BioCycle August 2010, Vol. 51, No. 8, p. 46
Several commercial-scale AD projects utilizing “batch-mode” dry digestion for residential and commercial SSO streams have been announced in North America, prompting this technology overview series. Part I
ANAEROBIC digestion is an age-old technology, but its use to process source separated organics (SSO) from the municipal solid waste stream (MSW) is relatively new, especially in North America. Aside from two installations in Ontario that use a wet digestion technology, no one has proceeded with an anaerobic digester (AD) system specifically designed to process separated MSW – versus an agricultural or municipal wastewater digester that has the capacity to receive SSO along with biosolids, manures and other “anchor” feedstocks.
That landscape is about to change. Recently, several new commercial-scale AD projects, processing 30,000 to 50,000 tons/year of SSO, have been announced. These projects will employ so-called “dry” AD technologies, or “high solids anaerobic digestion” (HSAD). Several wet digestion projects that will receive SSO are moving forward as well, including a Quasar Energy Group facility in Columbus, Ohio, and a facility on a dairy farm in Rutland, Massachusetts (with plans for expansion to six or more farms) that will codigest liquid dairy manure with liquefied SSO from the greater Boston area.
The engineering community more precisely utilizes the terms “low” and “high” solids for “wet” and “dry” AD systems. Generally, low solids means the organic matter is in a sufficient state of liquefaction to pump it as a slurry, such as more than 70 percent moisture content (less than 30% solids content), whereas high solids is typically comparable to the moisture content utilized for aerobic composting, around 55 to 70 percent (30-45% solids content).
Introduction of dry digestion systems in North America, and increasing use of wet digestion technologies to process SSO streams, prompted BioCycle to initiate this “Primer” article series. Part I is on the application of “batch-mode” high solids anaerobic digestion to SSO. Part II, in the next issue, will focus on wet digestion systems; Part III, in October, will cover continuous feed HSAD technologies.
SSO STREAM AND HSAD
SSO programs in the U.S. typically separate food waste, soiled paper/cardboard and yard trimmings from residential and commercial sources at the point of generation. Some residential programs include pet waste, or commingle the organics with yard trimmings. Implicit in all scenarios is a separate collection system of containers at the point of generation, such as the kitchen counter in the home, and the restaurant or supermarket’s food preparation area. Those materials are then transferred by employees to Toters or dedicated dumpsters at the curbside or the back of the business where the materials are picked up as part of a dedicated organic waste route and delivered to aerobic composting facilities.
One reason that batch-mode HSAD systems are gaining acceptance is that they are well-suited to the scale of the SSO programs being implemented. These programs tend to be relatively small in total tons of materials. Although there are numerous HSAD projects in some form of consideration in North America, BioCycle has identified four projects in development:
• University of Wisconsin, Oshkosh. 6,000 tons/year of food waste from campus, plus yard trimmings from community. BIOFerm Energy Systems.
• Fraser-Richmond Soil & Fibre, Richmond, British Columbia. 30,000 tons/year of food waste from Vancouver area, plus yard trimmings. Harvest Power, technology not announced.
• Cedar Grove Composting, Everett, Washington. 50,000 tons/year of food waste from Seattle area, plus yard trimmings. BIOFerm Energy Systems
• Zero Waste Energy Development Company (ZWED), San Jose, California. 50,0000 tons/year of food waste from City of San Jose and other regional generators, plus yard trimmings and the organic fraction remaining after processing recyclables and garbage at the GreenWaste Recovery, Inc. MRF in San Jose. ZWED, using Kompoferm technology.
Use of HSAD to process SSO is more widespread in Europe. As of 2009 there were 43 dry-batch AD facilities operating in Europe, with approximately 10 new plants coming on-line each year since 2005. Not all of those facilities process SSO; some operate on just yard trimmings, and others on only energy crops. In addition, as Figure 1 suggests, the individual systems are getting larger. For example, the average dry-batch system installed in 2009 was capable of digesting 24,600 tons/year of feedstocks.
The reasons that Europe has led North America in application of AD to SSO include a series of European Union landfill directives (requiring processing of organics before they can be disposed), higher electricity costs, and substantial subsidies. As with wind and solar power, it appears that AD is continuing its growth in Europe, and bringing with it the next generation of AD plants.
