aerobic reactors

ENVIRONMENTAL
BIOTECHNOLOGY
ANAEROBIC DIGESTION….AEROBIC
PROCESS…LAGOONS..
DEEPIKA R
PR14BT1008

ANAEROBIC DIGESTION
 Anaerobic digestion is a collection of processes by
which microorganisms break down biodegradable
material in the absence of oxygen.
 The process is used for industrial or domestic
purposes to manage waste and/or to produce fuels.
 Much of the fermentation used industrially to
produce food and drink products, as well as home
fermentation, uses anaerobic digestion.

 The digestion process begins with bacterial hydrolysis of
the input materials.
 Acidogenic bacteria then convert the sugars and amino
acids into carbon dioxide, hydrogen, ammonia, and
organic acids.
 These bacteria convert these resulting organic acids into
acetic acid, along with additional ammonia, hydrogen,
and carbon dioxide.
 Finally, methanogens convert these products to methane
and carbon dioxide.

ADVANTAGES
 Anaerobic digestion is widely used as a source of
renewable energy.
 The process produces a biogas, consisting of
methane, carbon dioxide and traces of other
‘contaminant’ gases.
 This biogas can be used directly as fuel, in
combined heat and power gas engines or upgraded
to natural gas-quality biomethane.
 The nutrient-rich digestate also produced can be
used as fertilizer.

CONFIGURATION OF DIGESTERS
 Anaerobic digesters can be designed and
engineered to operate using a number of different
process configurations:
 Batch or continuous
 Temperature: Mesophilic or thermophilic
 Solids content: High solids or low solids
 Complexity: Single stage or multistage

Batch or continuous:
 Anaerobic digestion can be performed as a batch
process or a continuous process.
 In a batch system biomass is added to the reactor at the
start of the process. The reactor is then sealed for the
duration of the process.
 In a typical scenario, biogas production will be formed
with a normal distribution pattern over time. Operators
can use this fact to determine when they believe the
process of digestion of the organic matter has completed.
There can be severe odour issues if a batch reactor is
opened and emptied before the process is well
completed.

 A more advanced type of batch approach has
limited the odour issues by integrating anaerobic
digestion with in-vessel composting. In this
approach inoculation takes place through the use of
recirculated degasified percolate. After anaerobic
digestion has completed, the biomass is kept in the
reactor which is then used for in-vessel composting
before it is opened

CONTINUOUS
 As the batch digestion is simple and requires less
equipment and lower levels of design work, it is
typically a cheaper form of digestion. Using more
than one batch reactor at a plant can ensure
constant production of biogas.
 In continuous digestion processes, organic matter
is constantly added (continuous complete mixed) or
added in stages to the reactor (continuous plug
flow; first in – first out).

 Here, the end products are constantly or periodically
removed, resulting in constant production of biogas. A
single or multiple digesters in sequence may be used.
 Examples of this form of anaerobic digestion include
continuous stirred-tank reactors, upflow anaerobic sludge
blankets, expanded granular sludge beds and internal
circulation reactors

OPERATING TEMPERATURE
 The two conventional operational temperature
levels for anaerobic digesters determine the
species of methanogens in the digesters:
 Mesophilic digestion takes place optimally around
30 to 38 °C, or at ambient temperatures between
20 and 45 °C, where mesophiles are the primary
microorganism present.
 Thermophilic digestion takes place optimally around
49 to 57 °C, or at elevated temperatures up to 70
°C, where thermophiles are the primary
microorganisms present.

DIGESTER COMPLEXITY
 Digestion systems can be configured with different
levels of complexity:
 In a single-stage digestion system (one-stage), all
of the biological reactions occur within a single,
sealed reactor or holding tank.
 Using a single stage reduces construction costs,
but results in less control of the reactions occurring
within the system.
 Acidogenic bacteria, through the production of
acids, reduce the pH of the tank. Methanogenic
bacteria, as outlined earlier, operate in a strictly
defined pH range

 Therefore, the biological reactions of the different species
in a single-stage reactor can be in direct competition with
each other. Another one-stage reaction system is an
anaerobic lagoon.
 These lagoons are pond-like, earthen basins used for the
treatment and long-term storage of manures.
 Here the anaerobic reactions are contained within the
natural anaerobic sludge contained in the pool.

