The document discusses biological treatment as a method for removing contaminants from wastewater. It describes how bacteria and microorganisms break down organic materials through assimilation. There are various physical, chemical, and biological treatment methods outlined, with biological treatment being the focus. The key types of biological treatment systems discussed are activated sludge treatment, trickling filtration, and constructed wetlands. The document provides details on the process, equipment, advantages, and output quality of biological wastewater treatment.

1. Biological ETP

2. Introduction:
There is a growing need for awareness regarding pollution
control systems in various industries. Surprisingly, most
operators of effluent treatment plants (ETP)/common effluent
treatment plants (CETP) are ignorant of basic engineering and
microbiological aspects of biological treatment of effluent. The
presence of organic substrates in wastewater necessitates
biological treatment. Very high BOD levels are combated with
up-flow anaerobic sludge blanker (UASB) treatment followed
by activated sludge process (ASP); moderate or low levels of
BOD are combated with only ASP.

3. Biological Treatment
Biological treatment is the use of bacteria and other
microorganisms to remove contaminants by
assimilating them has long been a mainstay of
wastewater treatment in the chemical process
industries (CPI). Because they are effective and
widely used, many bio- logical-treatment options
are available today. They are, however, not all
created equal, and the decision to install a
biological-treatment system requires ample
thought.

4. Some Physical, Chemical and Biological Wastewater
Treatment Methods
Physical
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Sedimentation (Clarification)
Screening
Aeration
Filtration
Flotation and Skimming
Degassification
Equalization

5. Chemical
Chlorination
Ozonation
Neutralization
Coagulation
Adsorption
Ion Exchange

6. Biological
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Aerobic
Activated Sludge Treatment Methods
Trickling Filtration
Oxidation Ponds
Lagoons
Aerobic Digestion
Anaerobic
Anaerobic Digestion
Septic Tanks
Lagoons

7. Biological
Chemical
Physical
Sedimentation
(Clarification)
Aerobic
Chlorination
Activated Sludge
Ozonation
Treatment Methods
Neutralization
Screening
Coagulation
Aeration
Trickling Filtration
Oxidation Ponds
Lagoons
Aerobic Digestion
Anaerobic
Anaerobic Digestion
Septic Tanks
Lagoons
Adsorption
Filtration
Ion Exchange
Flotation and
Skimming
Degassification
Equalization

8. Biological Treatment Process
The basic units needed for biological treatment are: screening; an equalization unit; a
pH control unit; an aeration unit; and a settling unit. A sludge dewatering unit may also
be included. Biological treatment plants require the presence of microorganisms that
are adapted to degrade the components of the effluent to be treated. Textile industry
waste will not contain suitable microorganisms so these must be added to the ETP
when it is set up. Traditionally in South Asia cow dung is used as a source of
microorganisms. While it may be useful to use cow dung it is unlikely to be the best
source of microbes for treatment of textile waste. If possible new reactors (either
activated sludge or fixed film systems) should be set up using activated sludge from an
existing ETP, preferably one treating a similar waste. If this is not possible polluted river
water is likely to be a good source of suitable microorganisms and can be used
together with cow dung or activated sludge. It is likely to take several months for the
microbial population to establish itself and successful treatment to result.

9. Figure : Typical Flow Diagram of a Biological Treatment Plant in Bangladesh

10. Untreated
effluent
Untreated effluent discharge from Textile dyeing & finishing
industry

11. Sedimentation
Tank
Dry Sludge

12. Sedimentation
Tank
Aeration Tank

13. Purpose:
The idea behind all biological methods of wastewater
treatment is to introduce contact with bacteria (cells), which
feed on the organic materials in the wastewater, thereby
reducing its BOD content. In other words, the purpose of
biological treatment is BOD reduction. Typically, wastewater
enters the treatment plant with a BOD higher than 200
mg/L, but primary settling has already reduced it to about
150 mg/L by the time it enters the biological component of
the system. It needs to exit with a BOD content no higher
than about 20-30 mg/L, so that after dilution in the nearby
receiving water body (river, lake),the BOD is less than 2-3
mg/L. Thus, the biological treatment needs to accomplish a
6-fold decrease in BOD.

