The Practical Handbook of Compost Engineering
About this book
The Practical Handbook of Compost Engineering by Roger T. Haug, published by Lewis Publishers (a CRC Press imprint) in 1993, remains one of the most thorough and systematic engineering treatments of the composting process ever compiled. At a time when composting was transitioning from a relatively informal agricultural practice to a regulated industrial operation handling municipal solid waste, sewage sludge, and industrial biosolids, Haug's volume provided operators, designers, and regulators with the quantitative framework needed to move from empirical rule-of-thumb to rational process engineering.
Haug defines composting as the biological decomposition and stabilization of organic substrates under conditions that permit the development of thermophilic temperatures as a result of biologically produced heat, with the goal of producing a stable, hygienic, humus-like product suitable for land application. This definition captures the three essential features that distinguish composting from simple biodegradation: deliberate management of thermal conditions, biological stabilization to reduce phytotoxicity and pathogen levels, and production of a marketable soil amendment rather than mere waste disposal. The handbook's central contribution is to provide the mathematical process models, mass and energy balances, and empirical correlations needed to design, size, and control composting systems in a systematic fashion.
The microbiology of composting is treated with considerable rigour. The active decomposer community in a compost pile is a succession of microbial guilds whose composition shifts as temperature rises through mesophilic (20–40 °C), thermophilic (40–70 °C), and cooling phases. In the thermophilic phase, which is the key to pathogen destruction and rapid organic matter transformation, heat-tolerant bacteria including members of Bacillus, Thermus, and related genera dominate.
Fungi and actinomycetes become more prominent during the cooling and maturation phases, producing the earthy odors and humic substances characteristic of mature compost. Haug presents kinetic models for microbial activity that relate decomposition rates to substrate availability, temperature, moisture, and oxygen concentration, enabling quantitative prediction of process performance under varying operating conditions. Moisture and oxygen control are identified as the two most critical operational variables.
Microbial activity requires free water for cellular metabolism; at moisture contents below approximately 40% decomposition rates drop sharply, while at contents above approximately 65% pore spaces become water-filled, restricting oxygen diffusion and promoting anaerobic conditions that produce malodorous compounds including hydrogen sulfide, mercaptans, and ammonia. The optimal range of 50–60% moisture must be maintained throughout the active composting period through careful feedstock blending and, in some systems, controlled water addition. Similarly, oxygen must be continuously replenished as microorganisms consume it in aerobic respiration; oxygen concentrations below approximately 5% in the pore gas trigger anaerobic metabolism.
The handbook provides equations relating aeration rate, pile geometry, porosity, and oxygen consumption to guide the engineering design of forced-aeration systems. Haug provides detailed treatment of three principal system configurations. Windrow composting arranges feedstock in elongated triangular or trapezoidal piles that are periodically turned by specialized machinery to expose fresh material to oxygen and redistribute heat.
In-vessel composting encloses the process in agitated tunnels, rotating drums, or enclosed reactors equipped with forced aeration and automated temperature monitoring, allowing much greater process control and reducing land area, odor emissions, and climate sensitivity. Aerated static pile systems represent an intermediate approach in which feedstock is formed into static piles over a network of perforated piping through which air is either blown or drawn, avoiding the mechanical turning requirement of windrows while achieving better aeration uniformity than passive piles. Process design equations covering pile dimensions, aeration pipe sizing, blower selection, moisture adjustment, and composting time are presented in accessible form, and the handbook addresses compost quality standards in terms of maturity indices, stability tests, heavy metal limits, and pathogen reduction verification requirements that were emerging in regulatory frameworks of the early 1990s.
The work's enduring value lies in its synthesis of biological science and engineering practice into a coherent design methodology that has informed generations of composting facility designers.