Environmental Engineering Reference
In-Depth Information
breakdown of chlorite to chloride (Cl
−
) and oxygen (O
2
) (Polk et al., 2001; Sartain and Craig,
2003; Beisel et al., 2004).
Fluidized bed reactors and packed bed reactors are two types of commercially available
bioreactors. Packed or ixed bed bioreactors are made up of static sand or plastic media
to support the growth of microbes. Fluidized bed bioreactors are made up of suspended
sand or granular-activated carbon media to support microbial activity and growth of bio-
mass. The activated carbon media are selected to produce a low-concentration efluent (i.e.,
parts per billion [ppb] levels). Fluidized systems provide larger surface area for growth
of microorganisms. The luidized bed expands with the increased growth of bioilms on
the media particles. The result of this biological growth is a system capable of additional
degradative performance for target contaminants in a smaller reactor volume than with
a ixed bed. However, the luidized bed reactors generally require greater pumping rates
than ixed beds (Evans et al., 2002; Polk et al., 2001; Hatzinger et al., 2000; NAVFAC, 2000;
Nerenberg et al., 2003).
Several factors affect the performance of a bioreactor. For instance, in the optimum range
of dissolved oxygen (DO) concentration of the inluent water, perchlorate reduction reaches
0.5-1.0 mg/L. When DO levels drop to <0.5 mg/L, anaerobic conditions develop that, in
the presence of sulfates, result in the formation of hydrogen sulide (USEPA, 2005). It has
been reported that removal of nitrate ions from the inluent water is required to achieve
complete reduction of perchlorate (NAVFAC, 2000). Consistent and adequate dosage of car-
bon source (electron donor) and nutrients are required for growth of microorganisms on
the reactor bed (FRTR, 2005; Evans et al., 2002). Furthermore, control of excessive microbial
growth with a backwash strategy is essential to eliminate short circuiting and low chan-
neling in the bioreactor system (Evans et al., 2002; Hatzinger et al., 2000; NAVFAC, 2000;
Nerenberg et al., 2003; Polk et al., 2001).
Normally, the treated efluent is suitable for discharge; however, for drinking water
treatment, the efluent from bioreactors might require further treatment to remove biosol-
ids present in the efluent (Evans et al., 2002). Fluidized bed bioreactors require a thorough
mixing and upward low of the luid inside the reactor. One key advantage of a luid-
ized bed system is the availability of a large surface area for growth of biomass. However,
to maintain required low inside the reactor vessel, relatively higher pumping rates are
required (USEPA, 2005). Moreover, because ixed-bed systems are more susceptible to
accumulation of biosolids, they require periodic back-lushing to avoid plugging or clog-
ging the bed (Evans et al., 2002; Hatzinger et al., 2000; Polk et al., 2001; NAVFAC, 2000;
Nerenberg et al., 2003).
32.2.4 Composting
Composting has been used infrequently to treat perchlorate in contaminated soil. It is also
a biological process that uses indigenous microorganisms to degrade perchlorate in the
presence of appropriate soil amendments that support microbial growth. This technology
has been found to reduce perchlorate concentrations in soil to as low as 0.1 mg/kg.
Under anaerobic, thermophilic conditions (54-65°C), soil contaminated with perchlorate
was composted. Heat produced by microorganisms during degradation of the contami-
nants in the waste increases the temperature of the compost pile (FRTR, 2005; Roote, 2001).
Additional information about perchlorate transformation and biodegradation, includ-
ing microbial degradation pathways, is presented above, under bioreactors. Monitoring
of moisture content and temperature are important for achieving maximum degradation
eficiency (FRTR, 2005; Cox et al., 2000).
