The Garbage Anomaly: What is a Dirty Murf and what can you do about it?

November 24, 2011 |

MSW is an obvious kick-off feedstock for the emerging bioenergy industry, but its numerous advantages remain largely overlooked.

by David Bransby, Professor, Energy Crops and Bioenergy, Auburn University, USA

There is widespread disappointment in the rate of progress towards commercialization of cellulosic biofuels, and industry is lagging substantially behind RFS2 annual targets. Feedstock limitations are often cited as one of the reasons for this: in particular, a reliable and adequate supply of low-cost, acceptable quality feedstock is required to launch the fledgling industry.

Use of agricultural crops or forest-based materials as bioenergy feedstocks involves production and/or harvesting, storage, and delivery of this biomass to processing plants, which are all costly. In addition, the necessary infrastructure is mostly not yet available, system optimization is still needed, and contracting hundreds of growers is a major challenge, especially with the future of the Biomass Crop Assistance Program (BCAP) in the balance. All this further increases costs and risk.

In contrast, infrastructure for collection of municipal solid waste (MSW) is already in place and paid for, and those who collect and dispose of it get paid for their services. This results in very low cost and low risk, making MSW a no-brainer feedstock for launching the cellulosic biofuels industry.

This being the case, it is difficult to understand why MSW continues to be mostly overlooked as a cellulosic feedstock: the original Billion-Ton Report, its very recent update and a report commissioned by Congress and just published on the status of the cellulosic biofuels industry, all totally ignore MSW as a potential feedstock, and relatively few start-up companies plan to use it in their first commercial plants.

MSW as a feedstock

Household garbage forms a large proportion of MSW. In the US some of this material is recycled, but most of it ends up in landfills, or is incinerated. Europe does a better job with recycling, but there is still room for improvement. Landfills and incineration are costly, and involve environmental risk.

Biogenic material (material of biological origin), such as paper, food scraps and yard waste, typically constitutes a little over 50% of MSW. And according to EPA regulations, it is classified as renewable and qualifies for RINs. In addition, because it contains a large proportion of paper and cardboard, the content of lignin is relatively low, resulting in better conversion efficiency for biochemical conversion technologies than is the case for agricultural and forest-based biomass.

On average, Americans generate about 4.3 pounds of MSW per person per day, or 260 million tons per year nationwide, of which a little over 130 million tons is biogenic. If only half of this were used to generate biofuels at an efficiency of 70 gallons per ton, we could produce 4.5 billion gallons of cellulosic biofuels from it every year.

True, relative to our total fuel consumption, or even to the amount of oil we import from OPEC, this is not much. However, we are not trying to build the entire industry on MSW, we are just trying to launch one – quickly. What we need to do that is a reliable supply of feedstock of adequate volume and quality for the first commercial plants, and at the lowest possible price.

Half the biogenic fraction of MSW could supply ninety 50-million gallon per year plants, which could well meet the volume side of the equation: this would provide all the cellulosic biofuels needed to meet the RFS2 target for 2016, or 28% of that needed to meet the 2022 target. But what about the quality and cost requirements?

The Dirty Murf

Sterilizing/removing contaminants and separation into biogenic, plastic, metal, glass and other fractions is often considered too expensive to make MSW economically viable. Materials recovery facilities (MRFs, often referred to as “dirty murfs”) in which manual labor is employed to sort filthy garbage by hand in an unhygienic work environment, currently constitute the most common method employed.

Indeed, for developed countries to even contemplate such facilities in the 21st century is hard to understand, especially with the availability of much cleaner autoclave technology to perform the same function at a lower cost and more efficiently.

The Autoclave process

The autoclave process involves treatment of equipment or materials with heat, pressure and steam in an autoclave vessel. It is commonly used to sterilize laboratory equipment in small lab-scale autoclave vessels, but it can also be used to treat MSW in much larger vessels.

The process sterilizes mixed garbage without the need for prior sorting, eliminates the unpleasant odor, and facilitates mechanical separation of the biogenic, plastic and metal fractions without direct use of manual labor. Furthermore, the biogenic fraction is homogenized by rotation of the autoclave vessel, leading to a very uniform feedstock. Therefore, the autoclave process essentially makes curbside separation and dirty MRFs obsolete, and solves the heterogeneity problem related to MSW as a feedstock.

There are several versions of the autoclave technology for processing MSW. Most of these involve high capital and operating costs. However,  a company in St. Louis, Missouri, CleanTech Biofuels ( has optimized their version of the process to a point where its cost is more than offset by tipping fees, in many cases by a lot ( in the US, tipping fees range from about $25/ton of MSW, to over $100/ton, with an average of about $40/ton).

CleanTech projects that it can customize its cellulosic feedstock product to suit its clients, including formatting into pellets, meeting moisture specs (up or down from the average 25% when it exits the autoclave vessel), and minimizing ash. And, in most cases, they expect to be able to do this at a price well below any other cellulosic feedstock.

It’s already commercial

The CleanTech autoclave technology is already in commercial use at Coffs Harbour in Australia (see, where it has been in operation for three years. Isn’t it sad that a technology invented in the United States is first commercialized down under?

Regardless of this, what is of particular interest is that the cellulosic material generated by the Coffs Harbour autoclave plant is not used to produce energy; it is disposed of as compost, the main objective being to avoid landfill in this beautiful resort environment ( And it still pays!

What about jobs? Yes, the facility employs about the same number of people as a dirty MRF, but under much cleaner and more hygienic conditions.

But will MSW not put other feedstocks out of business? No, to the contrary; for several reasons it will be synergistic with agricultural and forestry feedstocks. First, there is not nearly enough MSW for it to put other feedstocks out of business, there is just enough to launch the industry quickly. Secondly, its low cost and low risk can facilitate quicker commercialization of the industry, thus developing markets for other biomass sooner than would otherwise be the case.

Thirdly, and perhaps most important, some thermochemical conversion processes can use a mixture of feedstocks, in which case MSW could be mixed with agricultural and/or forest-based feedstocks to optimize cost and supply for projects that might otherwise not be viable.

So why are more conversion companies not planning to use MSW as the feedstock for their first commercial plants, when some of those that have are well on their way to commercialization? It’s an anomaly.

Contact the author: [email protected]

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