English Abstract In developing in situ remediation most of the focus
used to be on techniques using infiltration water to supply oxygen to the
location. Later, techniques were developed in which soil was flushed with
air to enhance the oxygen availability to microorganisms. The aim of the
study reported here was to examine the results of remediation where soil is
flushed with air, as reported in the literature, and to examine the costs of
such systems. In the study these techniques were compared with water-supply
systems, particularly those using hydrogen peroxide, e.g. bioremediation in
Asten (Noord-Brabant). The remediation techniques which use air to supply
oxygen to the soil are soil ventilation, soil vapour extraction, bioventing
and air sparging. Soil ventilation and soil vapor extraction are physical
remediation techniques which can be employed only in the vadoze zone for
removal of low molecular organic pollutants. Bioventing removes pollutants
mainly by microbiological conversions in the vadoze zone. Air sparging can
be employed only in the saturated zone, where the pollutants can be stripped
by air, as well as converted by microorganisms. In general, these
techniques are used on sandy soils, which are very permeable. The whole
range of hydrocarbons can be remediated. According to the literature,
remediations were generally kept in operation for two years. At some sites
high removal rates, up to 20 mg.kg-1 soil.day-1, were reached. The
literature did not mention anything about the remaining levels of pollution
dropping to target-value level. Although sometimes only 5% of the original
amount of pollution remains in the soil, the lowest levels obtained are
found just below the former B value. Therefore, the remediation levels
reached should always be viewed in connection with the remediation time and
the amount of pollution originally present. At the remediation site in
Asten, where a water-supply system with hydrogen peroxide dosing is used,
lower remaining levels were obtained after a longer remediation time. This
location could problably also be remediated using a bioventing technique if
the groundwater table were lowered. The remediation time would probably be
shorter because oxygen would be available to a greater extent. However, the
final pollution levels in the soil are expected to be higher. Due to the
lower soil-water content in the pores, the mineralization process would
terminate earlier. It is possible that these results would not be obtained
in the vicinity of the groundwater table. Another possibility might be
natural attenuation after lowering the contamination levels with an active
remediation technique. The Dutch target values can be expected to be
attained, but only after a very long remediation time. Bioventing appears
to be the most cost-effective remediation technique
available.
Rapport in het kort
Aan de hand van literatuurgegevens zijn resultaten en
kosten onderzocht van praktijksaneringen waarbij de bodem doorstroomd werd
met lucht. De saneringstechnieken waarbij zuurstof via de lucht aan de
bodem wordt toegevoerd, zijn bodemventilatie, bodemluchtextractie,
bioventing en persluchtinjectie. Deze technieken werden vergeleken met het
waterafgiftesysteem in Asten waarbij waterstofperoxyde toegepast is. De
locaties, waarop deze technieken toegepast worden, zijn over het algemeen
goed doorlaatbare zandgronden. De verontreiniging die zich op deze locaties
bevond, besloeg het hele scala koolwaterstoffen. Uit de resultaten die in
de literatuur gevonden werden, blijkt dat de saneringen over het algemeen
zo'n twee jaar in bedrijf gehouden worden. In sommige gevallen worden hoge
verwijderingssnelheden (tot ongeveer 20 mg.kg exp. -1 grond.dag exp. -1)
bereikt. Nergens in de literatuur wordt vermeld dat de streefwaarden
behaald konden worden. Bij de sanering in Asten zijn lagere restgehaltes
behaald, maar na een langere saneringsduur. Deze locatie had waarschijnlijk
ook gesaneerd kunnen worden met behulp van bioventing, waarbij de
grondwaterspiegel verlaagd had moeten worden. Waarschijnlijk zou de
saneringsduur korter zijn geweest omdat zuurstoflimitering een kleinere rol
zou hebben gespeeld. Verwacht wordt echter dat de uiteindelijke
restgehaltes hoger zouden liggen. Indien de kosten van de verschillende
technieken onderling vergeleken worden, blijkt bioventing de meest
kosteneffectieve saneringstechniek te zijn.