BioCycle January 2005, Vol. 46, No. 1, p. 28
Studies provide data to quantify principal differences between anaerobic and aerobic decomposition in large cells – and impact from organic matter “storage” on energy generated.
Nickolas J. Themelis

A WELL-KNOWN fact is that a large amount of heat is released during composting. For windrows and small composting cells, this heat is dissipated by natural convection and radiation from the surface of the cells. However, for large composting cells, most of this heat must be removed by means of water evaporation into the air flow that should be maintained through the cell. This air flow can be ten-fold higher than that required for the aerobic reaction.
Results of the analysis described in this article were validated by comparison to a state-of-the-art composting operation in Edmonton, Alberta. The study highlights a basic difference between large-scale aerobic and anaerobic digestion (AD): In AD, much of the chemical energy in the composted matter is “stored” in the generated methane gas.
CHEMICAL COMPOSITION OF FOOD AND YARD WASTES
Using the ultimate (atomic) analysis of mixed food and yard wastes (Tchobanoglous et al., 1993) and the atomic weights of the respective elements, the corresponding molecular formula is calculated to be as follows:
C6H9.6O3.5N0.28 S0.2
If one excludes the minor constituents of nitrogen and sulfur, the average molecular structure of mixed food and plant wastes can be approximated by the molecular composition C6H10O4, which corresponds to about ten organic compounds, such as ethyl butanedioic acid, succinic acid, adipic acid, ethylene glycol diacetate, and others.
BIOCHEMICAL REACTIONS IN COMPOSTING
In the presence of oxygen and moisture, the organic wastes undergo oxidation to carbon dioxide and water vapor (anaerobic bioreaction):
C6H10O4 + 6.5O2 = 6CO2 + 5H2O. . .(1)
From the heats of formation of the compounds in Equation 1, one can calculate that reaction (1) generates about 17.8 MJ per kilogram (7,660 Btu/lb) of dry organic matter (C6H10O4) oxidized. The same amount of heat would be generated if the organic materials were combusted in a waste-to-energy facility. The only difference is that combustion takes place in minutes and digestion in weeks. To avoid overheating of the composting cell, the rate of heat generation must be balanced by the heat losses of the cell:
o Heat loss by convection and radiation from the surface of the cell exposed to the atmosphere;
o Sensible heat carried in the air flow through the cell;
o Heat of evaporation of water from the composting mass to the air flowing through the cell.
In windrow and small composting cells, the surface to volume ratio of the windrow is large enough to allow for heat dissipation. However, for large composting cells, it is necessary to provide a sufficient air flow through the cell so as to carry away some of the generated heat.
It should be noted that the anaerobic digestion of food and yard wastes (i.e. composting in the absence of oxygen)
C6H10O4 + 1.5H2O = 3.25CH4 + 2.75CO2 . . . (2)
releases a very small amount of heat. Most of the chemical heat contained in the organic matter is transferred to the methane component of the biogas produced which consists of about 54 percent methane and 46 percent carbon dioxide.
RATE OF COMPOSTING REACTION
On the basis of reported experimental data on the rate of bioreaction of organic matter (Sesay (1998), Muller (1998), Murphy (1995)), Themelis and Kim (2002) estimated how decomposition is expected to proceed with days of operation, at different cell operating temperatures (Figure 1). At 50°C (122°F), it takes about four weeks for 50 percent degradation of the organic matter. The projected composting period is in fair agreement with the reported results of aerated in-vessel systems, such as the agitated bins of Longwood Manufacturing Corp. (King, 1999). Nearly the same composting period was reported for the treatment of biosolids mixed with wood chips and sawdust in aerated, static piles.
REMOVAL OF HEAT BY AIR FLOW AND WATER EVAPORATION
Let us now consider the required air flow for controlling the temperature of a large cell at 50°C (122°F). This cell is similar to the Longwood cell and is 60 meters (190 feet) long by 10 meters (33 feet) wide and 3 meters (10 feet) deep. It composts a load of 900 metric tons (990 short tons) of mixed food and green wastes containing 36 percent organic matter and 64 percent water. Air is injected through the bottom of the cell at 20°C (68°F) and 50 percent relative humidity, and leaves saturated with water vapor (100 percent relative humidity) at 50°C (122°F). For these entry and exit conditions, the water vapor table (psychrometric chart) shows that the increase in moisture in the air flow amounts to 0.07 kilograms of water per kilogram of inlet air (0.07 lb water/lb air).