 In a two-stage digestion system (multistage),
different digestion vessels are optimised to bring
maximum control over the bacterial communities
living within the digesters. Acidogenic bacteria
produce organic acids and more quickly grow and
reproduce than methanogenic bacteria.
Methanogenic bacteria require stable pH and
temperature to optimise their performance.

RESIDENCE TIME
 The residence time in a digester varies with the
amount and type of feed material, the configuration
of the digestion system, and whether it be one-
stage or two-stage.
 In the case of single-stage thermophilic digestion,
residence times may be in the region of 14 days,
which, compared to mesophilic digestion, is
relatively fast. In this event, digestate exiting the
system will be darker in colour and will typically
have more odour.
 In two-stage mesophilic digestion, residence time
may vary between 15 and 40 days

INHIBITORS INVOLVED IN DIGESTION
 The anaerobic digestion process can be inhibited
by several compounds, affecting one of more of the
bacterial groups responsible for the different
organic matter degradation steps.
 The degree of the inhibition depends, among other
factors, on the concentration of the inhibitor in the
digester. Potential inhibitors are ammonia, sulfide,
light metal ions (Na, K, Mg, Ca, Al), heavy metals,
some organics (chlorophenols, halogenated
aliphatics, N-substituted aromatics, long chain fatty
acids), etc.

APPLICATION
 Using anaerobic digestion technologies can help to
reduce the emission of greenhouse gases in a number
of key ways:
 Replacement of fossil fuels
 Reducing or eliminating the energy footprint of waste
treatment plants
 Reducing methane emission from landfills
 Displacing industrially produced chemical fertilizers
 Reducing vehicle movements
 Reducing electrical grid transportation losses
 Reducing usage of LP Gas for cooking

AEROBIC TREATMENT SYSTEM
 An aerobic treatment system or ATS, often called
(incorrectly) an aerobic septic system, is a small
scale sewage treatment system similar to a septic
tank system, but which uses an aerobic process for
digestion rather than just the anaerobic process
used in septic systems.
 These systems are commonly found in rural areas
where public sewers are not available, and may be
used for a single residence or for a small group of
homes.

 Unlike the traditional septic system, the aerobic
treatment system produces a high quality
secondary effluent, which can be sterilized and
used for surface irrigation. This allows much greater
flexibility in the placement of the leach field, as well
as cutting the required size of the leach field by as
much as half.

STAGES OF TREATMENT
 The ATS process generally consists of the following phases:
 Pre-treatment stage to remove large solids and other
undesirable substances from the wastewater; this stage acts
much like a septic system, and an ATS may be added to an
existing septic tank to further process the primary effluent.
 Aeration stage, where the aerobic bacteria digest the
biological wastes in the wastewater.
 Settling stage to allow any undigested solids to settle. This
forms a sludge which must be periodically removed from the
system.
 Disinfecting stage, where chlorine or similar disinfectant is
mixed with the water, to produce an antiseptic output.

TYPES OF AEROBIC TREATMENT SYSTEMS
 Small scale aerobic systems generally use one of
two designs, fixed-film systems, or continuous flow,
suspended growth aerobic systems (CFSGAS).
 The pre-treatment and effluent handling are similar
for both types of systems, and the difference lies in
the aeration stage

FIXED FILM SYSTEM
 Fixed film systems use a porous medium which
provides a bed to support the biomass film that
digests the waste material in the wastewater.
 Designs for fixed film systems vary widely, but fall
into two basic categories (though some systems
may combine both methods).
 The first is a system where the media is moved
relative to the wastewater, alternately immersing
the film and exposing it to air,
 while the second uses a stationary media, and
varies the wastewater flow so the film is alternately
submerged and exposed to air.

