Limitations to the application of compost to the soil

In this section, limitations are discussed related to public health that were not mentioned in the section on loading. Also covered are limitations pertaining to transport and to the mechanics of applying the compost on the field.

G1. PUBLIC health

Public health hazards arise when the product is composted human excrement or is composted industrial wastewater solids (i.e., sludge). Wastes from diseased animals could be ranked with human excrement as a hazard to public health, but perhaps to a lesser degree. Only those pathogens not exterminated during or after composting pose a hazard to public health.

It is important to keep in mind that passage through the compost process is not of itself a guarantee that all viable pathogens have been killed, and hence that the product is entirely free of viable pathogens. The reality is that the intervention of some factor may have prevented exposure of all pathogens to the bactericidal conditions associated with the compost process. Pathogens that escape these conditions can subsequently recontaminate “steriled” material. A single kilogram of insufficiently exposed infested or contaminated material can recontaminate an entire pile of compost.

For example, failure to expose all material in a composting pile to temperatures lethal to pathogens can result in pockets of contaminated material. Subsequent turning or other redistribution of material can lead to a recontamination of the entire composting mass. In systems that involve no turning, and in which sufficiently elevated temperatures seemingly should prevail throughout the composting mass, pockets of low temperatures might nevertheless exist. A factor that amplifies the effects of high temperature and may even compensate for incomplete exposure to high temperature is the collection of inhibitory phenomena that begin in the composting mass at the time compost conditions are imposed and end when the process is terminated. Because the phenomena are time-dependent, loosely speaking it might be said that passage of time more or less compensates for shortcomings in exposure to high temperature. Thus, passage of time allows: 1) destruction of pathogens, 2) competition with non-pathogenic microorganisms, 3) antibiosis, and 4) the development of a less than favourable environment. An important factor regarding pathogen survival and multiplication is the fact that the normal habitat of pathogens is the human body. Because of this, nutrient sources useful to pathogens are compounds usually found only in the human body. Therefore, only a relatively few compounds in a composting mass can be used as nutrient sources by pathogens. These compounds are rapidly destroyed under composting conditions.

Because they are adapted to the protective environment provided by their host, pathogens are at a disadvantage when competing with microbes indigenous to the habitat constituted by the composting material. Moreover, the presence of some antibiotics is ensured by the proliferation of actinomycetes and fungi normally characteristic of composting. For example, species of

Streptomyces (actinomycete) and Aspergillus (fungus) appear in substantial numbers during

composting. Therefore, all things considered, a year of storage should be ample for rendering a product safe for most uses.

G2. TRANSPORT and application mechanics

The bulky nature and low density of the dry compost product are responsible for most of the difficulties that pertain to transport and mechanics of applying the product. Because the volume- to-weight ratio is higher, a low-density product is more expensive to transport than is one of a high density. Inasmuch as compost generally is an uncompacted low-density product, the volumetric capacity of the transport vehicle is reached long before its weight capacity. The bulkiness and low density of compost necessitate the use of bulky equipment to spread it on the field. As previously indicated, a manure spreader can be used or adapted to apply compost to the field. With regard to manual application, more trips per unit mass of material are needed.

Nevertheless, these disadvantages are minor in comparison to the many benefits to be gained from the use of compost.

This chapter has provided the motivation for composting through a description of the many uses of the compost product. The efficient production of the product is the goal of Chapter VIII, Composting.

H. References

1. Ragen, N., “The Struggle for Israeli’s Environment”, Hadassah Magazine, November 1979. 2. Waksman, S.A., Humus (ed. 2), Williams-Wilkins Company, Baltimore, Maryland, USA,

1938.

3. Waksman, S.A., Soil Microbiology, John Wiley and Sons, New York, New York, USA, 1952.

4. Leeper, G.W., Managing the Heavy Metals on Land, Pollution Engineering and Technology Series, Marcel Dekker, Inc., New York, New York, USA, and Basel, Switzerland, 1978. 5. Filipi, C. and A. Pera, “The Role of Telluric Microflora in the Control of Fusarium Wilt of

Carnation Grown in Soils With Bark Composts”, Biol. Wastes, 27:271-279, 1989.

