Effects of sewage sludge treatments on aggregate slaking, clay dispersion and hydraulic conductivity of a semi-arid soil sample

Abstract

The semi-arid Zezia soil (fine, mixed, thermic Xerollic Calciorthid) is characterized by an unstable structure. Aggregate stability of sludge-amended samples was evaluated by measuring the optical transmission (%T) of two size fractions ( < 2 μm and < 5 μm) “dispersed” in NaCl/CaCl 2 solutions of decreasing electrolyte concentration at constant SAR values of 5, 10, 15, or 20. In addition, the hydraulic conductivity (HC) of reference and sludge-amended soil columns was measured by permeating similar electrolyte solutions of decreasing concentration at constant SAR of 5, 10, or 20. The sludge-amended samples were prepared by mixing sludge:soil portions (wt/wt) equivalent to 2, 4, and 8%. The moistened mixtures were incubated in the laboratory for 6 months after which the samples were air-dried and ground to pass a 2 mm sieve. For the < 2 and < 5 μm size fractions, %T of the soil suspensions increased linearly with increasing electrolyte concentration beyond a certain limiting value dependent on the solution SAR and sludge application rate. Only the 8% sludge treatment was effective in preventing extensive clay dispersion with decreasing electrolyte concentration at SAR ⩽ 10. The 8% sludge treatment also improved the aggregate stability as indicated by decreasing threshold concentration (corresponding to % T = 20) of the < 5 μm size fraction to levels lower than the respective values of the other sludge-treated samples. At a given SAR, the ratio of the < 5 μm-TC to the < 2 μm-TC was almost constant irrespective of the sludge treatments and, thus, supported the hypothesis that clay dispersion takes place at the ultimate stage of aggregate slaking. At any combination of SAR-electrolyte concentration of the permeating solutions, HCs of the 8% sludge-amended samples were greater than the corresponding values of the other samples. For all soil columns, minimum %T of the effluent solution was observed when permeating distilled water following any SAR solution series. The minimum %T of the distilled water effluent solutions decreased with increasing SAR for a given sludge-amended sample and increased with increasing sludge application rate at constant SAR. The limited clay dispersion in the distilled water effluent solutions indicated that the major cause for reductions in HC (e.g. ∼ 80% for the 8% sludge-amended sample permeated with distilled water following SAR 20 solution series) was aggregate slaking rather than clay dispersion and clogging of the conducting pores by the dispersed clay particles.