Organic Fertilizer Production from City Waste : A Model Approach in a Southeast Asian Urban Environment

According to projections of the United Nations, about 67 % of the overall world population will reside in urban areas by the year 2025 compared to 50 % only in 1995 (UNFPA, 1996) Particularly in low-income countries, city administrations are faced with huge challenges, among those the proper disposal of an abundance of refuse from households, commerce and industry by safe means, and the sufficient supply of cheap but nutritious food rich in vitamins and minerals to feed the population of these expanding urban centers. In European countries, the daily average production of garbage is about 3 to 5 kg per capita. In developing countries the amounts are smaller, about 1 to 1.5 kg daily. However, due to logistical constraints, garbage disposal is generally considered as a major source of ecological problems such as release of bad odor, occurrence of flies, as well as contamination of the underground aquifer by nitrate, heavy metals and bacteria. The percentage of organic materials in city waste of developing countries reaches up to 80%(Katzir, 1996) which can be used for organic fertilizer production if appropriate technologies are available. Composting is basically a process for decomposition of organic solid wastes (Airan and Bell, 1980). The decomposition process is accomplished by various microorganisms including bacteria, actinomycetes and fungi. In the process of composting, microorganisms break down organic matter and produce carbon dioxide, water, energy in forms of heat humus, and the relatively stable organic end product (Rynk et al., 1992) Under optimal conditions, composting proceeds through three phases: • the mesophilic, or moderate-temperature phase, • the thermophilic, or high-temperature phase, • a cooling and maturation phase. Different communities of microorganisms predominate during the various composting phases. Innitial decomposition is carried out by mesophilic microorganisms, which rapidly break down the soluble, readily degradable compounds. The heat they produce causes the compost temperature to rapidly rise. As the temperature rises above 40°C, the mesophilic microorganisms become less competitive and are replaced by others that are thermophilic. At temperatures of 55°C and above, many microorganisms that are human or plant pathogens are destroyed. During the thermophilic phase, high temperatures accelerate the breakdown of proteins, fats, and complex carbohydrates like cellulose and hemicellulose, that are the major structural molecules in plants. As the supply of