The Potential of Sewage Sludge and Composting in a Nitrogen Recycling Strategy for Agriculture

Abstract

A significant proportion of agricultural productivity is consumed by the human population and the nutrients thus exported from the farm will be found in human excreta. Nutrients present in human excreta can therefore make a substantial contribution to the nutrient demand in agriculture. The low cost of chemical fertilizers and a pricing policy supportive of high agricultural productivity not only stimulates high rates of fertilizer usage to the extent of causing increasing pollution of, in particular, aquatic ecosystems and drinking water supplies, it also discourages the development of a farming system based on the efficient use and recycling of its wastes. Efficient recycling of human excreta in agriculture is restricted in most industrialized countries by the contamination of sewage sludge by industrial wastes, in particular heavy metals. High costs of chemical fertilizers and the absence of industrial pollutants in night soil and sewage sludge in many developing countries, create an environment in which the recycling of human excreta could make a substantial contribution to agricultural productivity. Treatment of human excreta through composting eliminates the risk of dissemination of pathogenic organisms, and produces an aesthetically acceptable product for use in agriculture. Losses of nitrogen during the composting of nitrogen rich wastes such as sewage sludge are high, and can easily amount to 50%. Factors effecting the loss of nitrogen during composting and several strategies to reduce these losses are discussed.