Charles Egigian Nichols
BioCycle January 2004, Vol. 45, No. 1, p. 47
Anaerobic digestion technology development has been underway in Europe for food residuals for over 30 years, driven by the issue of dwindling landfill space. Three distinct phases of technology development have occurred during these three decades. First, European governments funded university and academic research to initiate anaerobic digestion technology advancements, which resulted in a wide range of laboratory testing over about ten years. Second, pilot and demonstration-scale facilities were developed in the 1980s and into the 1990s. Third, this work led to the commercialization phase that was launched in the 1990s and continuing to today.
 According to Luc De Baere from the O.W.S. Company based in Belgium, about 70 anaerobic digestion plants were operating in Europe by the end of the year 2002. The feasibility study conducted by Tetra Tech for the Seattle (Washington) Public Utilities (see December 2003 BioCycle, p.39) included an assessment of digestion systems available in Europe. The study encompassed those processes that fall within the market segment “source separated organic waste (SSOW) digestion” (in Europe mostly called “biowaste”) and “municipal solid waste (MSW) digestion,” including residential and commercial SSOW along with food-soiled paper. In addition to limited landfill capacities with increasing tipping fees, more and more stringent environmental regulatory requirements and recently introduced renewable energy laws with the objective of promoting and facilitating sustainable energy production have been driving the development of anaerobic digestion technologies for SSOW and MSW in Europe.
There are many different anaerobic digestion processes on the market for SSOW or MSW with the following primary characteristics: 1) Number of stages – Single-stage and two-stage; 2) Feed total solids (TS) content -Wet process (20% TS); 3) Operating temperature – Mesophilic process (approximately 93°F to 98°F or 34°C to 37°C) and Thermophilic process (approximately 131°F to 140°F or 55°C to 60°C); 4) Agitation -Gas Injection, Internal mechanical components (agitator) or Repumping/recirculation; 5) Reactor Type – Vertical positioning and Horizontal positioning; 6) Process Flow – Continuous process flow and Plug process flow.
AD TECHNOLOGIES ON THE EUROPEAN MARKET
Table 1 summarizes the anaerobic treatment technologies for SSOW and MSW currently available on the European market. It is important to note that the technologies and facilities summarized in this table do not utilize manure, which is a common feedstock and technology option in Europe. However, the feedstock and material management were considered sufficiently different, therefore Tetra Tech recommended not relying on manure technology comparisons. A total of 21 systems are included in Table 1.
Using the parameters of number of plants installed and of a size similar to that anticipated for the city of Seattle, the following six anaerobic digestion processes were investigated in more depth: BTA; Linde KCA/BRV; Valorga; WAASA; Kompogas; and DRANCO. With only a few exceptions, all waste treatment facilities relying on those processes are still in operation and have proven reliability. Though each plant has its specific design accommodating the individual needs at its location, the majority of the installed plants attained their performance targets within the first one to two years. Some of the installations have been exceeding their initial design capacities, demonstrating their reliability and flexibility in response to local demands.
A more detailed analysis of these six technology options follows.
BTA Biologische Abfallverwertung GmbH & Co KG: The BTA process was initially developed in a pilot plant in Garching (Germany) to gain experience testing a range of feedstocks and to fine-tune the technology. It was in operation from 1986 to 1988. The first plant on an industrial scale was built in Elsinore (Denmark) in 1990 with an annual capacity of 20,000 metric tons.
The process consists of two major steps: mechanical wet pretreatment and biological conversion. In the pulper, feedstock is mixed with recirculated process water. Contaminants like plastics, textiles, stones and metals are separated without any hand sorting by means of a rake and a heavy fraction trap. The process yields a thick, pumpable suspension (pulp) that is fed to the digester. An optional but essential component is the grit removal system, which separates the remaining finest matter like sand, little stones and glass splinters by passing the pulp through a grit removal system (hydrocyclone).
Various concepts of the biological conversion step are offered, including single stage, multistage and two-stage digestion. Single stage is mainly for comparatively small, decentralized waste management units. Multistage digestion is mainly for plants with capacity of more than 50,000 metric tons/year. The multistage involves separating the pulp in a solid mass and a liquid phase by using a dewatering aggregate. The liquid, already containing dissolved organic components, is pumped directly into a methane reactor with a two day retention time. The dewatered solid material, still containing undissolved organic components, is once more mixed with water and fed into a hydrolysis reactor. After four days, the mass is dewatered again and then the liquid is fed into the methane reactor.
