Discussion: Effect of pH on metal solubilization from sewage sludge: a neural-net-based approach

Abstract

Detoxification of sludge from municipal wastewater treatment plants has recently gained great importance in many cities in Europe and North America (Becker et al. 1990). The reasons are mainly to resolve the sludge disposal problem and increase the use of sludges as a soil amendment. Disposal sf sewage sludge has become a major problem because ocean disposal is banned almost everywhere; incineration and landfill face problems of cost and public acceptance. Removal of toxic chemicals, particularly heavy metals (e. g . , Zn, Cd, Cr, Ni, Cu, and Pb), from sewage sludges will, therefore, enhance its agricultural use and increase its value as fertilizer, and decrease the need for disposal. In one study where maerobically composted sewage sludge was used as soil conditioner and organic fertilizer, more vigorous crop establishment and higher yields were obtained (Bevacqua and Mellano 1993). The cumulative effect of multiple application of sludge over a 2-year period was clearly evident at the time of harvest. The use of sludge had positive effects in regulating soil pH, increasing the levels of the primary plant nutrients (e.g., nitrogen, phosphorus, and potassium) and water holding capacity of the soil. It also had the negative effects of increasing soluble salts such as Ca, Mg, Na, C1, and heavy metals. Accumulation of heavy metals in the soil poses the major concern in the use of sewage sludge on agricultural soils; these metals can build up with repeated applications. As metal concentrations increase, metal toxicity can suppress plant growth and metals can pass into the food chain. It was also demonstrated by the same team that, when the sludge contains low concentrations of heavy metals, the resulting buildup of heavy metals in the soil was small and was far below the amounts suggested for agricultural soils. Chenaical characteristics and suggested levels of heavy metals are shown in Table 1 (Bevacqua and Mellano 1993). It is a cost-effective approach to predict solubilization of metals from sewage sludge before treating the sludge for