-Eneigy Efficient Solid Waste Management The principles of waste management efficiency are outlined, with particular reference to planning, design and equipment best suited to objectives. CLARENCE G. GOLUEKE Senior Editor Compost Science/Land Utilization

ENERGY EFFICIENT solid waste management is another name for resource recovery. The principle difference in the terms is simply one of the underlying motivation. In the resource recovery of a few short years ago, the “motivation” or “rationale” was the conservation of non-renewable and renewable resources. The rationale was based on the finite nature of the supply of non-renewable resources and hence to stretch that supply as far as possible through prudent use and reuse to the fullest extent possible. With respect to renewable resources, the reasoning was along one of two or of both lines. One was that, if consumption were left unbridled, our resources would be used up more swiftly than they could be renewed. The alternative line is that the production of renewable resources involves the use of space, Le. land, which must also be used for the production of food. As the population expands, so does the demand for food and for the renewable resources. The result is that land becomes limiting to both. With the realization of the all too pervasive and universal energy shortage, the emphasis has shifted to energy conservation, a shift which is reflected in the designation “energy efficient waste management”. If one analyzes the term and its implications, the similarity between expressions “energy Conservation” and “energy efficient” waste management soon becomes apparent. Strictly speaking, energy efficient management is a form of resource conservation in that every resource that is conserved represents a reduction in energy consumption, and hence an increase in the energy efficiency of waste management. This is true because it is almost axiomatic that the 20

transformation of a discarded resource into a useable raw material requires less energy than is needed to process virgin material. Of course, the lower net usage of energy in the utilization of recovered resources is based upon the assumption that the total energy expended in collecting, separating, and processing the reclaimable resource does not exceed that required to process a virgin material. Where the meaning of the expression “energy efficient management” departs from that of “resource recovery” per se is in the extent of its coverage. Resource recovery is but one as-

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“Another aspect.. is conservation through use of low energy consuming technology.”

pect of energy efficient management. Equally important aspects are: 1. the manner in which the resource recovery is accomplished; and 2. the actual mechanics of the management activity itself. With respect to the second aspect, efficiency is attained through rational planning, satisfactory design, and the use of equipment best suited to the task to be accomplished. The planning activity should be more than an exercise in operations research or systems analysis. It should include inputs from sociology and political sci-

This paper was presented at the Tenth Annual Composting and Recycling Conference, sponsored by The JC Press, in Los Angeles, May5-7, 1980.

ence, and certainly from the field of public health. Design and the selection of appropriate equipment are activities best left to engineers qualified not only on the basis of education, but also on experience. Experience with solid waste management ranks tops among the essentials. Another aspect of energy efficient management, actually an extension of the mechanics of the management activity, is conservation through the use of a low energy consuming technology in the management of wastes. Here, all aspects of the technology are taken into consideration, i.e. the total energy involved in producing the associated equipment and expended in operating and maintaining that equipment. Taken in this overall sense, a generalization might be made to the effect that low technology approaches involve a lesser total expenditure than do high technology approaches. Whether the same relation extends to the efficiency of the utilization of energy is open to debate. Note the distinction, between total expenditure and efficiency of expenditure. A small amount of energy can be used inefficiently; whereas the reverse can be true in the c,onsumption of a large amount of energy, or vice versa. Since the usual corollary of low technology is “labor intensive”, the question becomes one of equating human energy and efficiency of usage with that of non-human energy and its consumption by machines. For our purposes in this presentation, composting and the reclamation of energy from wastes are two examples for an illustration of the bearing of resource recovery and conservation on energy efficient waste management. Compost SciencelLand Utilization

Composting The reclamation aspect of composting comes through the conversion of potential plant nutrients in organic wastes into a product that can be used in the production of food or other plant materials. Moreover, the process is not a simple reclamation of fertilizer elements. On the contrary, composting is, so to speak, the “packaging” of those elements into a product that has a utility in crop produ‘ction greatly in excess of the value of the reclaimed fertilizer elements. Since the conversion can be done in a manner that poses no hazards to human health nor deteriorates the environment, it constitutes a desirable means of waste processing, provided economic constraints are not ignored. Neither time nor the occasion warrant a detailed dissertation to exemplify and “prove” these glowing encomiums. Instead, one or possibly two notes of caution are discussed, and a few words of advice are offered. The cautionary notes deal with the danger of expending more energy and, not to be ignored, money than are necessary. The margin between energy and money expended in converting wastes into compost and those gained from the production is not great. Consequently, the amount of permissible expenditure is very little, even with making allowances for the increase in crop yield from the use of the compost product and for its role as a waste disposal mechanism. A practical lesson to be drawn from this note of caution is the need to avoid unnecessarily complex reactors when simple systems can suffice. Thus, monetary and energy savings from an apparent reduction in time required to produce the compost should be weighed against the expenditures required to bring about the reduction. For example, since the demand for the compost product is largely seasonal in nature, little is to be gained by accelerating the production process during the off-season. Of course, it could be argued that space requirements for storage are less than those for the active stages of the process. An argument in favor of speeding * the process is that by so doing, the required reactor volume is reduced and the period of intensive handling is shortened. But then it can be said that with a less hurried system, the total amount of handling is no greater, or

