MSW Processing Past, Present, and Future

Faced with the increasing political difficulty and economic cost of permitting and constructing landfills in expensive real estate markets, communities in high-population-density areas are giving new waste management technologies a serious look. Adoption of these future technologies may mean the end of the landfill’s monopoly on waste management operations. But before we can see where we’re going, let’s start by looking at where we have been and where we are now.

Pre–Subtitle D Waste Management Practices
Prior to the publication of the Subtitle D regulations governing MSW management and disposal, waste management techniques (collection, disposal, protection of health and the environment, etc.) were at best haphazard. Waste was deposited in unlined landfills that were often no more than pits excavated down until groundwater was encountered, which prevented further excavation. Waste would therefore be in direct contact with groundwater, creating extensive contaminant plumes. The waste was rarely covered on a regular basis, and open-dump burning of the waste was a regular practice resulting in air pollutants and blown debris. Burning was done to minimize airspace utilized by the deposited waste, and it was a form of volume reduction that was required since compaction of the waste was rarely done. When a landfill was closed, it received a simple soil cover of varying thickness on irregular slopes. Surface-water runoff was rarely controlled, and sheet flows often resulted in gully formation, serious erosion, and the carrying of sediment into local surface waters. When landfill gas was a noticeable problem (made noticeable not by monitoring but by a serious odor problem or the killing of large areas of vegetation), passive vents would be installed around the perimeter or in the middle of the dead vegetation. Environmental sampling and testing of groundwater, surface water, and leachate was unknown.

In addition to these serious environmental shortcomings, pre–Subtitle D landfills tended to be small and scattered dumps. Instead of being serviced by one big landfill or a few large landfills, many municipalities had multiple small dumps ringing their communities. Subtitle D’s impact was not just environmental; it was economic. The small mom-and-pop town dumps could not hope to raise the capital needed to construct and operate a landfill that met the new requirements. Subtitle D indirectly gave rise to the large regional and nationwide landfill and waste management companies that are the norm today.

Oddly enough, with fewer and larger waste companies, the industry is more cost-competitive than when we had many more—but smaller—operators. The small operators of pre–Subtitle D times lacked the capacity to increase the amount of daily waste intake. They lacked the volume, and they lacked the operational resources (staff and equipment) needed to manage increased waste volumes. They had no “surge capacity,” unlike a modern landfill. A large, modern landfill can compete for larger waste receipts (up to its permit limits), gearing up staff and equipment to handle the increased workload and having enough designed airspace to allow for the increased waste disposal without having to immediately expand the landfill’s permitted volume. This increased capacity is fed by improved waste hauling techniques, such as regional transfer stations and rail hauling from out of state or even cross-country.

Current Waste Management Practices
Subtitle D replaced the old haphazard methods of landfilling with a single, uniform, minimum standard for landfill construction and operation. The states have the option to make these federal regulations stricter (and many have), and many of these regulations have been tweaked and refined over the years by guidelines and rulings that have removed much of their original ambiguity.

The landfill industry now operates on a level playing field where every waste management firm has to meet the same standards. Each MSW landfill has to have a composite liner system consisting of compacted clay overlain by a flexible membrane liner, a layer of sand and piping installed above this liner to extract and remove accumulated leachate, and a final cap of similar construction. In addition to these structural elements, each landfill has to have mechanical systems designed to remove and incinerate explosive landfill gas, collect surface-water runoff and remove accumulated sediment prior to offsite discharge, pre-treat and dispose of extracted leachate, and reduce groundwater buildup. Lastly, each landfill has to meet a set of minimum monitoring requirements and sampling frequencies for leachate, groundwater, explosive gas, and surface water.

Disposal operations require the placement, spreading, and in-place compaction of the waste. This is usually performed with specially designed, heavy earthmoving equipment equipped with dozer blades and sheepsfoot rollers. Placement is performed in a controlled manner with tracking of airspace utilization and quantitative measurement of the compaction efforts. Landfill operations also take into account the need for environmental protection with the placement of daily cover material (either a layer of soil or a synthetic tarp cover) to minimize odors, vectors, blown debris, and the infiltration of precipitation. Modern landfill operations are marked by uniformity to nationally applied standards. Though they may vary significantly in size, configuration, and setting, each landfill’s component and operational tasks are shared in common with little variation.