14. Principle:
Simple bacteria (cells) eat the organic material present in the
wastewater. Through their metabolism, the organic material is
transformed into cellular mass, which is no longer in solution
but can be precipitated at the bottom of a settling tank or
retained as slime on solid surfaces or vegetation in the
system. The water exiting the system is then much clearer
than it entered it. A key factor is the operation of any
biological system is an adequate supply of oxygen.
Indeed, cells need not only organic material as food but also
oxygen to breathe, just like humans. Without an adequate
supply of oxygen, the biological degradation of the waste is
slowed down, thereby requiring a longer residency time of the
water in the system. For a given flow rate of water to be
treated, this translates into a system with a larger volume
and thus taking more space.

15. Advantages:
Like all biological systems, operation takes place at ambient
temperature. There is no need to heat or cool the water, which
saves on energy consumption. Because wastewater treatment
operations take much space, they are located outdoor, and
this implies that the system must be able to operate at
seasonally varying temperatures. Cells come in a mix of many
types, and accommodation to a temperature change is simply
accomplished by self adaptation of the cell population.
Similarly, a change in composition of the organic material (due
to people’s changing activities) leads to a spontaneous change
in cell population, with the types best suited to digest the new
material growing in larger numbers than other cell types.

17. Types of equipment for biological treatment:
There are two broad types of biological wastewater treatment:
those that include mechanical means to create contact between
wastewater, cells and oxygen, and those than don’t.
a) With mechanical means
b)
Without mechanical means

18. With mechanical means
1. Activated sludge
2. Trickling filter
3. Biological contactor

19. 1. Activated sludge:
This is the most common type. It consists in a set of two basins. In
the first, air is pumped through perforated pipes at the bottom of
the basin, air rises through the water in the form of many small
bubbles. These bubbles accomplish two things: they provide
oxygen form the air to the water and create highly turbulent
conditions that favor intimate contact between cells, the organic
material in the water and oxygen. The second basin is a settling
tank, where water flow is made to be very quiet so that the cellular
material may be removed by gravitational settling. Some of the
cell material collected at the bottom is captured and fed back into
the first basin to seed the process. The rest is treated
anaerobically (= without oxygen) until it is transformed into a
compost-type material (like soil). The cost of an activated-sludge
system is chiefly due to the energy required to pump air at high
pressure at the bottom of the aerator tank (to overcome the
hydrostatic pressure of the water). Another disadvantage is that

21. 2. Trickling
filter:
A trickling filter consists in a bed of fist-size rocks over which the wastewater is gently sprayed
by a rotating arm. Slime (fungi, algae) develops on the rock surface, growing by intercepting
organic material from the water as it trickles down. Since the water layer passing over the rocks
makes thin sheets, there is good contact with air and cells are effectively oxygenated. Worms
and insects living in this “ecosystem” also contribute to removal of organic material from the
water. The slime periodically slides off the rocks and is collected at the bottom of the
system, where it is removed. Water needs to be trickled several times over the rocks before it is
sufficiently cleaned. Multiple spraying also provides a way to keep the biological slimes from
drying out in
hours of low-flow conditions (ex. at night).

23. 3. Biological contactor:
This is essentially a variation on the trickling filter, with the difference being that solid
material on which slime grows is brought to the water rather than water being brought to
it. Rotating disks alternate exposure between air and water.

25. b) Without
mechanical means:
The wastewater is made to flow by gravity through a specially constructed
wetland. There, the water is brought into close contact with vegetation (ex.
reeds), which acts as a5 biological filter to the water. The organic material in the
wastewater is used as nutrient by the plants. Oxygen supply is passively
accomplished by surface aeration (contact with oxygen of the atmosphere).
Since water flow is slow in such system, to give ample time for the biological
activity to take place, there is almost no turbulence in the water and reaeration
is weak. Compared to mechanical systems, constructed wetlands occupy far
more real estate, but they may be aesthetically pleasing, especially if they are
well integrated in the local landscape. They emit no odor, but people should stay
away because of the danger posed by pathogens. Constructed wetlands have
also the least energy requirement. Energy is only needed to pump the
wastewater to the entrance of the system, from where gravity and biology do
the rest. A major disadvantage, however, is the highly reduced performance
during winter, especially in regions where ground freezes during some of
the winter months.

26. Output Effluent Quality
Evidence shows that output quality from biological treatment can satisfy
the national standards for most of the required parameters except
colour. According to Metcalf & Eddy (2003) a properly designed
biological ETP can efficiently satisfy BOD, pH, TSS, oil and grease
requirements. However, as already mentioned, the compounds in
industrial wastewater may be toxic to the microorganisms so
pretreatment may be necessary. Similarly most dyes are complex
chemicals and are difficult for microbes to degrade so there is usually
very little colour removal.