Although some find it frustrating that the U.S. and Canada lag behind Europe (and Asia for that matter), North America will hopefully be the beneficiary of a second, or third, generation of AD facilities. In all fairness to North American organic waste managers, one of the major impediments to widespread use of HSAD has been a dearth of SSO programs.
DRY DIGESTION TECHNOLOGIES
As noted in the introduction, dry technologies can be broken down into those that operate as continuous flow digesters and those that operate in dry-batch mode. Continuous flow digesters are common in Europe for slurries that have moisture contents in the 75 to 85 percent range.
There are some significant differences in these two approaches to high solids AD. The following factors should be considered when selecting a technology:
Preprocessing: Continuous systems typically preprocess feedstocks to less than 2-inch particles. High solids batch facilities may grind yard trimmings, but require mixing that material with food waste and some digestate. Usually there is no size reduction of food waste. As the batch systems essentially operate in static-pile configuration, adequate structural porosity is needed both for percolate penetration (see below) and for biogas extraction.
Inorganic Contamination: In continuous systems, to prevent clogging of pumps, the preprocessing stage often includes removal of inorganic materials such as ferrous metals, plastic, glass and stones. Batch systems, since they are not pumping the materials, are more tolerant of such contaminants, however, they can be a detriment to the quality of the digestate/compost. In addition, large pieces can impede the flow of percolate through the static pile, and very fine particles can clog percolate spray nozzles.
Effluent Discharges: Continuous systems operating at 20 percent solids content will have a greater quantity of liquid effluent than batch systems operating at higher solids content such as 35 percent.
Biogas Safety: Both systems require storage of biogas, piping systems, and monitoring and alarm systems.
In a batch process, the digester is loaded to approximately three-quarters of the fermenter height with fresh organic matter – typically mixed with a front-end loader – and closed with a gas- and liquid-tight door. The digester remains closed until the end of the desired retention time. It is then flushed with fresh air to reduce explosive gas concentrations, emptied and filled with new material – often a mixture of partially digested feedstocks that were just removed, and fresh, undigested material. The partially digested material acts a seed to restart the digestion process. Most systems operate at mesophilic temperature ranges, but there are higher temperature thermophilic systems for both continuous and batch processing.
To initiate gas generation, a liquid “percolate” is sprayed into the top of the digester to gravity drain through the digesting material. The percolate is typically preheated, and since it has already been through an active digester it contains requisite anaerobic microorganisms for production of methane. Once a digester has been reseeded and percolate has been pumped into the digester, gas production begins almost immediately. Over the retention time of the digester, the percolate is repeatedly drained and sprayed back into the digester.
The digester also can be flooded with percolate to rearrange the organic material inside the digester. This ensures good contact between the microorganisms and the digestible organic matter. Computer controls monitor each reactor as to gas generation, methane concentration, percolate water level, pressure and temperature, as well as the integrity of the gas seals.
Dry-batch digestion has several advantages:
• Because dry material is stackable, the digester can be filled with a front-end loader, or a stacking conveyor.
• When material is finished digesting, it still has a relatively high solids content and is typically removed with front-end loaders. This digestate can be mixed with yard trimmings or older digestate and composted without having to remove liquid from it. It may be necessary to add fresh, higher volatile solids material to the digestate to ensure that material meets required pathogen and vector attraction reduction requirements.
• Pumping liquid percolate with its lower solids content is generally easier than pumping higher solids content slurries of food waste, which is the case in most continuous digestion systems. Pumping a heavy slurry requires significant horsepower, and the pipes are susceptible to clogging by inorganic contaminants.
• To reduce the impact of reduced biogas production that occurs when the digester is refilled, multiple digester “cells” can be employed so that several are operating at capacity at any one time.
• If one digester cell has an upset condition such that gas production is significantly reduced, or the methane content is too low, fresh percolate from the other properly functioning cells can be used to restart a “stuck” cell. In an extreme case, the percolate can be completely isolated and removed from circulation, and the cell can be emptied and refilled with the proper mixture for gas generation.
