RECLAMATION OF NITROGEN AND OTHER NUTRIENTS
The two most important purposes for composting organic wastes are (a) reclamation or conservation of the nutrient and fertilizer values of waste, and (b) sanitary treatment and disposal to prevent the spread of disease. Of the major nutrients-nitrogen, phosphorus, and potash- the nitrogen conservation is the most important in most areas of the world because so often the shortage of nitrogen limits the amount of food that is produced. Nitrogen is also more difficult to conserve than phosphorus and potash, as are the micronutrients, which, owing to the chemical condition in which they are present, are lost only to leaching. Nitrogen may be lost by leaching, but the major loss comes from escape of ammonia or other volatile nitrogenous gases from compost material to the atmosphere.
There has been much research and writing on conserving nitrogen and other nutrients, particularly with respect to microbiology of the soil. Limited experience exists on nitrogen conservation and reclamation in the composting process. Results of investigations and studies on nitrogen utilization in the basic biological processes provide fundamental information on the control of nitrogen loss in composting.
Nitrogen loss as ammonia in aerobic composting is affected by the C:N ratio, pH, moisture content, aeration, temperature, and the form of nitrogen compounds at the start of the composting materials.
Since organisms use about 30 parts of carbon for each part of nitrogen, a C:N ratio in the raw compostable material of around 30:1 is best for good composting, would also seem satisfactory for tying up or binding nitrogen in biological cell material and thus preventing its escape.
Various research workers reported optimum ratios of C:N to avoid nitrogen loss under different conditions of from 26 to as high as 38. A ratio of available carbon to available nitrogen of about 30 or more permits minimum loss of nitrogen, but the ratio of carbon to nitrogen measured chemically is often not the ratio of available carbon to available nitrogen. Since most refuse contains considerable amounts of cellulose and lignins, which are resistant to biological decomposition, and since most of the nitrogen is usually in a readily available form, an actual C:N ratio of considerably over 30 may be necessary to provide maximum conservation of nitrogen. Also, studies indicated that nitrogen conservation decreased rapidly as the C:N ratio increased from 40 to 50. This rapid decrease is not entirely consistent with the fundamental aspects of bacterial decomposition. Above a C:N ratio of 50, nitrogen conservation remained uniform at about 70% of the optimum. Basically there should be little drop in nitrogen conservation below the maximum when the initial C:N ratio is above the ratio utilized by the organisms. When carbon is higher than the ideal C:N ratio, organisms will require all the nitrogen for decomposition of the carbonaceous materials. University of California studies found Nitrogen losses of around 50% when the C:N ratio was in the range 20 to 25. From about 30 upward, nitrogen losses were very small.
NITROGEN CONSERVATION IN RELATION TO C:N RATIO
||Initial C:N ratio
||Final percentage of nitrogen
||Nitrogen conservation %
This table shows a few examples of nitrogen conservation for different C:N ratios. It was found that in manure composts nitrogen was conserved only when the C:N ratio was adequate and when immediate decomposition set in, resulting in transformation of soluble forms of nitrogen into insoluble forms. Whenever decomposition was delayed, owing to too low or too high a temperature, losses of volatile forms of nitrogen occurred. From 85% to 90%, and possibly 95%, of the nitrogen in the raw materials can be conserved if the C:N ratio is high and other avenues for nitrogen loss are controlled.
There are three phases in the relation of nitrogen supply and conservation to available carbon in biological decomposition:
(a) When more nitrogen is available than necessary for organisms to use carbon, large quantities of ammonia and volatile forms of nitrogen are given off and lost;
(b) When the requisite amount of nitrogen to carbon for bacterial utilization is present, decomposition proceeds without appreciable loss of nitrogen;
(c) When nitrogen is low in relation to carbon, some of the organisms will die and their nitrogen will be recycled. Small additional amounts of nitrogen may be picked up by nitrogen fixation when conditions are satisfactory.
In all three phases there is a tendency to reach the same final amount of nitrogen, that which the bacteria can hold when the compost is in a stabilized condition. In the first phase nitrogen is lost; in the second, it is stabilized and conserved; and in the third, it is recycled, conserved, and sometimes accumulated. This illustrates that composting operations can be operated to conserve most nitrogen.
Ammonia escapes as ammonia hydroxide as the pH rises above 7.0. In the later stages of composting the pH may rise to between 8.0 and 9.0. At this time there should not be an excessive amount of nitrogen present as ammonia. Materials that contain large amounts of ash will have a high pH and may be expected to lose more nitrogen.
Some compost operators add lime to improve composting. This should be done only under rare circumstances, such as when raw material has a high acidity due to acid wastes or contains materials which give rise to highly acid conditions during composting. When the pH remains above 4.0 to 4.5, lime should not be added. The pH will be increased by biological action and nitrogen conserved.
The moisture content of compost affects nitrogen conservation less than the C:N ratio and the pH. Ammonia escape is greater when the moisture content is low. The water serves as a solvent and diluent for the ammonia, thereby reducing vapor pressure and volatilization. A moisture content range of 50% to 70%, satisfactory for other aspects of composting, will assist in conserving nitrogen.
Aeration and turning adversely affect nitrogen conservation.
If ammonia is present, it will escape more easily when material is disturbed and exposed to the atmosphere. However, if the initial C:N ratio is high enough, nitrogen losses during turning will be small. Since some ammonia may be present during the dynamic transitional phases of active decomposition, turn only as often as necessary to maintain aerobic conditions and control flies.
High temperatures increase volatilization and escape of ammonia. Since high temperatures are fundamental in aerobic composting and destruction of pathogen, not much can be done about controlling temperatures other than to avoid temperatures above 160o Fahrenheit, which retard bacterial activity and permit ammonia accumulation. Since the greatest ammonia loss occurs during early stages of active decomposition, only little conservation of nitrogen will be gained by reducing temperatures after the two turns or after the first 6 to 8 days of active decomposition.
The nitrogen initially present in the material may affect nitrogen conservation. If large amounts of ammonia are present in raw materials, some of this ammonia may be volatilized and lost before the organisms have had sufficient time to utilize and stabilize it, even though the C:N ratio is satisfactory for nitrogen conservation. This can be an important factor since much of the nitrogen loss occurs during the first few days of composting.
Some materials, such as cellulose and porous fibrous matter, have the capacity to absorb or hold moisture and volatile substances, thereby reducing the tendency to escape. Materials of this type play a part in reducing nitrogen loss from compost, which contain accumulated ammonia. Materials containing considerable quantities of horse or cow manure seemed to exhibit less nitrogen loss at low C:N ratio than other materials, and should be considered to be nitrogen carriers. This could have been due to the form of nitrogen, to the absorptive of nitrogen holding capacity, or to some other characteristic of the manures.
Also, addition of soil to compost with a high ammonia content absorbed some of the nitrogen.
Loss of nitrogen by leaching may occur in rainy weather or if the composting material has too high initial moisture content and excess liquid drains away. Loss by leaching depends on the amount of soluble nitrogen in the compost and on the amount of rainfall. Arranging compost piles so that water can't enter may minimize leaching.
The greatest nitrogen conservation may be accomplished by anaerobic digestion in water when liquids as well as the solids are conserved. In such cases, while nitrogen fixation would not be expected, there should be almost no nitrogen loss, since ammonia in low concentration in the liquid would not escape.
Conservation of phosphorus and potash in composting is not difficult since about the only loss occurs through leaching during rainy weather