Prolonged Effect of Land Disposal of Human Wastes on Soil Conditions

Abstract

Land disposal of human wastes is a comprehensive issue, where a series of aspects have to be considered. Few, if any, alternatives exist to this technology. No doubt future emphasis will be on sewage farming, which should result in the cultivation of new arid land. This paper addresses the prolonged effect of land disposal of human wastes on soil properties, as well as on the dissemination of enteric pathogens. In order to assess this impact, soil samples were collected from Gabal el Asfar sewage farm in Cairo to represent sandy soils irrigated solely, by surface flooding, with decanted sewage effluent for 0, 2.5, 5, 10, 15, 20, 30, 45 and 60 years. Sewage farming tended to build up the soil microbial population, particularly during the first five years. In all soils, human wastes increased total bacterial counts and promoted the proliferation of the physiological groups. This population would accelerate the oxidation of organic matter to available nutrients. It also resulted in a high increase in nitrogen, phosphorus, micro-nutrients, organic carbon and raised the water holding capacity and exchangeable cations. However, the prolonged use of sewage effluent would disturb the balance of nutrients in soil, as the pH shifted towards acidity. The most interesting observation in the present work is to record that neither soluble salts nor micro-nutrients reached any injurious level. But land disposal of human wastes should be excercised with caution and if it is intended to be applied, salts, pH and nutrient elements should be checked occasionally. From the hygienic point of view, faecal E. coli, which is considered to be an indicator for enteric pathogens, gave positive results in all sewaged soils. Hence, it is recommended, from the sanitary point of view, that no crops which come in contact with sewage effluent should be cultivated in a sewage farm.