Interactions on the soil-rainfall system of a lime-stabilized sludge as surface amendment

Sludge from a wastewater treatment plant can be applied to soil to make use of its fertilizing properties, nevertheless it is necessary to study all the effects of this deposition to ascertain any possible hazardous properties. Interactions among the soil, the water, and a lime-stabilized waste water sludge were studied, both in batch experiments, and in column percolation experiments. Firstly a physico-chemical characterization of the soil and the sludge used for the experiments was carried out. This analysis included pH, organic matter, cation exchange capacity, metals and cations. The kinetics and equilibrium of the sludge-water and leachate-soil systems were studied in batch experiments; the kinetics were fitted to a first order differential equation and distribution coefficients were found for the equilibrium. Experiments in columns (10 cm diameter x 50 cm height) were carried out with a bed of sludge over a bed of soil using different sludge/soil ratios and then 6 L of water (rainfall) was poured over the beds. Results showed good adsorption of Ca and a complete leaching of the aqueous phase of this ion after passing a volume of water equivalent to three volumes of the column. The concentration of heavy metals (Zn, Ni, Hg, Cd, Pb and Cr) in the leachates did not reach the admissible legal limits (Spanish and European regulations) in any case.