Two-stage digestion is mainly for plants with medium capacities; this process is based on the multistage concept but without a solid/liquid separation. The pulp is fed into a mixed hydrolysis reactor, which is connected with an also completely mixed fermentation reactor. To create optimal hydrolysis conditions, a part of the fermentation reactor content is fed back into the hydrolysis reactor.
For treatment of food residuals, an additional sanitation step is supplied. The digestion residue is dewatered and, in general, sent to aerobic after-treatment. The water demand of all process types is met by recirculating the process water, which is contained in the waste. Depending upon the waste composition and local requirements, excess water is sent to the sewage system, or will receive additional treatment on-site before it can be discharged.
Energy can be recovered from the generated biogas for use in gas engines or coheat and power (CHP) stations. Depending on the waste composition, the gas yield ranges between 80 and 120 m3/metric ton of waste feedstock.
Linde-KCA/BRV: Linde-KCA-Dresden GmbH is a fully owned subsidiary of Linde AG (Germany), and associated with Linde BRV Biowaste Technologies AG. It acquired the technologies and experience of the “Mechanical-Biological Waste Systems” product line of Austrian Energy & Environment, and has a broad range of biological treatment technologies for SSOW and MSW, including composting.
The first anaerobic digester plant based on wet digestion was built by Linde-KCA in 1985 treating 16,000 metric tons/year of manure. Linde-BRV’s technology based on dry digestion saw its first introduction to the market in 1994 with an installed annual treatment capacity of 6,000 tons of SSOW and 12,000 tons of yard trimmings.
Mechanical treatment and conditioning of feedstocks depend upon the type of digestion process. Up to now, wet processes have been primarily used for codigestion of SSOW with liquid manure and/or biosolids. Particularly for wastes with high grease content, the thermophilic process has been selected. The particular feature of the two-stage wet digestion process is the hydrolysis tank with an intermittent aeration option, and a downstream methanization unit. Characteristic of Linde’s wet digestion design are automatic removal of contaminants (heavy material and light matter) in the wet pretreatment step combining a waste dissolver and a downstream rotary screen, and a patented loop reactor designed for the recirculation of the material by gas injection into a centrally arranged drought tube that can double for the introduction of heat.
Dry digestion is either a thermophilic or mesophilic process that takes place in the rectangular shaped, horizontally arranged plug-flow fermenter with a concrete compartment. As a rule, preliminary aerobic composting or hydrolysis, as well as the adjustment of the acid concentration, takes place upstream of the fermenter. The digester is comprised of several agitators of transverse in-line arrangement; a conveyor is mounted to the digester bottom transporting the sediments to the fermenter’s discharging end. Wastes with a total solids content of 15 to 45 percent in the digestion feedstock can be treated in the reactor; a gas yield of greater than 100 m3/metric ton of feedstock can be achieved.
For the wet process, the fermented waste suspension is dewatered in a centrifuge. The residue may either be utilized or further treated. The filtrate is returned to the waste dissolver and/or is used to flush the rotary screen. The energy of the generated biogas can be recovered in gas engines or CHP stations.
Valorga: Based in France, Valorga was founded in 1981 to develop MSW treatment technologies. The anaerobic digestion process was introduced on an industrial scale in France in the mid 1980s at La Buisse and Amies for biological treatment of mechanically pretreated MSW. The process has been adapted to mixed MSW, source separated household waste (SSOW), and organic residual fraction after SSOW collection (grey waste). There are nine facilities in operation and three under construction.
Besides crushing, its trademark feature is the dry ballistic separation to remove the heavy fraction and other contaminants of the feedstock in the pretreatment step, before the waste is sent to the digesters. A blending pump is used to mix in process water and achieve the desired moisture content. Digestion occurs in a single stage within the mesophilic or thermophilic temperature range. Total solids content in the fermenter is approximately 25 to 35 percent by weight and has a residence time of18 to 25 days. The digestion reactor is designed as a vertical cylindrical tank allowing “extraction by gravity.” Due to the static pressure, the digested matter flows through a collector with two outlets – one to the dewatering press, and the other to the recirculation pump. Gravity ensures a constant flow without surge and a constant pressure at the feed of the dewatering press. To prevent erosion by inert particles in the waste stream, mixing of the mass is done with a patented high pressure biogas injection every 15 minutes though a network of nozzles located at the reactor’s base.
