Sludge residuals may also be biochemically digested, stabilized, and perhaps even partially or fully pasteurized using composting operations of the sort shown in Figure 16.49. When exposed to moisture and appropriate environmental conditions, organic, nutrient-rich surfaces will quickly become colonized by bacteria and other microorganisms. Since no organism can be 100% efficient in its metabolism, during the ensuing degradation of the organics, some chemical energy is wasted and given off as an exothermic heat release. Ordinarily, this release of heat would not be noticed, since it quickly dissipates into the environment. When solid-phase organic materials are held in a large pile, however, the pile itself acts as insulation and traps some of the heat. This effect can then lead to a noticeable increase in temperature of the material and is thus referred to as biological self-heating or autothermal metabolism. The intensity of self-heating can be surprising, particularly if the pile is sufficiently porous to allow oxygen penetration and if the available moisture content remains sufficiently high to sustain continued biodegradation. Temperatures of up to 80°C (176°F) can be reached with many materials, at which point subsequent chemical self-heating and eventual combustion may occur if moisture is still present. In fact, important early research on this process was completed in New Zealand during the 1960s, leading to documented reports of spontaneous ignition in piles of wool! In much the same fashion, spontaneous ignition is a familiar phenomenon
to farmers, who must, for example, carefully aerate and cool hay stored in barns to prevent disastrous fires.
This sort of heat production is, in fact, a natural occurrence with any biochemical system, and in our own case provides the thermochemical basis by which we intrinsically maintain an optimal body temperature. Exothermic biochemical energy is also released within aqueous-phase wastewater treatment processes, whether they be suspended-growth activated sludge or attached-growth biofilm processes, but the high thermal mass of the water and the cooling effect of aeration (by evaporation and conduction) typically offset the metabolic heat release to such a degree that it does not affect system temperature. In rather rare instances, high-strength industrial wastewater operations have experienced elevated temperatures due to their elevated levels of heat release. Similarly, the ATAD systems described previously are also designed to benefit specifically from this mechanism, in which aeration is maintained inside insulated reactors at rates close to stoichiometric levels such that their exothermic heat release is not excessively offset by off-gas heat loss.
As applied to the high-temperature degradation of wastes bearing high-level biodegradable solids, composting is an engineered process that utilizes self-heating for waste treatment purposes. By definition, it would be described as a mainly aerobic, self-heating, solid-phase biological treatment process. The goals of composting include stabilization, reduction, drying, and pathogen destruction. Traditionally, composting has been used to treat agricultural wastes such as crop residues and animal manures, and large-scale composting of separately collected yard wastes is a common practice in the United States, particularly in the northeast and midwest, where the fall leaf collection can be sizable (often 10 to 20% of the total MSW for the year!). In the broader context of environmental waste management, composting also has wide applicability for waste-water treatment sludges, municipal solid waste (MSW) fractions, and some industrial (including hazardous) wastes.
The primary objective of composting is to stabilize the waste material being treated. This results in a reduction of mass and volume as well as stabilization, destruction of putrescibles (rapidly degrading, odor-producing compounds). Another potential benefit of composting is that in many cases the final residue is a loamy, soil-like material with an earthy smell, called compost, which may be used beneficially as a soil amendment or surface mulch. The complementary fact that composting is capable of substantial pathogen and weed seed destruction is also quite beneficial. As a soil amendment, compost adds to the organic content of a soil, increasing its friability and its ability to adsorb water and nutrients. Uncomposted wastes are not as suitable for this purpose because they will degrade in the soil, depleting soil oxygen, which can harm plant roots.
Waste materials subjected to composting will typically undergo a succession of microorganisms in relation to progressive changes in waste character and environmental conditions. At first, mesophilic organisms originally present in the material, along with early invaders, will be dominant. This community may be highly diverse, including fungi, protozoans, and even invertebrate animals, such as earthworms, insects, and sow bugs, in addition to bacteria. However, as the ongoing heat release moves temperatures above 40°C, many of these original inhabitants are inhibited, and eventually, most are killed by the heat. At this point, having shifted into a thermophilic realm, small numbers of ther-mophiles that were present find suitable conditions and grow rapidly. This is a more select group, as few eukaryotes can survive at temperatures above 50°C. Above 62°C, the last fungi are unable to grow, and only bacteria (and perhaps archaea) are left. The known organisms in thermophilic composting include mainly Bacillus species, such as B. stear-othermophilus and B. coagulans, and some actinomycetes, although there is evidence from molecular techniques that other groups may also be present. Eventually, as substrate is used up, the material will cool again, be recolonized by the germination of spores that survived the high temperatures, and be reinvaded by mesophiles.
