Passive Technologies

Experience shows that all active technologies require homogenous geological conditions. If these conditions do not pertain, passive technologies may be of advantage. Furthermore, the low solubility of hydrophobic organic contaminants and the slow diffusion of contaminants that have been within soil micropores for decades may result in a low efficiency of remediation technologies based on induced groundwater flow. Passive technologies are used at or near the end of the contaminant plume. They consist of constructed zones (reactors) in which the contaminants are degraded. If the zones cover the complete cross section of the plume, the technologies are called activated zone, bioscreen, reactive wall, or reactive trench. The main difference between these techniques is the need for soil excavation. Whereas activated zones or bioscreens are arranged without any soil management, the reactive wall and reactive trench techniques require construction of a subsurface bioreactor. Activated zones can be arranged, for example, as a line of narrow wells perpendicular to the direction of groundwater flow (Fig. 12.7). Incompletely passive technologies comprise alternating pumping and reinfiltration of groundwater in closed, directly linked loops, combined with an in-line nutrient amendment system consisting of a

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Fig. 12.7 Process scheme of activated zones.

Fig. 12.7 Process scheme of activated zones.

water-driven proportional feeder and a reservoir [7]. In this system the autochthonous microbial population is stimulated to adapt to a new and suitable redox situation and to develop the appropriate contaminant-degrading activity [8]. In such systems the hydraulic conductivity of the activated zone is the same as in the surrounding aquifer. Any alteration, e.g., by iron hydroxide precipitation, may lead to a reduction of hydraulic conductivity and a changed groundwater flow regime, which may result in deficient contaminant treatment.

Completely passive systems were developed because such exfiltration-infiltration loops consume significant amounts of energy. The advantage of passive systems is the highest when the energy demand is the lowest. This is true of completely passive systems, in which the remediation time is long and the contaminant load to be treated is usually low. In these completely passive systems, the wells are, for example, charged with solid cylinders consisting of highly permeable structural material (sand/cement) and so-called oxygen-release compounds (ORC®), which represent a proprietary MgO2 formulation. This compound releases oxygen over a period of about 300 d. If the ORC is exhausted. the cylinders can be easily replaced with fresh ones.

Because degradation of numerous contaminants requires anaerobic conditions, it is necessary to supply electron donors (e.g., hydrogen) in completely passive systems. Hence, hydrogen-release compounds (HRC®) were developed. This product is used especially to enhance the in situ transformation of volatile highly chlorinated hydrocarbons. These viscous compounds must be supplied via high-pressure injections.

At present, many different substances are used for injection into the groundwater to construct so-called in situ reactive zones (IRZ), including H2O2 for aerobic biodegradation and molasses, whey, or chitin for anaerobic biodegradation. Current investigations are concerned with the electrochemical generation of hydrogen as electron donor (2 H2O ^ O2 + 2 H2). The cathode where H2 is generated can be located within an anaerobic treatment zone, but the anode where O2 is generated is located within an aerobic treatment zone.

Reactive walls or comparable systems are local zones in a natural porous medium exhibiting high contaminant retention capacity and increased bioactivity. For reactive walls, systems with high longevity and without significant maintenance or the necessity of nutrient replenishment are desirable. Reactive walls can be composed of a mixture of organic waste (compost, wood chips, sewage sludge, etc.) and of, e.g., limestone for pH correction. The organic waste serves as a nutrient source, a structural material to establish high permeability, and a source of bacteria. A carrier (e.g., activated carbon) coated with specific contaminant-degrading microorganisms can also be used. Reactive walls can be in place for the complete duration of the treatment. If so, the necessary amount of nutrients is calculated on the basis of mass balance. However, it is difficult to estimate the fraction of nutrient mass that will be available for contaminant removal. Alternatively, reactive walls can be constructed so that the wall material is exchangeable or restorable. The materials have to be homogenized prior to installation to avoid channeling within the wall. Online monitoring of wall permeability is necessary to avoid changes in the predicted groundwater flow regime. The thickness of the wall depends on the groundwater flow velocity within the wall, contaminant concentration, degradation rates, and the required concentrations at the downgradient side of the reactive wall. Long-time changes in the values of these parameters have to be considered. The use of a numeric groundwater flow model for designing the wall is helpful. At present, several types of reactive walls have been developed; however, experience on a technical scale is limited. Reactive walls are implemented as denitrification zones [9] or metal barriers with biopre-cipitation [10] (see Section 12.3.3.6). An adverse effect during metal precipitation is high concentrations of Fe(II) and Mn(II), because they consume most of the precipitation capacity. Furthermore, because the metals stay within the reactive wall, mainly as metal sulfides, the permeability of the wall may decrease with time. If environmental conditions will not change and the long-time stability of the insoluble metal sulfides is known, they may remain in the subsurface; otherwise, the wall material has to be removed.

In general, reactive walls can be used with all biochemical processes that eliminate pollutants. However, site-specific conditions always need to be considered to show which technology is feasible and also economical.

Funnel-and-Gate™ is a system that channels contaminated groundwater, usually at the front of a plume, by means of impermeable walls (the funnel) toward gates within the wall, where a reactor is located (Fig. 12.8). To design such systems, use of a groundwater model that also considers inhomogeneities of the subsurface is essential. Within the gates, the same bioprocesses may be installed as in reactive walls. However, the smaller width of the bioreactors is favorable, because constructive measures to exchange the reactor material (e.g. reactor material filled cassette) are easier to implement. However, the groundwater flow velocity is increased within the gates and the groundwater table may rise. Full scale experience with Funnel-and-Gate technology is still rare in Europe. In particular, the longevity can be calculated only by extrapolation of short-term monitored processes. The advantage of these passive systems is low operation costs over long periods, since only monitoring is necessary.

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