BioCycle Magazine

Mechanical biological treatment trends

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Composting mixed municipal solid waste has a long tradition in Germany. In 1953, the first large-scale facilities in Baden-Baden and Blaubeuren started operating, followed later by plants in Heidelberg, Duisburg and other locations across the western part of Germany. Facilities were being opened to provide alternatives to landfilling MSW, and to produce compost for agricultural use. The quality of the compost made from MSW was very poor, however, and the product was never accepted for use in agriculture, despite a significant amount of research and optimization of the waste treatment process.

Composting of source separated organic waste from households has not had as long a history in Germany. The first pilot test in Witzenhausen started in 1983; two years later, a total of 100,000 tons/year of source separated organics (SSO) were collected in all of Germany. The “brown bin,” dedicated to collection of SSO from households, started being rolled out across Germany in the early 1990s. Today, nearly half of all German households use the brown bin system for source separation of organics, capturing about 35 to 50 percent of all waste generated in these households, or about 9 million tons of SSO annually. There are about 800 composting facilities in Germany, with a total input capacity of nearly 10 million tons of organic waste per year. A little more than half (422) of those facilities take part in the quality assurance system of the German Composting Association, producing 5 million tons of high quality compost annually.

Although the separate collection system for organic waste is well developed in Germany, a larger portion of organics still remains in the mixed solid waste, collected in the grey bin. This is attributed to only about half of German households participating in SSO programs and sorting behavior in apartment buildings; local management of waste collection systems; and households not taking the extra steps to empty packaged food that is out of date, or separating food from containers that are not completely empty. This organic waste ends up in the grey bin.
TASi Rule Lays MBT Groundwork

In 1993, the German Bundesrat adopted the Technical Instructions for MSW (TASi), which requires pretreatment of all waste containing biodegradables prior to landfilling. (The European Union's Landfill Directive that phased in limits for landfilling unprocessed organic waste was adopted six years later, in 1999.) Few incinerators were available to accept this waste, as costs were high in comparison to landfilling. With the TASi rule becoming effective in 2005 - prohibiting landfilling of mixed waste collected in the grey bin without pretreatment -most waste management companies started looking for alternatives.

When TASi was adopted in 1993, mechanical biological treatment (MBT) was not recognized as meeting its requirements. But two new federal regulations later, MBT became an accepted alternative to incineration in 2001. An MBT system combines a sorting facility (like a materials recovery facility) with a form of biological treatment such as composting or anaerobic digestion to degrade the organic fraction. Between 2001 and 2005, a large number of MBT facilities and incineration plants started operating, just in time to meet the TASi deadline in June 2005. Today 50 MBT facilities with combined capacity of 6 million tons receive approximately 25 percent of the total MSW collected in Germany. The MBT facilities feature different biological technologies, e.g., two-step aerobic treatment or a combination of anaerobic treatment followed by aerobic treatment.

The TASi limits have to be met before the end product can be put into landfill. Typically, the end product of an MBT facility is not called compost, even if a composting process and composting technologies are used at the MBT facilities. The output product is ultimately landfilled.

The MBT plants faced a lot of technical problems right from the start, with low throughput capacity caused by machine overload or breakdown and higher operational costs than expected due to high demand for maintenance and service. There also were challenges getting rid of the high caloric fraction produced during mechanical separation. Many problems were solved, and nearly all of the facilities are running at their expected capacities. One weak point at some MBT facilities is the final biological treatment process, which is often designed like a composting process.

“We started our facility with a system of big heaps in the final treatment area,” explains Roland Greif, operations manager of the largest MBT plant in Germany, situated in Cröbern near Leipzig. “Fighting with the brand new machinery we learned a lot about the biological process but still could not get a constant and stable material in our output. We realized that even over a short period, the composition of the waste could change completely.”

Facility operators decided to put the processed material into windrows with smaller volumes that can be treated separately. The plant purchased a BACKHUS 6.68 turner. “Changing from heap to windrow, we can reach our targets faster,” adds Greif. “We can operate the turner according to the requirements of each single windrow.”

Christoph Hofmann, operations manager of the biological treatment process at the Rosenow MBT plant in Mecklenburg, Germany, had a similar experience. “It is hard to continuously meet the TASi limits for oxygen consumption (5 mg O2 /g DS), especially for dissolved organic carbon in eluate (DOCEluat = 300 mg/l),” he says. “We have a total of nine weeks of biological treatment to achieve these limits. Our first treatment step is tunnel composting with fully automatic filling and emptying as well as under floor aeration, followed by our second treatment step - five to six weeks of composting in heaps.”

During those last five weeks, the facility was seeing an increase of the DOC. “We presumed the reason for the increase is an adaptation of mostly fungi cultures that are capable of metabolizing long-chain organic molecules to soluble ones, while the further metabolism to produce CO2 and water runs more slowly,” explains Hofmann. “In contrast to the intensive composting process with active aeration, the post composting process resulted in instability of the end product. We were running out of processing capacity because we were not reaching the limits in time. We also were experiencing high wear costs and constant downtime of our two turning machines.” The plant decided to switch from heaps to windrows. Since switching to windrows, the MBT plant has been able to meet its output limits.

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