However, compared with the uniformity of the present, the future presents us with a variety of new methods for the safe management, disposal, and even beneficial reuse of MSW. So let’s stare into the crystal ball and take a look at those technologies most likely to impact the MSW management industry in the foreseeable future.

Plasma Arc Reduction
What if we could completely eliminate our waste while generating electricity and a combustible synthetic gas? The technique to do so is called plasma-arc reduction, and it takes classic waste incineration to the next level by vaporizing waste with temperatures hotter than the sun (over 10,000°F, or 5,500°C). A plasma arc is basically a controlled lightning bolt that passes through the waste, resulting in extremely high temperatures. Its intense heat breaks down organic waste (paper, plastics, food scraps, wood, etc.) into elemental hydrogen and carbon atoms, which combine to make natural gas. The inorganic materials (metals, glass, etc.) are melted down and vitrified into a hard slag material. After the waste is vaporized, only steam, natural gas, and the hard slag residue are left. The steam is vented off, while the gas is used to generate power and the slag is used as a building material. In theory, the process could be a net generator of electricity (the gas turbines producing more kilowatts than are needed to operate the plasma arc) and result in a near complete consumption/utilization of the waste material, making landfills unnecessary.

But as the saying goes, “We should all move to Theory, because everything works in Theory.” The jury is still out regarding the question of whether or not this technology works as advertised. Some pilot projects have ended badly. Furthermore, if chlorine is present in significant quantities, the process may result in dioxin emissions.

However, analysis of this (or any other) technology’s market potential should not be performed in isolation. Even if the process is a net consumer of electricity, if there is no market for the slag as building material, if the managing of dioxin emissions results in additional operational costs, or if the plasma-arc process is otherwise oversold and does not work as advertised, it may still have market viability. If its overall operational costs or losses still do not exceed the capital and operating costs of regional landfilling, plasma-arc waste reduction would be an attractive option to communities strapped for disposal airspace and unable to either expand existing landfills or build new ones.

St. Lucie County, FL, is one community that is deciding in favor of plasma-arc technology. The county intends to build a $425 million, 100,000-square-foot facility designed to vaporize 3,000 tons per day. Not only does it intend to divert its wastestream into this new facility instead of the county landfill, but it intends to dig up the waste in the landfill itself and eliminate it with plasma-arc reduction. The goal is to have the landfill completely eliminated and opened up for commercial development within 18 years. The steam generated by the process will be sold to a local juice plant to run its turbines. It is also likely that the projected value of the land (in a county experiencing skyrocketing real estate costs) that will be made available by elimination of the landfill was a factor in determining the economic viability of this project. Needless to say, these are ambitious goals, and it will be worth watching
the progress.

Waste Pyrolysis and Gasification
Similar in objectives to plasma-arc reduction but different in technique are pyrolysis and gasification of waste. As with the previous technology, pyrolysis and gasification aim to produce a useable source of clean energy while radically reducing the waste mass. Instead of incineration, the processes heat the waste under strictly controlled pressure, atmospheric, and moisture conditions that prevent combustion from occurring. Instead, the waste is converted to liquid and gaseous fuels.

Pyrolysis isn’t really a new process, being as old as the conversion of wood to charcoal. During pyrolysis, waste is degraded by intense heat in the absence of oxygen, which makes combustion impossible. What results is a mixture of solid chars, liquid oils, and synthetic gas. Gasification differs somewhat with the addition of limited and controlled amounts of oxygen. This breaks down hydrocarbons in the waste to make synthetic gas. Again, this is not strictly a new process, going back to coal gasification techniques, but it is a new application when used on waste. To put it simply, both processes substitute waste for the more traditional wood and coal feedstocks.

Photo: Eriez

Metal separation systems are in widespread use.

An apples-to-apples comparison would require matching pyrolysis and gasification to the established technology of mass incineration of waste. When waste is incinerated, it produces heat to make steam, exhaust byproducts that have to be scrubbed from emissions, and ash residue. The ash is typically landfilled, though it takes up only a tiny fraction of the original waste volume (usually less than 10%). By comparison, pyrolysis and gasification can provide a wide spectrum of recoverable byproducts. Not only can the liquids and gases be used as fuel, but they can also be further refined and used as chemical feedstock for industrial applications. A bonus byproduct of gasification is hydrogen, which can be sequestered and utilized as a source for fuel cells and other applications for the “hydrogen economy.” Though it requires the application of energy to create these fuels, they can run gas engines and turbines at much greater efficiencies compared with steam produced by direct incineration of the waste. If not used as the primary waste management option, pyrolysis and gasification at smaller scales can be used to manage residue waste from recycling operations or in conjunction with composting or traditional landfilling.