Columbus, Ohio sludge composting project uses Cobey machine in its extended aerated pile operation.

it is or has been used), and the quality of the compost product. An important aspect of quality of product is stability. All too often the product from certain mechanized systems is far from stable. Nor should instability be camouflaged by dehydration (dessication). An unstable product not only is a source of nuisances, it also could well be detrimental to crop production. Past and

effort is spread over different lengths of time. Another factor not to be overlooked is whether the acceleration is real or only apparent, and if real, just how much. In making the assessment, both the “active” and the “maturation” stages must be taken into consideration. A good compromise in terms of complexity is the system shown in Figure 1. It is a modification of one of the Buhler proposed systems. If one leans towards mechanical or enclosed systems, the Metro system as illustrated in Figure 2 is a good candidate. Other factors to weigh are past experience with the system of choice (history and status of operations in which Fig. 1

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a state in which it can be handled, stored, and/or applied to the land without adversely affecting the environment. Three bases of classification in composting that have practical significance are degree of aeration, temperature and technology. The resulting classes are (1) aerobic vs. anaerobic, (2) mesophilic vs. thermophilic and (3) mechanized vs. nonmechanized systems. Synonyms for the third classification are closed vs. open composting and mechanical vs. windrow composting. Aerobic composting is the designation given those compost processes that involve decomposition in the presence of air. (Conversely, anaerobic composting implies decomposition in the absence of air). Modern compost systems are aerobic for several reasons-aerobic processes are not characterized by objectionable odors; they are better for public health and crop safety due to the fact that an aerobic pile reaches levels about the thermal death point of most plant and animal pathogens and parasites; and aerobic composting is more rapid than anaerobic fermentation. Second, modern composting practice calls for the involvement of mesophilic followed by thermophilic conditions. The heat energy responsible for elevating the temperature originates in processes that are internal to the composting mass rather than from the deliberate application of heat from an external source. The classification mesophilic vs. thermophilic is based on the temperature range within which the process takes place. The last classification, mechanical vs. windrow, is based upon the technology involved in the compost operation. Mechanical composting involves the use of mechanized, enclosed units equipped to provide control of the major environmental factors. Open or windrow composting implies stacking the raw material in elongated piles (windrows) and allowing the composting process to proceed therein. Composting has the limitations of biological systems and the process is affected by the basic environmental conditions that influence biological activity. The presence of the proper environmental conditions is a requisite to the realization of the potential of the composting process. present experiences of others with the candidate system is of inestimable value in appraising the good and bad points of a proposed system, and of evaluating its utility in terms of its proposed use. Energy Reclamation and Production

The energy in wastes can be reclaimed in two broad ways: It can be reclaimed as energy, Le. direct production; or it, the waste, can be converted into an energy source, Le. a fuel. A third possibility is to use the transformed energy as a chemical feedstock, thereby conserving energy. The direct production of energy is done thermally. Common methods are mass burning, incineration, and the use of refuse derived fuel (RDF). Indirect production can be done either thermally or biologically. Pyrolysis is the most common method of indirect production. Pyrolysis systems presently proposed actually are hybrid systems in that they include a pyroli22

zation step and a combustion step. Biological processes fall into two broad classes, namely “biogasification” and alcohol fermentation. Our interest is in the biological processes. *. Alcohol Fermentation: Alcohol fermentation is a two-step process: cellulose hydrolysis to produce glucose, followed by fermentation of the glucose to ethanol. From the preceding sentence, one perceives that ethanol fermentation is limited to cellulosic and carbohydrate fractions of the waste stream. The hydrolysis step can be accomplished either biologically or non-biologically (i.e. chemically and physically). The fermentation of glucose (and other sugars) is by way of straightforward conventional fermentation. Ethanol production from solid waste has far to go before it assumes a meaningful place in everyclay waste I management. Biogasijcation: “Biogasification” is a commonplace term, and as almost everyone knows, it is applied to the production of methane through the

