Biodegradation of dead plant, animal, and other residues has played an important role on our planet's surface since the inception of life eons ago, recycling valuable nutrients back into our biosphere while negating or at least reducing the shear physical burden of this never-ending natural debris stream. At least in theory, therefore, nature provides a highly useful example of a means by which we might manage our own solid waste residuals, taking full advantage of much the same aerobic and anaerobic degradative mechanisms.
The level of success presently realized in using biology effectively as a management process for human solid waste residuals has, however, considerable room for improvement. At the low end of this spectrum of biochemically engineered solid waste management strategies, the majority of municipal detritus generated by the world's current leader in per-capita solid waste production (i.e., the United States) follows a least-cost disposal
pathway with little, if any, thought given to promoting, let alone maximizing, its biological degradation, resulting in the consequent loss of potential resources, including nutrients, organic material, and energy. Indeed, expedient burial in subsurface landfills (Figure 16.56) has in this case become the U.S. norm, with emphasis given to prior source reduction and recycling of various fractions of the original waste as a means of reducing the shear magnitude of the problem.
However, there are areas in the world that exhibit clear and escalating evidence of a national shift toward biochemically engineered solid waste processing systems involving controlled biological strategies. Europe and Asia represent two regions where escalating land constraints that limit landfilling options are matched both by a national motivation to pursue environmentally friendly disposal strategies and the financial wherewithal and willingness to accept technical solutions considerably more expensive than mere burial.
Commensurate with the continuing escalation of our world's population and its attendant increase in human solid waste production, future generations will no doubt be aggressively challenged to pursue a better universal means of managing their solid waste problems. Although biology may, at best, be presently considered a minor, hidden aspect of solid waste processing, at least for much of the world, the opportunity at hand to employ renewable metabolic strategies beneficially will probably continue to attract increased attention (Palmisano and Barlaz, 1996).
One of the fundamental issues with solid waste residues is that of characterizing its nature and source, principally in terms of how degradable or recalcitrant it might be. The term solid waste truly covers a wide range of high-volume residues, including not only municipal wastes of the sort that might be generated in your own home to industrial (e.g., food-processing residues, spent foundry sands, slag), agricultural (e.g., manure, bedding, plant residues), power (e.g., fly ash, bottom ash), and even mining (e.g., overburden) wastes. Only two of these fractions, the municipal and agricultural groups, include putrescible materials, and even in the latter case, a large segment of these wastes (e.g., manures) are already being biochemically recycled for the beneficial purpose of rejuvenating farmland productivity.
The municipal solid waste (MSW) segment itself is composed of many different fractions whose putrescibility varies widely. At one end of this spectrum, food scraps have the highest level of potential degradability, due to both their organic makeup and their high water content. Cellulose-rich residues, whether discarded as lawn clippings or paper products, would also be amenable to biochemical degradation, although perhaps at a somewhat slower rate. Extending beyond these two segments, though, municipal solid waste includes many other materials whose composition will not be amenable, and possibly even antagonistic or inhibitory, to biochemical degradation. For example, MSW generated within affluent countries includes a sizable proportion of plastic, for which the vast majority will have no susceptibility to biochemical breakdown. Similarly, affluent countries generate municipal solid wastes with a proportionately higher percentage of metals (e.g., cans, batteries, used appliances) whose presence may actually lead to the release via leaching of soluble heavy (e.g., cadmium, chromium, nickel, lead) and transition (e.g., arsenic, selenium) metal ions that are detrimental to desired biochemical activity.
Yet another key issue is that of available moisture, since there must be adequate water present to facilitate and sustain the growth of biological populations. Contrasted against this necessity, though, modern landfills are provided with overlying physical caps (i.e., built with impervious clay and/or plastic liners) that intentionally limit the influx of water into their buried wastes, such that limited moisture presence within the subsurface waste matrix could well become a constraining factor. Indeed, solid waste materials excavated from decades-old landfills have in many instances proven to contain a remarkable amount of undegraded putrescible residuals (e.g., corncobs, newspapers) whose lingering structural integrity reflects what appears to be a desiccating condition within the waste vs. a moisture-bearing environment conducive to microbial degradation.
