The Practical Handbook of Compost Engineering

RogerTim Haug

• Editorial: Routledge
• Año: 2018
• ISBN:
  9781351409513
• Edición: 1
• Categoría: Biological Sciences
• Categoría: Environmental Science

The Practical Handbook of Compost Engineering presents an in-depth examination of the principles and practice of modern day composting. This comprehensive book covers compost science, engineering design, operation, principles, and practice, stressing a fundamental approach to analysis throughout. Biological, physical, chemical, thermodynamic, and kinetic principles are covered to develop a unified analytical approach to analysis and an understanding of the process. A brief history of the development of composting systems, which leads to descriptions of modern processes, is presented. The Practical Handbook of Compost Engineering also discusses the elements of successful odor management at composting facilities, including state-of-the-art odor treatment and enhanced atmospheric dispersion. The book is excellent for all engineers, practitioners, plant operators, scientists, researchers, and students in the field.

CHAPTER 1

Introduction

COMPOSTING — DEFINITIONS AND OBJECTIVES

There is no universally accepted definition of composting. This text uses a practical definition of the process. Composting is the biological decomposition and stabilization of organic substrates, under conditions that allow development of thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens and plant seeds, and can be beneficially applied to land. Thus, composting is a form of waste stabilization, but one that requires special conditions of moisture and aeration to produce thermophilic temperatures. The latter are generally considered to be above about 113°F (45°C). Maintenance of thermophilic temperatures is the primary mechanism for pathogen inactivation and seed destruction.

Most biological stabilization and conversion processes deal with dilute aqueous solutions, and only limited temperature elevations are possible. Thermophilic temperatures in aqueous solutions can be achieved if substrate concentrations are high and special provisions for aeration are employed. Aside from such special cases, composting is usually applied to solid or semisolid materials, making composting somewhat unique among the biological stabilization processes used in sanitary and biochemical engineering.

Aerobic composting is the decomposition of organic substrates in the presence of oxygen (air). The main products of biological metabolism are carbon dioxide, water, and heat. Anaerobic composting is the biological decomposition of organic substrates in the absence of oxygen. Metabolic end products of anaerobic decomposition are methane, carbon dioxide, and numerous low molecular weight intermediates such as organic acids and alcohols. Anaerobic composting releases significantly less energy per weight of organic decomposed compared to aerobic composting. Also, anaerobic composting has a higher odor potential because of the nature of many intermediate metabolites. For these reasons almost all engineered compost systems are aerobic. Mass transfer limitations, however, may cause anaerobic zones in otherwise aerobic systems. Such subtlety aside, this book will deal primarily with aerobic systems because of their commercial importance to man.

The objectives of composting have traditionally been to biologically convert putrescible organics into a stabilized form and to destroy organisms pathogenic to humans. Composting is also capable of destroying plant diseases, weed seeds, insects, and insect eggs. Odor potential from compost is greatly reduced because organics that remain after proper composting are relatively stable with low rates of decomposition. Composting can also effect considerable drying, which has particular value with wet substrates such as municipal and industrial sludges. Decomposition of substrate organics together with drying during composting can reduce the cost of subsequent handling and increase the attractiveness of compost for reuse or disposal.

Organic composts can accomplish a number of beneficial purposes when applied to the land. First, compost can serve as a source of organic matter for maintaining or building supplies of soil humus, necessary for proper soil structure and moisture holding capacity. Second, compost can improve the growth and vigor of crops in commercial agriculture and home related uses. Stable compost can reduce plant pathogens and improve plant resistance to disease. Colonization by beneficial microorganisms during the latter stages of composting appears to be responsible for inducing disease suppression. Third, compost contains valuable nutrients including nitrogen, phosphorus, and a variety of essential trace elements. The nutrient content of compost is related to the quality of the original organic substrate. However, most composts are too low in nutrients to be classified as fertilizers. Their main use is as a soil conditioner, mulch, top dressing, or organic base with fertilizer amendments. On the other hand, nutrients such as nitrogen are organically bound and slowly released throughout the growing season, making them less susceptible to loss by leaching than soluble fertilizers.

Like the process of composting, there is no universal agreement of what makes something a compost. The following is a workable definition as used in this text. Compost is an organic soil conditioner that has been stabilized to a humus-like product, that is free of viable human and plant pathogens and plant seeds, that does not attract insects or vectors, that can be handled and stored without nuisance, and that is beneficial to the growth of plants.

ENGINEERING WITH MICROBES

The most common engineering application of microbes is to treat or convert substrates in aqueous solution. Suspended growth reactors, such as the activated sludge process, or fixed film reactors, such as the trickling filter, anaerobic filter, and rotating biological contactor, are widely used for treatment of municipal and industrial liquid wastes. Suspended cultures of microbes are used for fermentations to produce ethanol, antibiotics, and medicines. Biochemical engineering is well developed and it is possible to design and operate such systems using a reasoned, engineered approach.

There are a number of biological processes used on solid or semisolid materials including fermentation and ripening of cheese, production of silage, and, of course, composting. At least in the case of composting, a reasoned, fundamental approach to analysis and design of new facilities and operation of existing ones has not been fully developed. The first edition of this book, published in 1980, presented a “first generation” approach to biochemical engineering analysis of the process. This second edition continues the evolution and considerably advances the tools available for analysis compared to the first edition.

Almost every book on the subject of composting begins with the statement that composting is an ancient art, practiced by man since before the dawn of recorded history. Although the evidence does suggest that man has had a long affair with composting, fundamental scientific studies of the process have generally occurred in the past four decades. Our abilities to engineer the process and to work with the numerous competing forces within a composting matrix are only now developing. In other words, the theory of composting may be understood and most of the forces involved may be known, yet engineering is still often conducted using a “handbook” approach with little knowledge of how to control these forces to achieve the final end product. It is a goal of this book to develop a more fundamental approach to analysis and design, one that relies as much as possible on first principles of physics, chemistry, biology, thermodynamics, and kinetics.

COMPOSTING SUBSTRATES

The quantity of substrates potentially suitable for composting is indeed large. Klass¹ estimated the solid and semisolid organic wastes generated and collected ...