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218 Waste Management Practices: Municipal, Hazardous, and Industrial
8.5.3 NUTRIENTS
The organic fraction of the MSW feedstock possesses a range of microbial substrates including pro-
teins, lipids, sugars, starch, amino sugars, chitin, cellulose, lignin, crude fiber, and other compounds
(see Chapter 4) that will vary in terms of nutrient content, energy content, and availability to
microbes.
Nitrogen, P, K, Mg, S, Fe, Ca, Mn, Zn, Cu, Co, and Mo are integral to the protoplasmic struc-
ture of the microbial cell. These nutrients along with C, H, and O are essential for proper cell syn-
thesis. Nutrients obviously must be present in sufficiently large concentrations in a substrate;
however, they must also occur in a form that can be easily assimilated by the microbial cell.
Availability is partly a function of the enzymatic production by the microbe. Thus, certain microbes
possess enzymes that permit them to attack and utilize the organic matter within the raw feedstock,
whereas others can utilize only intermediate products. The significance is that decomposition and,
hence, the composting of MSW is the result of the activities of a dynamic succession of different
groups of microorganisms, in which one group prepares the local environment for its successors.
Another aspect of nutrient availability in composting is that certain organic molecules are resist-
ant to microbial attack, even to those that possess the required enzyme systems. Such refractory
materials are therefore broken down slowly, even with all other environmental conditions set at an
optimum level. Common examples of such materials are lignin (wood) and chitin (exoskeletons).
Cellulose C is unavailable to most bacteria, although it is readily available to certain fungi. Nitrogen
is easily available when in the amino acid form whereas N present in chitin is relatively unavail-
able. Many sugars and starches are readily decomposed, being a common unit of assimilable sub-
strate. Similarly, many fats and fatty acids are relatively available to microorganisms.
The availability of nutrients is also influenced by pH of the feedstock. In the circumneutral pH
range, trace metals (e.g., Cu, Ni, Zn) are typically soluble and therefore available in sufficient
quantities. In contrast, excess quantities, for example under acid pH regimes, will prove toxic and
inhibitory. Also, at neutral pH, phosphorus is maximally available. A pH of 5.5–8 is therefore gen-
erally considered optimal for composting (see below).

8.5.4 C:N RATIO
The ratio of carbon content to nitrogen content in the feedstock strongly affects the rate of micro-
bial activity. With few exceptions, all other nutrients are present in organic waste in adequate
amounts and ratios.
Carbon and nitrogen are used by microbes to obtain energy and for the synthesis of new cellu-
lar material. A large percentage of the carbon substrate is oxidized to CO during metabolic activi-
2
ties. The remaining carbon is converted into cell wall or membrane, protoplasm, and storage
products. The principal use of nitrogen is in the synthesis of protoplasm (e.g., proteins, amino acids,
nucleic acids). Much more carbon is required than nitrogen for adequate microbial growth.
After much empirical research, the optimum C:N ratio for soil and compost microorganisms has
been established at approximately 25:1. A ratio much higher than this will slow down the decom-
position; if the initial ratio is over 35, the microbial consortium must pass through many life cycles,
oxidizing the excess carbon to CO until a more suitable ratio is attained. On the other hand, if the
2
C:N ratio is lower than about 20:1, composting will be inhibited due to low-energy supplies and
nitrogen will be lost both by leaching and volatilization as ammonia (NH ). A low C:N ratio in
3(g)
composts and soils is typically rare, however. Of course, these ratios may vary widely depending
on the type of carbonaceous materials initially present.
If the initial C:N ratio of a waste is too high, adding a nitrogenous waste (e.g., blood meal) can
bring it to acceptable levels. If the ratio is too low, a carbonaceous waste (straw, wood shavings,
sawdust, shredded paper) can be added. The nitrogen contents and C: N ratios of various wastes and
other materials are listed in Table 8.2.