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most a half century in the treatment of wastewater solids. However, the use in wastewater management has been and probably continues to be centered more on the treatment aspect of the process than on the methane production aspect. In terms of energy reclamation, biogasification systems may be divided into two very broad groups, namely those involving the use of reactors designed to promote biogasification; and those which are designed to capture the methane generated in suitably landfilled wastes. For convenience of reference, the first group probably could be designated as being “active”, whereas the second group (landfill) would fit under the heading “passive”. The biological processes in the two groups are identical with each other. Because of time limitations, a description and a discussion of the principles of biogasification are not given in this presentation. Anyway, most of /” the attendees at this Conference are sufficiently conversant with the process and its many peculiarities. More appropriately, our attention is directed now to the factors which determine energy potential. Factors Which Determine Energy Potential

Factors which to a large extent determine the energy potential of a biogasification enterprise are efficiency of conversion by the reactor, quality of the gas, and the climatic conditions of the locale of the operation. The conversion efficiency of the digester is expressed in terms of the percentage conversion of potential energy (inherent energy of the input biomass) converted into a combustible gas. Because of the nature of the biogasification process, the conversion efficiency is always less than with combustion processes. This is especially true when municipal refuse is concerned. The difference between potential conversion efficiencies of biogasification and those of thermal processes lessens as the moisture content of refuse or of any other waste increases. The lessening is due to the expenditure of energy to bring the biomass to the degree of dryness required for combustion. The limitation on the energy conversion efficiency of Compost SciencelLand Utilization

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biogasification stems from two main factors: 1) Not all of the carbon is utilized to produce methane. Some is oxidized to carbon dioxide, and some is used in the synthesis of new cellular mass. 2) The second factor is that not all of the carbon in the wastes is accessible to microbes, and thus a portion of the energy in the wastes remains untapped. The amount untapped remains a function of the percentage of biologically resistant (refractory) substances in the wastes. The refractory material becomes a more or less major constituent of the residue. The second factor pertains not only to the completely inert fraction, but also to that which is but slowly broken down. An example of the latter is wood. Theoretically, wood could be anaerobically digested, but the rate of breakdown would be so slow and the resulting detention time so long as to make the process economically, if not technologically, unfeasible. While it is true that the digestibility of the refractory fraction of the waste can be increased significantly by suitable processing (e.g. high temperature plus acid or alkaline exposure), or the undigested residue can be combusted, the energetic and economic feasibilities of the two approaches remain to be demonstrated. An idea of the conversion efficiencies may be gained from the data in Table 1. The efficiencies listed in the table agree in the main with those reported in the literature. The point of interest is that only from 25 to 35 percent of the energy in the input material was converted into the energy of methane. Judging from the low yields of methane from the digestion of cattle manure, the conversion efficicy would be lower than that with municipal ref-

ENERGY EFFICIENCIES WITH MlXTuRiES OF SLUDGE AUD REFlBE’~2

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‘Reference: Diaz, L.F. Energy Recovery Through Biogasification of Municipal Solid Waste. . . . Doctoral Disseflation, Univ. California 1973

use. The further reduction in efficiency is due in part to the fact that the material has already been partially digested in the animals’ rumens. Quality of the Gas

It need not be pointed out that quality of the biogas determines its utility. In this case, quality is a function of the heat content and of the amount of the contaminants in the gas. The methane content of the gas determines its heat value. The methane in gas from a properly functioning digester should constitute from 60 to 70 percent of the total gas produced. Carbon dioxide makes up most of the remainder, and hydrogen sulfide in t h e amount of 0.1 percent together with trace amounts of hydrogen, ammonia, and oxides of nitrogen may also be found. In general, the relative percentages of carbon dioxide and methane are functions of the feed material. The heat value of biogas is relatively low, namely, on the order of 18,630 kJ to 26,080kJ/m3 (500 to 700 BTU/cu ft). The principal impurities responsible for the low heat value, and which also would accompany its use as a fuel are the carbon dioxide and hydrogen sulfide. Use of the gas as a fuel in internal combustion engines is attended by difficulties in the form of corrosion in the engine due to the oxidation of the hydrogen sulfide to sulfurous and sulfuric acid in it. Furthermore, the gas is not of pipeline quality and therefore can not be economically transported more than one or two kilometers. However, the gas can be upgraded such as to make it a satisfactory substitute for natural gas. For example, dehydration accompanied by carbon dioxide and hydrogen sulfide removal results in a heat value of 22,360 to 26,000 kJ/m3. Landfill Gas