The oxygen requirement for 50 percent decomposition of the feed (36 percent organic matter) to the cell is calculated from Equation 1 to be 0.95 tons of air per ton of feed material. However, a much higher air flow is required to remove the heat generated by the biochemical reaction during composting. The overall heat loss by means of natural convection and radiation from the surface of the bed and the outer surface of the walls of the cell during a four-week composting period was calculated (Themelis and Kim 2000) to be only 24 percent of the heat generated by the composting reaction. The rest of the heat must be removed by means of the air flow and water evaporation through the bed. Using the chemical rate data of Figure 1, it is possible to calculate the heat generation and the required air flow during the entire composting period. The results are shown in Figure 2, where the air flow through the composting bed is expressed as superficial velocity (i.e., volumetric air flow rate divided by the cross sectional area of the cell, length x width).
At the cell temperature of 50°C (122°F), the average superficial velocity of air through the cell is 0.67 cm/s and the peak air velocity 1.1 cm/s. Over the 28-day composting period, the average superficial velocity of 0.67 cm/s corresponds to 11.7 tons of air per ton of feed material to the cell. In other words, the requirement for cooling air is twelve times greater than the chemical requirement for oxygen. Also, since the air flow carries away 0.07 kilograms of water per ton of air flow, it is necessary to keep adding water during the operation of the cell. Overall, to close the heat balance, it is necessary to evaporate 0.7 tons of water per ton of feed to the cell. The material and energy balances for a composting period of 28 days at 50°C (122°F) are as follows:
Material balance/Material flow to cells: Air-10,700 tons or 87%; Organic feed-900 tons or 7.3%; Water-700 tons or 5.7%.
Energy Balance/Heat Dissipation: Heat in water vapor-63.1%; Heat in air flow-13.4%; Heat loss of cell: -23.6%.
COMPARISON WITH AN OPERATING COMPOSTING CELL
The above calculations were based on theoretical considerations and on experimental data. They were validated by comparing them with a composting facility in Edmonton, Alberta. The composting period in both cases is about 28 days. Table 1 compares the actual operating conditions at one of the three operating cells in Edmonton (Lapp 2003) with the hypothetical cell discussed above. Air flows through the Edmonton cell at a superficial velocity of 0.82 centimeters per second vs. the calculated 0.67 cm/s of the “theoretical” cell. Also, the total air flow during the composting period in the Edmonton cell corresponds to 20.5 tons of air per ton of feed to the cell, vs. the 11.88 tons computed for the “theoretical” cell. The higher air consumption per ton of feed in the Edmonton cell is due to the fact that the bed height is only two-thirds that of the hypothetical cell.
CONCLUSIONS
Theory and data from several experimental studies were used to estimate the air and water flow requirements for a hypothetical large composting cell. It was found that the calculated heat losses by natural convection and radiation from the exposed surfaces of the cell amounted to only 25 percent of the generated chemical heat. The rest of the heat must be removed by means of water evaporation into a large air flow through the cell. This air input is nearly twelve times that required for chemical reaction. The power consumption to deliver this amount of air was estimated to be on the order of 120 kWh per ton of feed material. The bulk of the heat is removed by means of water evaporation that nearly saturates the air flow and carries about 63 percent of the chemical heat from the cell. These analytical results were validated by comparison with a composting operation in Edmonton, Alberta.
Most of the composting in the U.S. is carried out in small-scale cells (e.g. “windrows”), or by simply spreading grass cuttings, etc., on areas to be fertilized. For composting large quantities of food and plant wastes, it is necessary to build large-scale industrial plants such as the Edmonton facility. This study has quantified the principal difference between anaerobic and aerobic decomposition in large-scale cells: In anaerobic reaction, most of the chemical energy of the organic matter is “stored” in the methane gas produced, which then can be used as a fuel. In aerobic bioconversion, most of this energy must be carried out in a continuous and relatively large air/water vapor flow through the cell.
Nickolas J. Themelis is with the Earth Engineering Center and Department of Earth and Environmental Engineering at Columbia University in New York City.
References
Keener, H.M., D.L. Elwell, et al. (1998) Specifying Design/Operation of Composting Systems Using Pilot Scale Data, Applied Engineering in Agriculture; Keener, H M., R.C. Hansen, et al. (1997) Airflow Through Compost: Design and Cost Implications, Applied Engineering in Agriculture, 13(3): 377-384.
King, M.A. (August 1999), An Amazing 93% Reuse for Biosolids, BioCycle, 42-45.
Lapp, J., Compost Operations Technologist, City of Edmonton, Private Communication, Jan . 2, 2003.
Murphy, R. J., D. E. Jones, et al. (1995), Relationship of Microbial Mass and Activity in Biodegradation of Solid Waste, Waste Management & Research, 13: 485-497.
Sesay, A.A., Lasaridi, K.E., and Stentiford, E.I. (1998) Aerated static pile composting of municipal solid waste, Waste Management and Research, 16:3, p. 264-272.
Tchobanoglous, G., Theisen, H., and Vigil, S. (1993) Integrated Solid Waste Management, Chapter 4, McGraw-Hill, New York.
Themelis, N.J. and Kim Y.H., Material and Energy Balances in an Anaerobic Bioconversion Cell, p.234-242, 2002.