF (anaerobic filter). Some are better for water balance, high loading, low loading and fertilizer production (rather than composting).
Determining Scale
Another key question is: What scale makes sense in terms of tons per year? To address the lower and more affordable entry point to this technology, Impact Bioenergy, Shoreline,Wash., is offering three separate but complementary small-scale technologies for sale to organizations that want to produce renewable energy and create valuable, carbon-rich products locally while producing benefits globally. They are:
- Biomethane production via AD;
- Soil and heat production via composting; and
- Charcoal, syngas and heat production via gasification.
The primary beneficiaries will be communities of 5,000 to 50,000 people that want to avoid disposal and recycling services as well as utility and petroleum-based energy use. This includes campuses, small cities, large city demonstration projects, islands, zoos, parks, resorts, military bases, correctional facilities and similar organizations. The objective is to simultaneously reduce disposal costs and overall environmental footprint at a local level.
The state of the art for purpose-built urban AD facilities is to build regional urban facilities for food waste, soiled paper and possibly yard waste at a typical scale of 5,000 to 150,000 tons per year. Likewise, the state of the art for centralized composting is to build regional urban facilities for yard waste and food waste at a typical scale of 35,000 to 350,000 tons per year. Separable or isolated communities are interested in self-sufficiency and are not well-served by the organics recycling industry today.
Impact Bioenergy is specifically engineered and focused on 1,000 to 5,000 tons per year of food waste and paper products for digestion and digested food waste, grass clippings, leaves, woody materials and similar high-carbon organic materials for composting. Charcoal production is focused on 2,500 to 3,000 tons per year of screenings from compost production, wood chips, brush, branches and trees.
This is a different approach because it focuses on standard, off-the-shelf, low-cost prefabricated assemblies to drive down the scale and capital cost of renewable energy and biocarbon systems. This is an interesting strategy because it reduces cost in a low-impact way.
These systems extract heat, renewable natural gas or syngas to make electricity, heat or vehicle fuel from urban organic waste and convert the rest into compost, soil products and biocarbon. The engineering is based on off-the-shelf, pre-engineered, prefabricated systems that can be sold and installed quickly. Delivery and installation can be accomplished in about six months versus concept-to-startup development cycles that require two to four years today.
Developing a Strategy
Several beneficial-use strategies can be considered for the renewable energy produced. In the case of biomethane, the first consideration should be on-site offsets for heating (natural gas, propane, other fuels) and electricity consumption. The first order of business is to avoid buying energy. In some states, net metering up to 100 kilowatts (kW) is possible as well (where the utility is obligated to project up to 100 kW sent backward through the utility meter). The other popular topic right now is CNG and liquefied natural gas (LNG) vehicle fueling. These are the two primary strategies. There are others however, such as district heating, carbon-footprint reduction, and protection from energy-price fluctuation (volatility risk).
Once the energy strategy is in place, an owner/operator can consider beneficial use of the other outputs. AD does not consume the feedstock, so a finishing system is necessary. For solids, the most common pathways are composting, drying into fertilizer and land application. For liquids, the pathways are land application, wastewater treatment and evaporation. In most cases connection to the food supply will eventually be a success factor. An article in National Geographic titled “Urban Farming Is Growing a Green Future” recently stated this:
“By growing what we need near where we live, we decrease the ‘food miles’ associated with long-distance transportation. We also get the freshest produce money can buy and are encouraged to eat in season. Another benefit of urban farming is that it can add greenery to cities, reducing harmful runoff, increasing shading and countering the unpleasant heat island effect.”
The challenge for every project will be to select the strategy and scale to meet the waste disposal, energy and soil needs in a cost-effective way.
Each customer can potentially have multiple drivers that support a purchase—especially in urban and isolated locations. Economics and return on investment are stronger when disposal fees or energy rates are high. Interested parties should begin an evaluation process to confirm economic rate of return, sustainability, social and environmental expectations. For existing composters already in the business, the process is straightforward and quick, so moving to implementation can occur rapidly.
The author is president of Impact Bioenergy, Shoreline, Wash., and can be contacted at janallenorg@gmail.com.