Extending beyond water presence, a number of additional ambient environmental conditions come into play. Nutrient availability will certainly be an issue, not only in regard to the macroscale distribution of carbon and nitrogen (i.e., for which a C/N ratio of ^20: 1 is typically considered optimal), but also in terms of the available presence of lower-level essential elements (e.g., phosphorus, sulfur, potassium, iron, calcium). While the pH of the constitutive moisture must also be suitably conducive to microbial activity, fermentative metabolism will progressively release weak organic acids that could well shift the pH to a more acidic, and less optimal, state. In fact, a downward shift in pH of this sort might accordingly escalate the undesired rate of trace metals leaching from the waste matrix, thereby leading to inhibitory metal toxicity.
Finally, the relative combination of high solids and low moisture found in solid waste streams effectively yields a high-level specific energy content (i.e., cal/kg) that is higher than that of most other biodegraded waste (e.g., greater than wastewater and sludge), with the sole exception of agricultural manures. Commensurate with effective biodegradation of these wastes, therefore, the heat release per mass of degraded solids would not only be considerably higher, but also apt to be trapped inside the high-density waste, due to its insulating nature. On the one hand, this heat release and temperature increase could help to accelerate the ongoing biochemical process. However, as is the case with composting, this thermal buildup could accelerate evaporative water loss to a degree that eventually retards the desired activity.
There are several engineering and operational aspects of the current design and management of MSW landfills that are frankly contrary to a goal of optimizing their biological degradation. For example, restricting water egress into and out of this buried waste is routinely considered a beneficial environmental goal for landfill operations, even though this effort could well end up slowing down biochemical degradation. By reducing the hydraulic pressure gradient that might accrue with an interior water buildup, the transport of waterborne contaminants into the adjacent groundwater table can subsequently be reduced or obviated. At no point during this process, either during active filling of the landfill or after final closure, will the moisture content of the material ever come anywhere close to an optimal value relative to maximal microbial activity. Unlike composting, therefore, where moisture control is addressed routinely, landfills have no management plans that involve steps to proactively increase moisture content. Furthermore, efforts taken to heavily compress the waste to minimize its bulk volume, or of using a daily cover of —15 cm soil to restrict rodent and other vector access, work against desired microbial degradation, as these measures impose physical constraints that degrade process homogeneity. In fact, the mere circumstance of allowing homeowners to isolate much of their daily MSW within plastic bags contributes yet another complication, further exacerbating the waste's isolation into discrete, semi-isolated packets.
Given these inherent shortcomings with landfills in the context of biological MSW degradation, several alternative engineering strategies have been developed to secure higher levels of success. These systems are typically charged with a presorted waste stream whose degradable content has been enhanced by way of separating out a large fraction of the relatively nondegradable content (e.g., plastics, glass, metal). The engineering features of these systems subsequently vary in relation to operational solids content. In-vessel digester reactors are maintained with the lowest solids content and the highest water contents, including both low-solids units at 4 to 8% solids and high-solids units with solids values in the low- to mid-20% range. MSW is shredded and slurried before introduction to these reactors. These digesters include both aerobic and anaerobic options, much like sludge digestion vessels.
A second MSW processing option, composting systems, is again quite similar to the equivalent strategies used for sludge treatment, with waste solids levels approximately double that of high-solids MSW digesters (i.e., 40 to 50%). Both aerobic and anaerobic composting strategies are available, maintained in static and windrowed piles held either in open or in-vessel configurations. Yet another option is that of codisposal systems, treating a blended mixture of MSW and sludge. As compared to MSW digestion options, though, and their relative focus on volatile solids destruction, MSW composting reactors provide an overall volume reduction tied both to volatiles destruction and moisture loss allowed to accelerate at the end of the processing cycle during a final curing phase. Volume reductions on the order of 50% have been observed with both aerobic and anaerobic composting systems at processing times on the order to 3 to 4 weeks.
Finally, it is worth mentioning a process in which animals are the basic organism. Vermiculture (raising of earthworms) or vermicomposting uses earthworms to help decompose organic material, especially food-processing wastes (Datar et al., 1997). Vermicomposting produces stabilized waste similar to ordinary composting, although the process must be maintained at lower temperatures. The process has been applied mostly at small scale, including household use, although large-scale facilities exist that process up to 20,000 metric tons of organic waste per year.
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