Dry-batch technologies currently being marketed in North America include Bekon, BIOFerm, Gicon, Kompoferm and Solum.
MASS NATURAL FEASIBILITY STUDY
Mass Natural Fertilizer Co., Inc. is a 25-acre, family-owned, commercial composting facility located in Westminster, Massachusetts on a 240-acre farm. It was established in 1987, to recycle manure from the farm’s chicken egg business, which was eventually closed, leaving the composting operation as the primary business. When Mass Natural started operating as a composting facility in 1987 it was the first large-scale organics recycling facility permitted by the Commonwealth of Massachusetts.
The site consists of a series of composting pads and a detention basin for runoff control. Composting of industrial food waste, paper mill sludge and animal manures is accomplished by static pile and turned windrows. Mass Natural recently purchased two in-vessel composting systems that have expanded the range of materials composted. It also has a vermicomposting operation to produce vermicompost, as well as red wiggler worms.
With funding from Massachusetts Technology Collaborative, Mass Natural retained Steve Brunner, an anaerobic digestion engineer with the Brendle Group in Fort Collins, Colorado, to conduct portions of a feasibility study of anaerobic digestion of industrial food processing waste. (go to www.masstech.org and search on report, “Mass Natural Fertilizer Co., Inc. Feasibility Study Of Anaerobic Digestion Of Industrial Organic Waste Using Dry Fermentation Technology,” January 2010). The author of this article also worked on the feasibility study, particularly the waste stream assessment and regulatory requirements.
As a permitted solid waste composting facility, Mass Natural can receive SSO from commercial and residential sources. However, because only a few SSO collection programs are operating in the central Massachusetts and southern New Hampshire region, and Mass Natural already processes a variety of industrial food wastes, a decision was made to conduct the feasibility study based on processing food manufacturing by-products and yard trimmings in a 20,000 tons/year facility. The study identified numerous potential sources of industrial food waste in Mass Natural’s service area.
Initially, wet digestion technologies were investigated. For Mass Natural, the moisture content of the digester effluent would be too high to compost without a dewatering step. Since Mass Natural does not have municipal sewer service, storage and removal of the excess moisture from the site would have been prohibitively expensive. Although land application of liquid digestate is an option, it too would have required construction of very large storage tanks, and then contracting for land application on land owned by other parties. Mass Natural also anticipated that permitting requirements for a wet AD facility would have been more onerous if millions of gallons of liquid digestate per year had to be stored and land applied. Therefore, the project focused on dry-batch digestion options.
Mass Natural Fertilizer is a nearly ideal site for an anaerobic digester for multiple reasons: Already permitted to compost organics; local electrical distribution line is capable of receiving considerable current from an on-site engine generator; existing equipment (front-end loader, rotary drum aerobic compost vessel, agitated bay compost system, deck screens, trucks, etc.) and personnel to handle large volumes of organic material; many years of experience with, and understanding of, the local organics market; and can compost the dry fraction of the digested effluent to add value to its current product.
Proposals were solicited from two German companies, Bekon and BIOFerm. Each has more than a dozen operating digesters in Europe and is constructing more each year. Both technologies share a nearly identical process, similar to that described above: 1) Digestible, organic matter is loaded into a long, narrow gas-tight building; 2) Once the building is sealed, material is saturated with percolate that contains anaerobic microorganisms from an active digester; 3) Percolate is periodically drained and resprayed into the digester over a period of approximately 28 days; 4) Biogas is collected and combusted in an engine genset to produce electricity; 5) Waste heat from the engine is used to maintain optimum temperature in the digesters; 6) After prescribed retention time, digester is opened, organic matter is removed, and one-third to one-half is mixed with fresh organic material and loaded back inside the digester; and 7) Remaining digested material is actively composted.
Reciprocating engine gensets that act as combined heat and power (CHP) units are incorporated into both companies’ design. Heat is captured from the water loop that circulates through the engine jacket to keep it cool. Less than 10 percent of the available jacket thermal energy is utilized to maintain digester temperatures, leaving considerable thermal energy available for other on-site purposes. Additionally, all of the exhaust gas thermal energy is available for further exploitation. Typically, less than 10 percent of the electrical output is used for operation of the digesters.
