primary
treatment - physical separation that removes 20-30% of
"particulate" biochemical oxygen demand (BOD)
accomplished by filtration through grates and screens,
followed by sedimentation of suspended solids using alum and
other coagulants during settling
sludge (mixture of particulates and biomass) is thus
generated
most pathogens are killed on the way to treatment plant,
due to their susceptibility to metabolic products of the
dominant microbes and their inability to compete with them for
nutrients, etc.
secondary
treatment - microbial action further reduces BOD by 90-95%
(and kills most of the remaining pathogens) by removing organics
in a complex series of digestive and fermentative reactions
depending on interspecies hydrogen transfer reactions during which
organics are degraded to their components (carbon dioxide,
hydrogen, methane, ammonia, nitrate, sulfate, phosphate) and
producing more sludge; frequently starts with aerobic
decomposition processes, then reduces the amount of the resulting
sludge by anaerobic decomposition
when operated at low oxygen levels,
Sphaerotilus, Thiothrix, and other
filamentous microbes predominate, generating bulking
sludge, which causes problems with floc formation and
settling
anaerobic
decomposition process - used for insoluble organics,
such as fiber and cellulose, or for certain concentrated
industrial wastes
initial digestion of macromolecules by extracellular
enzymes
soluble(ized) organics (glucose, etc.) are then
fermented to form fatty acids (butyric, propionic, lactic,
succinic and acetic acids), ethanol, hydrogen, and carbon
dioxide by various facultative and obligate anaerobes
including Clostridium, Bacteroides,
Peptococcus, Peptostreptococcus,
Eubacterium, and Lactobacillus
fatty acids and ethanol are then fermented to form
acetate, carbon dioxide, hydrogen by bacteria such as
Syntrophomonas, Syntrophobacter, and
Acetobacterium
acetate, carbon dioxide and hydrogen are then converted
to methane by methanogenic bacteria
(Methanobrevibacter, Methanomicrobium,
Methanogenium, Methanobacterium,
Methanococcus, Methanospirillum)
after 2-4 weeks, the non-degradable material (still
called sludge) is settled, then dried and burned, buried or
used for fetlilizer (if it doesn't contain a lot of heavy
metals or other toxic materials)
tertiary
treatment - removes environmentally-damaging substances
including potential pathogens and eutrophic agents
(especially inorganic nutrients generated during secondary
treatment) heavy metals, and hard-to-digest organic compounds)
precipitation and filtration remove most of the
inorganics, especially nitrate and phosphate
chlorination
eliminates any remaining harmful microbes
Drinking Water Treatment
derived from rain,
stored as surface or ground water
purified by a
combination of methods that include:
filtration or sedimentation (in settling basin) -
removes sedimentable solids (optional)
coagulation (floculation) - addition of alum or lime
aggregates suspended solids (microbes, organic matter, toxic
contaminants) which are then removed by sedimentation in a
settling basin
filtration - removes turbidity and up to 99% of
remaining microbes
carbon treatment - removes unpleasant odor, taste,
and coloration (optional)
disinfection - 0.2 to 0.3 parts per million (ppm)
free chlorine or ozone kills pathogens
fluoridation - 1 ppm of fluoride helps prevent tooth
decay in children (optional)
indicators of fecal
contamination (problems)
indicators are allochthonous (introduced) organisms which
are constantly being added to natural environments, but do not
always survive due to maladaptation to the extremes of
temperature, nutrients, competition, Bdellovibrio predation,
etc.
Vibrio cholerae has been clearly shown to be present
in the natural environment in a viable but nonculturable form -
perhaps the same is true of other microbes
Biodegradation and
Bioremediation
Bioremediation is also called Biodegradation Enhancement and
includes any purposful use of microbes to degrade unwanted substances
in the environment
natural products
petroleum - certain bacteria (some cyanobacteria,
pseudomonads, corynebacteria, mycobacteria), green algae and
fungi (several molds and yeasts) oxidize hydrocarbons at
aerobic water/oil interfaces (with optimal conditions, up to
80% removal within 1 year after a spill)
biodegradable plastics
photobiodegradable - structure of polymer altered
by UV light in sunlight so that it is now amenable to
biodegradation
biochemically biodegradable starch-linked
polymers - starch-digesting bacteria in soil attack the
starch, releasing polymer fragments which are degraded by
other microbes
xenobiotics - chemically
synthesized compounds not found in nature (pesticides, synthetic
polymers, etc.) and thus would seem unlikely to be degradable by
naturally existing microorganisms; these
products tend to be persistent in nature, and many nations are
working to ban the use of many of them; microbes that can
degrade xenobiotics are rather diverse and typically include both
bacteria and fungi
PCBs - certain Pseudomonas species have been
engineered to accelerate breakdown of polychlorinated biphenyls
(formerly used by electric industry as transformer
insulation)
PAHs (poly aromatic hydrocarbons) can be difficult
to degrade, but there are microbes in the environment that can
accomplish this task, especially when working together
pesticides - herbicides, insecticides and fungicides
these are typically rather complex molecules
some xenobiotics are good carbon sources and electron
donors for soil microbes, so they are more readily degraded
than others
other xenobiotics, such as chlorinated insecticides, are
recalcitrant to degradation; thus they have rather long
persistence times in the environment
lindane - 3 years for 75-100% disappearance
DDT - 4 years for 75-100% disappearance
chlordane - 5 years for 75-100% disappearance
to degrade these xenobiotics, microbes may employ
co-metabolism, in which an organic material other than the
xenobiotic is used as the primary energy source and the
xenobiotic is degraded as a secondary process
Genetically
engineered microbes in bioremediation
Microbes can be "engineered" to carry out the
biochemical processes needed for bioremediation
concerns about long-term the effects of
genetically engineered microbes on the environment are
manifold
fate of genetically engineered microbes is
similar to that of other allochthonous organisms, but even
more extreme because they are typically engineered to
require nutrients, etc. not naturally present in the
environments into which they may be introduced ... so they
will die out when the material they were engineered to
degrade has been removed from the area
Naturally-occurring microbes bioremediate just as well
as engineered microbes in many cases ... it is important to
adjust environmental conditions to favor their growth,
however