.Much attention is being directed to the recovery of the methane generated in landfilled municipal waste. Typically, if all of the organic carbon in a fill were transformed into gas, the theoretical yield would range from 0.43 to 0.51 m3/kg of wet waste. However, in practice, the reported yields have been much lower, namely on the order of 0.006 to 0.38 m3/kg wet waste/yr. The reasons for the low actual yields are several, not the least of which are boundary losses, collection losses, and gas generation before and after the

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period in a landfill depends upon the composition of the waste, internal moisture content, temperature, and other environmental factors. In general, a mature landfill that has reached its maximum gas generation rate, variously estimated to occur between one and five years after completion of the fill, will produce gas at a high rate for at least six to ten years, and at much lesser rates from 30 to 100 years. According to Bowerman, the practical collection period is about five years, and ten years at the longest. The minimum size or volume of a landfill for economic collection is dependent upon the economics of gas recovery wells and the collection system. processing equipment, delivery system, and final gas use. In general, assuming proper design and operation, the recommended minimum size landfill would contain no less than about two million tons of municipal solid waste. At near peak generation rates, from 28.32 to 33.98 m3/min of raw gas would be generated per day. The heat equivalent would be 759 MM kJ/day (720 MM Btu/day).

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The smallest landfill area at which a gas system would have strong economic viability would be from 11.3 ha for a fill with an average depth of 45.7 m to about 3 1.6 ha for one with an average depth of 15.2 m. Fills of these two sizes would generate about 50,976 m3/day of landfill gas at a unit cost of about $1.00 per 1.054 MMkj of dehydrated product. Proposed System for “Total” Resource Recovery

Several years ago, a group of researchers who now make up the nucleus of Cal Recovery Systems, Inc. (C.R.S.) developed and put into practice on an experimental scale, the concept of a system designed to recover practically all of the discarded resources that make up municipal solid wastes. The system embodies both thermal and biological methods of energy recovery, as well as the reclam-tinn

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modular in make up and is thoroughly flexible in application. Consequently, the degree of mechanization can be varied to suit local conditions. Excepting for the compost and anaerobic digester units, a working model of the complete (Le. integrated) system capable of processing four tonnes of domestic refuse taken di-

rectly from the collection truck was constructed and operated by the researchers at the University of California Richmond Field Station under contracts and grants from the U.S. E.P.A. It should be pointed out that the shredder and air classifier units have a throughput capacity of 10 tonnes per hour. Composting was tried on

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present, another full scale plant is in the course of being designed. A flow diagram of the system is shown in Figure 3. As the figure shows, the mixed paper stream has three different outputs. Because of this characteristic, all paper fiber can be used to best advantage. An aluminum separation step is included in the diagram, although at present no completely successful device is available. The authors obtained fairly good concentration of the metal by differential screening, but the product did contain an unsatisfactory amount of “impurities”. Since, as may be assumed from the flow diagram, the amount of residue destined for the landfill is very small, it can be concluded that the system meets one of the important criteria for energy efficient solid waste management, namely, maximum resource recovery. Whether or not it would meet other requirements if applied to a fullscale operation, remains to be demon- 2” strated. However, results of a variety of studies conducted with the use of . the system furnish a basis for a reasonable amount of optimism.

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INCREASING DEMAND FOR ORGANIC SOIL CONDITIONERS

Zook & Ranck, Inc. is located in the heart of Pennsylvania Dutch country in the town of Gap, offering a range of organic soil conditioners. “We form composted products according to specified formulas,” explains their sales brochure, “so as to lose as few natural nutritional and trace elements as possible and produce humus of highest quality.” Across the country, 15 miles northeast of Oakland, California Organics operates a 40-acre composting yard to which local companies deliver about 200 cubic yards of waste materials daily. Coffee and tea processing plants and a couple of paper mills provide raw materials as coffee grounds, tea leaves and paper sludge that the company turns into soil conditioners and potting mixes. Says James O’Neal of California Organics: “Ten years ago, we began in a small way to compost waste materials and produce soil conditioners. As the demand grew, we gradually increased our production capacity. We originally operated a small Royer shredder-mixer to process the decomposed materials. Today, our annual sales of $3 million are based on the output of two of Royer’s largest units.” The Whittier Fertilizer Co. of Pic0 Rivera, California was founded 50 years ago, and today produces two million bags of such products 24

Whittier Fertilizer’s Bob Osborn checks bags and sees “rosy future.”

as Superior Brand Compost in 1% and 2 cubic foot sizes. Raw materials include steer manure, wood shavings and sawdust. Comments Whittier Fertilizer head Bob Osborn: “With transportation costs rising out of sight, we see only a rosy future for our locally-produced soil conditioners and potting mixes.” Compost SciencelLand Utilization