Mass Natural established a goal of generating 848 kW from approximately 19,000 tons/year of identified feedstocks. Table 1 summarizes the biogas output/ton of potential feedstocks that Mass Natural would process annually. Both technology providers supplied data on expected biogas and methane production from these various feedstocks to supply 848 kW. While there are differences in the biogas output per ton of individual feedstocks, the weighted-average output did not vary considerably.
One of the critical questions this answered was whether or not the feedstocks can produce the desired 848 kW. Based on the biogas potential and the methane content, the calculated feedstock requirement for the scenarios ranged from 40 to 49 tons/day of feedstock. The sources of potential total feedstock flow is 52 tons/day, adequate to produce the desired electrical output.
As a further check, Mass Natural’s feedstock amount and desired electrical output was compared to the combined data from reference plants from both technology providers. While Mass Natural’s goal of 848 kW derived from the suggested feedstock amount of approximately 19,000 tons/year is greater than the average electrical output per ton of feedstock, it is within reasonable performance parameters of existing European plants. It should be noted that the feedstocks identified in Table 1 were chosen specifically for their potential methane output, whereas many European plants are designed with waste disposal as a primary goal and energy production as a secondary goal.
The Jenbacher JS3 316 genset was evaluated for this study. These units are known for their electrical efficiency as well as their ability to easily capture waste heat from the intercooler, oil cooler, engine jacket cooler and exhaust stack. For Mass Natural, electrical efficiency is the most critical of these, but the site is interested in capturing thermal energy from the engine to dry materials on site.
At full output, the genset has a heat rate of approximately 9,400 Btu/kWh, which translates to an electrical efficiency of 36.3 percent. Given typical operations and maintenance schedules, one can expect an engine genset to operate at full capacity a maximum of 90 percent of the hours in a year. In addition, a dry digestion system will typically use about 7.5 percent of the rated electrical output to run the pumps and controls of the plant. Therefore, Mass Natural can expect to export a maximum of 6,128,496 kWh/yr.
The following infrastructure would be incorporated into the design of a facility at Mass Natural: Truck scale; enclosed receiving and mixing building; building air collection system with biofiltration for odor control; dry fermentation reactors and associated pumps and tanks; electrical generation system and connection to the grid; storage tank for excess percolate; and enclosed composting system to process digestate and for bypass materials.
COSTS, FINANCING AND ROI
Capital cost data was obtained from technology providers directly, and from research on other plants built by the providers. The data was based on assumptions that no major site upgrades such as off-site utility line upgrades or major geotechnical accommodations were needed. Estimates include CHP units, as they are integral to the design and operation. The capital cost estimates for the Mass Natural project range from $5.5 to $8.4 million. As a check, Brendle compared these cost estimates to data compiled in two reports about anaerobic digestion capital costs written several years ago, and concluded they are appropriate.
Operating costs were estimated by the Brendle report to be approximately $130,000/year assuming 1.5 full-time equivalent employees. Costs for the CHP operation are estimated at $137,000/year, for a total operating cost of $267,000/year.
Project revenue is estimated at approximately $1.9 million in the first year of full operation. That revenue is based on an average tipping fee of $45/ton of feedstock, and receipt of $0.10/kWh of electricity generated.
Assumptions for project financing were based on 35.5 percent of total costs financed with solid waste bonds, 23 percent by investment tax credit, and the balance by equity investment. With this combination of revenue and financing, the estimated return on investment, EBITDA (earnings before interest, taxes, depreciation and amortization), divided by predebt capital costs, is 13.4 percent. This yields a payback period of 7.5 years.
The Mass Natural feasibility study concluded that a high-solids anaerobic digester would be complementary to the company’s existing composting operation. Ancillary benefits would include over 3.8 MMBtu/hr of thermal energy for on-site use and over 3,250 greenhouse gas emissions credits for sale. However, for a 20,000 tons/year facility, this was determined by Mass Natural to be a marginal investment. The company intends to conduct further evaluation to identify less costly high solids dry fermentation options.
Robert Spencer is an environmental planning consultant based in Vernon, Vermont (email@example.com). He works part-time for Mass Natural, and is also a BioCycle Contributing Editor.