Earlier, some composting enthusiasts seemed to believe "the hotter, the better.'' However, it is now well established that if not controlled properly, most composting materials will overheat, killing or severely inhibiting even the thermophilic microorganisms. This has led to the failure of many composting facilities, since the subsequent rate of degradation slows dramatically, and the remaining putrescibles lead to odor problems. Thus, an important goal of modern composting technologies is to maintain temperatures at a desirable level of ^60°C (140°F) to maintain the desired high rates of microbial activity. As a point of comparison, most home hot-water heaters are set at <130°F; reaching into the interior of an actively compost pile would lead to a serious burn!
It is also important that the composting material be kept mainly aerobic. Only aerobic metabolism releases energy rapidly enough to sustain this degree of self-heating. Also, avoiding extensive anaerobic conditions helps to minimize odor production. Usually, maintenance of at least a 10% oxygen partial pressure in the pore spaces within a pile (compared to the 21% oxygen in ambient air; Figure 16.50) will be adequate, depending
Pile Depth (cm)
Figure 16.50 Biosolids composting pile oxygen tension relative to pile depth.
Pile Depth (cm)
Figure 16.50 Biosolids composting pile oxygen tension relative to pile depth.
on the material and method used. To facilitate the desired transport of oxygen into these actively composting systems, therefore, the composting solids must be maintained in a suitably porous form. If the starting waste materials are too wet, as is the case with most raw sludges, they may need to be partially dewatered and may also need to be mixed with other materials to improve porosity (bulking agents, such as finished compost, MSW, or wood chips). If the pile is too wet, the water filling the pore spaces severely limits oxygenation, and anaerobic conditions develop. On the other hand, materials such as municipally collected leaves commonly start out too dry for rapid microbial growth and thus benefit from water addition. Generally, moisture contents in the range 50 to 70% will give the best results, but this depends on both the material and the type of composting system used. Many of the bulking agents used in composting (e.g., wood chips, shredded tire chips, bark) have an original bulk cost that warrants an attempt to secure their recovery and reuse. As shown in Figure 16.51, it may be possible to screen and recover these
supplemental bulking agents from the finished, composted product so that they might then be reused yet again with fresh sludge. This effort not only saves money by reducing the necessary volume of bulking material but also provides an initial seeding of the coblended material with thermophilic microorganisms, which then promotes faster startup times.
The relative masses of carbon and nitrogen (the C/N ratio) can be important in composting, with a ratio of about 30: 1 often considered desirable. Materials such as dry leaves may have C/N ratios of 80: 1; under such conditions, nitrogen becomes limiting and composting rates are slowed. This is normally acceptable for leaf composting, and N addition is not recommended, but this might be an issue for some industrial wastes. Municipal sewage sludge, though, tends to have a C/N ratio of <8:1, indicating excess nitrogen. This will not decrease composting rates, but can lead to the release of nitrogen as either ammonia gas (making odors more of a problem) or as a potential water pollutant. Addition of a carbonaceous material may therefore be desirable. However, although a bulking agent such as wood chips will increase the calculated C/N ratio, most of the C is unavailable to microorganisms and hence may not produce the full beneficial effect expected.
As with other microbially based treatment systems, the presence of a large number and variety of microorganisms is desirable for rapid composting. However, waste materials typically already contain a high concentration and diversity of appropriate organisms, and under proper conditions their growth will be very rapid. In the few cases where the deliberate addition of microorganisms may be warranted, such as for some pasteurized food-processing residues, this is usually best accomplished by adding small amounts of finished compost or soil to the initial mixture. There is no scientific evidence that the addition of commercially available inocula or ''compost starters'' is beneficial.
On the other hand, waste materials may start out with numerous undesirable biological agents present, such as pathogens, parasites, and weed seeds. Composting can be extremely effective (better even than chemical disinfection, and probably second only to incineration among treatment processes) at inactivation of these undesirables. This is a result mainly of the high temperatures achieved (e.g., Salmonella will be reduced significantly within an hour or so at temperatures much above 60°C), but is also aided by the vigorous microbial activity that occurs. Thus, decreases of well above 99.99% are expected in properly run systems. In fact, a common criterion, maintenance of 55°C for 3 days, is predicted to give a minimum of 15 "9's" (i.e., 99.9999999999999%) reduction of even the most resistant pathogens. Federal (i.e., 503 Rule) regulations in the United States for pathogen control with composting systems stipulate specifically that the temperature of these piles must be held above 40°C or higher for a period of 5 days. This standard also requires that a temperature of 55°C or higher must be reached for a period of at least 4 hours in such piles to maintain the necessary reduction in pathogens.
