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.

FLOW PROCESS

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. Methanogenicbacteria, 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)

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