Waste to Oil
Operating at temperatures and pressures much lower than those required for plasma arcs, pyrolysis, or gasification, thermo-depolymerization (TDP) works to accelerate the natural process of organic decomposition and production of hydrocarbons. Instead of a complete breakdown of the organic waste materials into elemental atoms, TDP causes the materials’ long polymer chains to break down and disassociate into shorter, simpler molecules. A sudden venting of steam causes a rapid drop in pressure. Distillation then results in crude oil and natural gas. Earlier TDP techniques utilized direct incineration, but the modern method substitutes the injection of a stream of water under high pressure.

Most famously, this process has been referred to as “turkey guts to oil” since its first large-scale application has been at a facility rendering turkey entrails and other waste from a slaughterhouse. Biological waste is not the only material that can be treated by TDP. Scrap tires, organic sludges, agricultural waste, wood pulp, paper, and other organic wastes can be fed into the process. Like plasma-arc reduction, the economic viability of this process remains to be seen. While the sale of the resultant crude oil can defray costs somewhat, it is still not known if the overall net costs will keep it competitive with standard landfilling operations or other new technologies. Additional operating costs will also be required for odor controls and exhaust scrubbing. While not a magic bullet, TDP does have the potential to become the waste management option of choice for regions with high landfill operating costs.

While these processes primarily manage organic wastes, they still represent a potentially significant change in waste management techniques. Approximately 75% of the MSW stream consists of organic materials. While much of this is already diverted from landfills by mandatory composting programs in many states, these techniques could potentially cut in half the amount of waste that needs to be landfilled. They also represent a high-tech fix to the problem of waste disposal. Some decidedly low-tech options are available that involve changes in landfill site operations.

Landfill Mining
In addition to the various components of the MSW stream, a typical landfill consists of 5% to 25% dirt by volume. This dirt is cover soil used in daily cover and intermediate closure operations. Landfill mining and reclamation involves the excavation of previously deposited waste. The excavators (clamshells, backhoes, front-end loaders, and hydraulic excavators) load the partially decomposed waste into transporters (hauling trucks or conveyors) that take the waste to be filtered (by shredders, vibratory screens, sieves and trommels). The goal is to recover the up to 10% of the landfill mass that consists of potentially valuable metals and other recoverable materials with sufficient market value.

Given the environmental and health hazards that result from the exposure of partially decomposed waste (explosive gases, disease vectors, organic contaminants, etc.), landfill mining should be limited to those landfills old enough to be sufficiently stabilized. Through stabilization, the percentage of the landfill volume consisting of organic materials is reduced to its minimum. Once started, landfill excavation isn’t much different than strip mining. Neither the equipment nor the method is new. Where possible, any significant strata of soil cover are excavated and stockpiled separately.

Other materials and the soil mixed up in the waste present a more unique challenge. Pre-shredding the waste material into small particles allows for the efficient use of sorting screens. Smaller soil and organic particles pass through the screens, leaving behind larger objects consisting mostly of metals (primarily aluminum). Eddy-current separators (using rapidly spinning magnetic rotors) create an electrical current even in nonferrous (but conductive) aluminum, creating a localized magnetic field that allows for easy separation of the material. Other metals, such as steel and copper, are also recovered.

For most market conditions, the landfill-mining and waste-separation process isn’t cost-effective, though it does result in secondary cost savings. For example, the cost of separating aluminum from excavated waste is significantly lower than the cost of processing aluminum from ore. Another indirect cost saving results from the freeing up of additional airspace. For every cubic yard of airspace made available by landfill mining, there is less need to permit, construct, and operate either an expansion to the existing landfill or a brand-new landfill. By extending the life of existing landfills, significant capital costs can be postponed or completely avoided. Still, only very high scrap-metal prices or prohibitive land costs allow for the economical use of landfill mining. Of the two, the most common justification of landfill mining is high real estate costs and the need to avoid building more landfill areas.