One special concern with pathogens is that a few may actually grow during some composting processes. The best known example is Aspergillus fumigatus. This thermotolerant fungus is cellulolytic (degrades cellulose) and thus very common in nature and agriculture in soil and decaying vegetative material. However, it produces large numbers of spores that can cause a mild to severe allergic reaction in susceptible people. In a few cases, it is also able to opportunistically invade people with severely weakened immune systems, leading to potentially lethal infections. Sludge composting operations using wood chips as a bulking agent may release very high levels of A. fumigatus spores during the final screening step to remove the wood chips from the compost. Reuse of the wood chips then serves to heavily reinocúlate the new pile with A. fumigatus. Elevated levels have also been observed at some leaf composting sites during the turning of windrows.
The overall composting reaction can be described using a modified form of the basic equation for aerobic respiration:
organic matter + O2 ! CO2 + H2O + compost + heat (16.5)
From this expression it can be seen that the rate of organic matter stabilization is proportional to the rate of heat production. Thus, maximizing the rate of heat production will maximize the rate of stabilization. However, if the material becomes too hot, rates slow dramatically. Once active self-heating occurs, therefore, it is necessary to remove heat at approximately the same rate it is released, so as to avoid exceeding 60°C in the material. This should be a major concern in system design, as the amount of heat requiring removal can be substantial. The amount of heat released per mass of oxygen consumed is approximately 14,000 J/g and is very nearly constant for a wide variety of different organic materials. At the same time, it would be necessary to provide adequate oxygen to reach this oxidative heat release, and to keep moisture and other parameters within a desirable range.
The goal of a composting system is to stabilize the particular material being treated in an efficient, economical, and environmentally sound manner. For some materials, such as leaves, a low-cost system can be used, even though the conditions it provides do not come close to maximizing composting rates. This is because the facility, if it is large and isolated enough, can simply allow extra time for completion (e.g., 6 to 18 months), and the materials can be managed so as not to cause problems during this time. Other materials, such as sludges, however, usually demand closer control and require composting systems that much more nearly achieve maximum rates. Otherwise, problems are likely to occur, and costs may soar.
Although there are many variations, there are really three approaches to large-scale composting: mixing; forced aeration; and both. Any of the three potentially can be done out in the open, under a roof, or in an enclosed reactor, although some combinations are more common and/or logical. Interestingly, almost all systems are operated as batch processes rather than continuous feed, as is the case with most other waste treatment systems.
Probably the most common type of composting, such as that shown in Figure 16.49, is that of windrowing. The sludge-plus-bulking agent windrow piles are constructed in an elongated, haystack shape (in cross section) up to perhaps ~1.3 to 2 m (4 to 6 ft) high and 4 to 5 m (12 to 15 ft) wide, and lengths reaching up to and beyond 100 m (^300 ft). Periodically (e.g., twice a week initially, monthly later) it is mixed, or turned, using a front-end loader or specialized turning machine (as can be seen in the background of Figure 16.49). Windrowing is virtually the only method used for yard waste composting and is also used occasionally for sludges and solid wastes in areas that are sufficiently isolated, handling small volumes, or with other special circumstances. Also, it is commonly used in a curing stage, a low-rate finishing step after a more active composting phase. In some cases, the material is enclosed in bins, and mixed there: essentially, "wind-rowing" within an isolated reactor.
One disadvantage of static piles and other unmixed systems is that stratification occurs with gradients of temperature and moisture within the pile. Thus, some portions of the material may not heat sufficiently for pathogen kill, whereas others may dry or overheat and become inactive. Periodic windrow mixing or turning, therefore, helps to ensure uniform solids breakdown. Although some heat is lost during turning, and some oxygen is incorporated in the pile, the increase in microbial activity spurred by the mixing quickly (within hours) reheats the material and depletes this added oxygen. The height and width of these windrows must therefore be held to values of less than a few meters, such that oxygen may adequately diffuse into the pile interior from the surface. In fact, the ongoing release of heat from windrow piles helps to facilitate this aeration process, whereby the physical air current induced by heat rising upward and out of these hot piles essentially drafts or pulls in cooler air, and oxygen, at the pile base.