CONTINUOUS FLOW, SUSPENDED GROWTH
AEROBIC SYSTEMS
 CFSGAS systems, as the name imply, are designed
to handle continuous flow, and do not provide a bed
for a bacterial film, relying rather on bacteria
suspended in the wastewater.
 The suspension and aeration are typically provided
by an air pump, which pumps air through the
aeration chamber, providing a constant stirring of
the wastewater in addition to the oxygenation.
 A medium to promote fixed film bacterial growth
may be added to some systems designed to handle
higher than normal levels of biomass in the
wastewater.

RETROFIT OR PORTABLE AEROBIC SYSTEMS
 Another increasingly common use of aerobic
treatment is for the remediation of failing or failed
anaerobic septic systems, by retrofitting an existing
system with an aerobic feature.
 This class of product, known as aerobic
remediation, is designed to remediate biologically
failed and failing anaerobic distribution systems by
significantly reducing the biochemical oxygen
demand (BOD5) and total suspended solids (TSS)
of the effluent.
 The reduction of the BOD5 and TSS reverses the
developed bio-mat. Further, effluent with high
dissolved oxygen and aerobic bacteria flow to the
distribution component and digest the bio-mat.

LAGOONS
 An aerated lagoon or aerated basin is a holding
and/or treatment pond provided with artificial
aeration to promote the biological oxidation of
wastewaters.
 There are many other biological processes for
treatment of wastewaters, for example activated
sludge, trickling filters, rotating biological contactors
and biofilters.
 They all have in common the use of oxygen (or air)
and microbial action to biotreat the pollutants in
wastewaters.

TYPES OF AERATED LAGOONS OR BASINS
 Suspension mixed lagoons, where there is less
energy provided by the aeration equipment to keep
the sludge in suspension.
 Facultative lagoons, where there is insufficient
energy provided by the aeration equipment to keep
the sludge in suspension and solids settle to the
lagoon floor. The biodegradable solids in the settled
sludge then degrade anaerobically.

SUSPENSION MIXED LAGOONS
 Suspension mixed lagoons flow through activated
sludge systems where the effluent has the same
composition as the mixed liquor in the lagoon.
 Typically the sludge will have a residence time or
sludge age of 1 to 5 days.
 This means that the chemical oxygen demand
(COD) removed is relatively little and the effluent is
therefore unacceptable for discharge into receiving
waters.
 The objective of the lagoon is therefore to act as a
biologically assisted flocculator which converts the
soluble biodegradable organics in the influent to a
biomass which is able to settle as a sludge.

 There are many methods for aerating a lagoon or
basin:
 Motor-driven submerged or floating jet aerators
 Motor-driven floating surface aerators
 Motor-driven fixed-in-place surface aerators
 Injection of compressed air through submerged
diffusers

FLOATING SURFACE AERATORS
 A Typical Surface-Aerated Basin (using motor-driven
floating aerators)
 Ponds or basins using floating surface aerators achieve
80 to 90% removal of BOD with retention times of 1 to
10 days.[5] The ponds or basins may range in depth
from 1.5 to 5.0 metres.[5]
 In a surface-aerated system, the aerators provide two
functions: they transfer air into the basins required by
the biological oxidation reactions, and they provide the
mixing required for dispersing the air and for contacting
the reactants (that is, oxygen, wastewater and
microbes).

 Typically, the floating surface aerators are rated to deliver
the amount of air equivalent to 1.8 to 2.7 kg O2/kWh.
 However, they do not provide as good mixing as is
normally achieved in activated sludge systems and
therefore aerated basins do not achieve the same
performance level as activated sludge units.
 Biological oxidation processes are sensitive to
temperature and, between 0 °C and 40 °C, the rate of
biological reactions increase with temperature. Most
surface aerated vessels operate at between 4 °C and 32
°C

SUBMERGED DIFFUSED AERATION
 Submerged diffused air is essentially a form of a
diffuser grid inside a lagoon.
 There are two main types of submerged diffused
aeration systems for lagoon applications: floating
lateral and submerged lateral.
 Both these systems utilize fine or medium bubble
diffusers to provide aeration and mixing to the
process water.
 The diffusers can be suspended slightly above the
lagoon floor or may rest on the bottom. Flexible
airline or weighted air hose supplies air to the
diffuser unit from the air lateral (either floating or
submerged)

aerobic reactors

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aerobic reactors