Bioreactor Landfills
Bioreactor landfills use a more indirect approach to reduce existing landfill volume. Most landfills are “dry tomb” landfills where waste is deposited and decomposes very slowly over a very long period of time. The relative dryness is caused by the efficient removal of leachate and waste contact liquids by the landfill’s leachate-collection and extraction piping system. One of the interesting things about dry landfill waste is how little of it actually decomposes. Organics, especially food products, tend to “mummify” over long periods. Newspapers and magazines can still be read decades
after disposal.

The bioreactor landfill takes the opposite approach by purposely introducing large quantities of water into the landfill in order to accelerate the typically slow decomposition process. An additional aid to decomposition is provided by the injection of air into the waste mass, greatly increasing the rate of aerobic decomposition. Instead of being tightly packed in place, waste is kept loose and often shredded to facilitate decomposition. The result is a rapid decomposition of organic waste. This in and of itself is a good thing, but it can have an important side benefit as a precursor to other alternate landfill operations. For example, decomposition of the greater part of a landfill’s waste would make subsequent landfill-mining operations simpler and less costly.

There are actually three styles of bioreactor landfills; each utilizes a different method for decomposition. Aerobic bioreactors utilize oxygen-based decomposition, with the removal of leachate from the bottom of the landfill, the storage of the leachate in adjacent surge tanks, and the recirculation of the leachate back into the waste mass via controlled injection. Simultaneously, air is injected into the waste by vertical well fields or horizontal pipe arrays to promote aerobic bacterial activity. Anaerobic bioreactors recirculate the leachate but without the injection of air, leading to biodegradation in the absence of oxygen. The goal here is to maximize the production of landfill gas (methane) as an energy source. A hybrid bioreactor utilizes a combined aerobic-anaerobic treatment cycle, with aerobic processes applied to the upper portions of the landfill to promote rapid decomposition and anaerobic processes used in the lower half of the landfill to promote landfill-gas production.

Bioreactors occupy a nebulous area under most state environmental regulations. Few state agencies have any regulations directly governing the design and operation of bioreactors. The EPA has an ongoing program of data collection that it will use to establish operating standards and ascertain the advantages and disadvantages of the bioreactor approach.

Onsite Energy
Bioreactors produce methane as a side product, but what about really harnessing the energy potential of a landfill? Waste has always been a source of energy, beginning with the old practice of open-dump burning (though this particular energy was never captured and put to use) and continuing with the extraction of methane as a fuel for making steam to drive small turbines. Now comes a new approach, the conversion of methane into the liquid fuel methanol.

Landfill gas typically consists of methane (50%–55%), carbon dioxide (40%–45%), and trace levels of volatile organic compounds (VOCs, <1%). But before it can be used to make methanol, landfill gas has to be scrubbed of its impurities. First it passes through a scrubber that removes hydrogen sulfide, and then it passes through a distiller that liquefies and removes water vapor. Lastly it passes through a carbon-dioxide “wash” tower that refrigerates and removes carbon dioxide in liquid form along with other volatiles. What is left is almost pure methane. The technology for making methanol from methane has been around for a long time, but this new process makes it economical.

The purified landfill-gas fuel is combined with steam in a catalytic reformer and then sent through a catalytic methanol synthesizer and an optional purifier. As a result, 3 million cubic feet of landfill gas can be converted into 15,000 gallons of methanol. Conversely, the landfill gas can be passed through a shift reactor to produce hydrogen for fuel cells. The EPA estimates that about 500 landfills nationwide are large enough to produce enough landfill gas to make this process worthwhile. It has the potential to provide significant quantities of fuel and chemical feedstocks for local markets.

Pushing Operations
“Fewer” and “larger” are the words that will describe landfill operations in the future. For landfill customers, these words translate into “distance” and “time” as they scramble to find landfills within a reasonable distance capable of handling their wastestreams.

Multimodal transport (the hauling of waste by different modes, such as road, rail, or canal) is the answer to the problem of distance. This has been an established part of waste hauling since the first trash barge or transfer station, and it is based on the simple observation that transport becomes economical only when there is a higher tonnage-to-vehicle ratio. The cost of using waste collection trucks for long-hauling waste is prohibitively high.

Typically, in multimodal-transport operations, waste gets funneled into fewer and larger transport vehicles. Collection trucks haul wastes locally to transfer stations, where the waste is transferred into large, open-topped semitrailer trucks for regional transport. The semitrailers hau