Bushra Khatoon

Subject: Environmental biotechnology

Year: 4th (Semester: 7th)
Unit: II
Topic: Biological waste water treatment and Biogas production

Biological wastewater treatment method, also known as the conventional method, is a common and
widely used method of treatment. It takes into account biodegradation bleaching by taking aid of
several micro-organisms, fungi, bacteria, yeasts, and algae. In biological waste water treatment,
wastewater is treated with microorganisms instead of chemicals in order to prevent the adverse effects
caused by chemical treatment of wastewater such as chemical accumulation in water bodies or algal
blooming.
In biological wastewater treatment, many aerobic as well as anaerobic microorganisms can be used in
various methods to reduce different types of pollutants present in water and to reduce biochemical
oxygen demand of water. Biological wastewater treatment is an important and integral step of
wastewater treatment system and it treats wastewater coming from either residential buildings or
industries etc. It is often called as Secondary Treatment process which is used to remove any
contaminants that left over after primary treatment. This is a cheap and easy process that goes through
a combination of aerobic and anaerobic processes.

Purpose of Biological wastewater treatment

The purpose of biological wastewater treatment is to create a system that can easily collect and dispose of
decomposition results. Biological treatments are used around the world because they are more effective and
economical than many mechanical or chemical processes.

The three key wastewater treatment steps are:

1. Primary treatment
The primary step of the wastewater treatment process handles the removal of solids. This typically
happens in a large holding tank where heavier solids sink and lighter solids float. Suspended solids are
collected mechanically and the liquid is sent along to the next step in the process.

2. Secondary treatment (Biological waste water treatment)

This step is sometimes referred to as biological wastewater treatment, as it involves the use of
bacteria, microorganisms and other biological agents to break down biodegradable contaminants to
safe levels, which enables the water to be released or reused.

3. Tertiarytreatment
This third step encompasses any additional treatment that might be necessary to remove pollutants
such as heavy metals and chemical compounds that biological treatment methods cannot remove.

Factors to be considered while choosing a secondary waste water treatment method:

 The characteristics of the wastewater

 Chemical oxygen demand (COD)
 Biological oxygen demand (BOD)
 How much space and time you need to treat the water
 Sludge production
 Costs
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 Desired microbial concentration

 Desired quality of the discharge
Types of Biological wastewater treatment

Based on the process, biological treatment of waste water methods are majorly classified into two
types and are as follows:

1. Biological Aerobic Treatment (in presence of oxygen)

2. Biological Anaerobic Treatment (in absence of oxygen)

1. Biological Aerobic Treatment: Aerobic wastewater treatment is a biological process that takes
place in the presence of oxygen. It is the rapid and the most efficient biological waste treatment which
remove up to 98% of organic contaminants. This process causes effective breakdown of organic
pollutants and yields a cleaner water effluent than anaerobic treatment. Aerobic biological treatment
processes include many processes such as activated sludge process, Fixed bed bioreactors, moving
bed bioreactors, membrane bioreactors, trickling filter, aerated lagoons and oxidation ponds
etc. Activated sludge process is the most widely used process for domestic and industrial wastewater.
Aerobic biological treatment will remain efficient and stable in all conditions.

a. Activated Sludge Process: The activated sludge process is the most widely used biological waste
treatment in secondary stage of wastewater treatment. An activated sludge process refers to a multi-
chamber reactor unit that makes use of highly concentrated microorganisms to degrade organics and
remove nutrients from wastewater to produce a high-quality effluent. In this method, the sewage
containing organic matter with the microorganisms is aerated (by a mechanical aerator) in an aeration
tank. This process speeds up waste decomposition. Aeration in an activated sludge process is based on
pumping air into a tank, which promotes the microbial growth in the wastewater. The effluent from
the aeration tank containing the flocculent microbial mass, known as sludge, is separated in a settling
tank, sometimes called a secondary settler or a clarifier.

Advantages:

The activated sludge process is a very compact, low-cost and an efficient biological treatment system
for sewage/waste water treatment.

Figure: Activated sludge process

b. Fixed-bed bioreactors (FBBRs)

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Fixed-bed bioreactors or FBBRs are a technology that was developed in the 1970s and 80s. FBBRs
comprise a series of tanks with multiple chambers, packed with a porous material such as ceramic,
foam or plastic.

The wastewater flows through the various chambers, with contaminants being eaten by microbes
along the way. The design of the chambers and porous material allows FBBRs to hold more microbes
in the same space, making this a space-saving technology when compared with other options. It’s also
energy efficient, and ideal for treating wastewater at all BOD levels.