In lieu of intermittently mixed windrow systems, there are also units designed to provide nearly continuous mixing and/or aeration for composting wastes. Forced static pile aeration, as shown in Figure 16.51, represents yet another method developed by the Agricultural Research Service (U.S. Department of Agriculture) at Beltsville, Maryland. An aeration system (ducts, or a perforated false floor) in the lower portion of, or under, the pile is used, and control systems can be provided to either increase aeration (if the pile starts to get too hot) or decrease aeration (if the pile starts to get too cool), as needed. In some cases (including that of the piles seen in Figure 16.51), this supplemental airflow is drawn into and through the pile by applying a vacuum to the ductwork, with the exhaust air then being routed through a secondary smaller pile of aged compost material in order to screen out potentially problematic gas-phase odors, fungal spores, and so on. However, forced blower aeration directly into the pile interior has also proven to be effective, particularly in terms of securing direct oxygen entry to the pile's hottest and most active zone. These types of simple, feedback aeration schemes, based on a thermostatically regulated vacuum or blower, have a number of interesting features. First, the pile itself demands the amount of aeration it needs to remove enough heat to keep temperatures in the desirable range. This is important because the rate of activity (heat generation) changes with time as the readily degradable organics are depleted. Second, most of the cooling results from evaporation of water within the pile into the supplied air, at which point the pile starts to dry out. With some materials pile drying may be so extensive that composting rates decrease and supplemental water will need to be added. Often, however, the drying is beneficial, as it substantially reduces the remaining mass and volume and leads to a more stable final product that is more easily stored, transported, and ultimately used with gardening, land application, and so on, measures.
A third feature stems from the fact that the amount of oxygen required for biodegradation of the organic material is closely related to the amount of heat released [as can be seen from equation (16.5)]. Thus, an air function ratio can be defined for forced aeration systems as the amount of air required to remove the heat produced compared to the stoi-chiometric amount required to provide the oxygen necessary for the oxidation reaction that releases it. This ratio will vary slightly based on materials, ambient conditions, and pile temperature, but is typically about 8.5 to 9.0. This ensures that sufficient—in fact, considerably excess—oxygen will automatically be provided by the aeration required for cooling. It is interesting to note that the air function ratio must in fact be greater than 1.0 for self-heating to occur. As a practical matter, some aeration, typically provided by a timer, also is needed to supply oxygen before and after the phase of the process during which aeration for cooling is required: the come-up and cool-down stages.
The most advanced composting systems combine forced aeration for temperature control and oxygenation with mixing to increase rates and uniformity. Such systems include agitated beds and "mushroom tunnels'' (so-called because they were developed in the mushroom industry to provide the high-quality compost needed for the commercial growing of mushrooms). Tunnels are enclosed, with a small headspace above the material, and a portion of the used air can be reused for aeration. This requires some cooling of the air and condensation of the water vapor present but can greatly reduce the amount of fresh air needed (since based on the air function ratio, little of the oxygen is utilized). This in turn reduces the amount of spent gas that must be vented, making control of odors and other volatile compounds simpler.
Example 16.7: Composting Air Function Ratio Sample Calculation Based on equation (16.5) and the typical energy release of 14,000 J/g O2 consumed, an air function ratio can be calculated for specific composting conditions. Assume, for example, that the ambient air is at 20°C and 50% relative humidity (RH), and the air exiting the composting pile is at 60°C and 100% RH. From thermodynamic data (found in psychrometric charts) it can be determined that this represents a change in enthalpy (heat energy) of 362 J/g dry air (part from the increase in temperature, and part from the increase in the amount of water vapor). Thus, the amount of dry air required to remove the heat released from 1 g of O2 consumption is
362 J=g dry air 2
The amount of dry air required to provide 1 g of O2 is only 4.31 g. Thus, the ratio is air required for cooling 38.7g/g air function ratio =-----=-= 8.98
air required for providing oxygen 4.31 g/g
The air function ratio will vary slightly depending mainly on the temperature and RH of the inlet and outlet air. However, it is usually between 8.5 and 9.0 for conditions that are likely to be encountered during thermophilic composting.
A number of other composting system designs have also been of initial interest because of their materials handling approaches, including semicontinuous feed systems such as silos (in which materials were added to the top and finished material was removed from the bottom) and rams (in which material was pushed along in a "tunnel"). However, these particular approaches greatly compacted the material, destroying the porosity that was essential to movement of air through it. It is important to keep in mind that although efficient materials handling approaches are important for cost-effectiveness, the system will fail if the basic biological requirements of the composting process are not sufficiently met.
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