Remediation can be classified according to contaminant type, soil phase and the core transformation process, as shown in Chapter 1. The goal of remediation is to reduce adverse effects and the resulting environmental and human risks posed by the contaminants. This goal can be achieved by their elimination, degradation or other transformation to non-hazardous or less hazardous and non-mobile forms. This chapter applies the reactor approach, an interpretation common in chemical and bioengineering, to describe the overall remediation technology. This description is based on a transformation process that (i) is characterized by material balances; (ii) takes place in reactor volumes; (iii) is equipped with machinery to perform certain operations; and (iv) aims to design, operate, and maintain “the plant,” i.e. the processes in the reactor.

What this means in the context of soil remediation is that processes take place in real reactors or in quasi-reactors. Quasi-reactors have no walls or other built boundaries, but they are limited by the boundaries of the impact volume of operation, usually by an engineered volume – with in situ soil remediation as an example. The entire technology, equipment, processes, material transport, inputs to and outputs from the reactor, and optimal technological parameters and their control, including mass balances, should be planned and implemented as for any other technology. Reactors can be characterized by size, type, and the process taking place in them. They can be closed, semi-closed, or entirely open, but always containing a certain volume of soil or range of operation.

Soil and groundwater remediation covers a sequence of operations, the mass transport route within the reactor or from one reactor to another in a cascade arrangement, and the mass balance of the individual steps and the whole process.

Remediation as a “reactor” differs from other engineering devices in its extreme complexity and heterogeneity and the limited accessibility of the solid material in which the transformation takes place, which hampers predictability and design. Pilot testing, high versatility, and a combination of technologies, suitable monitoring, and control may ensure efficient soil treatment.

Soil remediation is redefined in this chapter based on a generic engineering approach: a controlled transformation produces valuable end products in a reactor, and the processes can be characterized by mass and energy balances.

The author believes that this approach is necessary to clarify confusion regarding terminology and the way of thinking that still prevails in environmental remediation. Today’s practice often uses terms of processes or operations instead of those for treatment technologies and confuses technological considerations with managerial aspects. The reactor approach will help in situ natural or near-natural bio- and ecotechnologies to obtain recognition and will contribute to their spread and acceptance.