The methane yield is between 220 and 270 m3/metric ton of total volatile solids fed to the digester, or between 80 and 160 m3/metric ton of feedstock, depending on the waste. It should be noted that the Valorga design is ill-suited for relatively wet wastes, since sedimentation of heavy particles inside the reactor occur at total solid contents less than 20 percent.
A screw press dewaters the digested residue. The filtrate has a dry matter content of approximately ten percent, and a dewatered solid residue (digestate) with a dry matter content of approximately 40 percent. The digestate receives a short aerobic composting treatment (about two weeks). Most of the produced biogas is transformed into electricity and heat in an internal combustion engine coupled with a heat exchanger. A gas buffer optimizes the energy management.
WAASA: The WAASA process, developed by CITEC (Finland), was installed at one of the largest MSW digestion plants in Vagron/Groningen (Netherlands), processing 230,000 metric tons/year of MSW, and resulting in 92,000 metric tons of organic fraction to be treated in four anaerobic digester rectors of 2,750 m3 each annually. This plant started operating in May 2000. Initially designed as a wet, mesophilic and single-stage technology, the WAASA process is operated now at both thermophilic and mesophilic temperatures. At a 15,000 mt/year plant in Vaasa, Finland, the mesophilic and thermophilic systems are run in parallel, with the thermophilic process showing a hydraulic retention time of ten days compared to the 20 days in the mesophilic design. (The Vaasa plant also included the DBA-Wabio anaerobic digestion system, which uses a technology similar to WAASA. That equipment is no longer being actively marketed.)
The process has been tested on a number of waste types, including a mixture of mechanically separated MSW and biosolids, and operates in a solids range of ten to 15 percent. The vertical reactor consists of a single vessel although it has been subdivided internally to create a predigestion chamber. (The company refers to this as a two-stage “Twinreactor” preventing a short circuit circulation of the medium.) Mixing is carried out by injection of biogas at the base of the reactor. The operational performance indicates that gas production is in the range of 100 to 150 m3/metric ton of SSOW, with volume reduction of 60 percent, weight reduction of 50 to 60 percent, and 20 to 30 percent internal consumption of biogas. The digestate can be further treated by composting, but this is highly dependent on the type of waste treated.
Kompogas: The Swiss company Kompogas was established at the end of the 1980s. After many months of studying relevant literature and carrying out his own experiments on his balcony at home, the firm’s founder Walter Schmidt was convinced that he could turn organic waste into gas to create a valuable source of energy. He used a small test fermenter to observe, improve, and perfect the fermentation process. With financial support from the Swiss government and the Canton of Zurich, the first solid-fuel fermentation plant embarked upon a trial phase in Rümlang, Switzerland in 1991. The official start-up followed in 1992. A number of plants using the technology now operate around the world.
To produce energy from yard trimmings and SSOW, contaminants with high solids content are removed prior to the subsequent dry, single stage, thermophilic digestion system. Feedstock is loaded daily into cylindrical reactors; the plug-flow occurs horizontally, aided by slowly rotating and intermittently acting impellers inside the reactor (which also serve to homogenize, degas, and resuspend heavier particles). This system requires careful adjustment of the total solids content to around 23 to 28 percent within the reactor. At lower TS values, heavy particles such as sand and glass tend to sink and accumulate inside the reactor; higher values cause excessive resistance to the flow. The digested material is discharged at the far end of the reactor after a retention time of approximately 15 to 20 days. During this time, undesirable germs and weed seeds are eliminated. Due to mechanical constraints, the volume of the Kompogas reactor is limited, and the capacity of the plant is accommodated by installing several reactors in parallel, each with a capacity of either 15,000 or 25,000 metric tons/year.
The biogas produced during the degradation process is converted in CHP stations into electrical and thermal energy, ensuring self-sufficient operation, as well as generating considerable surplus energy. The biogas may be upgraded to natural gas standards for fuelling cars and/or for being fed into the public natural gas works. Approximately 130 m3 of biogas are extracted from one metric ton of organic waste, corresponding to about 70 liters of gasoline. The digestate is dewatered and composted. Some of the filtrate is used as an inoculum source; the surplus is sent to an anaerobic wastewater treatment facility, which also produces biogas.
