Environmental Engineering Reference
In-Depth Information
3.4
Anaerobic Treatment
Anaerobic treatment is a biological process carried out in the absence of oxy-
gen for the stabilization of organic materials by conversion to methane (CH 4 )
and inorganic end products such as carbon dioxide (CO 2 ) and ammonia (NH 3 ).
Anaerobic conversion occurs in three steps:
Hydrolysis . In hydrolysis, water directly participates and breaks the chemi-
cal bond within the molecules. Complex organic materials such as protein,
carbohydrates, and lipids are converted to simple compounds such as amino
acids, sugars and long-chain fatty acids, respectively, by the action of
enzymes excreted by fermentative bacteria.
The next step involves bacterial conversion of high-molecular-weight com-
pounds into low-molecular-weight intermediate compounds for use as a
source of energy and cell tissue.
The final step involves conversion of intermediate compounds into simpler
end products such as methane and carbon dioxide by methane-generating
bacteria.
The successful operation of an anaerobic reactor depends on maintaining the
environmental factors close to the comfort of the microorganisms involved in
the process. Since methane generation is the rate-limiting step in anaerobic
treatment of wastewater, the major environmental factors are governed by the
methane-producing bacteria. Some of the important environmental factors are
temperature, pH, availability of nutrients, presence of sufficient alkalinity, and
absence of toxic and inhibitory compounds in the influent.
Best candidates of industrial wastewaters for anaerobic treatment are petro-
chemical wastes, pulp and paper wastes, alcohol production, brewery waste, and
chemical wastes (LaGrega et al. 2001). Low-rate and high-rate anaerobic treat-
ment processes are used to treat hazardous wastes. High-rate anaerobic systems
are able to retain very high concentrations of active biomass in the reactor. There-
fore, these systems can maintain very high SRT levels. Some of the high-rate
anaerobic treatment processes are anaerobic contact process, anaerobic filter,
up-flow anaerobic filter, down-flow anaerobic filter, and up-flow anaerobic sludge
blanket.
3.5
Trickling Filters
Trickling filters are attached-growth treatment systems where wastewater passes
over a fixed bed of packed media covered with biofilm. The media are either
rock or plastic, and the wastewater flows over the media where bacteria grow
and creates the biofilm or biological slime layer. As the wastewater flows over
the biofilm, organic matter (substrate), oxygen, and nutrients diffuse across the
boundary layer. Organic matter removal is a function of the available biological
slime surface and the time of contact of the wastewater with that surface. The
 
 
 
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