Wastewater effects on montmorillonite suspensions and hydraulic propertiesof sandy soils

Recycled wastewater eluent is an important source of irrigation water in ar id and semiarid regions. In these regions, however, irrigation water qualit y is one of the main factors limiting plant growth Wastewater effluents gen erally contain high concentrations of suspended and dissolved solids, both organic and inorganic. Inorganic dissolved solids me only minimally removed from the effluent during conventional sewage treatment (secondary or terti ary). As a result, most of the salts added during domestic and industrial u sage remain in the irrigation wafer and may eventually reach the soil, A nu mber of researchers have reported reduced hydraulic conductivity for soils to which treated wastewater has been applied. In this research, the influen ce of dissolved organic matter (DOM) contained in reclaimed wastewater effl uents (mainly humic substances) on the flocculation of montmorillonite and on the hydraulic properties of sobs was studied. Flocculation values (FVs) for Na-montmorillonite increased with increasing concentrations of DOM at a ll pH levels analyzed (5.5, 7.0, and 8.5), Maximum FV levels were exhibited for Na-montmorillonite at the highest DONI concentrations. The effect of D OM on FV can be explained by the mechanisms of edge-charge reversal and mut ual flocculation. The hydraulic conductivity (HC) of a sandy soil was deter mined in the laboratory by leaching columns with an electrolyte solution ch emically similar to that of the wastewater effluent (but without DOM). In c olumns treated with wastewater effluent, the HC exhibited a-sharp decrease to only 20% of its initial value. The adverse effect of DOM on HC was evide nt for this soil despite a relatively low exchangeable sodium percentage (E SP). The reduction in KC is likely to be the result of decreased soil pore- size, which reflects two processes: (i) retention of part of the DOM during water percolation; and (ii) a change in pore-size distribution due to swel ling and dispersion of clay particles. The latter may result from a higher percentage of adsorbed sodium combined with the presence of humic substance s from the wastewater effluent.