Advantages:

 Sludge disposal costs are low and FBBRs have a long lifespan too.
 Water can flow through the system without plugging or channeling.

c. Moving-bed bioreactors

Moving-bed bioreactors or MBBRs are now being used in many countries for treating both domestic
wastewater and industrial wastewater. A typical MBBR set-up comprises aeration tanks filled with
small biofilm carriers of different shapes and sizes. As the biofilm carriers are MBBRs are suspended
and moving, this allows high BOD wastewater to be treated in a smaller area.There’s usually a second
stage in the process after a MBBR is used. Excess sludge settles into a slurry which can be removed
by vacuum, or solids can be pressed into solids for removal and disposal. MBBRs are typically used
in the first step of the treatment process, or used in situations where the quality of the effluent is of
lesser importance. MBBRs are typically used to treat wastewater from food and drink factories, meat
processing or packing plants, oil refineries or petrochemical sites.

d. Membrane bioreactors (MBRs)

Membrane modules are submerged into the aeration tank, and air is used to scour the submerged
membrane to keep them clear. This is an advanced treatment method which combines conventional
activated sludge and membrane filtering to remove solids rather than depending on sedimentation.
MBRs produce a higher quality of effluent when compared with conventional activated sludge plants,
and take up far less space.

e. Trickling filters

This is the second commonly using type of aerobic treatment which is also called as percolating or
sprinkling filters. Biological trickling filters can be used to remove organic contamination from either
wastewater or air. The air or water is passed through some type of medium which will allow biofilm
to collect on its surface. This biofilm, composed of both anaerobic and aerobic bacteria, will break
down organic contamination. Gravel, foam, sand or ceramics might be used to create these systems.
These filters are more commonly found in water treatment plants but can be very effective in any
situation where keeping smells to a minimum is essential.
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Figure: Trickling filter for waste water treatment

f. Aerated Lagoons: It is one of the aerobic biological wastewater or waste treatment process. An
aerated lagoon is a treatment pond that is provided with mechanical aeration that introduces oxygen
into the pond in order to promote the biological oxidation of the wastewater. The effluent of
aeratedponds may be reused or used for recharge, but settled sludge requires a further treatment.

g. Oxidation Pond: The ponds involve an interaction between bacteria, algae and other organisms
which feed on the organic matter received from primary effluent. These ponds are also productive,
because it generates effluent that can be used for other applications. Overall the process is slow and
requires large areas of land. Typically oxidation ponds are used in areas with small populations where
land is readily available.

Advantages of aerobic wastewater treatment

 These include reduced odor (due to non-production of hydrogen sulfide or methane),

 Better nutrient removal efficacy (facilitating direct discharge into surface waters or
disinfection).
 Simple technology and facilities
 Low investment costs.

2. Biological Anaerobic Treatment: This treatment process is effectively utilized to treat high
strength waste water and it employs organisms that function in the absence of oxygen and it will
typically treat high-strength waste water to a level that will permit discharge to a municipal sewer
system. Here, the amount of sludge produced is very small when we compared to aerobic treatment.
Anaerobic treatment is a slow process and it occurs in many different stages. Anaerobic digestion is
biological process which is used in wastewater treatment plants for sludge degradation and
stabilization. Once the process is completed, the wastewater can undergo many additional treatments.

The benefits of using anaerobic bacteria and the anaerobic wastewater treatment are the production of
biomass, carbon dioxide, and methane gas as the by-products. This methane gas known as biogas
is one of the best alternative sources of energy. The biogas production is of immense significance as it
can be used to run the anaerobic wastewater treatment process itself which consumes much energy.
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The anaerobic treatment of waste consists of primarily four stages where different microorganisms act
in an oxygen-free environment and produce methane-CH4 and leads to the production of biogas. A
digester tank is a place where all the process takes place. The main goal is to produce biogas, clean-up
water before its discharge and sometimes nutrients or organic fertilizers are also produced.
The four phases includes in bio gas production: –
i) Hydrolysis – In this phase, complex organic molecules are degraded into smaller particles
constituting sugars, amino acids, and fatty acids. In other words, non-soluble biopolymers get
converted into soluble organic compounds.
ii) Acidogenesis – The acidogenic bacteria release enzymes in the acidogenesis (fermentation)phase
and converts soluble organic compounds into volatile fatty acids and carbon dioxide.
iii) Acetogenesis – The acetogenic bacteria further degrade the particles into acetic acid and H 2.
iv) Methanogenesis – Methanogenic bacteria use intermediate products from previous stages and
convert them into methane, carbon dioxide, and water.

Figure: Anaerobic waste water treatment

There are certain environmental factors and conditions which have to be maintained for the process to
be completely successful in creating methane gas. The major factors that affect the anaerobic process
are temperature, alkalinity, pH, and toxicity. Depending upon the processing capacity of microbes, the
loading rate of anaerobic reactors needs large bacterial mass and long retention time.

Advantages of anaerobic wastewater treatment

The advantages of anaerobic wastewater treatment are as follows: –

– The process itself consumes very little energy, because of no aeration equipment

– Sludge production on a COD basis is low when compared to an aerobic process

- Biogas produced during anaerobic process can be used as a source of renewable energy

-It helps significantly in sludge de-watering, waste stabilization and generally provides end product
without odour.

– The construction cost of an anaerobic system is low

– The space requirement is low compared to other systems.