Environmental Engineering Reference
In-Depth Information
The effectiveness of SCWO has been demonstrated at the laboratory and pilot scale with
a wide broad range of feedsctocks, such as pig manure (Rulkenes et al., 1989), a variety of
biomass slurries including pulp mill sludge (Modell, 1990), and sewage sludge (General
Atomics, 1997). It has been demonstrated that complete oxidation of virtually any organic
material, including hazardous wastes such as hexachlorobenzene, could be achieved by
SCWO. A supercritical water oxidation sludge processing plant has been installed at
Harlingen, Texas to process up to 9.8 dry tonnes per day of municipal sludge (Griffith and
Raymond, 2002). An environmental assessment was conducted on the Harlingen plant and
found large environmental gains from recovery of heat thereby reducing natural gas
consumption for heat generation (Svanström et al., 2004). Hydrothermal oxidation, in
particular SCWO, is currently being considered by various research and waste management
organizations as an alternative treatment option (Stark et al., 2006).
2.6. Anaerobic Digestion
Since a large number of the enteric bacteria and viral pathogens presented in untreated
sewage are associated with wastewater solids, many are not completely removed during
sewage treatment processes and are merely transferred to wastewater sludge (Farrah and
Bitton, 1983). Anaerobic digestion processes are widely recognized as particularly suitable
for highly polluted wastewater treatment and for the stabilization of primary and secondary
sludges. Anaerobic digestion occurs in the absence of oxygen while in the presence of
bacterial activity, producing bio-gas (mainly methane). The methane gas produced can be
used to generate power by fueling a biogas engine connected to an electric generator. The
digested and separated solids can undergo further processing and potentially be used as a
fertilizer or soil conditioner for land application (Kumar, 2000; Gavala et al., 2003), while the
treated water may be used for irrigation (Kumar, 2000). Anaerobic digestion of municipal or
pulp/paper bio-solids could reduce solid wastes by 30-70% with the benefit of energy
recovery through methane production. Generally about half of the organic matter in sludge is
susceptible to anaerobic biodegradation into the formation of biogas (Elliot and Mahmood,
2007).
The microbiology of anaerobic digestion is complicated and involves several bacterial
groups forming a complex interdependent food web. However, four major steps can be
distinguished. In the first hydrolysis step, both solubilization of insoluble particulate matter
and biological decomposition of organic polymers to monomer or dimmers take place.
Acidogensis and acetogenesis follow in the second and third step while in the fourth and final
step methane is produced by menthanogenic archaea (Gavala et al., 2003). Figure 9 outlines a
flow diagram of anaerobic digestion of secondary sewage sludge for energy production.
Secondary sludge is fed into the hydrolysis tank. In conventional single-stage anaerobic
digestion processes, hydrolysis is regarded as the rate-limiting step in the degradation of
complex organic compounds, such as sewage sludge. Two-stage systems have been proposed
to enhance this process (Ponsá et al., 2008). The first stage digests the solids, and the second
stage separates the undigested solids from the liquid to form carbon dioxide, methane and
water. There are two typical operating temperatures for anaerobic digesters determined by the
desired species of methanogens. For mesophilic processes, the optimum operating
temperature is 37
o
C, while 55
o
C is desirable for thermophilic processes (Song et al., 2004).
