Integrated surfactant enhanced soil washing and physicochemical/biological treatment of produced wastewaters

Surfactant-enhanced soil washing has become a very important option for the remediation of soils contaminated with a variety of compounds and elements, i.e., petroleum hydrocarbons, pesticides, dense nonaqueus-phase liquids, metals, and others. Soil washing enhanced by surfactants is a feasible remediation technique when there is no chance for biodegradation-based methodologies. When time and/or space are scarce, soil washing is an excellent cost/benefit option. This chapter reviews the state of the art for surfactant enhanced soil washing developments at laboratory, pilot plant, and real scale levels. Aspects such as the selection of the best surfactant and doses and the use of natural surfactants, alone or in combination with synthetic ones, are discussed. On the other hand, aspects concerning operational practice, such as the relationship between the initial concentration of contaminants in soils and removal efficiency, a rule of the thumb for calculating how many washing steps are necessary to get a desired final contaminant concentration, and the experience developed in the washing tons of contaminated soils, are presented also. Finally, aspects related with the design of an industrial washing process and estimation of costs are reviewed and discussed. Once that soils have been washed, an effluent containing petroleum hydrocarbons, surfactants, and small amounts of salts, humic acids, and other molecules lixiviated from the soil is originated. A very general approximation is to produce 3-5 liters of wastewaters per kilogram of washed soil. It is necessary to treat the produced wastewaters in order to: 1) recycle them to the washing process or 2) at the end of the process, to release it to be drained. Many techniques have been reported for the treatment of these wastewaters, such as coagulation-flocculation, biological systems, and other physicochemical processes. Recently, advanced oxidation processes have shown a great potential for the treatment of these wastewater streams, in combination with other treatments (such as coagulation-flocculation or biological processes). It is important to remark that information regarding the produced wastewater quality and volume is very scarce. This chapter pretends to approach the integrated process of wastewater generation and treatment of the produced effluents, based on the experience at a pilot plant and with real scale field work. Technical and economic features are also reviewed and discussed.