Microbially Derived Off-Flavor from Geosmin and 2-Methylisoborneol: Sources and Remediation - Reviews of Environmental Contamination and Toxicology - page 13

et al. 2004). Ozonation followed by biological filtration has the potential to provide
effective treatment, as shown by Elhadi et al. (2004) in bench-scale experiments
using granular activated carbon and sand for the removal of geosmin and MIB.
Persson et al. (2007) used biofiltration to investigate differences between adsorp-
tion and biodegradation. They suppressed microbial activity by adding azide in
granular activated carbon crushed in expanded clay. It was found that granular acti-
vated carbon still removed gesomin and MIB nearly unaffectedly, whereas in the
clay biofilter, removal of both odorants ceased completely. Other oxidation proc-
esses using chlorine, chloramines, and potassium permanganate are ineffective for
reducing geosmin and MIB as these oxidants cause only cell damage and the release
of intracellular MIB into the water (Tung et al. 2004). These results are similar to
those of Glaze et al. (1990). Peterson et al. (1995) also found that chlorine and per-
manganate caused extensive damage to algal cells, inducing the release of geosmin
and dissolved organic carbon. Ashitani et al. (1988) observed an increase of MIB
and geosmin concentrations in water following prechlorination at a water treatment
plant. Jung et al. (2004) studied removal of geosmin and MIB by oxidation (O 3 , Cl 2 ,
ClO 2 ) and adsorption. They observed higher removal efficiency with increased
ozone dosage and, in the case of pulverized activated carbon, adsorption efficiency
of geosmin was superior to MIB. As an alternative to these synthetic algicides, nat-
ural compounds and extracts from plants are being screened for use in catfish
aquaculture (Schrader et al. 2003; Meepagala et al. 2005).

C Environmentally Safe Plant-Derived Algicides

The discovery of eco-friendly, selective algicides that suppress the growth of the
cyanobacteria implicated in musty off-flavor in pond-cultured catfish would be
beneficial for the aquaculture industry. Green algae do not produce such undesira-
ble odors, are good oxygenators of the water, and form a base for periphytic food
growth in catfish production (Paerl and Tucker 1995); thus, the discovery of safe
selective compounds that kill cyanobacteria would be beneficial for the aquaculture
industry. Previous research (Schrader and Harries 2001; Schrader et al. 1998a,b)
has identified several natural compounds that are selectively toxic toward O. peror-
nata . 9,10-Anthraquinone, found in plant tannin extracts (Robinson 1967), has a
high degree of selective toxicity toward O. perornata (Schrader et al. 1998a, b) and
also inhibits its photosynthesis (Schrader et al. 2000). Previous studies shows that
in comparison with copper-based products and diuron (half-life, 2 wk in pond
water), anthraquinone-59 derived from the natural compound 9,10-anthraquinone
has much lower persistence in pond water (half-life 19 hr) and also has greater
selective toxicity toward cyanobacteria than other phytoplankton (Tucker 2000). In
addition, the application of anthraquinone-59 in food fish production is advanta-
geous in view of the public's negative perception of diuron.
Meepagala et al. (2005) extracted rutacridone epoxide from Ruta graveolens
roots, which has potent selective algicidal activity toward the MIB-producing blue-
green alga Oscillatoria perornata . Rutacridone epoxide is reported as a direct-acting