DRANCO: Organic Waste Systems (O.W.S) developed the DRANCO process (dry anaerobic composting) for the anaerobic treatment of MSW and industrial organic waste. The first facility on an industrial scale began operating in 1992, processing a mix of 20,000 mt/yr of SSOW and paper waste. O.W.S. also developed a sorting, digestion and separation system (SORDISEP) to recover recyclables and energy. Like Kompogas, DRANCO features only one anaerobic digestion process type – a thermophilic, single step anaerobic fermentation, followed by a short aerobic maturation phase.
The process takes place in an enclosed vertical digester capable of treating a wide range of incoming material with a dry matter content from 15 to 40 percent. It operates without the addition of water or structure material and is flexible in its requirements for feedstocks. Characteristic for the DRANCO digestion is the “stationary” fermentation feeding the reactor once a day from the top of the reactor. The mixing occurs via re-circulation of the wastes extracted at the bottom end, combining with fresh wastes (usually one part fresh wastes for six parts digested wastes), and pumping the material to the top of the reactor. Apart from feeding and removal of the residue, there is no further mixing or agitation needed in the vessel, which works at very low pressure (less than 50 mbar). The process has been shown to be effective for treatment of wastes ranging from 20 to 50 percent TS. The reactor retention time is 15 to 30 days, and the biogas yield ranges between 100 and 200 m3/metric ton of waste feedstock.
Residue is extracted from the digester, dewatered to about 50 percent, and then stabilized and sanitized aerobically during a period of approximately two weeks. Collected biogas can be stored temporarily then further purified before it is used in gas engines or sent to CHP stations.
COMPARISON OF TECHNICAL FEATURES, ECONOMICS
The commercially available digestion processes considered in this study have specific characterizing features. The advantages and disadvantages of the main features of anaerobic treatment technologies for SSOW and MSW are shown in Table 2. All of the processes are well suited technically for all feedstocks of the MSW type that were the focus of this study for the city of Seattle. The optimum application areas for these processes are separately collected organic waste or household waste, as well as commercial waste. Only a “fine” fraction of MSW or commercial waste is directed to the digestion stage followed by separation through mechanical pretreatment. The mechanical wet treatment required for wet digestion provides the advantage of removal of virtually all contaminants, resulting in higher quality compost.
In general, wet digestion processes require a relatively high outlay for process equipment. To prevent formation of sediments and layers of floating material, mechanical wet preparation of SSOW and MSW is essential. The quantity of process water flowing in a closed circuit is appreciably greater than for dry digestion processes. The number of plant components needed is even greater if wet digestion is designed in two stages. However, depending upon individual quality requirements of the AD products, this statement may be less justifiable.
As illustrated in Table 1, the range of size of facilities of the principal suppliers encompasses many different sizes, components and technologies. The net effect is that there is actually no such thing as, for example, a single Linde-process, and likewise no single Valorga process. Therefore, a direct economic comparison isolated from individual cases is not practical.
As a general trend, the wet two-stage anaerobic digestion systems require higher capital costs than dry single-stage anaerobic digestion systems. The outlay for equipment as well as material flow handled is higher. Moreover, wet system operating costs are negatively affected by the electricity consumption of pumps and other equipment. Higher quality products may offset this additional cost.
Which of the mentioned factors, stated as examples, will play a more or less dominant role can only be determined for specific case studies on the basis of an accurate understanding of the waste type and quantities, as well as the constraints and conditions at the facility site. Table 3 below provides an overview of capital and operating costs of anaerobic digestion plants, broken down by its annual range of throughput. The substantial ranges are attributed to the variety of processes, as noted above.
It should be noted that the costs provided in this table are based on 1995 study data and thus do not entirely reflect the current cost schedule. Since that time, facilities have been brought on-line with an annual capacity greater than 50,000 metric tons, and, in some instances, even exceeding an annual capacity of 100,000 metric tons. Therefore, costs for capital, as well as for operations, have seen some further decline on a unit basis The figures of Table 3 clearly indicate that both capital and operating costs are decreasing as the capacity of the facilities is increasing, with some further cost reduction potential on the horizon.
Charles Egigian-Nichols is senior manager with Tetra Tech, Inc. Additional assistance on this project was supplied by Larry Sasser, now with Biorecycling Corp., and Joerg Blischke with Metcalf and Eddy.