Environmental Engineering Reference
In-Depth Information
Anaerobic systems
The best way to cut direct and indirect GHG emissions and energy consumption from
wastewater treatment is using anaerobic treatment in enclosed systems. Anaerobic wastewater
treatment is performed by anaerobic microorganisms in the absence of oxygen. The anaerobic
process produces methane and carbon dioxide as the main gases and traces of other gases
including hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. These systems
require less energy to operate than the traditional aerobic ones because oxygenation is not
needed to promote the development of bacteria and as a result produce less indirect emissions
of GHGs. Another advantage of anaerobic reactors is that most of the solids removed from the
wastewater stream are transformed into methane and other gases with around 10 percent of
residual sludge. In comparison, aerobic systems produce around 50 percent of sludge.
A consequence of reduced formation of sludge is the lower need for nutrients for bacterial
development. In aerobic systems, the typical nutrient requirement on mass basis is 100:5:1
(chemical oxygen demand, nitrogen, phosphorus), and in anaerobic systems is 350:7:1 for
highly loaded and 1000:7:1 for lightly loaded wastewaters (Hansen and Cheong, 2007).
Methane produced during the anaerobic fermentation can be captured and used to produce
energy or flared, which is not recommended when protecting the environment is a concern. In
addition, anaerobic treatment is effective for effluents with high contents of fats and oils that
normally present a challenge in traditional aerobic counterparts.
The main advantage of anaerobic wastewater treatment is that the treatment system can be a
net energy producer instead of a consumer as in aerobic equivalents. The emission of GHGs can
be reduced from 2.4 to 1.0 kg CO
2
-eq/kg of chemical oxygen demand removed when going from
a traditional aerobic system to an anaerobic one (Keller and Hartley, 2003). It is important to
remember that chemical oxygen demand is not equivalent to biological oxygen demand. For
municipal wastewater biological oxygen demand is about 0.64 to 0.68 of chemical oxygen
demand (Russell, 2006), and in the case of effluents from food plants, it varies depending on the
industry.
Anaerobic systems, however, need higher concentration of solids to produce lower emis-
sion than anaerobic systems. Cakir and Stenstrom (2005) reported that a concentrations of
solid has to be at least 300 mg/L for the anaerobic systems to emit less GHG than anaerobic
counterparts. The authors attribute these estimates to the fact that some methane exits in the
anaerobic bioreactor dissolved in water, and their research suggests that a system to recover
this methane from solution would make anaerobic systems to emit less GHG than aerobic
ones across all concentrations of dissolved solids.
Anaerobic wastewater treatment has some disadvantages that need to be taken into consid-
eration and managed properly. Some disadvantages include: longer start-up time (in terms of
months) compared to few days in aerobic systems, need for additional treatment to remove
inorganic nutrients and pathogens, need for pH correction by addition of base (alkalinization),
not accommodating to load changes or composition of effluents, and susceptibility to toxic
compounds (e.g., heavy metals) (Tchobanoglous et al., 2004). Also, anaerobic fermentation
needs thermal energy to reach the ideal temperatures required by mesophilic or thermophilic
bacteria. The heating needs, however, can be met with energy from low-grade recovered heat
or by burning the methane produced by the bioreactor.
The anaerobic process
The anaerobic process takes place in four phases: (1)
hydrolysis
, (2)
acidogenesis
, (3)
acetogenesis
,
and (4)
methanogenesis
, and they are driven by different types of microorganisms.
