“Our latest waste audit shows that as much as 56.1 percent of the 178,000 tons of material landfilled yearly is suitable for anaerobic digestion (AD),” Hogan explains. “At a capture rate of 50 percent of that total, we can project some 50,000 tons/year. Coincidentally, that is the size of the AD facility in Brecht, Belgium which we inspected last fall on an information gathering trip.” With a combination of the two methods — anaerobic digestion (50,000 tpy) and aerobic composting (118,000 tpy), Bluestem would be recycling almost 170,000 tpy of its organic residuals. “That’s why a feasibility study is so important to us, and why the DNR is so interested,” Hogan says enthusiastically.
Last month, the DNR approved a Memorandum of Agreement which would provide $150,000 to Bluestem to investigate the benefits of anaerobic digestion for the Cedar Rapids region. “The Bluestem Solid Waste Agency is facing an imminent local landfill space shortage,” comments Liz Christiansen, director of Iowa’s Waste Management Assistance Division (WMAD). Based on the objectives of former DNR director, Paul Johnson, Christiansen’s division has pledged to assist Bluestem in pursuing “hyperwaste reduction” — a diversion percentage beyond 50 percent. “With an abundance of industrial organics available — Cedar Rapids is a grain processing center and a successful composting history — Bluestem’s project is the largest municipally owned operation in the Midwest, the agency is well poised to consider further organics recycling,” sums up Christiansen.
In addition, Cedar Rapids has a flourishing curbside recycling program and is the largest Iowa community with pay-as-you-throw collection. Several years ago, she continues, Linn County banned landfilling corrugated cardboard, which added to the percentage of organics in the MSW stream. A recent waste characterization estimated food residuals in the county’s garbage at 16 percent.
Christiansen emphasizes DNR’s commitment to the continued investigation of organics recycling alternatives. “We must also initiate a new conversation about crafting a system that offers incentives to local governments and organizations to provide waste diversion services. The investigation and testing of the anaerobic digestion technology is an imperative part of our new approach. We can help Governor Vilsack make Iowa the food capital of the world through sustainable management of organic residuals.”
Following are descriptions of three anaerobic digestion facilities that were visited last fall by representatives of the DNR and the Bluestem Solid Waste Agency:
Two to four trucks per day deliver source separated food and garden residuals from residents and restaurants to the (Kaufbeuren) facility that is located in a converted sewage treatment plant (no proprietary technology was used). While 3,000 metric tons/year are permitted, the plant could handle 10,000 mt/yr. Material is dumped from screw-type auger packer collection trucks onto a covered tip floor. Material resembled shredded yard trimmings, high in straw-like content, containing bread loaves, vegetables and fruits with few contaminants. A small tractor with a bucket loaded material into a slow speed grinder; then loosely ground feedstock was mixed with water and pulped, while nonpulped rejects were removed. Pulped material (8 mm minus) is pumped to a vessel where it is heated to 55°C, then pumped into the top of the main reactor vessel (digester) where it begins a 50-day digestion period. Feedstock is then removed from the bottom of the digester and dewatered in a centrifuge, with water treated at the plant across the road. “About 1,000 net tons/year of ‘compost’ are removed through the centrifuge and placed in a rolloff container,” write Christiansen and Hogan in their facility reports. “The material — 50-plus day old digested solids — looks very mature, resembling a dark soil. Methane produced in the digester is piped to the wastewater treatment plant and offsets 30 percent of energy used, saving about $1,500 (US)/month.” The facility reports having a vibrant market for the digested solids.
The digester, which serves 12 communities, is part of a very large industrial-type installation that processes virtually all the region’s solid waste and wastewater. The solid waste handling facility in Salzburg has three main flow paths:
1. Grey residuals (essentially the trash/ nonrecyclable fraction — 80,000 MT/yr) are shredded to 10 cm, mixed with sewage sludge in three rotating drums (500 tons capacity each) for 36 hours at 45°C, composted on a large aerated (vacuum) floor for volume reduction, landfilled as cover. Experimental screen to 10 mm was being tested to yield a higher end compost or soil replacement; 2. Green (yard and wood) waste composting for high grade compost; and 3. Digestion of food and yard organics (purpose of the Iowa inspection).
Side flow materials handling paths included a comprehensive residential drop off for recyclables and household hazardous waste; commercial hazardous waste; and a MRF for bulky materials such as plastic (three days/week), regular nongrey, nongreen container wastes, and ICI (two days/week). Materials recovered from the MRF are marketed and rejects sent to the landfill.
Composted grey waste and rejects from the MRF total approximately 165,000 tons going to the landfill. (Addition of sewage sludge impacts this tonnage total.) Tipping fees range from $65 to $130/ton (US) depending upon material. There are 500 trucks daily entering the facility delivering 220,000 MT/yr. The facility uses the Dranco (Dry Anaerobic Compost) process developed by Organic Waste Systems. The comprehensive facility manages virtually all materials in the waste stream on-site. “It does, however, have some drawbacks,” notes Hogan. Much of the material is handled several times; three different composting methods are used, mainly to separate finished products; and a significant waste fraction (grey waste) still does not go through the digestion line and therefore ends up in the landfill in a partially composted (semi-unstable) condition.
This facility — the first unit built in 1993 and the new unit finished last year — is rated by Hogan as the one “that most closely fits our situation.” The new plant, which also uses the Dranco process, serves a population of 300,000 to 400,000 from 24 communities, and is operated by three persons /shift (two shifts/day) — handling 50,000 tpy. The plant closes overnight and for the weekend. Approximately 165 tons/day of food residuals and special feedstocks (sausage casing trimmings, nonrecyclable paper, garden trimmings) are received.
Yard and food organic materials are unloaded onto an enclosed tipping floor and fed onto a walking floor-type bunker that slowly feeds to a conveyor into the screening/mixing room. The screens and mixers separate plastics and other contaminants from the feedstock, conditioning it for further mixing. Two trommel-styled chambers receive the crudely premixed organics which stay in the first chamber for two to three hours, then reside in the second for another six to eight hours. Each chamber rotates, moves and breaks up the organics into a 10 mm size product. Oversized materials are screened off the end of the second chamber and dumped into large rolloff boxes for disposal.
Screened and mixed materials are then fed into a dosing unit that injects steam to reach 55°C (pathogen kill) and mixes in previously digested materials for conditioning. Material is pumped to the top of the reactor (digester) unit in the same way as the other two plants. Residence times in the new plant reactor are around 14 days, which is an improvement over previous designs. The facility uses a generator set to power the plant and sell electricity to the power grid.
53 PLANTS . . . AND COUNTING
Currently, there are 53 anaerobic digestion plants in Europe that accept at least 3,000 tpy with at least ten percent solid waste as a feedstock. Only a few of these facilities are primarily for solid waste. Observes Hogan based on his European inspection tour with colleagues from Bluestem and the Iowa DNR: “Most AD plants were developed within the last ten years. This trend should be studied since there must be valid reasons for the popularity of the technology.”