BioCycle September 2009, Vol. 50, No. 9, p. 36
High solids anaerobic digesters developed in Europe are an attractive technology option, consuming less energy and tolerating higher levels of contaminants inside the reactor vessel.
Ron Alexander and Alexander MacFarlane
ANAEROBIC digestion of source separated organic (SSO) solid waste has long seemed like a win-win solution to landfill diversion. With anaerobic digestion (AD), compost-based soil amendments and fertilizers are created, massive tonnage from landfills and incinerators diverted, renewable biomathane produced, and long distance hauling of municipal solid waste (MSW) reduced.
Generating two revenue streams – renewable electricity, pipeline quality biomethane or compressed natural gas (CNG) vehicle fuel, and soil products – can improve project economics as compared to aerobic composting (which can be somewhat energy intensive).
So why aren’t more anaerobic digestion facilities processing SSO in the U.S. today? The three key barriers to widespread deployment of this technology for organics recycling have been, in order of importance: 1) Lack of policies supporting source separation of organic waste; 2) Cumbersome renewable electricity sale options for smaller generators, e.g. < 5 MW; and 3) Excessive complexity and cost of anaerobic digester technology offerings.
Given that SSO diversion policies and feed-in tariffs (as well as other helpful distributed energy sales options) are now in effect in several regions of the U.S., an appropriate technology solution has the potential to open the gates for an AD infrastructure. This article addresses one technology option for the North American market – high solids AD, or “dry fermentation” as Europeans refer to the process.
Considering that energy revenue is so critical to the successful performance of an AD plant, it is important to remember the following algebra: Energy Revenue = (Quantity of Energy Produced — Quantity of Energy Consumed) x Energy Sales Price. In other words, minimizing process energy consumption is equal in importance to the rate of energy production. High solids AD techniques were developed with this equation in mind, allowing for up to 28-day retention times in static piles, while not sacrificing biogas yield. High solids AD technologies emerged in Western Europe over the last two decades, and for the most part they resemble high solids versions of wastewater or manure treatment digestion systems in which the feedstock and inoculant mixture is pumped through a reactor vessel. These agitated systems are strongly net energy positive, yet require much of the same feedstock pretreatment, dewatering and pumping as wet digestion, thereby driving up operating costs.
More recently, a new type of high solids anaerobic digester has emerged in which the digestion substrates are static within the digester and initial solids content of the feedstock can reach 50 percent. Instead of mechanical agitation inside the digester, upfront substrate mixing with a front-end loader and of percolation of moisture through the static pile are the primary methods of process control and nutrient and bacteria mobility. These static batch digesters consume minute amounts of energy, tolerate higher levels of contamination inside the reactor vessel and require minimal if any feedstock size reduction or screening.
A high solids AD technology, BEKON, has recently been introduced in the U.S. by Harvest Power, Inc. Harvest Power’s strategy is to develop AD facilities in partnership with existing composting operations, building upon the experience and operational capabilities of the market while bringing financial, engineering and management capacity into the mix. A facility using the BEKON technology, located in Munich, Germany, is operated by the City of Munich Waste Management Company (AWM). It processes approximately 25,000 metric tons/year of biowaste, yard trimmings and household food residuals, using AD followed by composting. This volume of incoming organics generates approximately 3.8 million kWh of electricity and an equivalent amount of thermal energy. (This is actually considered a lower amount of energy generated by the system, caused by the processing of lower volumes of food residuals.) There are 10 digesters (built in two phases) and a turned windrow composting facility, which is under cover (a shed). The entire plant is on a concrete pad.
The technology is designed to process high solids feedstocks, typically 30 to 50 percent total solids entering the digester. However, feedstocks with minimum total solids of 25 percent may be processed as long as they are stackable within the digester to a height of four meters (13 feet). Process parameters for the incoming materials are similar to that of aerobic composting: 40 to 50 percent porosity, 30 to 50 percent moisture content and 625 kg/m3 (39 lbs/ft3) or less bulk density.
Incoming materials at the Munich plant are stored in a negatively aerated reception hall. They are not ground prior to digestion. On Mondays and Thursdays, the digesters are unloaded and reloaded to a height of approximately four meters (13 feet). If necessary, the feedstock is blended with screened overs (15 to 40 mm, or 3/5 to 1-3/5 inch) to improve porosity, which in turn improves biogas generation.
Once filled, the digester is sealed for approximately four weeks. After methane production starts to fall, the residual methane in the digester is completely flushed out and the digester is opened. About half of the processed feedstock (digestate) is remixed with the incoming new materials for inoculation and pH buffering. The rest is placed into piles until it can be formed into windrows where it is composted for 6 to 8 weeks. With forced aeration, compost finishing requires only two to four weeks. The finished compost is screened through a 15 mm (3/5 inch) screen and sold to a media blender.
The methane concentration in the biogas is 55 to 60 percent. No biogas storage tanks are used at the Munich facility; all biogas goes directly to a combined heat and power (CHP) facility to produce electric and thermal energy. The production of methane is managed using multiple digesters to keep a steady state and minimize the need for biogas storage. The electricity generated at the facility is fed into the grid, with 4 to 5 percent used to operate the plant. The heat generated at the facility cannot be sold off-site, so it is used to dry off the screening residuals destined for the landfill (since they pay on a per ton basis to dispose of them at a local incinerator). Approximately 15 percent of the heat generated is used to keep the digesters at the optimum mesophilic temperature range for biogas production. Four full time staff operate this facility, but similar privately operated BEKON facilities utilize two to three full time staff. Tip fees in the region are €30 to 50 (42.7 to 71 USD) per metric ton for source separated organics and €100 to 140 (142 to 199 USD) per metric ton for MSW. The facility cost approximately €5.5 million (7.8 million USD) to construct.
Ron Alexander is president of R. Alexander Associates, Inc., in Apex, North Carolina (www.alexassoc.net). Alexander MacFarlane is with Harvest Power, Inc., in Waltham, Massachusetts (www.harvestpower.com), which uses the process developed by BEKON Energy GmbH of Munich, Germany.