6. Rendig, V.V. and H.M. Taylor, Principles of Soil-Plant Interrelationships, McGraw-Hill Publishing Company, New York, New York, USA, 1989.

7. Anonymous, Quality Criteria Application and Recommendations for Compost Derived from

Solid Waste or Solid Waste/Sludge, (Compost Standards of West Germany, “Merkblatt 10”),

Document No. TVAB/18.Lfg. II. 85, (Translation from German), 1984.

8. The Waste and Resources Action Programme, Comparison of Compost Standards Within the

EU, North America and Australasia, Main Report, June 2002.

Notes 1

To legally qualify as a fertilizer in the United States, the NPK content must be at least 6%.

2

Because of grave hazards to public health, human excreta must be treated in a manner such that pathogens and parasites are destroyed before applying in the field.

CHAPTER VIII. COMPOSTING

A. Introduction

In economically developing countries, constraints related to economics, technology, and qualified personnel have narrowed the choice of acceptable solid waste management, treatment, and disposal options. Viable options include minimisation, recycling, composting, incineration, and sanitary landfilling. Composting is the option that, with few exceptions, best fits within the limited resources available in developing countries. A characteristic that renders composting especially suitable is its adaptability to a broad range of situations, due in part to the flexibility of its requirements. As a result, there is a composting system for nearly every situation; i.e., simple systems for early stages of industrial development to relatively complex, mechanised systems for advanced industrial development.

The compost option affords the many advantages of biological systems: lower equipment and operating costs; in harmony with the environment; and results in a useful product. On the other hand, composting is sometimes attributed with disadvantages often associated with biological systems -- namely, a slow reaction rate and some unpredictability. Regarding the attributed disadvantages, slow reaction rate may be justified, in that retention times are in terms of weeks and months. However, the attribution of unpredictability is not justified. If all conditions are known, applied, and maintained, the course of a given process will be predictable.

Among the major prerequisites for successful composting are a satisfactory understanding and application of the basic principles of the process. Without this understanding, inadequacies of design and operation are practically inevitable. An understanding of the biology rests upon a knowledge of the basic principles of the process. Such a knowledge enables a rational evaluation of individual compost technologies and utilisation of those technologies. An obvious benefit of the knowledge is the ability to select the system most suited to an intended undertaking. An accompanying benefit is the ability to critically evaluate claims made on behalf of candidate systems.

B. Definitions

Two definitions of composting are presented. The first is a definition in the strict sense of the term, which differentiates composting from all other forms of decomposition. The second one is an ecological definition.

B1. DEFINITION in the strict sense

A definition that distinguishes composting from other biological processes is:

“Composting is the biological decomposition of biodegradable solid waste under controlled predominantly aerobic conditions to a state that is sufficiently stable for nuisance-free storage and handling and is satisfactorily matured for safe use in agriculture”.

The terms and phrases that collectively differentiate composting from other decomposition processes are: “biological decomposition”, “biodegradable”, “under controlled predominantly aerobic conditions”, “sufficiently stable”, and “matured”. The phrase “biological decomposition” implies that the decomposition is accomplished by living organisms. “Biodegradable” refers to the substrate and it requires that the substance be susceptible to decomposition attack by certain living organisms, e.g., bacteria and fungi. Such substances are organic compounds formed either by living organisms or by way of chemical synthesis (e.g., halogenated hydrocarbons).

Decomposition of synthetic organics generally involves the activity of certain microorganisms under special conditions. The phrase “under controlled predominantly aerobic conditions” has a twofold significance: 1) it differentiates composting from the random biological decomposition that takes place in nature (e.g., open dump, forest, field, etc.); and 2) it distinguishes composting from anaerobic digestion (biogasification). The criterion for “stable” is safe and nuisance-free storage. The criterion for “sufficiently mature” is oriented to use in agriculture.

B2. ECOLOGICAL definition

An “ecological definition” is as follows:

“Composting is a decomposition process in which the substrate is progressively broken down by a succession of populations of living organisms. The breakdown products of one population serve as the substrate for the succeeding population. The succession is initiated by way of the breakdown of the complex molecules in the raw substrate to simpler forms by microbes indigenous to the substrate”.