Safety Use of Sewage Sludge as Soil Conditioner

Abstract

A series of field and laboratory experiments were conducted during 1994–996 to study the seasonal changes in sewage sludge characteristics and to evaluate the effect of sludge treatment processes and their application to sandy soil on soil-plant heavy metal contents and the numbers of some pathogenic microorganisms in both treated sludge and sludge provided soils. Results of seasonal production of sludge showed wide variation in their chemical and microbiological characteristics. Lime application resulted in an increase in the pH values and temperature degrees. The maximal values of temperature and pH were obtained when sludge treated with 20% lime. No big difference was observed between the sludge treatments received lime at rates of 10, 15, and 20% lime. The 10% limed-sludge treatment was the best for reducing concentrations of heavy metals and numbers of bacterial pathogens in sludge. Stoping addition of sludge to soil for one cropping season after continuous sludge addition for four seasons sharply decreased the soil heavy metal contents and subsequently decreased their accumulation in the edible parts of plants.

Keywords:

• Heavy metals
• Pathogens
• Plant
• Sewage sludge
• Treatment processes